WorldWideScience

Sample records for ac quantum transport

  1. Modeling A.C. Electronic Transport through a Two-Dimensional Quantum Point Contact

    Energy Technology Data Exchange (ETDEWEB)

    Aronov, I.E.; Beletskii, N.N.; Berman, G.P.; Campbell, D.K.; Doolen, G.D.; Dudiy, S.V.

    1998-12-07

    We present the results on the a.c. transport of electrons moving through a two-dimensional (2D) semiconductor quantum point contact (QPC). We concentrate our attention on the characteristic properties of the high frequency admittance ({omega}{approximately}0 - 50 GHz), and on the oscillations of the admittance in the vicinity of the separatrix (when a channel opens or closes), in presence of the relaxation effects. The experimental verification of such oscillations in the admittance would be a strong confirmation of the semi-classical approach to the a.c. transport in a QPC, in the separatrix region.

  2. Dynamics of energy transport and entropy production in ac-driven quantum electron systems

    Science.gov (United States)

    Ludovico, María Florencia; Moskalets, Michael; Sánchez, David; Arrachea, Liliana

    2016-07-01

    We analyze the time-resolved energy transport and the entropy production in ac-driven quantum coherent electron systems coupled to multiple reservoirs at finite temperature. At slow driving, we formulate the first and second laws of thermodynamics valid at each instant of time. We identify heat fluxes flowing through the different pieces of the device and emphasize the importance of the energy stored in the contact and central regions for the second law of thermodynamics to be instantaneously satisfied. In addition, we discuss conservative and dissipative contributions to the heat flux and to the entropy production as a function of time. We illustrate these ideas with a simple model corresponding to a driven level coupled to two reservoirs with different chemical potentials.

  3. AC transport in p-Ge/GeSi quantum well in high magnetic fields

    Energy Technology Data Exchange (ETDEWEB)

    Drichko, I. L.; Malysh, V. A.; Smirnov, I. Yu.; Golub, L. E.; Tarasenko, S. A. [A.F. Ioffe Physical Technical Institute of Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation); Suslov, A. V. [National High Magnetic Field Laboratory, Tallahassee, FL 32310 (United States); Mironov, O. A. [Warwick SEMINANO R and D Center, University of Warwick Science Park, Coventry CV4 7EZ (United Kingdom); Kummer, M.; Känel, H. von [Laboratorium für Festkörperphysik ETH Zürich, CH-8093 Zürich (Switzerland)

    2014-08-20

    The contactless surface acoustic wave technique is implemented to probe the high-frequency conductivity of a high-mobility p-Ge/GeSi quantum well structure in the regime of integer quantum Hall effect (IQHE) at temperatures 0.3–5.8 K and magnetic fields up to 18 T. It is shown that, in the IQHE regime at the minima of conductivity, holes are localized and ac conductivity is of hopping nature and can be described within the “two-site” model. The analysis of the temperature and magnetic-field-orientation dependence of the ac conductivity at odd filing factors enables us to determine the effective hole g-factor, |g{sub zz}|≈4.5. It is shown that the in-plane component of the magnetic field leads to a decrease in the g-factor as well as increase in the cyclotron mass, which is explained by orbital effects in the complex valence band of germanium.

  4. Spin Polarized Transport in an AC-Driven Quantum Curved Nanowire

    Directory of Open Access Journals (Sweden)

    Walid A. Zein

    2011-01-01

    Full Text Available Using the effective-mass approximation method, and Floquet theory, we study the spin transport characteristics through a curved quantum nanowire. The spin polarization, P, and the tunneling magnetoresistance, TMR, are deduced under the effect of microwave and infrared radiations of wide range of frequencies. The results show an oscillatory behavior of both the spin polarization and the tunneling magnetoresistance. This is due to Fano-type resonance and the interplay between the strength of spin-orbit coupling and the photons in the subbands of the one-dimensional nanowire. The present results show that this investigation is very important, and the present device might be used to be a sensor for small strain in semiconductor nanostructures and photodetector.

  5. Ac response of a coupled double quantum dot

    Institute of Scientific and Technical Information of China (English)

    Xu Jie; W.Z. Shangguan; Zhan Shi-Chang

    2005-01-01

    The effect of phase-breaking process on the ac response of a coupled double quantum dot is studied in this paper based on the nonequilibrium Green function formalism. A general expression is derived for the ac current in the presence of electron-phonon interaction. The ac conductance is numerically computed and the results are compared with those in [Anatram M P and Datts S 1995 Phys. Rev. B 51 7632]. Our results reveal that the inter-dot electron tunnelling interplays with that between dots and electron reservoirs, and contributes prominently to the ac current when inter-dot tunnelling coupling is much larger than the tunnelling coupling between dots and electron reservoirs. In addition, the phase-breaking process is found to have a significant effect on the ac transport through the coupled double dot.

  6. A dry-cooled AC quantum voltmeter

    Science.gov (United States)

    Schubert, M.; Starkloff, M.; Peiselt, K.; Anders, S.; Knipper, R.; Lee, J.; Behr, R.; Palafox, L.; Böck, A. C.; Schaidhammer, L.; Fleischmann, P. M.; Meyer, H.-G.

    2016-10-01

    The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65 mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 1010.

  7. Vibration enhanced quantum transport

    CERN Document Server

    Semião, F L; Milburn, G J

    2009-01-01

    In this paper, we study the role of a collective vibrational motion in the phenomenon of electronic energy transfer (EET) between chromophores with different electronic transition frequencies. Previous experimental work on EET in conjugated polymer samples has suggested that the common structural framework of the macromolecule introduce correlations in the energy gap fluctuations which cause coherent EET. We present a simple model describing the coupling between the chromophores and a common vibrational mode, and find that vibration can indeed lead to an enhancement in the transport of excitations across the quantum network. Furthermore, in our model phase information is partially retained in the transfer process from a donor to an acceptor, as experimentally demonstrated in the conjugated polymer system. Consequently, this mechanism of vibration enhanced quantum transport might find applications in quantum information transfer of qubit states or entanglement.

  8. Crossover of the Hall-voltage distribution in AC quantum Hall effect

    OpenAIRE

    Akera, H.

    2011-01-01

    The distribution of the Hall voltage induced by low-frequency AC current is studied theoretically in the incoherent linear transport of quantum Hall systems. It is shown that the Hall-voltage distribution makes a crossover from the uniform distribution to a concentrated-near-edges distribution as the frequency is increased or the diagonal conductivity is decreased. This crossover is also reflected in the frequency dependence of AC magnetoresistance.

  9. Crossover of the Hall-voltage distribution in AC quantum Hall effect

    OpenAIRE

    Akera, Hiroshi

    2009-01-01

    The distribution of the Hall voltage induced by low-frequency AC current is studied theoretically in the incoherent linear transport of quantum Hall systems. It is shown that the Hall-voltage distribution makes a crossover from the uniform distribution to a concentrated-near-edges distribution as the frequency is increased or the diagonal conductivity is decreased. This crossover is also reflected in the frequency dependence of AC magnetoresistance.

  10. Noise-induced quantum transport

    OpenAIRE

    Ghosh, Pulak Kumar; Barik, Debashis; Ray, Deb Shankar

    2004-01-01

    We analyze the problem of directed quantum transport induced by external exponentially correlated telegraphic noise. In addition to quantum nature of the heat bath, nonlinearity of the periodic system potential brings in quantum contribution. We observe that quantization, in general, enhances classical current at low temperature, while the differences become insignificant at higher temperature. Interplay of quantum diffusion and quantum correction to system potential is analyzed for various r...

  11. Spin-polarized currents in double and triple quantum dots driven by ac magnetic fields

    OpenAIRE

    Busl, Maria; Platero, Gloria

    2010-01-01

    We analyze transport through both a double quantum dot and a triple quantum dot with inhomogeneous Zeeman splittings in the presence of crossed dc and ac magnetic fields. We find that strongly spin-polarized current can be achieved by tuning the relative energies of the Zeeman-split levels of the dots, by means of electric gate voltages: depending on the energy level detuning, the double quantum dot works either as spin-up or spin-down filter. We show that a triple quantum dot in series under...

  12. BITLLES: Electron Transport Simulation with Quantum Trajectories

    CERN Document Server

    Albareda, Guillermo; Benali, Abdelilah; Alarcón, Alfonso; Moises, Simeon; Oriols, Xavier

    2016-01-01

    After the seminal work of R. Landauer in 1957 relating the electrical resistance of a conductor to its scattering properties, much progress has been made in our ability to predict the performance of electron devices in the DC (stationary) regime. Computational tools to describe their dynamical behavior (including the AC, transient and noise performance), however, are far from being as trustworthy as would be desired by the electronic industry. While there is no fundamental limitation to correctly modeling the high-frequency quantum transport and its fluctuations, certainly more careful attention must be paid to delicate issues such as overall charge neutrality, total current conservation, or the back action of the measuring apparatus. In this review, we will show how the core ideas behind the Bohmian formulation of quantum mechanics can be exploited to design an efficient Monte Carlo algorithm that provides a quantitative description of electron transport in open quantum systems. By making the most of traject...

  13. Quantum Transport in Semiconductor Nanostructures

    OpenAIRE

    Beenakker, C. W. J.; Houten, van, H.

    2004-01-01

    I. Introduction (Preface, Nanostructures in Si Inversion Layers, Nanostructures in GaAs-AlGaAs Heterostructures, Basic Properties). II. Diffusive and Quasi-Ballistic Transport (Classical Size Effects, Weak Localization, Conductance Fluctuations, Aharonov-Bohm Effect, Electron-Electron Interactions, Quantum Size Effects, Periodic Potential). III. Ballistic Transport (Conduction as a Transmission Problem, Quantum Point Contacts, Coherent Electron Focusing, Collimation, Junction Scattering, Tunn...

  14. Effect of the magnetic material on AC losses in HTS conductors in AC magnetic field carrying AC transport current

    Science.gov (United States)

    Wan, Xing-Xing; Huang, Chen-Guang; Yong, Hua-Dong; Zhou, You-He

    2015-11-01

    This paper presents an investigation on the AC losses in several typical superconducting composite conductors using the H-formulation model. A single superconducting strip with ferromagnetic substrate or cores and a stack of coated conductors with ferromagnetic substrates are studied. We consider all the coated conductors carrying AC transport currents and simultaneously exposed to perpendicular AC magnetic fields. The influences of the amplitude, frequency, phase difference and ferromagnetic materials on the AC losses are investigated. The results show that the magnetization losses of single strip and stacked strips have similar characteristics. The ferromagnetic substrate can increase the magnetization loss at low magnetic field, and decrease the loss at high magnetic field. The ferromagnetic substrate can obviously increase the transport loss in stacked strips. The trends of total AC losses of single strip and stacked strips are similar when they are carrying current or exposed to a perpendicular magnetic field. The effect of the frequency on the total AC losses of single strip is related to the amplitude of magnetic field. The AC losses decrease with increasing frequency in low magnetic field region while increase in high magnetic field region. As the phase difference changes, there is a periodic variation for the AC losses. Moreover, when the strip is under only the transport current and magnetic field, the ferromagnetic cores will increase the AC losses for large transport current or field.

  15. Coherent transport through interacting quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Hiltscher, Bastian

    2012-10-05

    The present thesis is composed of four different works. All deal with coherent transport through interacting quantum dots, which are tunnel-coupled to external leads. There a two main motivations for the use of quantum dots. First, they are an ideal device to study the influence of strong Coulomb repulsion, and second, their discrete energy levels can easily be tuned by external gate electrodes to create different transport regimes. The expression of coherence includes a very wide range of physical correlations and, therefore, the four works are basically independent of each other. Before motivating and introducing the different works in more detail, we remark that in all works a diagrammatic real-time perturbation theory is used. The fermionic degrees of freedom of the leads are traced out and the elements of the resulting reduced density matrix can be treated explicitly by means of a generalized master equation. How this equation is solved, depends on the details of the problem under consideration. In the first of the four works adiabatic pumping through an Aharonov-Bohm interferometer with a quantum dot embedded in each of the two arms is studied. In adiabatic pumping transport is generated by varying two system parameters periodically in time. We consider the two dot levels to be these two pumping parameters. Since they are located in different arms of the interferometer, pumping is a quantum mechanical effect purely relying on coherent superpositions of the dot states. It is very challenging to identify a quantum pumping mechanism in experiments, because a capacitive coupling of the gate electrodes to the leads may yield an undesired AC bias voltage, which is rectified by a time dependent conductance. Therefore, distinguishing features of these two transport mechanisms are required. We find that the dependence on the magnetic field is the key feature. While the pumped charge is an odd function of the magnetic flux, the rectified current is even, at least in

  16. Quantum transport in carbon nanotubes

    DEFF Research Database (Denmark)

    Laird, Edward A.; Kuemmeth, Ferdinand; Steele, Gary A.;

    2015-01-01

    Carbon nanotubes are a versatile material in which many aspects of condensed matter physics come together. Recent discoveries, enabled by sophisticated fabrication, have uncovered new phenomena that completely change our understanding of transport in these devices, especially the role of the spin...... and valley degrees of freedom. This review describes the modern understanding of transport through nanotube devices. Unlike conventional semiconductors, electrons in nanotubes have two angular momentum quantum numbers, arising from spin and from valley freedom. We focus on the interplay between the two....... In single quantum dots defined in short lengths of nanotube, the energy levels associated with each degree of freedom, and the spin-orbit coupling between them, are revealed by Coulomb blockade spectroscopy. In double quantum dots, the combination of quantum numbers modifies the selection rules of Pauli...

  17. Charge transport and ac response under light illumination in gate-modulated DNA molecular junctions

    Science.gov (United States)

    Zhang, Yan; Zhu, Wen-Huan; Ding, Guo-Hui; Dong, Bing; Wang, Xue-Feng

    2015-05-01

    Using a two-strand tight-binding model and within nonequilibrium Green's function approach, we study charge transport through DNA sequences {{(GC)}{{NGC}}} and {{(GC)}1}{{(TA)}{{NTA}}}{{(GC)}3} sandwiched between two Pt electrodes. We show that at low temperature DNA sequence {{(GC)}{{NGC}}} exhibits coherent charge carrier transport at very small bias, since the highest occupied molecular orbital in the GC base pair can be aligned with the Fermi energy of the metallic electrodes by a gate voltage. A weak distance dependent conductance is found in DNA sequence {{(GC)}1}{{(TA)}{{NTA}}}{{(GC)}3} with large NTA. Different from the mechanism of thermally induced hopping of charges proposed by the previous experiments, we find that this phenomenon is dominated by quantum tunnelling through discrete quantum well states in the TA base pairs. In addition, ac response of this DNA junction under light illumination is also investigated. The suppression of ac conductances of the left and right lead of DNA sequences at some particular frequencies is attributed to the excitation of electrons in the DNA to the lead Fermi surface by ac potential, or the excitation of electrons in deep DNA energy levels to partially occupied energy levels in the transport window. Therefore, measuring ac response of DNA junctions can reveal a wealth of information about the intrinsic dynamics of DNA molecules.

  18. Quantum thermal transport in stanene

    Science.gov (United States)

    Zhou, Hangbo; Cai, Yongqing; Zhang, Gang; Zhang, Yong-Wei

    2016-07-01

    By way of the nonequilibrium Green's function simulations and analytical expressions, the quantum thermal conductance of stanene is studied. We find that, due to the existence of Dirac fermion in stanene, the ratio of electron thermal conductance and electric conductance becomes a chemical-potential-dependent quantity, violating the Wiedemann-Franz law. This finding is applicable to any two-dimensional (2D) materials that possess massless Dirac fermions. In strong contrast to the negligible electronic contribution in graphene, surprisingly, the electrons and phonons in stanene carry a comparable heat current. The unusual behaviors in stanene widen our knowledge of quantum thermal transport in 2D materials.

  19. Quantum transport and electroweak baryogenesis

    International Nuclear Information System (INIS)

    We review the mechanism of electroweak baryogenesis. The main focus of the review lies on the development of quantum transport equations from first principles in the Kadanoff-Baym framework. We emphasize the importance of the semi-classical force that leads to reliable predictions in most cases. Besides, we discuss the status of electroweak baryogenesis in the light of recent electric dipole moment probes and collider experiments in a variety of models.

  20. Quantum transport and electroweak baryogenesis

    Energy Technology Data Exchange (ETDEWEB)

    Konstandin, Thomas

    2013-02-15

    We review the mechanism of electroweak baryogenesis. The main focus of the review lies on the development of quantum transport equations from first principles in the Kadanoff-Baym framework. We emphasize the importance of the semi-classical force that leads to reliable predictions in most cases. Besides, we discuss the status of electroweak baryogenesis in the light of recent electric dipole moment probes and collider experiments in a variety of models.

  1. Quantum Pumping and Adiabatic Transport in Nanostructures

    NARCIS (Netherlands)

    Wakker, G.M.M.

    2011-01-01

    This thesis consists of a theoretical exploration of quantum transport phenomena and quantum dynamics in nanostructures. Specifically, we investigate adiabatic quantum pumping of charge in several novel types of nanostructures involving open quantum dots or graphene. For a bilayer of graphene we fin

  2. AC transport in graphene-based Fabry-Perot devices

    OpenAIRE

    Rocha, Claudia G; Torres, Luis E. F. Foa; Cuniberti, Gianaurelio

    2009-01-01

    We report on a theoretical study of the effects of time-dependent fields on electronic transport through graphene nanoribbon devices. The Fabry-P\\'{e}rot interference pattern is modified by an ac gating in a way that depends strongly on the shape of the graphene edges. While for armchair edges the patterns are found to be regular and can be controlled very efficiently by tuning the ac field, samples with zigzag edges exhibit a much more complex interference pattern due to their peculiar elect...

  3. Transport and Dissipation in Quantum Pumps

    OpenAIRE

    Avron, J. E.; Elgart, A.; Graf, G. M.; Sadun, L.

    2003-01-01

    This paper is about adiabatic transport in quantum pumps. The notion of ``energy shift'', a self-adjoint operator dual to the Wigner time delay, plays a role in our approach: It determines the current, the dissipation, the noise and the entropy currents in quantum pumps. We discuss the geometric and topological content of adiabatic transport and show that the mechanism of Thouless and Niu for quantized transport via Chern numbers cannot be realized in quantum pumps where Chern numbers necessa...

  4. AC-field-induced quantum phase transitions in density of states

    Science.gov (United States)

    Yang, Kai-Hua; Liu, Kai-Di; Wang, Huai-Yu; Qin, Chang-Dong

    2016-02-01

    We investigate the joint effects of the intralead electron interaction and an external alternating gate voltage on the time-averaged local density of states (DOSs) of a quantum dot coupled to two Luttinger-liquid leads in the Kondo regime. A rich dependence of the DOSs on the driving amplitude and intralead interaction is demonstrated. We show that the feature is quite different for different interaction strengths in the presence of the ac field. It is shown that the photon-assisted transport processes cause an additional splitting of the Kondo peak or dip, which exhibits photon-assisted single-channel (1CK) or two-channel Kondo (2CK) physics behavior. The phase transition between photon-assisted 1CK and 2CK physics occurs when the interaction strength is moderately strong. The inelastic channels associated with photon-assisted electron tunneling can dominate electron transport for weak interaction when the ac amplitude is greater than the frequency by one order of magnitude. In the limit of strong interaction the DOSs scale as a power-law behavior which is strongly affected by the ac field.

  5. Electron transport in quantum dots

    CERN Document Server

    2003-01-01

    When I was contacted by Kluwer Academic Publishers in the Fall of 200 I, inviting me to edit a volume of papers on the issue of electron transport in quantum dots, I was excited by what I saw as an ideal opportunity to provide an overview of a field of research that has made significant contributions in recent years, both to our understanding of fundamental physics, and to the development of novel nanoelectronic technologies. The need for such a volume seemed to be made more pressing by the fact that few comprehensive reviews of this topic have appeared in the literature, in spite of the vast activity in this area over the course of the last decade or so. With this motivation, I set out to try to compile a volume that would fairly reflect the wide range of opinions that has emerged in the study of electron transport in quantum dots. Indeed, there has been no effort on my part to ensure any consistency between the different chapters, since I would prefer that this volume instead serve as a useful forum for the...

  6. Quantum transport in semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Kubis, Tillmann Christoph

    2009-11-15

    The main objective of this thesis is to theoretically predict the stationary charge and spin transport in mesoscopic semiconductor quantum devices in the presence of phonons and device imperfections. It is well known that the nonequilibrium Green's function method (NEGF) is a very general and all-inclusive scheme for the description of exactly this kind of transport problem. Although the NEGF formalism has been derived in the 1960's, textbooks about this formalism are still rare to find. Therefore, we introduce the NEGF formalism, its fundamental equations and approximations in the first part of this thesis. Thereby, we extract ideas of several seminal contributions on NEGF in literature and augment this by some minor derivations that are hard to find. Although the NEGF method has often been numerically implemented on transport problems, all current work in literature is based on a significant number of approximations with often unknown influence on the results and unknown validity limits. Therefore, we avoid most of the common approximations and implement in the second part of this thesis the NEGF formalism as exact as numerically feasible. For this purpose, we derive several new scattering self-energies and introduce new self-adaptive discretizations for the Green's functions and self-energies. The most important improvements of our NEGF implementation, however, affect the momentum and energy conservation during incoherent scattering, the Pauli blocking, the current conservation within and beyond the device and the reflectionless propagation through open device boundaries. Our uncommonly accurate implementation of the NEGF method allows us to analyze and assess most of the common approximations and to unveil numerical artifacts that have plagued previous approximate implementations in literature. Furthermore, we apply our numerical implementation of the NEGF method on the stationary electron transport in THz quantum cascade lasers (QCLs) and answer

  7. Quantum Transport Calculations Using Periodic Boundary Conditions

    OpenAIRE

    Wang, Lin-Wang

    2004-01-01

    An efficient new method is presented to calculate the quantum transports using periodic boundary conditions. This method allows the use of conventional ground state ab initio programs without big changes. The computational effort is only a few times of a normal ground state calculations, thus is makes accurate quantum transport calculations for large systems possible.

  8. Dynamic Localization Condition of Two Electrons in a Strong dc-ac Biased Quantum Dot Molecule

    Institute of Scientific and Technical Information of China (English)

    WANG Li-Min; DUAN Su-Qing; ZHAO Xian-Geng; LIU Cheng-Shi

    2004-01-01

    @@ We present a perturbation investigation of dynamic localization condition of two electrons in a strong dc-ac biased quantum dot molecule. By reducing the system to an Hubbard-type effective two-site model and by applying Floquet theory, we find that the dynamical localization phenomenon occurs under certain values of the large strength of the dc and ac field. This demonstrates the possibility of using appropriate dc-ac fields to manipulate dynamical localized states in mesoscopic devices, which is an essential component of practical schemes for quantum information processing. Our conclusion is instructive to the field of quantum function devices.

  9. Time dependent quantum transport through Kondo correlated quantum dots

    OpenAIRE

    Goker, Ali; Gedik, Elif

    2013-01-01

    In this article, we review recent work about time dependent quantum transport through a quantum dot in Kondo regime. This represents a major step towards designing next generation transistors that are expected to replace current MOSFET's in a few years. We first discuss the effects of the density of states of gold contacts on the instantaneous conductance of an asymmetrically coupled quantum dot that is abruptly moved into Kondo regime via a gate voltage. Next, we investigate the effect of st...

  10. Spin transport through quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Lima, A.T. da Cunha; Anda, Enrique V. [Pontificia Univ. Catolica do Rio de Janeiro (PUC-Rio), RJ (Brazil)

    2003-07-01

    Full text: We investigate the spin polarized transport properties of a nanoscopic device constituted by a quantum dot connected to two leads. The electrical current circulates with a spin polarization that is modulated via a gate potential that controls the intensity of the spin-orbit coupling, the Rashba effect. We study a polarized field-effect transistor when one of its parts is constituted by a small quantum dot, which energies are controlled by another gate potential operating inside the confined region. The high confinement and correlation suffered by the charges inside the dot gives rise to novel phenomena. We show that through the manipulation of the gate potential applied to the dot it is possible to control, in a very efficient way, the intensity and polarization of the current that goes along the system. Other crucial parameters to be varied in order to understand the behavior of this system are the intensity of the external applied electric and magnetic field. The system is represented by the Anderson Impurity Hamiltonian summed to a spin-orbit interaction, which describes the Rashba effect. To obtain the current of this out-of-equilibrium system we use the Keldysh formalism.The solution of the Green function are compatible with the Coulomb blockade regime. We show that under the effect of a external magnetic field, if the dot is small enough the device operates as a complete spin filter that can be controlled by the gate potential. The behavior of this device when it is injected into it a polarized current and modulated by the Rashba effect is as well studied. (author)

  11. Quantum transport in semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Kubis, Tillmann Christoph

    2009-11-15

    The main objective of this thesis is to theoretically predict the stationary charge and spin transport in mesoscopic semiconductor quantum devices in the presence of phonons and device imperfections. It is well known that the nonequilibrium Green's function method (NEGF) is a very general and all-inclusive scheme for the description of exactly this kind of transport problem. Although the NEGF formalism has been derived in the 1960's, textbooks about this formalism are still rare to find. Therefore, we introduce the NEGF formalism, its fundamental equations and approximations in the first part of this thesis. Thereby, we extract ideas of several seminal contributions on NEGF in literature and augment this by some minor derivations that are hard to find. Although the NEGF method has often been numerically implemented on transport problems, all current work in literature is based on a significant number of approximations with often unknown influence on the results and unknown validity limits. Therefore, we avoid most of the common approximations and implement in the second part of this thesis the NEGF formalism as exact as numerically feasible. For this purpose, we derive several new scattering self-energies and introduce new self-adaptive discretizations for the Green's functions and self-energies. The most important improvements of our NEGF implementation, however, affect the momentum and energy conservation during incoherent scattering, the Pauli blocking, the current conservation within and beyond the device and the reflectionless propagation through open device boundaries. Our uncommonly accurate implementation of the NEGF method allows us to analyze and assess most of the common approximations and to unveil numerical artifacts that have plagued previous approximate implementations in literature. Furthermore, we apply our numerical implementation of the NEGF method on the stationary electron transport in THz quantum cascade lasers (QCLs) and answer

  12. Quantum and classical thermoelectric transport in quantum dot nanocomposites

    Science.gov (United States)

    Zhou, Jun; Yang, Ronggui

    2011-10-01

    Quantum dot nanocomposites are potentially high-efficiency thermoelectric materials, which could outperform superlattices and random nanocomposites in terms of manufacturing cost-effectiveness and material properties because of the reduction of thermal conductivity due to the phonon-interface scattering, the enhancement of Seebeck coefficient due to the formation of minibands, and the enhancement of electrical conductivity due to the phonon-bottleneck effect in electron-phonon scattering for quantum-confined electrons. In this paper, we investigate the thermoelectric transport properties of quantum dot nanocomposites through a two-channel transport model that includes the transport of quantum-confined electrons through the hopping mechanism and the semiclassical transport of bulk-like electrons. For the quantum-confined electrons whose wave functions are confined in the quantum dots with overlapping tail extending to the matrix, we develop a tight-binding model together with the Kubo formula and the Green's function method to describe the transport processes of these electrons. The formation of minibands due to the quantum confinement and the phonon-bottleneck effect on carrier-phonon scattering are considered. For transport of bulk-like electrons, a Boltzmann-transport-equation-based semiclassical model is used to describe the multiband transport processes of carriers. The intrinsic carrier scatterings as well as the carrier-interface scattering of these bulk-like electrons are considered. We then apply the two-channel transport model to predict thermoelectric transport properties of n-type PbSe/PbTe quantum dot nanocomposites with PbSe quantum dots uniformly embedded in the PbTe matrix. The dependence of thermoelectric transport coefficients on the size of quantum dots, interdot distance, doping concentration, and temperature are studied in detail. Due to the formation of minibands and the phonon-bottleneck effect on carrier-phonon scattering, we show that

  13. Crossover from quantum to classical transport

    Science.gov (United States)

    Morr, Dirk K.

    2016-01-01

    Understanding the crossover from quantum to classical transport has become of fundamental importance not only for technological applications due to the creation of sub-10-nm transistors - an important building block of our modern life - but also for elucidating the role played by quantum mechanics in the evolutionary fitness of biological complexes. This article provides a basic introduction into the nature of charge and energy transport in the quantum and classical regimes. It discusses the characteristic transport properties in both limits and demonstrates how they can be connected through the loss of quantum mechanical coherence. The salient features of the crossover physics are identified, and their importance in opening new transport regimes and in understanding efficient and robust energy transport in biological complexes are demonstrated.

  14. An ac quantum voltmeter based on a 10 V programmable Josephson array

    Science.gov (United States)

    Lee, Jinni; Behr, Ralf; Palafox, Luis; Katkov, Alexander; Schubert, Marco; Starkloff, Michael; Böck, Andreas Charles

    2013-12-01

    An ac quantum voltmeter based on a 10 V programmable Josephson array that is simple to use, provides dc and ac calibration up to kHz range for equipment widely used in metrology, and ensures direct traceability to a quantum-based standard, is developed. This ac quantum voltmeter is proven to match conventional Josephson standard systems at dc and extends its advantages up to 10 kHz in the low-frequency ac range. The ac quantum voltmeter is capable of performing calibrations up to 7 VRMS in the frequency range from dc to 10 kHz completely under software control. A direct comparison at dc has demonstrated an uncertainty better than 2 parts in 1010 (k = 2). The uncertainty at 1 kHz is better than 1.7 µV V-1 (k = 2) for a measurement time of 1 min. The ac quantum voltmeter is a robust and practical system that fulfils the needs of general metrology laboratories for quantum-based voltage calibrations.

  15. Electron Transport in Quantum Dots and Heat Transport in Molecules

    DEFF Research Database (Denmark)

    Kirsanskas, Gediminas

    and to perform electrical transport experiments at temperatures below one Kelvin (1 K), and thus to address such question. In this thesis we are concerned with the theoretical description of one kind of such devices called quantum dots. As the name suggest a quantum dot is a system where particles are confined...

  16. On quantum hydrodynamic and quantum energy transport models

    OpenAIRE

    Degond, Pierre; Gallego, Samy; Mehats, Florian

    2007-01-01

    In this paper, we consider two recently derived models: the Quantum Hydrodynamic model (QHD) and the Quantum Energy Transport model (QET). We propose different equivalent formulations of these models and we use a commutator formula for stating new properties of the models. A gauge invariance lemma permits to simplify the QHD model for irrotational flows. We finish by considering the special case of a slowly varying temperature and we discuss possible approximations which will b...

  17. Dielectric relaxation and ac conductivity behaviour of polyvinyl alcohol–HgSe quantum dot hybrid films

    International Nuclear Information System (INIS)

    Here we report a comparative study on the dielectric relaxation and ac conductivity behaviour of pure polyvinyl alcohol (PVA) and PVA–mercury selenide (HgSe) quantum dot hybrid films in the temperature range 298 K ⩽ T ⩽ 420 K and in the frequency range 100 Hz ⩽ f ⩽ 1 MHz. The prepared nanocomposite exhibits a larger dielectric constant as compared to the pure PVA. The real and imaginary parts of the dielectric constants were found to fit appreciably with the modified Cole–Cole equation, from which temperature-dependent values of the relaxation times, free charge carrier conductivity and space charge carrier conductivity were calculated. The relaxation time decreases with the quantum dot's inclusion in the PVA matrix and with an increase in temperature, whereas free charge carrier conductivity and space charge carrier conductivity increases with an increase in temperature. An increase in ac conductivity for the nanocomposites has also been observed, while the charge transport mechanism was found to follow the correlated barrier hopping model in both cases. An easy-path model with a suitable electrical equivalent circuit has been employed to analyse the temperature-dependent impedance spectra. The imaginary part of the complex electric modulus spectra exhibit an asymmetric nature and a non-Debye type of behaviour, which has been elucidated considering a generalized susceptibility function. The electric modulus spectra of the nanocomposite demonstrate a smaller amplitude and broader width, as compared to the pure PVA sample. (paper)

  18. A quantum photonic dissipative transport theory

    Science.gov (United States)

    Lei, Chan U.; Zhang, Wei-Min

    2012-05-01

    In this paper, a quantum transport theory for describing photonic dissipative transport dynamics in nanophotonics is developed. The nanophotonic devices concerned in this paper consist of on-chip all-optical integrated circuits incorporating photonic bandgap waveguides and driven resonators embedded in nanostructured photonic crystals. The photonic transport through waveguides is entirely determined from the exact master equation of the driven resonators, which is obtained by explicitly eliminating all the degrees of freedom of the waveguides (treated as reservoirs). Back-reactions from the reservoirs are fully taken into account. The relation between the driven photonic dynamics and photocurrents is obtained explicitly. The non-Markovian memory structure and quantum decoherence dynamics in photonic transport can then be fully addressed. As an illustration, the theory is utilized to study the transport dynamics of a photonic transistor consisting of a nanocavity coupled to two waveguides in photonic crystals. The controllability of photonic transport through the external driven field is demonstrated.

  19. Spin-related transport phenomena in HgTe-based quantum well structures

    Energy Technology Data Exchange (ETDEWEB)

    Koenig, Markus

    2007-12-15

    Within the scope of this thesis, spin related transport phenomena have been investigated in HgTe/Hg{sub 0.3}Cd{sub 0.7}Te quantum well structures. In our experiments, the existence of the quantum spin Hall (QSH) state was successfully demonstrated for the first time and the presented results provide clear evidence for the charge transport properties of the QSH state. Our experiments provide the first direct observation of the Aharonov-Casher (AC) effect in semiconductor structures. In conclusion, HgTe quantum well structures have proven to be an excellent template for studying spin-related transport phenomena: The QSH relies on the peculiar band structure of the material and the existence of both the spin Hall effect and the AC effect is a consequence of the substantial spin-orbit interaction. (orig.)

  20. Transport, shot noise, and topology in AC-driven dimer arrays

    Science.gov (United States)

    Niklas, Michael; Benito, Mónica; Kohler, Sigmund; Platero, Gloria

    2016-11-01

    We analyze an AC-driven dimer chain connected to a strongly biased electron source and drain. It turns out that the resulting transport exhibits fingerprints of topology. They are particularly visible in the driving-induced current suppression and the Fano factor. Thus, shot noise measurements provide a topological phase diagram as a function of the driving parameters. The observed phenomena can be explained physically by a mapping to an effective time-independent Hamiltonian and the emergence of edge states. Moreover, by considering quantum dissipation, we determine the requirements for the coherence properties in a possible experimental realization. For the computation of the zero-frequency noise, we develop an efficient method based on matrix-continued fractions.

  1. Quantum transport of energy in controlled synthetic quantum magnets

    Science.gov (United States)

    Bermudez, Alejandro; Schaetz, Tobias

    2016-08-01

    We introduce a theoretical scheme that exploits laser cooling and phonon-mediated spin–spin interactions in crystals of trapped atomic ions to explore the transport of energy through a quantum magnet. We show how to implement an effective transport window to control the flow of energy through the magnet even in the absence of fermionic statistics for the carriers. This is achieved by shaping the density of states of the effective thermal reservoirs that arise from the interaction with the external bath of the modes of the electromagnetic field, and can be experimentally controlled by tuning the laser frequencies and intensities appropriately. The interplay of this transport window with the spin–spin interactions is exploited to build an analogue of the Coulomb-blockade effect in nano-scale electronic devices, and opens new possibilities to study quantum effects in energy transport.

  2. Quantum transport in Sierpinski carpets

    Science.gov (United States)

    van Veen, Edo; Yuan, Shengjun; Katsnelson, Mikhail I.; Polini, Marco; Tomadin, Andrea

    2016-03-01

    Recent progress in the design and fabrication of artificial two-dimensional (2D) materials paves the way for the experimental realization of electron systems moving on complex geometries, such as plane fractals. In this work, we calculate the quantum conductance of a 2D electron gas roaming on a Sierpinski carpet (SC), i.e., a plane fractal with Hausdorff dimension intermediate between 1 and 2. We find that the fluctuations of the quantum conductance are a function of energy with a fractal graph, whose dimension can be chosen by changing the geometry of the SC. This behavior is independent of the underlying lattice geometry.

  3. Anomalous Thermal Transport in Quantum Wires

    OpenAIRE

    Fazio, Rosario; Hekking, F. W. J.; Khmelnitskii, D. E.

    1997-01-01

    We study thermal transport in a one-dimensional quantum wire, connected to reservoirs. Despite of the absence of electron backscattering, interactions in the wire strongly influence thermal transport. Electrons propagate with unitary transmission through the wire and electric conductance is not affected. Energy, however, is carried by bosonic excitations (plasmons) which suffer from scattering even on scales much larger than the Fermi wavelength. If the electron density varies randomly, plasm...

  4. Conformational Influence on Quantum Transport in Nanostructures

    OpenAIRE

    Maul, Robert

    2010-01-01

    In the present thesis we have studied the interplay of conformational and electronic transport properties in metallic and organic nano-structures. Characterization of the influence of thermal, electrostatic or fabrication-induced structural rearrangement on the conductance characteristics gives new insights into the functionality of nano-scale systems, such as quantum point contacts, nano-wires and nano-particles.

  5. Quantum transport in semiconductor nanowires

    NARCIS (Netherlands)

    Van Dam, J.

    2006-01-01

    This thesis describes a series of experiments aimed at understanding the low-temperature electrical transport properties of semiconductor nanowires. The semiconductor nanowires (1-100 nm in diameter) are grown from nanoscale gold particles via a chemical process called vapor-liquid-solid (VLS) growt

  6. Quantum transport in carbon nanotubes

    NARCIS (Netherlands)

    Jarillo-Herrero, P.D.

    2005-01-01

    Electronic transport through nanostructures can be very different from trans- port in macroscopic conductors, especially at low temperatures. Carbon na- notubes are tiny cylinders made of carbon atoms. Their remarkable electronic and mechanical properties, together with their small size (a few nm in

  7. Quantum transport in carbon nanotubes

    NARCIS (Netherlands)

    Laird, E.A.; Kuemmeth, F.; Steele, G.A.; Grove-Rasmussen, K.; Nygard, J.; Flensberg, K.; Kouwenhoven, L.P.

    2015-01-01

    Carbon nanotubes are a versatile material in which many aspects of condensed matter physics come together. Recent discoveries have uncovered new phenomena that completely change our understanding of transport in these devices, especially the role of the spin and valley degrees of freedom. This revie

  8. Semiconductor Nanostructures Quantum States and Electronic Transport

    CERN Document Server

    Ihn, Thomas

    2009-01-01

    This textbook describes the physics of semiconductor nanostructures with emphasis on their electronic transport properties. At its heart are five fundamental transport phenomena: quantized conductance, tunnelling transport, the Aharonov-Bohm effect, the quantum Hall effect, and the Coulomb blockade effect. The book starts out with the basics of solid state and semiconductor physics, such as crystal structure, band structure, and effective mass approximation, including spin-orbit interaction effects important for research in semiconductor spintronics. It contains material aspects such as band e

  9. Quantum transport in a ring of quantum dots

    International Nuclear Information System (INIS)

    Full text: Quantum dots play a central role in the recent technological efforts to build efficient devices to storage, process and transmit information in the quantum regime [1]. One of the reasons for this interest is the relative simplicity with which its control parameters can be changed by experimentalists. Systems with one, two and even arrays of quantum dots have been intensively studied with respect to their efficiency in processing information carried by charge, spin and heat [1]. A particularly useful realization of a quantum dot is a ballistic electron cavity formed by electrostatic potentials in a two-dimensional electron gas. In the chaotic regime, the shape of the dot is statistically irrelevant and the ability to change its form via external gates can be used to generate members of an ensemble of identical systems. From a theoretical point of view, such quantum dots are ideal electron systems in which to study theoretical models combining phase-coherence, chaotic dynamics and Coulomb interactions. In this work, we use the Keldysh non-linear sigma model [2] with a counting field to study electron transport through a ring of four chaotic quantum dots pierced by an Aharonov-Bohm flux. This system is particularly well suited for studying ways to use the weak-localization effect to process quantum information. We derive the quantum circuit equations for this system from the saddle-point condition of the Keldysh action. The results are used to build the action of the corresponding supersymmetric (SUSY) non-linear sigma model. The connection with the random scattering matrix approach is then made via the color-flavor transformation. In the perturbative regime, where weak-localization effects appear, the Keldysh, SUSY and random scattering matrix approaches can be compared by means of independent analytical calculations. We conclude by pointing out the many advantages of our unified approach. [1] For a review, see Yu. V. Nazarov, and Ya. M. Blanter, Quantum

  10. Quantum transport in semiconductor nanowires

    OpenAIRE

    Van Dam, J.

    2006-01-01

    This thesis describes a series of experiments aimed at understanding the low-temperature electrical transport properties of semiconductor nanowires. The semiconductor nanowires (1-100 nm in diameter) are grown from nanoscale gold particles via a chemical process called vapor-liquid-solid (VLS) growth. The huge versatility of this material system (e.g. in size and materials) results in a wide range of potential applications in (opto-)electronics. During the last few years many important proofs...

  11. Calculating Quantum Transports Using Periodic Boundary Conditions

    OpenAIRE

    Wang, Lin-Wang

    2004-01-01

    An efficient new method is presented to calculate the quantum transports using periodic boundary conditions. This new method is based on a method we developed previously, but with an essential change in solving the Schrodinger's equation. As a result of this change, the scattering states can be solved at any given energy. Compared to the previous method, the current method is faster and numerically more stable. The total computational time of the current method is similar to a conventional gr...

  12. Quantum transport in carbon-based nanostructures

    OpenAIRE

    Nemec, Norbert

    2007-01-01

    The electronic structure and the quantum transport properties of graphene, carbon nanotubes and graphene nanoribbons are studied using analytical and numerical tools. Special care is taken in considering fundamental questions of high experimental relevance and in relating the results to experiments. The main focus of the work is on numerical calculations based on the tight-binding description of electrons, also integrating the results of microscopic ab initio calculations a...

  13. Transport ac losses in Bi-2223 multifilamentary tapes - conductor materials aspect

    International Nuclear Information System (INIS)

    Transport ac losses in technical superconductors based on Bi-2223 tape material are influenced by many parameters. The major factors that define the ac performance of such conductors are the following: the size and number of filaments, their geometrical arrangement in the cross-section of the conductor, the twist pitch length, the resistivity of the matrix, the presence of oxide barriers around the filaments and deformation procedures such as sequential pressing or rolling followed by appropriate thermal treatment. In the present paper the above aspects are addressed from the viewpoint of the materials science of technical conductor design. Transport ac losses at power frequencies in different types of Bi-2223 conductor are presented and analysed. The results of conductor design analysis with respect to the coexistence of the superconductor with other materials in the conductor structure are presented. New concepts for minimization of the transport ac losses are discussed in detail. (author)

  14. Quantum Transport: The Link between Standard Approaches in Superlattices

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka

    1998-01-01

    Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: (i) transport in a miniband; (ii) hopping between Wannier-Stark ladders; and (iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium...

  15. Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR

    OpenAIRE

    Morello, A.; Bakharev, O. N.; Brom, H. B.; de Jongh, L.J.

    2002-01-01

    We present an ultra-low temperature study (down to T = 20 mK) of the nuclear spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet Mn12-ac. The nuclear spins act as local probes for the electronic spin fluctuations, due to thermal excitations and to tunnelling events. In the quantum regime (below T = 0.75 K), the nuclear SLR becomes temperature-independent and is driven by fluctuations of the cluster's electronic spin due to the quantum tunnelling of magnetization in the gr...

  16. Quantum inductive circuits under ac and dc fields: Current manifestations of charge discreteness

    International Nuclear Information System (INIS)

    It is well known that the electrical current of a quantum inductive circuits with charge discreteness qe displays Bloch-like oscillations (frequency ωB=qeεd-bar ) under a dc external voltage (εd). Here we consider the effect of a superposed ac voltage in the circuit. Resonances are explicitly found. In the limit of small external frequency (ω-bar ωB), the electrical (one-period-averaged) current exist and has always the same sign. This gives us an experimental method to measure discrete charge effects in (quantum) nanometric circuits since the established current is depending on charge discreteness

  17. Quantum spin transport in semiconductor nanostructures

    International Nuclear Information System (INIS)

    In this work, we study and quantitatively predict the quantum spin Hall effect, the spin-orbit interaction induced intrinsic spin-Hall effect, spin-orbit induced magnetizations, and spin-polarized electric currents in nanostructured two-dimensional electron or hole gases with and without the presence of magnetic fields. We propose concrete device geometries for the generation, detection, and manipulation of spin polarization and spin-polarized currents. To this end a novel multi-band quantum transport theory, that we termed the multi-scattering Buettiker probe model, is developed. The method treats quantum interference and coherence in open quantum devices on the same footing as incoherent scattering and incorporates inhomogeneous magnetic fields in a gauge-invariant and nonperturbative manner. The spin-orbit interaction parameters that control effects such as band energy spin splittings, g-factors, and spin relaxations are calculated microscopically in terms of an atomistic relativistic tight-binding model. We calculate the transverse electron focusing in external magnetic and electric fields. We have performed detailed studies of the intrinsic spin-Hall effect and its inverse effect in various material systems and geometries. We find a geometry dependent threshold value for the spin-orbit interaction for the inverse intrinsic spin-Hall effect that cannot be met by n-type GaAs structures. We propose geometries that spin polarize electric current in zero magnetic field and analyze the out-of-plane spin polarization by all electrical means. We predict unexpectedly large spin-orbit induced spin-polarization effects in zero magnetic fields that are caused by resonant enhancements of the spin-orbit interaction in specially band engineered and geometrically designed p-type nanostructures. We propose a concrete realization of a spin transistor in HgTe quantum wells, that employs the helical edge channel in the quantum spin Hall effect.

  18. Quantum spin transport in semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Schindler, Christoph

    2012-05-15

    In this work, we study and quantitatively predict the quantum spin Hall effect, the spin-orbit interaction induced intrinsic spin-Hall effect, spin-orbit induced magnetizations, and spin-polarized electric currents in nanostructured two-dimensional electron or hole gases with and without the presence of magnetic fields. We propose concrete device geometries for the generation, detection, and manipulation of spin polarization and spin-polarized currents. To this end a novel multi-band quantum transport theory, that we termed the multi-scattering Buettiker probe model, is developed. The method treats quantum interference and coherence in open quantum devices on the same footing as incoherent scattering and incorporates inhomogeneous magnetic fields in a gauge-invariant and nonperturbative manner. The spin-orbit interaction parameters that control effects such as band energy spin splittings, g-factors, and spin relaxations are calculated microscopically in terms of an atomistic relativistic tight-binding model. We calculate the transverse electron focusing in external magnetic and electric fields. We have performed detailed studies of the intrinsic spin-Hall effect and its inverse effect in various material systems and geometries. We find a geometry dependent threshold value for the spin-orbit interaction for the inverse intrinsic spin-Hall effect that cannot be met by n-type GaAs structures. We propose geometries that spin polarize electric current in zero magnetic field and analyze the out-of-plane spin polarization by all electrical means. We predict unexpectedly large spin-orbit induced spin-polarization effects in zero magnetic fields that are caused by resonant enhancements of the spin-orbit interaction in specially band engineered and geometrically designed p-type nanostructures. We propose a concrete realization of a spin transistor in HgTe quantum wells, that employs the helical edge channel in the quantum spin Hall effect.

  19. AC shot noise through a quantum dot in the Kondo regime

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Kai-Hua, E-mail: khybjut@yahoo.com.cn [College of Applied Sciences, Beijing University of Technology, Beijing 100124 (China); Wu, Yan-Ju; Chen, Yang [College of Applied Sciences, Beijing University of Technology, Beijing 100124 (China)

    2011-08-01

    The photon-assisted shot noise through a quantum dot in the Kondo regime is investigated by applying time-dependent canonical transformation and non-crossing approximation technique. A basic formula for the photon-assisted shot noise is obtained. The rich dependence of the shot noise on the external ac field and temperature is displayed. At low temperature and low frequencies, the differential shot noise exhibits staircase behavior. When the temperature increases, the steps are rounded. At elevated frequencies, the photon-assisted tunneling becomes more obvious. We have also found that the Fano factor is enhanced as the ac frequency is enhanced. -- Highlights: → The explicitly photon-assisted shot noise formula through strongly correlated quantum dot is obtained. → The time-dependent canonical transformation and non-crossing approximation technique. → The rich dependence of the shot noise on the external ac field and temperature is displayed. → The Fano factor is enhanced as the ac frequency is enhanced.

  20. Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles

    Energy Technology Data Exchange (ETDEWEB)

    Villavicencio, J [Facultad de Ciencias, Universidad Autonoma de Baja California, Ensenada (Mexico); Maldonado, I [Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (Mexico); Cota, E [Centro de Nanociencias y Nanotecnologia, Universidad Nacional Autonoma de Mexico, Ensenada (Mexico); Platero, G, E-mail: villavics@uabc.edu.mx [Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid (Spain)

    2011-02-15

    The dynamics of electrons in ac-driven double quantum dots have been extensively analyzed by means of Floquet theory. In these systems, coherent destruction of tunneling has been shown to occur for certain ac field parameters. In this work we analyze, by means of Floquet theory, the electron dynamics of a triple quantum dot in series attached to electric contacts, where the central dot position oscillates. In particular, we analyze the quasienergy spectrum of this ac-driven nanoelectromechanical system as a function of the intensity and frequency of the ac field and of external dc voltages. For strong driving fields, we derive, by means of perturbation theory, analytical expressions for the quasienergies of the driven oscillator system. From this analysis, we discuss the conditions for coherent destruction of tunneling (CDT) to occur as a function of detuning and field parameters. For zero detuning, and from the invariance of the Floquet Hamiltonian under a generalized parity transformation, we find analytical expressions describing the symmetry properties of the Fourier components of the Floquet states under such a transformation. By using these expressions, we show that in the vicinity of the CDT condition, the quasienergy spectrum exhibits exact crossings which can be characterized by the parity properties of the corresponding eigenvectors.

  1. Series Solution for Localization and Entanglement of an Exciton in a Quantum Dot Molecule by an ac Electric Field

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    The Schr(o)dinger equation involving the phenomenon of the localization and entanglement for an exciton in a quantum dot molecule by an ac electric field is analytically investigated. New exact series solutions for the Schr(o)dinger equation have been obtained for the first time. The analytical expressions can further describe the dynamical behaviors of an interacting electron-hole pair in a double coupled quantum dot molecule under an ac electric field accurately.

  2. Transport Properties of the Universal Quantum Equation

    Institute of Scientific and Technical Information of China (English)

    A.I.Arbab

    2012-01-01

    The universal quantum equation (UQE) is found to describe the transport properties of the quantum particles.This equation describes a wave equation interacting with constant scalar and vector potentials propagating in spacetime.A new transformation that sends the Schr(o)dinger equation with a potential energy V =-1/2mc2 to Dirac's equation is proposed.The Cattaneo telegraph equation as well as a one-dimensional UQE are compatible with our recently proposed generalized continuity equations.Furthermore,a new wave equation resulted from the invariance of the UQE under the post-Galilean transformations is derived.This equation is found to govern a Klein Gordon's particle interacting with a photon-like vector field (ether) whose magnitude is proportional to the particle's mass.

  3. Quantum transport in coupled resonators enclosed synthetic magnetic flux

    Science.gov (United States)

    Jin, L.

    2016-07-01

    Quantum transport properties are instrumental to understanding quantum coherent transport processes. Potential applications of quantum transport are widespread, in areas ranging from quantum information science to quantum engineering, and not restricted to quantum state transfer, control and manipulation. Here, we study light transport in a ring array of coupled resonators enclosed synthetic magnetic flux. The ring configuration, with an arbitrary number of resonators embedded, forms a two-arm Aharonov-Bohm interferometer. The influence of magnetic flux on light transport is investigated. Tuning the magnetic flux can lead to resonant transmission, while half-integer magnetic flux quantum leads to completely destructive interference and transmission zeros in an interferometer with two equal arms.

  4. Nonlocal edge state transport in the quantum spin Hall state

    OpenAIRE

    Roth, Andreas; Bruene, Christoph; Buhmann, Hartmut; Molenkamp, Laurens W.; Maciejko, Joseph; Qi, Xiao-Liang; Zhang, Shou-Cheng

    2009-01-01

    We present direct experimental evidence for nonlocal transport in HgTe quantum wells in the quantum spin Hall regime, in the absence of any external magnetic field. The data conclusively show that the non-dissipative quantum transport occurs through edge channels, while the contacts lead to equilibration between the counter-propagating spin states at the edge. We show that the experimental data agree quantitatively with the theory of the quantum spin Hall effect.

  5. Scaling theory for anomalous semiclassical quantum transport

    International Nuclear Information System (INIS)

    Quantum transport through devices coupled to electron reservoirs can be described in terms of the full counting statistics (FCS) of charge transfer. Transport observables, such as conductance and shot-noise power are just cumulants of FCS and can be obtained from the sample’s average density of transmission eigenvalues, which in turn can be obtained from a finite element representation of the saddle-point equation of the Keldysh (or supersymmetric) nonlinear sigma model, known as quantum circuit theory. Normal universal metallic behavior in the semiclassical regime is controlled by the presence of a Fabry–Pérot singularity in the average density of transmission eigenvalues. We present general conditions for the suppression of Fabry–Pérot modes in the semiclassical regime in a sample of arbitrary shape, a disordered conductor or a network of ballistic quantum dots, which leads to an anomalous metallic phase. Through a double-scaling limit, we derive a scaling equation for anomalous metallic transport, in the form of a nonlinear differential equation, which generalizes the ballistic-diffusive scaling equation of a normal metal. The two-parameter stationary solution of our scaling equation generalizes Dorokhov’s universal single-parameter distribution of transmission eigenvalues. We provide a simple interpretation of the stationary solution using a thermodynamic analogy with a spin-glass system. As an application, we consider a system formed by a diffusive wire coupled via a barrier to normal-superconductor reservoirs. We observe anomalous reflectionless tunneling, when all perfectly transmitting channels are suppressed, which cannot be explained by the usual mechanism of disorder-induced opening of tunneling channels. (paper)

  6. Transport through multiply connected quantum wires

    OpenAIRE

    Das, Sourin; Rao, Sumathi

    2003-01-01

    We study transport through multiply coupled carbon nano-tubes (quantum wires) and compute the conductances through the two wires as a function of the two gate voltages $g_1$ and $g_2$ controlling the chemical potential of the electrons in the two wires. We find that there is an {\\it equilibrium} cross-conductance, and we obtain its dependence on the temperature and length of the wires. The effective action of the model for the wires in the strong coupling (equivalently Coulomb interaction) li...

  7. Quantum transport of the single metallocene molecule

    Science.gov (United States)

    Yu, Jing-Xin; Chang, Jing; Wei, Rong-Kai; Liu, Xiu-Ying; Li, Xiao-Dong

    2016-10-01

    The Quantum transport of three single metallocene molecule is investigated by performing theoretical calculations using the non-equilibrium Green's function method combined with density functional theory. We find that the three metallocen molecules structure become stretched along the transport direction, the distance between two Cp rings longer than the other theory and experiment results. The lager conductance is found in nickelocene molecule, the main transmission channel is the electron coupling between molecule and the electrodes is through the Ni dxz and dyz orbitals and the s, dxz, dyz of gold. This is also confirmed by the highest occupied molecular orbital resonance at Fermi level. In addition, negative differential resistance effect is found in the ferrocene, cobaltocene molecules, this is also closely related with the evolution of the transmission spectrum under applied bias.

  8. Quantum Transport in Strongly Correlated Systems

    DEFF Research Database (Denmark)

    Bohr, Dan

    2007-01-01

    In the past decade there has been a trend towards studying ever smaller devices. Improved experimental techniques have made new experiments possible, one class of which is electron transport through molecules and artificially manufactured structures like quantum dots. In this type of systems...... spinless resonant 7 site chain, studying the effect of repulsive interaction inside the chain. We demonstrate that both weak and strong interactions inside the chain lead to Coulomb blockade renormalization of the resonances in the conductance spectrum. Additionally the strongly interacting case sharpens...... the resonances significantly, such that strong interaction inside the chain tends to suppress the off-resonance transport. Next we consider interacting resonant level models, studying the effect of repulsive interaction on the contact links. We demonstrate that even a small leak of the interaction in the system...

  9. Quantum transport in superlattice and quantum dot structures

    CERN Document Server

    Murphy, H M

    2000-01-01

    manifestation of oscillations in the current -voltage characteristics of superlattices in the Wannier-Stark transport regime when strong lateral confinement is provided due either to gaps in the folded phonon spectrum or phonon momentum meeting the condition for Bragg reflection. Current-voltage measurements are shown in this chapter for superlattice devices in the Wannier-Stark regime for a range of electric and magnetic fields (B//I). Many oscillations are clearly observed in the I(V) data presented, the possible origins of which are then fully explored. Moving away from superlattices, data involving tunnelling through quantum dots embedded in the barrier of a GaAs/AIAs/GaAs resonant tunnelling diode are presented in chapter 5. Quasi-hydrostatic pressure is used to tune tunnelling through the dots. These results lead to a new picture for the conduction band potential profile of this device, and give us important new information relating to devices incorporating self-assembled quantum dots. More importantly,...

  10. Controlling Quantum Transport with a Programmable Nanophotonic Processor

    Science.gov (United States)

    Harris, Nicholas; Steinbrecher, Gregory; Mower, Jacob; Lihini, Yoav; Prabhu, Mihika; Baehr-Jones, Tom; Hochberg, Michael; Lloyd, Seth; Englund, Dirk

    Recent experimental and theoretical work has revealed emergent, counter-intuitive quantum transport effects in a range of physical medial including solid-state and biological systems. Photonic integrated circuits are promising platforms for studying such effects. A central goal in for photonic quantum transport simulators has been the ability to rapidly control all parameters of the transport problem. Here, we present a large-scale programmable nanophotonic processor composed of 56 Mach-Zehnder interferometers that enables control over modal couplings and differential phases between modes--enabling observations of Anderson localization, environment-assisted quantum transport, ballistic transport, and a number of intermediate quantum transport regimes. Rapid programmability enables tens of thousands of realizations of disordered and noisy systems. In addition, low loss makes this nanophotonic processor a promising platform for many-boson quantum simulation experiments.

  11. Dynamic Localization of a One-Dimensional Quantum Dot Array in an ac Electric Field

    Institute of Scientific and Technical Information of China (English)

    罗莹; 段素青; 范文斌; 赵宪庚; 王立民; 马本堃

    2002-01-01

    We investigate the dynamics of two interaction electrons confined to one-dimensional quantum dot array in an acelectric field. We find that initially localized electrons will remain localized in the absence of Coulomb interactionif the ratio of the ac field magnitude to the frequency is a root of the ordinary zero-order Bessel function. Incontrast to the case without Coulomb interaction, no matter what the value is, the electrons are delocalized andthe delocalization effect depends on the ratio U/ω and eaE/ω, where U is the strength of Coulomb interaction,a is the lattice constant, and E and ω are the ac field amplitude and frequency, respectively.

  12. Linear ac transport in graphene semiconducting nanosystem with normal-metal electrodes

    Science.gov (United States)

    Ye, En-Jia; Sun, Yun-Lei; Lan, Jin; Shi, Yi-Jian

    2016-03-01

    Linear ac transport properties are investigated in a graphene semiconducting nanosystem, with the effect of normal-metal electrodes taken into account. We use a tight-binding approach and ac transport theory to study the dc conductance and ac emittance in normal-metal/graphene (NG) and normal-metal/graphene/normal-metal (NGN) systems with armchair-edge graphene. We find that the resonant and semiconducting behaviors in NG and NGN systems are closely related to the spatial-resolved local density of states. Furthermore, features of the size-dependent emittances in the NGN system are investigated. The results suggest a positive correlation between the width and capacitive response, and the capacitive response is robust as the size of the system increases proportionally.

  13. Frequency-dependent critical current and transport ac loss of superconductor strip and Roebel cable

    Energy Technology Data Exchange (ETDEWEB)

    Thakur, Kailash Prasad [Landcare Research, Palmerston North 4442 (New Zealand); Raj, Ashish [Computer Science in Radiology, Weill Medical College, Cornell University, NY 10022 (United States); Brandt, Ernst Helmut [Max-Planck-Institut fuer Metallforschung, PO B 800665, D-70506 Stuttgart (Germany); Kvitkovic, Jozef; Pamidi, Sastry V, E-mail: thakurk@landcareresearch.co.nz, E-mail: asr2004@med.cornell.edu, E-mail: ehb@mf.mpg.de, E-mail: kvitkovic@caps.fsu.edu, E-mail: pamidi@caps.fsu.edu [Center for Advanced Power System, Florida State University, Tallahassee, FL 32310 (United States)

    2011-06-15

    The frequency-dependent critical current of a superconductor strip and Roebel cable has been studied using a 2D finite element simulation. It is shown that the critical current of the superconductor increases with frequency as f{sup 1/n}, where n is the exponent of the power law flux creep model. Transport ac loss in a superconductor strip decreases with frequency as f{sup -2/n} when the amplitude of the applied ac current is far less than its critical current. However, when the applied current is large and becomes comparable to the critical current, the transport ac loss decreases with frequency as 1/f. The analytical results are substantiated with available experimental data and the results of a 2D finite element simulation.

  14. Plasmon assisted transport through disordered array of quantum wires

    OpenAIRE

    Chudnovskiy, A. L.

    2004-01-01

    Phononless plasmon assisted thermally activated transport through a long disordered array of finite length quantum wires is investigated analytically. Generically strong electron plasmon interaction in quantum wires results in a qualitative change of the temperature dependence of thermally activated resistance in comparison to phonon assisted transport. At high temperatures, the thermally activated resistance is determined by the Luttinger liquid interaction parameter of the wires.

  15. Transport through Zero-Dimensional States in a Quantum Dot

    NARCIS (Netherlands)

    Kouwenhoven, Leo P.; Wees, Bart J. van; Harmans, Kees J.P.M.; Williamson, John G.

    1990-01-01

    We have studied the electron transport through zero-dimensional (0D) states. 0D states are formed when one-dimensional edge channels are confined in a quantum dot. The quantum dot is defined in a two-dimensional electron gas with a split gate technique. To allow electronic transport, connection to t

  16. Harnessing quantum transport by transient chaos.

    Science.gov (United States)

    Yang, Rui; Huang, Liang; Lai, Ying-Cheng; Grebogi, Celso; Pecora, Louis M

    2013-03-01

    Chaos has long been recognized to be generally advantageous from the perspective of control. In particular, the infinite number of unstable periodic orbits embedded in a chaotic set and the intrinsically sensitive dependence on initial conditions imply that a chaotic system can be controlled to a desirable state by using small perturbations. Investigation of chaos control, however, was largely limited to nonlinear dynamical systems in the classical realm. In this paper, we show that chaos may be used to modulate or harness quantum mechanical systems. To be concrete, we focus on quantum transport through nanostructures, a problem of considerable interest in nanoscience, where a key feature is conductance fluctuations. We articulate and demonstrate that chaos, more specifically transient chaos, can be effective in modulating the conductance-fluctuation patterns. Experimentally, this can be achieved by applying an external gate voltage in a device of suitable geometry to generate classically inaccessible potential barriers. Adjusting the gate voltage allows the characteristics of the dynamical invariant set responsible for transient chaos to be varied in a desirable manner which, in turn, can induce continuous changes in the statistical characteristics of the quantum conductance-fluctuation pattern. To understand the physical mechanism of our scheme, we develop a theory based on analyzing the spectrum of the generalized non-Hermitian Hamiltonian that includes the effect of leads, or electronic waveguides, as self-energy terms. As the escape rate of the underlying non-attracting chaotic set is increased, the imaginary part of the complex eigenenergy becomes increasingly large so that pointer states are more difficult to form, making smoother the conductance-fluctuation pattern.

  17. Quantum Transport Simulations of Nanoscale Materials

    KAUST Repository

    Obodo, Tobechukwu Joshua

    2016-01-07

    Nanoscale materials have many potential advantages because of their quantum confinement, cost and producibility by low-temperature chemical methods. Advancement of theoretical methods as well as the availability of modern high-performance supercomputers allow us to control and exploit their microscopic properties at the atomic scale, hence making it possible to design novel nanoscale molecular devices with interesting features (e.g switches, rectifiers, negative differential conductance, and high magnetoresistance). In this thesis, state-of-the-art theoretical calculations have been performed for the quantum transport properties of nano-structured materials within the framework of Density Functional Theory (DFT) and the Nonequilibrium Green\\'s Function (NEGF) formalism. The switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes is investigated. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond, and thus lowers the conductance. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. Also examined is current-induced migration of atoms in nanoscale devices that plays an important role for device operation and breakdown. We studied the migration of adatoms and defects in graphene and carbon nanotubes under finite bias. We demonstrate that current-induced forces within DFT are non-conservative, which so far has only been shown for model systems, and can lower migration barrier heights. Further, we investigated the quantum transport behavior of an experimentally observed diblock molecule by varying the amounts of phenyl (donor) and pyrimidinyl (acceptor) rings under finite bias. We show that a tandem configuration of

  18. Temperature Dependence of Abnormal Fano Resonance in Photon-Assisted Transport Through a Side-Coupled Quantum Dot

    Institute of Scientific and Technical Information of China (English)

    HU Yin; SONG Hong-Yan; DONG Zheng-Chao; WU Liu-Po; SHI Yao-Ming; ZHOU Shi-Ping

    2008-01-01

    We investigate transport through a perfect quantum wire with a side-coupled quantum dot under an ac find. Time-averaged complex conductance is formulated by using the nonequilibrium Green function (NGF) method. We find that the electron-photon interaction together with the quantum interference of Nectron wave function can lead to anti-resonance in the conductance, which is then useful for tuning coherence and phases of Nectrons. Meanwhile, we study the temperature dependence of the conductance. Interestingly, a peak-structure can be developed at the Fano resonance levels with increasing temperatures.

  19. Numerical and theoretical evaluations of AC losses for single and infinite numbers of superconductor strips with direct and alternating transport currents in external AC magnetic field

    Energy Technology Data Exchange (ETDEWEB)

    Kajikawa, K., E-mail: kajikawa@sc.kyushu-u.ac.j [Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Funaki, K. [Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Shikimachi, K.; Hirano, N.; Nagaya, S. [Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522 (Japan)

    2010-11-01

    AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean's critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.

  20. Numerical and theoretical evaluations of AC losses for single and infinite numbers of superconductor strips with direct and alternating transport currents in external AC magnetic field

    Science.gov (United States)

    Kajikawa, K.; Funaki, K.; Shikimachi, K.; Hirano, N.; Nagaya, S.

    2010-11-01

    AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean’s critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.

  1. Quantum Spin Transport in Mesoscopic Interferometer

    Directory of Open Access Journals (Sweden)

    Zein W. A.

    2007-10-01

    Full Text Available Spin-dependent conductance of ballistic mesoscopic interferometer is investigated. The quantum interferometer is in the form of ring, in which a quantum dot is embedded in one arm. This quantum dot is connected to one lead via tunnel barrier. Both Aharonov- Casher and Aharonov-Bohm e ects are studied. Our results confirm the interplay of spin-orbit coupling and quantum interference e ects in such confined quantum systems. This investigation is valuable for spintronics application, for example, quantum information processing.

  2. Limited Quantum Helium Transportation through Nano-channels by Quantum Fluctuation

    Science.gov (United States)

    Ohba, Tomonori

    2016-01-01

    Helium at low temperatures has unique quantum properties such as superfluidity, which causes it to behave differently from a classical fluid. Despite our deep understanding of quantum mechanics, there are many open questions concerning the properties of quantum fluids in nanoscale systems. Herein, the quantum behavior of helium transportation through one-dimensional nanopores was evaluated by measuring the adsorption of quantum helium in the nanopores of single-walled carbon nanohorns and AlPO4-5 at 2–5 K. Quantum helium was transported unimpeded through nanopores larger than 0.7 nm in diameter, whereas quantum helium transportation was significantly restricted through 0.4-nm and 0.6-nm nanopores. Conversely, nitrogen molecules diffused through the 0.4-nm nanopores at 77 K. Therefore, quantum helium behaved as a fluid comprising atoms larger than 0.4–0.6 nm. This phenomenon was remarkable, considering that helium is the smallest existing element with a (classical) size of approximately 0.27 nm. This finding revealed the presence of significant quantum fluctuations. Quantum fluctuation determined the behaviors of quantum flux and is essential to understanding unique quantum behaviors in nanoscale systems. PMID:27363671

  3. Limited Quantum Helium Transportation through Nano-channels by Quantum Fluctuation.

    Science.gov (United States)

    Ohba, Tomonori

    2016-01-01

    Helium at low temperatures has unique quantum properties such as superfluidity, which causes it to behave differently from a classical fluid. Despite our deep understanding of quantum mechanics, there are many open questions concerning the properties of quantum fluids in nanoscale systems. Herein, the quantum behavior of helium transportation through one-dimensional nanopores was evaluated by measuring the adsorption of quantum helium in the nanopores of single-walled carbon nanohorns and AlPO4-5 at 2-5 K. Quantum helium was transported unimpeded through nanopores larger than 0.7 nm in diameter, whereas quantum helium transportation was significantly restricted through 0.4-nm and 0.6-nm nanopores. Conversely, nitrogen molecules diffused through the 0.4-nm nanopores at 77 K. Therefore, quantum helium behaved as a fluid comprising atoms larger than 0.4-0.6 nm. This phenomenon was remarkable, considering that helium is the smallest existing element with a (classical) size of approximately 0.27 nm. This finding revealed the presence of significant quantum fluctuations. Quantum fluctuation determined the behaviors of quantum flux and is essential to understanding unique quantum behaviors in nanoscale systems. PMID:27363671

  4. Transport AC loss characteristics of a nine strand YBCO Roebel cable

    Science.gov (United States)

    Jiang, Zhenan; Badcock, R. A.; Long, N. J.; Staines, Mike; Thakur, K. P.; Lakshmi, L. S.; Wright, A.; Hamilton, K.; Sidorov, G. N.; Buckley, R. G.; Amemiya, Naoyuki; Caplin, A. D.

    2010-02-01

    Transport AC loss in a short length of 9/2 YBCO Roebel cable (nine 2 mm wide strands) is measured. The AC loss data are compared with those in a 5/2 YBCO Roebel cable (five 2 mm wide strands) as well as that in a single strand. All the strands composing the cables and the single strand are insulated and cut from the same stock material. The validity of the measurement method was reconfirmed by results at a range of frequencies. At a wide range of It/Ic, the normalized AC losses in the Roebel cable were around 6.2-6.7 times of those in the single strand. This is less than the nine times predicted for a tight bundle of nine conductors. The normalized transport AC losses in the 5/2 Roebel cable are much smaller than those in the 9/2 Roebel. This should be due to larger superposition of magnetic field in the 9/2 Roebel. The Ic of the 9/2 and 5/2 Roebel cables is determined by serial connection of the strands. This eliminates the effect where differing resistances in the current terminations cause uneven current sharing between strands when the strands are connected in parallel.

  5. Quantum dissipation theory and applications to quantum transport and quantum measurement in mesoscopic systems

    Science.gov (United States)

    Cui, Ping

    The thesis comprises two major themes of quantum statistical dynamics. One is the development of quantum dissipation theory (QDT). It covers the establishment of some basic relations of quantum statistical dynamics, the construction of several nonequivalent complete second-order formulations, and the development of exact QDT. Another is related to the applications of quantum statistical dynamics to a variety of research fields. In particular, unconventional but novel theories of the electron transfer in Debye solvents, quantum transport, and quantum measurement are developed on the basis of QDT formulations. The thesis is organized as follows. In Chapter 1, we present some background knowledge in relation to the aforementioned two themes of this thesis. The key quantity in QDT is the reduced density operator rho(t) ≡ trBrho T(t); i.e., the partial trace of the total system and bath composite rhoT(t) over the bath degrees of freedom. QDT governs the evolution of reduced density operator, where the effects of bath are treated in a quantum statistical manner. In principle, the reduced density operator contains all dynamics information of interest. However, the conventional quantum transport theory is formulated in terms of nonequilibrium Green's function. The newly emerging field of quantum measurement in relation to quantum information and quantum computing does exploit a sort of QDT formalism. Besides the background of the relevant theoretical development, some representative experiments on molecular nanojunctions are also briefly discussed. In chapter 2, we outline some basic (including new) relations that highlight several important issues on QDT. The content includes the background of nonequilibrium quantum statistical mechanics, the general description of the total composite Hamiltonian with stochastic system-bath interaction, a novel parameterization scheme for bath correlation functions, a newly developed exact theory of driven Brownian oscillator (DBO

  6. Quantum transport through 3D Dirac materials

    International Nuclear Information System (INIS)

    Bismuth and its alloys provide a paradigm to realize three dimensional materials whose low-energy effective theory is given by Dirac equation in 3+1 dimensions. We study the quantum transport properties of three dimensional Dirac materials within the framework of Landauer–Büttiker formalism. Charge carriers in normal metal satisfying the Schrödinger equation, can be split into four-component with appropriate matching conditions at the boundary with the three dimensional Dirac material (3DDM). We calculate the conductance and the Fano factor of an interface separating 3DDM from a normal metal, as well as the conductance through a slab of 3DDM. Under certain circumstances the 3DDM appears transparent to electrons hitting the 3DDM. We find that electrons hitting the metal-3DDM interface from metallic side can enter 3DDM in a reversed spin state as soon as their angle of incidence deviates from the direction perpendicular to interface. However the presence of a second interface completely cancels this effect

  7. Relativistic quantum transport theory for electrodynamics

    CERN Document Server

    Zhuang, P; Zhuang, P; Heinz, U

    1995-01-01

    We investigate the relationship between the covariant and the three-dimensional (equal-time) formulations of quantum kinetic theory. We show that the three-dimensional approach can be obtained as the energy average of the covariant formulation. We illustrate this statement in scalar and spinor QED. For scalar QED we derive Lorentz covariant transport and constraint equations directly from the Klein-Gordon equation rather than through the previously used Feshbach-Villars representation. We then consider pair production in a spatially homogeneous but time-dependent electric field and show that the pair density is derived much more easily via the energy averaging method than in the equal-time representation. Proceeding to spinor QED, we derive the covariant version of the equal-time equation derived by Bialynicki-Birula et al. We show that it must be supplemented by another self-adjoint equation to obtain a complete description of the covariant spinor Wigner operator. After spinor decomposition and energy averag...

  8. Quantum transport through 3D Dirac materials

    Energy Technology Data Exchange (ETDEWEB)

    Salehi, M. [Department of Physics, Sharif University of Technology, Tehran 11155-9161 (Iran, Islamic Republic of); Jafari, S.A., E-mail: jafari@physics.sharif.edu [Department of Physics, Sharif University of Technology, Tehran 11155-9161 (Iran, Islamic Republic of); Center of Excellence for Complex Systems and Condensed Matter (CSCM), Sharif University of Technology, Tehran 1458889694 (Iran, Islamic Republic of)

    2015-08-15

    Bismuth and its alloys provide a paradigm to realize three dimensional materials whose low-energy effective theory is given by Dirac equation in 3+1 dimensions. We study the quantum transport properties of three dimensional Dirac materials within the framework of Landauer–Büttiker formalism. Charge carriers in normal metal satisfying the Schrödinger equation, can be split into four-component with appropriate matching conditions at the boundary with the three dimensional Dirac material (3DDM). We calculate the conductance and the Fano factor of an interface separating 3DDM from a normal metal, as well as the conductance through a slab of 3DDM. Under certain circumstances the 3DDM appears transparent to electrons hitting the 3DDM. We find that electrons hitting the metal-3DDM interface from metallic side can enter 3DDM in a reversed spin state as soon as their angle of incidence deviates from the direction perpendicular to interface. However the presence of a second interface completely cancels this effect.

  9. Quantum transport through 3D Dirac materials

    Science.gov (United States)

    Salehi, M.; Jafari, S. A.

    2015-08-01

    Bismuth and its alloys provide a paradigm to realize three dimensional materials whose low-energy effective theory is given by Dirac equation in 3+1 dimensions. We study the quantum transport properties of three dimensional Dirac materials within the framework of Landauer-Büttiker formalism. Charge carriers in normal metal satisfying the Schrödinger equation, can be split into four-component with appropriate matching conditions at the boundary with the three dimensional Dirac material (3DDM). We calculate the conductance and the Fano factor of an interface separating 3DDM from a normal metal, as well as the conductance through a slab of 3DDM. Under certain circumstances the 3DDM appears transparent to electrons hitting the 3DDM. We find that electrons hitting the metal-3DDM interface from metallic side can enter 3DDM in a reversed spin state as soon as their angle of incidence deviates from the direction perpendicular to interface. However the presence of a second interface completely cancels this effect.

  10. Quantum kinetics in transport and optics of semincionductors

    CERN Document Server

    Haug, H

    2008-01-01

    Nanoscale miniaturization and femtosecond laser-pulse spectroscopy require a quantum mechanical description of the carrier kinetics that goes beyond the conventional Boltzmann theory. On these extremely short length and time scales the electrons behave like partially coherent waves. This monograph deals with quantum kinetics for transport in low-dimensional microstructures and for ultra-short laser pulse spectroscopy. The nonequilibrium Green function theory is described and used for the derivation of the quantum kinetic equations. Numerical methods for the solution of the retarded quantum kinetic equations are discussed and results are presented for high-field transport and for mesoscopic transport phenomena. Quantum beats, polarization decay, and non-Markovian behaviour are treated for femtosecond spectroscopy on a microscopic basis. Since the publishing of the first edition in 1996 the nonequilibrium Green function technique has been applied to a large number of new research topics, and the revised edition...

  11. Transport and AC loss properties of the repaired multifilamentary REBCO superconducting tapes

    Science.gov (United States)

    Yamasaki, S.; Iwakuma, M.; Funaki, K.; Kato, J.; Chikumoto, T.; Tanabe, K.; Nakao, K.; Izumi, T.; Yamada, Y.; Shiohara, Y.; Saito, T.

    2010-11-01

    For near-future applications of REBa 2Cu 3O 7 (REBCO) coated conductors to electric power cables, transformers and Superconducting Magnetic Energy Storage (SMES), the long taped wires with high performance in the transport properties have been designed and fabricated. Moreover, in order to drastically reduce AC losses in perpendicular field configuration, advanced multifilament YBCO coated conductors (MFYCCs) fabricated with technique of a laser scribing process have been also developed. In the present study, from engineering viewpoints to utilize such advanced conductors, we evaluated the transport and AC loss properties of short MFYCCs with a repaired part or a joint by a diffusion joint technique with the saddle-shaped pickup coil method.

  12. Transport and AC loss properties of the repaired multifilamentary REBCO superconducting tapes

    Energy Technology Data Exchange (ETDEWEB)

    Yamasaki, S. [Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Iwakuma, M., E-mail: iwakuma@sc.kyushu-u.ac.j [Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Funaki, K. [Research Institute of Superconductor Science and Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Kato, J.; Chikumoto, T.; Tanabe, K.; Nakao, K.; Izumi, T.; Yamada, Y.; Shiohara, Y. [International Superconductivity Technology Center, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062 (Japan); Saito, T. [Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135-8512 (Japan)

    2010-11-01

    For near-future applications of REBa{sub 2}Cu{sub 3}O{sub 7} (REBCO) coated conductors to electric power cables, transformers and Superconducting Magnetic Energy Storage (SMES), the long taped wires with high performance in the transport properties have been designed and fabricated. Moreover, in order to drastically reduce AC losses in perpendicular field configuration, advanced multifilament YBCO coated conductors (MFYCCs) fabricated with technique of a laser scribing process have been also developed. In the present study, from engineering viewpoints to utilize such advanced conductors, we evaluated the transport and AC loss properties of short MFYCCs with a repaired part or a joint by a diffusion joint technique with the saddle-shaped pickup coil method.

  13. Parallel transport quantum logic gates with trapped ions

    CERN Document Server

    de Clercq, Ludwig; Marinelli, Matteo; Nadlinger, David; Oswald, Robin; Negnevitsky, Vlad; Kienzler, Daniel; Keitch, Ben; Home, Jonathan P

    2015-01-01

    Quantum information processing will require combinations of gate operations and communication, with each applied in parallel to large numbers of quantum systems. These tasks are often performed sequentially, with gates implemented by pulsed fields and information transported either by moving the physical qubits or using photonic links. For trapped ions, an alternative approach is to implement quantum logic gates by transporting the ions through static laser beams, combining qubit operations with transport. This has significant advantages for scalability since the voltage waveforms required for transport can potentially be generated using micro-electronics integrated into the trap structure itself, while both optical and microwave control elements are significantly more bulky. Using a multi-zone ion trap, we demonstrate transport gates on a qubit encoded in the hyperfine structure of a beryllium ion. We show the ability to perform sequences of operations, and to perform parallel gates on two ions transported t...

  14. Chaotic transport of particles in two-dimensional periodic potentials driven by ac forces

    OpenAIRE

    Guantes, R.; Miret-Artés, Salvador

    2003-01-01

    The diffusive and directed transport of particles in a two-dimensional peridoic potential was studied. The model represents diffusion of atoms adsorbed on metal surfaces under an applied ac electric field in the low-temperature limit. The results show that the second dimension and the potential energy coupling play an important role on both diffusion and net currents, depending on the direction of the drive.

  15. Numerical Evidence for Robustness of Environment-Assisted Quantum Transport

    CERN Document Server

    Shabani, A; Rabitz., H; Lloyd, S

    2014-01-01

    Recent theoretical studies show that decoherence process can enhance transport efficiency in quantum systems. This effect is known as environment-assisted quantum transport (ENAQT). The role of ENAQT in optimal quantum transport is well investigated, however, it is less known how robust ENAQT is with respect to variations in the system or its environment characteristic. Toward answering this question, we simulated excitonic energy transfer in Fenna-Matthews-Olson (FMO) photosynthetic complex. We found that ENAQT is robust with respect to many relevant parameters of environmental interactions and Frenkel-exciton Hamiltonian including reorganization energy, bath frequency cutoff, temperature, and initial excitations, dissipation rate, trapping rate, disorders, and dipole moments orientations. Our study suggests that the ENAQT phenomenon can be exploited in robust design of highly efficient quantum transport systems.

  16. AC transport current loss analysis for a face-to-face stack of superconducting tapes

    Energy Technology Data Exchange (ETDEWEB)

    Yoo, Jaeun [Dept. of Physics, Chonbuk National University, Jeonju (Korea, Republic of); Youm, Dojun [Dept. of Physics, Korea Advanced Institute of Science and Technology, Daejeon (Korea, Republic of); Oh, Sang Soo [Superconducting Materials Research Group, KERI, Changwon (Korea, Republic of)

    2013-06-15

    AC Losses for face to face stacks of four identical coated conductors (CCs) were numerically calculated using the H-formulation combined with the E-J power law and the Kim model. The motive sample was the face to face stack of four 2 mm-wide CC tapes with 2 μm thick superconducting layer of which the critical current density, J{sub c}, was 2.16 x 10{sup 6} A/cm{sup 2} on IBAD-MgO template, which was suggested for the mitigation of ac loss as a round shaped wire by Korea Electrotechnology Research Institute. For the calculation the cross section of the stack was simply modeled as vertically aligned 4 rectangles of superconducting (SC) layers withE=E{sub o}(J(x,y,t)/J{sub c}(B)){sup n} in x-y plane where E{sub o} was 10{sup -6} V/cm, J{sub c} (B) was the field dependence of current density and n was 21. The field dependence of the critical current of the sample measured in four-probe method was employed for J{sub c} (B) in the equation. The model was implemented in the finite element method program by commercial software. The ac loss properties for the stacks were compared with those of single 4 cm-wide SC layers with the same critical current density or the same critical current. The constraint for the simulation was imposed in two different ways that the total current of the stack obtained by integrating J(x,y,t) over the cross sections was the same as that of the applied transport current: one is that one fourth of the external current was enforced to flow through each SC. In this case, the ac loss values for the stacks were lower than those of single wide SC layer. This mitigation of the loss is attributed to the reduction of the normal component of the magnetic field near the SC layers due to the strong expulsion of the magnetic field by the enforced transport current. On the contrary, for the other case of no such enforcement, the ac loss values were greater than those of single 4cm-wide SC layer and. In this case, the phase difference of the current flowing

  17. Quantum thermometry using the ac Stark shift within the Rabi model

    CERN Document Server

    Higgins, Kieran D B; Gauger, Erik M

    2012-01-01

    A quantum two level system coupled to a harmonic oscillator represents a ubiquitous physical system. New experiments in circuit QED and nano-electromechanical systems (NEMS) achieve unprecedented coupling strength at large detuning between qubit and oscillator, thus requiring a theoretical treatment beyond the Jaynes Cummings model. Here we present a new method for describing the qubit dynamics in this regime, based on an oscillator correlation function expansion of a non-Markovian master equation in the polaron frame. Our technique yields a new numerical method as well as a succinct approximate expression for the qubit dynamics. We obtain a new expression for the ac Stark shift and show that this enables practical and precise qubit thermometry of an oscillator.

  18. Quantum chemistry and charge transport in biomolecules with superconducting circuits

    Science.gov (United States)

    García-Álvarez, L.; Las Heras, U.; Mezzacapo, A.; Sanz, M.; Solano, E.; Lamata, L.

    2016-01-01

    We propose an efficient protocol for digital quantum simulation of quantum chemistry problems and enhanced digital-analog quantum simulation of transport phenomena in biomolecules with superconducting circuits. Along these lines, we optimally digitize fermionic models of molecular structure with single-qubit and two-qubit gates, by means of Trotter-Suzuki decomposition and Jordan-Wigner transformation. Furthermore, we address the modelling of system-environment interactions of biomolecules involving bosonic degrees of freedom with a digital-analog approach. Finally, we consider gate-truncated quantum algorithms to allow the study of environmental effects. PMID:27324814

  19. Quantum chemistry and charge transport in biomolecules with superconducting circuits

    Science.gov (United States)

    García-Álvarez, L.; Las Heras, U.; Mezzacapo, A.; Sanz, M.; Solano, E.; Lamata, L.

    2016-06-01

    We propose an efficient protocol for digital quantum simulation of quantum chemistry problems and enhanced digital-analog quantum simulation of transport phenomena in biomolecules with superconducting circuits. Along these lines, we optimally digitize fermionic models of molecular structure with single-qubit and two-qubit gates, by means of Trotter-Suzuki decomposition and Jordan-Wigner transformation. Furthermore, we address the modelling of system-environment interactions of biomolecules involving bosonic degrees of freedom with a digital-analog approach. Finally, we consider gate-truncated quantum algorithms to allow the study of environmental effects.

  20. Transport through quantum wells and superlattices on topological insulator surfaces.

    Science.gov (United States)

    Song, J-T; Li, Y-X; Sun, Q-F

    2014-05-01

    We investigate electron transmission coefficients through quantum wells and quantum superlattices on topological insulator surfaces. The quantum well or superlattice is not constituted by general electronic potential barriers but by Fermi velocity barriers which originate in the different topological insulator surfaces. It is found that electron resonant modes can be renormalized by quantum wells and more clearly by quantum superlattices. The depth and width of a quantum well and superlattice, the incident angle of an electron, and the Fermi energy can be used to effectively tune the electron resonant modes. In particular, the number N of periodic structures that constitute a superlattice can further strengthen these regulating effects. These results suggest that a device could be developed to select and regulate electron propagation modes on topological insulator surfaces. Finally, we also study the conductance and the Fano factor through quantum wells and quantum superlattices. In contrast to what has been reported before, the suppression factors of 0.4 in the conductance and 0.85 in the Fano factor are observed in a quantum well, while the transport for a quantum superlattice shows strong oscillating behavior at low energy and reaches the same saturated values as in the case of a quantum well at sufficiently large energies. PMID:24759077

  1. Long-distance quantum transport dynamics in macromolecules

    Science.gov (United States)

    Schneider, E.; Faccioli, P.

    2014-04-01

    Using renormalization group methods, we develop a rigorous coarse-grained representation of the dissipative dynamics of quantum excitations propagating inside open macromolecular systems. We show that, at very low spatial resolution, this quantum transport theory reduces to a modified Brownian process, in which quantum delocalization effects are accounted for by means of an effective term in the Onsager-Machlup functional. Using this formulation, we derive a simple analytic solution for the time-dependent probability of observing the quantum excitation at a given point in the macromolecule. This formula can be used to predict the migration of natural or charged quantum excitations in a variety of molecular systems, including biological and organic polymers, organic crystalline transistors, or photosynthetic complexes. For illustration purposes, we apply this method to investigate inelastic electronic hole transport in a long homo-DNA chain.

  2. Quantum coherence in ion channels: Resonances, Transport and Verification

    CERN Document Server

    Vaziri, A

    2010-01-01

    Recently it was demonstrated that long-lived quantum coherence exists during excitation energy transport in photosynthesis. It is a valid question up to which length, time and mass scales quantum coherence may extend, how to one may detect this coherence and what if any role it plays for the dynamics of the system. Here we suggest that the selectivity filter of ion channels may exhibit quantum coherence which might be relevant for the process of ion selectivity and conduction. We show that quantum resonances could provide an alternative approch to ultrafast 2D spectroscopy to probe these quantum coherences. We demonstrate that the emergence of resonances in the conduction of ion channels that are modulated periodicallly by time dependent external electric fields can serve as signitures of quantum coherence in such a system. Assessments of experimental feasibility and specific paths towards the experimental realization of such experiments are presented. We show that this may be probed by direct 2-D spectroscop...

  3. Transport of quantum states of periodically driven systems

    OpenAIRE

    Breuer, H. P.; Dietz, K.; Holthaus, M.

    1990-01-01

    We discuss the transport of quantum states on quasi-energy surfaces of periodically driven systems and establish their non-trivial structure. The latter is shown to be caused by diabatic transitions at lines of narrow avoided crossings. Some experimental consequences pertaining to adiabatic transport and Landau-Zener transitions among Floquet states are briefly sketched.

  4. Incoherent transport in clean quantum critical metals

    CERN Document Server

    Davison, Richard A; Hartnoll, Sean A

    2015-01-01

    In a clean quantum critical metal, and in the absence of umklapp, most d.c. conductivities are formally infinite due to momentum conservation. However, there is a particular combination of the charge and heat currents which has a finite, universal conductivity. In this paper, we describe the physics of this conductivity $\\sigma_Q$ in quantum critical metals obtained by charge doping a strongly interacting conformal field theory. We show that it satisfies an Einstein relation and controls the diffusivity of a conserved charge in the metal. We compute $\\sigma_Q$ in a class of theories with holographic gravitational duals. Finally, we show how the temperature scaling of $\\sigma_Q$ depends on certain critical exponents characterizing the quantum critical metal. The holographic results are found to be reproduced by the scaling analysis, with the charge density operator becoming marginal in the emergent low energy quantum critical theory.

  5. Hidden symmetries enhance quantum transport in Light Harvesting systems

    CERN Document Server

    Zech, Tobias; Wellens, Thomas; Buchleitner, Andreas

    2012-01-01

    For more than 50 years we have known that photosynthetic systems harvest solar energy with almost unit {\\it quantum efficiency}. However, recent experimental evidence of {\\it quantum coherence} during the excitonic energy transport in photosynthetic organisms challenges our understanding of this fundamental biological function. Currently, and despite numerous efforts, the causal connection between coherence and efficiency is still a matter of debate. We show, through the study of extensive simulations of quantum coherent transport on networks, that three dimensional structures characterized by centro-symmetric Hamiltonians are statistically more efficient than random arrangements. Moreover, we demonstrate that the experimental data available for the electronic Hamiltonians of the Fenna-Mathew-Olson (FMO) complex of sulfur bacteria and of the crypophyte PC645 complex of marine algae are consistent with this strong correlation of centro-symmetry with quantum efficiency. These results show that what appears to b...

  6. Scattering matrix approach to non-stationary quantum transport

    CERN Document Server

    Moskalets, Michael V

    2012-01-01

    The aim of this book is to introduce the basic elements of the scattering matrix approach to transport phenomena in dynamical quantum systems of non-interacting electrons. This approach admits a physically clear and transparent description of transport processes in dynamical mesoscopic systems promising basic elements of solid-state devices for quantum information processing. One of the key effects, the quantum pump effect, is considered in detail. In addition, the theory for a recently implemented new dynamical source - injecting electrons with time delay much larger than the electron coherence time - is offered. This theory provides a simple description of quantum circuits with such a single-particle source and shows in an unambiguous way that the tunability inherent to the dynamical systems leads to a number of unexpected but fundamental effects.

  7. Background charges and quantum effects in quantum dots transport spectroscopy

    OpenAIRE

    Pierre M.; Hofheinz M.; Jehl X.; Sanquer M.; Molas G.; Vinet M.; Deleonibus S.

    2009-01-01

    We extend a simple model of a charge trap coupled to a single-electron box to energy ranges and parameters such that it gives new insights and predictions readily observable in many experimental systems. We show that a single background charge is enough to give lines of differential conductance in the stability diagram of the quantum dot, even within undistorted Coulomb diamonds. It also suppresses the current near degeneracy of the impurity charge, and yields negative differential lines far ...

  8. DC and AC transport properties on La0.8Sr0.2MnO3

    Institute of Scientific and Technical Information of China (English)

    Zhantao Wei; Xinsheng Yang; Li Lv; Min Zhang; Yong Zhang

    2014-01-01

    Magnetoresistive sensor can be widely used in modern transportation field, such as the vehicle positioning and navigation system, vehicle detection system, and intelligent transportation system. In order to improve the efficiency of magnetoresistive sensor, we synthesized La0.8Sr0.2MnO3 polycrystalline bulks at different sintering temperatures and investigated their DC and AC transport properties in this work. As a result, all samples showed insulator-metal (I-M) phase transition, and the transition temperature (TI-M) shifted to higher temperature with the increase of sintering temperature. The TI-M measured at different AC frequencies was smaller than that measured at DC condition, which implied that the I-M phase transition was suppressed at AC frequencies. The resistivity mea-sured at high AC frequencies was larger than that measured at low AC frequencies, which could be attributed to the change of the magnetic penetration depth (d). However, the room-temperature AC-magnetoresistance (MR) at low frequencies was much larger than that at high frequencies and room-temperature DC-MR. These findings demon-strate that reducing the AC frequency is an effective way for enhancing the room-temperature MR, which can be used to promote the efficiency of magnetoresistive sensor.

  9. Spin and edge channel dependent transport through quantum dots

    International Nuclear Information System (INIS)

    We investigate the influence of spin polarized currents and non-equilibrated edge channels on the transport properties of a single quantum dot. Polarized currents are realized by the manual depletion of edge channels in high magnetic fields via a metallic top gate covering the source contact in the system. We observe a suppression and enhancement in the conductance of the quantum dot dependent on the edge channel configuration in the leads.

  10. Spin and edge channel dependent transport through quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Ridder, T; Rogge, M C; Haug, R J [Institut fuer Festkoerperphysik, Gottfried Wilhelm Leibniz Universitaet Hannover, Appelstrasse 2, D-30167 Hannover (Germany)], E-mail: ridder@nano.uni-hannover.de

    2008-11-12

    We investigate the influence of spin polarized currents and non-equilibrated edge channels on the transport properties of a single quantum dot. Polarized currents are realized by the manual depletion of edge channels in high magnetic fields via a metallic top gate covering the source contact in the system. We observe a suppression and enhancement in the conductance of the quantum dot dependent on the edge channel configuration in the leads.

  11. Control of exciton transport using quantum interference

    Science.gov (United States)

    Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.

    2015-12-01

    It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.

  12. Nonequilibrium Electron Transport Through a Quantum Dot from Kubo Formula

    Institute of Scientific and Technical Information of China (English)

    L(U) Rong; ZHANG Guang-Ming

    2005-01-01

    Based on the Kubo formula for an electron tunneling junction, we revisit the nonequilibrium transport properties through a quantum dot. Since the Fermi level of the quantum dot is set by the conduction electrons of the leads, we calculate the electron current from the left side by assuming the quantum dot coupled to the right lead as another side of the tunneling junction, and the other way round is used to calculate the current from the right side. By symmetrizing these two currents, an effective local density states on the dot can be obtained, and is discussed at high and low temperatures, respectively.

  13. Structure of Quantum Chaotic Wavefunctions Ergodicity, Localization, and Transport

    CERN Document Server

    Kaplan, L

    1999-01-01

    We discuss recent developments in the study of quantum wavefunctions and transport in classically ergodic systems. Surprisingly, short-time classical dynamics leaves permanent imprints on long-time and stationary quantum behavior, which are absent from the long-time classical motion. These imprints can lead to quantum behavior on single-wavelength or single-channel scales which are very different from random matrix theory expectations. Robust and quantitative predictions are obtained using semiclassical methods. Applications to wavefunction intensity statistics and to resonances in open systems are discussed.

  14. Topological Effects on Quantum Phase Slips in Superfluid Spin Transport

    Science.gov (United States)

    Kim, Se Kwon; Tserkovnyak, Yaroslav

    2016-03-01

    We theoretically investigate effects of quantum fluctuations on superfluid spin transport through easy-plane quantum antiferromagnetic spin chains in the large-spin limit. Quantum fluctuations result in the decaying spin supercurrent by unwinding the magnetic order parameter within the easy plane, which is referred to as phase slips. We show that the topological term in the nonlinear sigma model for the spin chains qualitatively differentiates the decaying rate of the spin supercurrent between the integer versus half-odd-integer spin chains. An experimental setup for a magnetoelectric circuit is proposed, in which the dependence of the decaying rate on constituent spins can be verified by measuring the nonlocal magnetoresistance.

  15. Centrosymmetry enhances quantum transport in disordered molecular networks

    Science.gov (United States)

    Zech, Tobias; Mulet, Roberto; Wellens, Thomas; Buchleitner, Andreas

    2014-05-01

    For more than 50 years we have known that photosynthetic systems harvest solar energy with almost unit quantum efficiency. However, recent experimental evidence of quantum coherence during the excitonic energy transport in photosynthetic organisms challenges our understanding of this fundamental biological function. Currently, and despite numerous efforts, the causal connection between coherence and efficiency is still a matter of debate. We show, through extensive simulations of quantum coherent transport on networks, that three dimensional structures characterized by centro-symmetric Hamiltonians are statistically more efficient than random arrangements. Moreover, a strong correlation of centro-symmetry with quantum efficiency is also observed under the coherent transport dynamics induced by experimentally estimated electronic Hamiltonians of the Fenna-Mathew-Olson complex of sulfur bacteria and of the cryptophyte PC645 complex of marine algae. The application of a genetic algorithm results in a set of optimized Hamiltonians only when seeded from the experimentally estimated Hamiltonian. These results suggest that what appears to be geometrically disordered complexes may well exhibit an inherent hidden symmetry which enhances the energy transport between chromophores. We are confident that our results will motivate research to explore the properties of nearly centro-symmetric Hamiltonians in realistic environments, and to unveil the role of symmetries for quantum effects in biology. The unravelling of such symmetries may open novel perspectives and suggest new design principles in the development of artificial devices.

  16. Effects of bias on dynamics of an AC-driven two-electron quantum-dot molecule

    Institute of Scientific and Technical Information of China (English)

    Wang Li-Min; Duan Su-Qing; Zhao Xian-Geng; Liu Cheng-Shi

    2005-01-01

    The effects of bias on the dynamical localization of two interacting electrons in a pair of coupled quantum dots driven by external AC fields have been numerically investigated. With an effective two-site model and Floquet formalism,the time-dependent Schrodinger equation is numerically solved and the Pmin, the minimum of the population evolution of the initial state within a certain time period, is used to quantify the degree of the dynamical localization. Results indicate that the bias can change the energy of the initial state and break the dynamical symmetry of the system with a pure AC field. And the amplitude of the AC field with dynamical localization phenomenon changes with bias. All the numerical results are explained by the perturbation theory and two-level approximation.

  17. Transport ac loss of elliptical thin strips with a power-law E(J) relation

    Science.gov (United States)

    Jia, Chen-Xi; Chen, Du-Xing; Li, Shuo; Fang, Jin

    2015-10-01

    The transport ac loss Q of an elliptical thin strip of critical current I c with a power-law relation E\\propto {J}n is accurately computed as a function of current amplitude I m and frequency f. The resulting Q({I}m) is normalized to q({i}m) following the Norris critical-state formula, and converted to {q}*({i}m*) at a critical frequency f c based on a transport scaling law. Having a set of {q}*({i}m*) at several values of n as a base, a general expression of {q}*({i}m*,n) is obtained, which can be used to easily calculate q({i}m) for any practical purposes.

  18. Transport ac loss in a rectangular thin strip with power-law E(J) relation

    Energy Technology Data Exchange (ETDEWEB)

    Li, Shuo [School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044 (China); Chen, Du-Xing, E-mail: chendx3008@hotmail.com [Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona (Spain); Fan, Yu; Fang, Jin [School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044 (China)

    2015-01-15

    Highlights: • Transport ac loss in a thin strip with power-law E(J) is systematically computed. • The scaled results can be accurately used for strips with any critical current and frequency. • Experiments may be unambiguously compared with modeling results at a critical frequency. - Abstract: Transport ac losses of a rectangular thin strip obeying relation E/E{sub c}=(J/J{sub c}){sup n} with a fixed critical current I{sub c} and n=5,10,20,30, and 40 are accurately computed at a fixed frequency f as functions of the current amplitude I{sub m}. The results may be interpolated and scaled to those at any values of I{sub c},f, and 5⩽n⩽40. Normalized in the same way as that in Norris’ analytical formula derived from the critical-state model and converting f to a critical frequency f{sub c}, the modeling results may be better compared with the Norris formula and experimental data. A complete set of calculated modeling data are given with necessary formulas to be easily used by experimentalists in any particular case.

  19. Theory of quantum transport at nanoscale an introduction

    CERN Document Server

    Ryndyk, Dmitry A

    2016-01-01

    This book is an introduction to a rapidly developing field of modern theoretical physics – the theory of quantum transport at nanoscale. The theoretical methods considered in the book are in the basis of our understanding of charge, spin and heat transport in nanostructures and nanostructured materials and are widely used in nanoelectronics, molecular electronics, spin-dependent electronics (spintronics) and bio-electronics. The book is based on lectures for graduate and post-graduate students at the University of Regensburg and the Technische Universität Dresden (TU Dresden). The first part is devoted to the basic concepts of quantum transport: Landauer-Büttiker method and matrix Green function formalism for coherent transport, Tunneling (Transfer) Hamiltonian and master equation methods for tunneling, Coulomb blockade, vibrons and polarons. The results in this part are obtained as possible without sophisticated techniques, such as nonequilibrium Green functions, which are considered in detail in the...

  20. Quantum Model of Energy Transport in Collagen Molecules

    Institute of Scientific and Technical Information of China (English)

    XIAO Yi; LIN Xian-Zhe

    2001-01-01

    A semi-quantum model for energy transport in collagen molecules is presented. Soliton-like dynamics of this model is investigated numerically without and with the temperature effect taking into account. It is found that in both the cases energy can transport for a long distance along the collagen chain. This indicates that collagen molecules can be taken as a candidate for the acupuncture channel.

  1. Number-resolved master equation approach to quantum measurement and quantum transport

    Science.gov (United States)

    Li, Xin-Qi

    2016-08-01

    In addition to the well-known Landauer-Büttiker scattering theory and the nonequilibrium Green's function technique for mesoscopic transports, an alternative (and very useful) scheme is quantum master equation approach. In this article, we review the particle-number ( n)-resolved master equation ( n-ME) approach and its systematic applications in quantum measurement and quantum transport problems. The n-ME contains rich dynamical information, allowing efficient study of topics such as shot noise and full counting statistics analysis. Moreover, we also review a newly developed master equation approach (and its n-resolved version) under self-consistent Born approximation. The application potential of this new approach is critically examined via its ability to recover the exact results for noninteracting systems under arbitrary voltage and in presence of strong quantum interference, and the challenging non-equilibrium Kondo effect.

  2. Nonlinearly-enhanced energy transport in many dimensional quantum chaos

    KAUST Repository

    Brambila, D. S.

    2013-08-05

    By employing a nonlinear quantum kicked rotor model, we investigate the transport of energy in multidimensional quantum chaos. This problem has profound implications in many fields of science ranging from Anderson localization to time reversal of classical and quantum waves. We begin our analysis with a series of parallel numerical simulations, whose results show an unexpected and anomalous behavior. We tackle the problem by a fully analytical approach characterized by Lie groups and solitons theory, demonstrating the existence of a universal, nonlinearly-enhanced diffusion of the energy in the system, which is entirely sustained by soliton waves. Numerical simulations, performed with different models, show a perfect agreement with universal predictions. A realistic experiment is discussed in two dimensional dipolar Bose-Einstein-Condensates (BEC). Besides the obvious implications at the fundamental level, our results show that solitons can form the building block for the realization of new systems for the enhanced transport of matter.

  3. Quantum quasi-steady states in current transport

    Science.gov (United States)

    D'Agosta, Roberto; Zwolak, Michael; di Ventra, Massimiliano

    2007-03-01

    We investigate quasi-steady state solutions to transport in quantum systems by finding states which at some time minimize the change in density throughout all space and have a given current density flowing from one part of the system to another [1]. Contrary to classical dynamics, in a quantum mechanical system there are many states with a given energy and particle number which satisfy this minimization criterion. Taking as an example spinless fermions on a one-dimensional lattice, we explicitly show the phase space of a class of quasi-steady states. We also discuss the possibility of coherent and incoherent mixing of these steady state solutions leading to a new type of noise in quantum transport. [1] M. Di Ventra and T.N. Todorov J. Phys. Cond. Matt. 16, 8025 (2004).

  4. Effects of spin-orbit coupling on quantum transport

    NARCIS (Netherlands)

    Bardarson, Jens Hjorleifur

    2008-01-01

    The effect of spin-orbit coupling on various quantum transport phenomena is considered. The main topics discussed are: * How spin-orbit coupling can induce shot noise through trajectory splitting. * How spin-orbit coupling can degrade electron-hole entanglement (created by a tunnel barrier) by mo

  5. The Landauer-Büttiker formula and resonant quantum transport

    DEFF Research Database (Denmark)

    Cornean, Horia Decebal; Jensen, Arne; Moldoveanu, Valeriu

    We give a short presentation of two recent results. The firrst one is a rigorous proof of the Landauer-Büttiker formula, and the second one concerns resonant quantum transport. The detailed results are in [2]. In the last section we present the results of some numerical computations on a model...

  6. The Landauer-Büttiker formula and resonant quantum transport

    DEFF Research Database (Denmark)

    Cornean, Horia; Jensen, Arne; Moldoveanu, Valeriu

    2006-01-01

    We give a short presentation of two recent results. The first one is a rigorous proof of the Landauer-Büttiker formula, and the second one concerns resonant quantum transport. The detailed results are in [2]. In the last section we present the results of som numerical computations on a model system...

  7. Chaotic Dynamics and Transport in Classical and Quantum Systems

    Energy Technology Data Exchange (ETDEWEB)

    NONE

    2003-07-01

    The aim of this summer school is to provide a set of extended and pedagogical lectures, on the major present-day topics in dynamical systems and statistical mechanics including applications. Some articles are dedicated to chaotic transport in plasma turbulence and to quantum chaos. This document gathers the summaries of some presentations.

  8. Observation of quantum interference in molecular charge transport

    DEFF Research Database (Denmark)

    Guedon, Constant M.; Valkenier, Hennie; Markussen, Troels;

    2012-01-01

    for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid p-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface...

  9. What is novel in quantum transport for mesoscopics?

    Indian Academy of Sciences (India)

    Mukunda P Das; Frederick Green

    2006-07-01

    The understanding of mesoscopic transport has now attained an ultimate simplicity. Indeed, orthodox quantum kinetics would seem to say little about mesoscopics that has not been revealed – nearly effortlessly – by more popular means. Such is far from the case, however. The fact that kinetic theory remains very much in charge is best appreciated through the physics of a quantum point contact. While discretization of its conductance is viewed as the exclusive result of coherent, single-electron-wave transmission, this does not begin to address the paramount feature of all metallic conduction: dissipation. A perfect quantum point contact still has finite resistance, so its ballistic carriers must dissipate the energy gained from the applied field. How do they manage that? The key is in standard many-body quantum theory, and its conservation principles.

  10. Efficient calculation of dissipative quantum transport properties in semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Greck, Peter

    2012-11-26

    We present a novel quantum transport method that follows the non-equilibrium Green's function (NEGF) framework but side steps any self-consistent calculation of lesser self-energies by replacing them by a quasi-equilibrium expression. We termed this method the multi-scattering Buettiker-Probe (MSB) method. It generalizes the so-called Buettiker-Probe model but takes into account all relevant individual scattering mechanisms. It is orders of magnitude more efficient than a fully selfconsistent non-equilibrium Green's function calculation for realistic devices, yet accurately reproduces the results of the latter method as well as experimental data. This method is fairly easy to implement and opens the path towards realistic three-dimensional quantum transport calculations. In this work, we review the fundamentals of the non-equilibrium Green's function formalism for quantum transport calculations. Then, we introduce our novel MSB method after briefly reviewing the original Buettiker-Probe model. Finally, we compare the results of the MSB method to NEGF calculations as well as to experimental data. In particular, we calculate quantum transport properties of quantum cascade lasers in the terahertz (THz) and the mid-infrared (MIR) spectral domain. With a device optimization algorithm based upon the MSB method, we propose a novel THz quantum cascade laser design. It uses a two-well period with alternating barrier heights and complete carrier thermalization for the majority of the carriers within each period. We predict THz laser operation for temperatures up to 250 K implying a new temperature record.

  11. Hydrodynamic transport functions from quantum kinetic theory

    CERN Document Server

    Calzetta, E A; Ramsey, S

    2000-01-01

    Starting from the quantum kinetic field theory [E. Calzetta and B. L. Hu, Phys. Rev. D37, 2878 (1988)] constructed from the closed-time-path (CTP), two-particle-irreducible (2PI) effective action we show how to compute from first principles the shear and bulk viscosity functions in the hydrodynamic-thermodynamic regime. For a real scalar field with $\\lambda \\Phi ^{4}$ self-interaction we need to include 4 loop graphs in the equation of motion. This work provides a microscopic field-theoretical basis to the ``effective kinetic theory'' proposed by Jeon and Yaffe [S. Jeon and L. G. Yaffe, Phys. Rev. D53, 5799 (1996)], while our result for the bulk viscosity reproduces their expression derived from linear response theory and the imaginary-time formalism of thermal field theory. Though unavoidably involved in calculations of this sort, we feel that the approach using fundamental quantum kinetic field theory is conceptually clearer and methodically simpler than the effective kinetic theory approach, as the success...

  12. SymGF: A Symbolic Tool for Quantum Transport Theory

    Science.gov (United States)

    Feng, Zi Min

    In this thesis, I report the development and application of a symbolic derivation tool named "SymGF'' - standing for Symbolic Green's Function, that can automatically and analytically derive quantum transport expressions and the associated Keldysh nonequilibrium Green's functions (NEGF). Quantum transport happens in open systems consisting of a scattering region coupled to external electrodes. When there are strong electron-electron interactions in the scattering region, analytical derivations of the Green's functions can be very tedious and error prone. Running on a personal computer, SymGF derives the necessary analytical formulas at a level of correlation specified by the user, using the equation of motion (EOM) method. The input to SymGF are the second quantized form the device Hamiltonian, the (anti)commutators of the operators that appear in the Hamiltonian, and a truncation rule for the correlators which determines the accuracy of the final outcome. The output of SymGF are the analytical expressions of transport properties such as electric current and conductance in terms of various Green's functions; as well as the Green's functions themselves in terms of the unperturbed non-interacting Green's functions that can be obtained straightforwardly. For systems where electron-electron interaction can be neglected, the transport problems can be easily solved and SymGF is not necessary - even though SymGF gives the same answer; but for interacting systems SymGF drastically reduces the mathematical burden of analytical derivations. We have tested SymGF for several transport problems involving Kondo resonances where analytical derivations were done by humans: exactly the same results were obtained by SymGF but in a tiny fraction of time. We have applied SymGF to new and very hard problems that resist analytical derivations by hand, including quantum transport in a double quantum dot system; transport through a single quantum dot in parallel to a direct lead

  13. Physical and electrical models for interpreting AC and DC transport measurements in polymer solar cells

    Science.gov (United States)

    McIntyre, Max; Tzolov, Marian; Cossel, Raquel; Peeler, Seth

    We have fabricated and studied bulk heterojunction solar cells using a mixture of the low bandgap material PCPDTBT and PCBM-C60. Our transport studies show that the devices in dark have good rectification and they respond to AC voltage as a simple RC circuit. The illumination causes an additional contribution to the impedance, which varies with the level of illumination. One proposed model is that photo-generated charges can become trapped in potential wells. These charges then follow a Debye relaxation process, which contributes to a varying dielectric constant. Another proposed model is based on a RC circuit model with two capacitors which can describe the varying capacitance behavior. The physical mechanism for this model is that photo-generated charges become accumulated at the interface between PCPDTBT and PCBM-C60 and form an additional layer of charge. We will show that our circuit models and their analogous physical models can predict the AC and DC responses of polymer solar cells.

  14. Charge transport through a semiconductor quantum dot-ring nanostructure

    International Nuclear Information System (INIS)

    Transport properties of a gated nanostructure depend crucially on the coupling of its states to the states of electrodes. In the case of a single quantum dot the coupling, for a given quantum state, is constant or can be slightly modified by additional gating. In this paper we consider a concentric dot–ring nanostructure (DRN) and show that its transport properties can be drastically modified due to the unique geometry. We calculate the dc current through a DRN in the Coulomb blockade regime and show that it can efficiently work as a single-electron transistor (SET) or a current rectifier. In both cases the transport characteristics strongly depend on the details of the confinement potential. The calculations are carried out for low and high bias regime, the latter being especially interesting in the context of current rectification due to fast relaxation processes. (paper)

  15. Edge-state blockade of transport in quantum dot arrays

    Science.gov (United States)

    Benito, Mónica; Niklas, Michael; Platero, Gloria; Kohler, Sigmund

    2016-03-01

    We propose a transport blockade mechanism in quantum dot arrays and conducting molecules based on an interplay of Coulomb repulsion and the formation of edge states. As a model we employ a dimer chain that exhibits a topological phase transition. The connection to a strongly biased electron source and drain enables transport. We show that the related emergence of edge states is manifest in the shot noise properties as it is accompanied by a crossover from bunched electron transport to a Poissonian process. For both regions we develop a scenario that can be captured by a rate equation. The resulting analytical expressions for the Fano factor agree well with the numerical solution of a full quantum master equation.

  16. Hydrodynamic transport in strongly coupled disordered quantum field theories

    CERN Document Server

    Lucas, Andrew

    2015-01-01

    We compute direct current (dc) thermoelectric transport coefficients in strongly coupled quantum field theories without long lived quasiparticles, at finite temperature and charge density, and disordered on long wavelengths compared to the length scale of local thermalization. Many previous transport computations in strongly coupled systems are interpretable hydrodynamically, despite formally going beyond the hydrodynamic regime. This includes momentum relaxation times previously derived by the memory matrix formalism, and non-perturbative holographic results; in the latter case, this is subject to some important subtleties. Our formalism may extend some memory matrix computations to higher orders in the perturbative disorder strength, as well as give valuable insight into non-perturbative regimes. Strongly coupled metals with quantum critical contributions to transport generically transition between coherent and incoherent metals as disorder strength is increased at fixed temperature, analogous to mean field...

  17. Quantum transport in magnetic topological insulator thin films.

    Science.gov (United States)

    Lu, Hai-Zhou; Zhao, An; Shen, Shun-Qing

    2013-10-01

    The experimental observation of the long-sought quantum anomalous Hall effect was recently reported in magnetically doped topological insulator thin films [Chang et al., Science 340, 167 (2013)]. An intriguing observation is a rapid decrease from the quantized plateau in the Hall conductance, accompanied by a peak in the longitudinal conductance as a function of the gate voltage. Here, we present a quantum transport theory with an effective model for magnetic topological insulator thin films. The good agreement between theory and experiment reveals that the measured transport originates from a topologically nontrivial conduction band which, near its band edge, has concentrated Berry curvature and a local maximum in group velocity. The indispensable roles of the broken structure inversion and particle-hole symmetries are also revealed. The results are instructive for future experiments and transport studies based on first-principles calculations.

  18. Directed transport in quantum star graphs

    Science.gov (United States)

    Yusupov, Jambul; Dolgushev, Maxim; Blumen, Alexander; Mülken, Oliver

    2016-04-01

    We study the quantum dynamics of Gaussian wave packets on star graphs whose arms feature each a periodic potential and an external time-dependent field. Assuming that the potentials and the field can be manipulated separately for each arm of the star, we show that it is possible to manipulate the direction of the motion of a Gaussian wave packet through the bifurcation point by a suitable choice of the parameters of the external fields. In doing so, one can achieve a transmission of the wave packet into the desired arm with nearly 70 % while also keeping the shape of the wave packet approximately intact. Since a star graph is the simplest element of many other complex graphs, the obtained results can be considered as the first step to wave packet manipulations on complex networks.

  19. Non-Equilibrium Quantum Transport of Bosons through a Quantum Dot

    Institute of Scientific and Technical Information of China (English)

    CHEN Zuo-Zi; L(U) Rong; ZHAI Hui; CHANG Lee

    2006-01-01

    @@ The quantum dot coupled to reservoirs is known as a typical mesoscopic setup to manifest the quantum characteristics of particles in transport. In analogue to many efforts made on the study of electronic quantum dots in the past decades, we study the transport of bosons through such a device. We first generalize the formula which relates the current to the local properties of dot in the bosonic situation. Then, as an illustrative example, we calculate the local density of state and lesser Green function of the localized boson with a bosonic Fano-Anderson model The current-voltage (Ⅰ-Ⅴ) behaviour at zero temperature is presented, and in the bosonic dot it is the Ⅰ-Ⅴ curve, in contrast to the differential conductance in the electronic dot, which is found to be proportional to the spectral function.

  20. Spin-polarized quantum transport through an Aharonov-Bohm quantum-dot-ring

    Institute of Scientific and Technical Information of China (English)

    Wang Jian-Ming; Wang Rui; Liang Jiu-Qing

    2007-01-01

    In this paper the quantum transport through an Aharonov-Bohm (AB) quantum-dot-ring with two dot-array arms described by a single-band tight-binding Hamiltonian is investigated in the presence of additional magnetic fields applied to the dot-array arms to produce spin flip of electrons. A far richer interference pattern than that in the charge transport alone is found. Besides the usual AB oscillation the tunable spin polarization of the current by the magnetic flux is a new observation and is seen to be particularly useful in technical applications. The spectrum of transmission probability is modulated by the quantum dot numbers on the up-arc and down-arc of the ring, which, however, does not affect the period of the AB oscillation.

  1. Fractional quantum mechanics on networks: Long-range dynamics and quantum transport.

    Science.gov (United States)

    Riascos, A P; Mateos, José L

    2015-11-01

    In this paper we study the quantum transport on networks with a temporal evolution governed by the fractional Schrödinger equation. We generalize the dynamics based on continuous-time quantum walks, with transitions to nearest neighbors on the network, to the fractional case that allows long-range displacements. By using the fractional Laplacian matrix of a network, we establish a formalism that combines a long-range dynamics with the quantum superposition of states; this general approach applies to any type of connected undirected networks, including regular, random, and complex networks, and can be implemented from the spectral properties of the Laplacian matrix. We study the fractional dynamics and its capacity to explore the network by means of the transition probability, the average probability of return, and global quantities that characterize the efficiency of this quantum process. As a particular case, we explore analytically these quantities for circulant networks such as rings, interacting cycles, and complete graphs. PMID:26651751

  2. Kondo effect for electron transport through an artificial quantum dot

    Institute of Scientific and Technical Information of China (English)

    Sun Ke-Wei; Xiong Shi-Jie

    2006-01-01

    We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper.We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases.The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case.It increases as the external magnetic field increases.We obtain the relation between the coupling coefficient and conductance.If the interaction is big enough to prevent conduction electrons from tunnelling through the dot,the dispersion effect is dominant in this case.In the Kondo temperature regime,we obtain the conductivity of a quantum dot system with Kondo correlation.

  3. Parallel Transport Quantum Logic Gates with Trapped Ions.

    Science.gov (United States)

    de Clercq, Ludwig E; Lo, Hsiang-Yu; Marinelli, Matteo; Nadlinger, David; Oswald, Robin; Negnevitsky, Vlad; Kienzler, Daniel; Keitch, Ben; Home, Jonathan P

    2016-02-26

    We demonstrate single-qubit operations by transporting a beryllium ion with a controlled velocity through a stationary laser beam. We use these to perform coherent sequences of quantum operations, and to perform parallel quantum logic gates on two ions in different processing zones of a multiplexed ion trap chip using a single recycled laser beam. For the latter, we demonstrate individually addressed single-qubit gates by local control of the speed of each ion. The fidelities we observe are consistent with operations performed using standard methods involving static ions and pulsed laser fields. This work therefore provides a path to scalable ion trap quantum computing with reduced requirements on the optical control complexity. PMID:26967401

  4. I-V curve of Bi-2223/Ag tapes in overload conditions determined from AC transport data

    International Nuclear Information System (INIS)

    The influence of dynamics on the dissipation at electrical charge transport in superconductors can be investigated by comparing the resistive part of AC transport loss with the prediction deduced from DC I-V curve. The complication is that in the AC regime a hysteretic loss is present, generating a voltage that is similar to that produced by the I-V curve. We present how the signal due to hysteresis loss can be eliminated from the total voltage measured with AC transport current, and the DC I-V curve deduced from its fundamental and higher harmonics can be constructed. The experiments were performed on two samples prepared from the same tape. The voltage was monitored using contact as well as contact-less method. Its harmonics (up to 7th) were decomposed to the parts that are out of phase and in phase with transport current, using a lock-in amplifier. The experiments were focused on the over critical current regime up to AC currents with amplitudes 3 times exceeding the tape critical current. We show how from these data the basic parameters characterising the I-V curve of the composite tape, i.e. its critical current, the slope and the normal state resistivity, can be determined

  5. Distribution of tunnelling times for quantum electron transport

    Science.gov (United States)

    Rudge, Samuel L.; Kosov, Daniel S.

    2016-03-01

    In electron transport, the tunnelling time is the time taken for an electron to tunnel out of a system after it has tunnelled in. We define the tunnelling time distribution for quantum processes in a dissipative environment and develop a practical approach for calculating it, where the environment is described by the general Markovian master equation. We illustrate the theory by using the rate equation to compute the tunnelling time distribution for electron transport through a molecular junction. The tunnelling time distribution is exponential, which indicates that Markovian quantum tunnelling is a Poissonian statistical process. The tunnelling time distribution is used not only to study the quantum statistics of tunnelling along the average electric current but also to analyse extreme quantum events where an electron jumps against the applied voltage bias. The average tunnelling time shows distinctly different temperature dependence for p- and n-type molecular junctions and therefore provides a sensitive tool to probe the alignment of molecular orbitals relative to the electrode Fermi energy.

  6. Electron transport in coupled double quantum wells and wires

    Energy Technology Data Exchange (ETDEWEB)

    Harff, N.E.; Simmons, J.A.; Lyo, S.K. [and others

    1997-04-01

    Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.

  7. Effects of spin-orbit coupling on quantum transport

    OpenAIRE

    Bardarson, Jens Hjorleifur

    2008-01-01

    The effect of spin-orbit coupling on various quantum transport phenomena is considered. The main topics discussed are: * How spin-orbit coupling can induce shot noise through trajectory splitting. * How spin-orbit coupling can degrade electron-hole entanglement (created by a tunnel barrier) by mode mixing. * Mesoscopic Spin Hall effect: longitudinal charge current leads to transverse spin currents in a chaotic electron cavity which has universal fluctuations around a zero mean. * How smooth d...

  8. Quantum Interference Effects in Electronic Transport through Nanotube Contacts

    OpenAIRE

    Buia, Calin; Buldum, Alper; Lu, Jian Ping

    2002-01-01

    Quantum interference has dramatic effects on electronic transport through nanotube contacts. In optimal configuration the intertube conductance can approach that of a perfect nanotube ($4e^2/h$). The maximum conductance increases rapidly with the contact length up to 10 nm, beyond which it exhibits long wavelength oscillations. This is attributed to the resonant cavity-like interference phenomena in the contact region. For two concentric nanotubes symmetry breaking reduces the maximum intertu...

  9. Quantum transport through STM-lifted single PTCDA molecules

    OpenAIRE

    Pump, Florian; Temirov, Ruslan; Neucheva, Olga; Soubatch, Serguei; Tautz, Stefan; Rohlfing, Michael; Cuniberti, Gianaurelio

    2008-01-01

    Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip-molecule-surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip substrat...

  10. Quantum effective potential, electron transport and conformons in biopolymers

    Energy Technology Data Exchange (ETDEWEB)

    Dandoloff, Rossen [Laboratoire de Physique Theorique et Modelisation, Universite de Cergy-Pontoise, F-95302 Cergy-Pontoise (France); Balakrishnan, Radha [The Institute of Mathematical Sciences, Chennai 600113 (India)

    2005-07-08

    In the Kirchhoff model of a biopolymer, conformation dynamics can be described in terms of solitary waves, for certain special cross-section asymmetries. Applying this to the problem of electron transport, we show that the quantum effective potential arising due to the bends and twists of the polymer enables us to formalize and quantify the concept of a conformon that has been hypothesized in biology. Its connection to the soliton solution of the cubic nonlinear Schroedinger equation emerges in a natural fashion.

  11. Quantum Transport Phenomena Of Two-Dimensional Mesoscopic Structures

    OpenAIRE

    Szaszkó-Bogár Viktor

    2015-01-01

    The dissertation is strongly related to quantum theory of spin systems. Spintronics (or spin electronics) is a promising field that has a kind of multidisciplinary nature in solid state physics. The aim of the research in this rapidly developing field is the control and manipulation of spin degrees of freedom in various material samples. Spintronics concentrates on the basic physical principles underlying the generation of carrier spin-polarization, spin dynamics, and spin-polarized transport...

  12. Quantum transport in the cylindrical nanosize silicon-based MOSFET

    OpenAIRE

    Balaban, S. N.; Pokatilov, E. P.; Fomin, V. M.; Gladilin, V. N.; Devreese, J. T.; Magnus, W.; W. Schoenmaker; Van Rossum, M.; Soree, B.

    2000-01-01

    A model is developed for a detailed investigation of the current flowing through a cylindrical nanosize MOSFET with a close gate electrode. The quantum mechanical features of the lateral charge transport are described by Wigner distribution function which is explicitly dealing with electron scattering due to acoustic phonons and acceptor impurities. A numerical simulation is carried out to obtain a set of I-V characteristics for various channel lengths. It is demonstrated that inclusion of th...

  13. Dynamics of heat and mass transport in a quantum insulator

    Science.gov (United States)

    Łącki, Mateusz; Delande, Dominique; Zakrzewski, Jakub

    2015-04-01

    The real-time evolution of two pieces of quantum insulators, initially at different temperatures, is studied when they are glued together. Specifically, each subsystem is taken as a Bose-Hubbard model in a Mott insulator state. The process of temperature equilibration via heat transfer is simulated in real time using the minimally entangled typical thermal states algorithm. The analytic theory based on quasiparticle transport is also given.

  14. Cooperative emission in transport setting through a quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Schuetz, Martin J.A.; Kessler, Eric M.; Giedke, Geza; Cirac, Juan Ignacio [Max-Planck-Institut fuer Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching (Germany)

    2012-07-01

    We theoretically show that intriguing features of coherent many-body physics can be observed in electron transport through a quantum dot (QD). In particular, we show that electron transport in the Pauli-blockade regime is coherently enhanced by hyperfine interaction (HF) with the nuclear spin ensemble in the QD. For an initially polarized nuclear system this leads to a strong current peak in close analogy with superradiant emission of photons from atomic ensembles. This effect could be observed with realistic experimental parameters and would provide clear evidence of coherent HF dynamics of nuclear spin ensembles in QDs.

  15. Electronic transport through a quantum-dot molecule

    Energy Technology Data Exchange (ETDEWEB)

    Coutinho, Renato Maximo; Souza, Fabricio Macedo de [Universidade Federal de Uberlandia (UFU), MG (Brazil). Inst. de Fisica. Grupo de Nanociencia

    2012-07-01

    Full text: Electron transport through quantum dot (QD) systems has been extensively studied both experimentally and theoretically. Many fascinating phenomena have emerged, such as the periodic oscillations of linear conductance as a function of gate voltage [1], the characteristic I-V in the nonlinear transport regime and the Kondo effect observed in a system composed of a quantum dot coupled to leads [2]. In this work we study the electronic transport in a array of quantum dots coupled to each other and to two electron reservoirs. An external bias voltage is applied along the structure in order to drive the system out of equilibrium. In the present work we apply nonequilibrium Green function technique [3] to calculate current, transmission coefficient, shot-noise and Fano factor in a quantum-dot molecular array attached to a left and to a right lead [4,5]. In the presence of an external source-drain bias voltage, a charge current flows in the system, thus generating shot-noise. We pay particular attention on the relation between molecular geometry and the noise signal. We note that depending on the molecular configuration, the shot-noise can be suppressed to values further below the characteristic 0.5 observed in tunneling junctions. This indicates that the molecular configuration gives rise to an enhancement of the charge transport correlation. In particular, as the molecular sites becomes randomly distributed the transport correlation tends to increase, with Fano factors reaching values close to 0.4 . The present results provides an alternative way to figure out the molecular structure based on the shot-noise signature. References: [1] J.H.F. Scott-Thomas, et al, Phys. Rev. Lett. 62 (1989) 583. [2] M.Pustilnik, I.I. Glazman, Phys. Rev. Lett. 87 (2001) 216601. [3] H. Haug, A.P. Jauho, Quantum Kinects in Transport and Optics of Semiconductors, Springer, Berlin, 1996. [4] Yu Liu, Yisong Zheng, Weijiang Gong, Tianquan Lu, Phys. Lett. A 360 (2006) 154-163. [5] W

  16. Quantum and Ionic Transport Across Superconductor-based Heterostructures

    Science.gov (United States)

    Nayfeh, Osama; Dinh, Son; Taylor, Benjamin; de Andrade, Marcio; Swanson, Paul; Offord, Bruce; de Escobar, Anna Leese; Claussen, Stephanie; Kassegne, Sam

    2015-03-01

    We present analysis of quantum and ionic transport across superconductor/barrier/ionic/barrier/superconductor (SBIBS) heterostructures. Calculations for various ionic configurations demonstrate modification of the quantum transport coherence length and energy profile with moderate ionic transport away from the superconductor-barrier interface. The effect of electric field and cryogenic temperature on the stability of the ionic configurations for quantum information state storage is examined. Characterization and analysis of constructed Al and Nb-based device structures are presented. Acknowledgements: We acknowledge the support of the SSC Pacific In-house Laboratory Independent Research Science and Technology Program managed by Dr. Dave Rees, the Naval Innovative Science and Engineering Program managed by Mr. Robin Laird, and the ONR Summer Faculty Research Program. Interactions with Dr. Van Vechten (ONR) and Dr. Manheimer (IARPA) are appreciated. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of SPAWAR or the U.S. Government. Approved for Public Release; distribution is unlimited.

  17. The quantum Goldilocks effect: on the convergence of timescales in quantum transport

    CERN Document Server

    Lloyd, Seth; Shabani, Alireza; Rabitz, Herschel

    2011-01-01

    Excitonic transport in photosynthesis exhibits a wide range of time scales. Absorption and initial relaxation takes place over tens of femtoseconds. Excitonic lifetimes are on the order of a nanosecond. Hopping rates, energy differences between chromophores, reorganization energies, and decoherence rates correspond to time scales on the order of picoseconds. The functional nature of the divergence of time scales is easily understood: strong coupling to the electromagnetic field over a broad band of frequencies yields rapid absorption, while long excitonic lifetimes increase the amount of energy that makes its way to the reaction center to be converted to chemical energy. The convergence of the remaining time scales to the centerpoint of the overall temporal range is harder to understand. In this paper we argue that the convergence of timescales in photosynthesis can be understood as an example of the `quantum Goldilocks effect': natural selection tends to drive quantum systems to the degree of quantum coheren...

  18. Quantum Simulation via Filtered Hamiltonian Engineering: Application to Perfect Quantum Transport in Spin Networks

    Science.gov (United States)

    Ajoy, Ashok; Cappellaro, Paola

    2013-05-01

    We propose a method for Hamiltonian engineering that requires no local control but only relies on collective qubit rotations and field gradients. The technique achieves a spatial modulation of the coupling strengths via a dynamical construction of a weighting function combined with a Bragg grating. As an example, we demonstrate how to generate the ideal Hamiltonian for perfect quantum information transport between two separated nodes of a large spin network. We engineer a spin chain with optimal couplings starting from a large spin network, such as one naturally occurring in crystals, while decoupling all unwanted interactions. For realistic experimental parameters, our method can be used to drive almost perfect quantum information transport at room temperature. The Hamiltonian engineering method can be made more robust under decoherence and coupling disorder by a novel apodization scheme. Thus, the method is quite general and can be used to engineer the Hamiltonian of many complex spin lattices with different topologies and interactions.

  19. Photonic quantum transport in a nonlinear optical fiber

    CERN Document Server

    Hafezi, Mohammad; Gritsev, Vladimir; Demler, Eugene; Lukin, Mikhail D

    2009-01-01

    We theoretically study the transmission of few-photon quantum fields through a strongly nonlinear optical medium. We develop a general approach to investigate non-equilibrium quantum transport of bosonic fields through a finite-size nonlinear medium and apply it to a recently demonstrated experimental system where cold atoms are loaded in a hollow-core optical fiber. We show that when the interaction between photons is effectively repulsive, the system acts as a single-photon switch. In the case of attractive interaction, the system can exhibit either anti-bunching or bunching, associated with the resonant excitation of bound states of photons by the input field. These effects can be observed by probing statistics of photons transmitted through the nonlinear fiber.

  20. Photonic quantum transport in a nonlinear optical fiber

    Science.gov (United States)

    Hafezi, M.; Chang, D. E.; Gritsev, V.; Demler, E. A.; Lukin, M. D.

    2011-06-01

    We theoretically study the transmission of few-photon quantum fields through a strongly nonlinear optical medium. We develop a general approach to investigate nonequilibrium quantum transport of bosonic fields through a finite-size nonlinear medium and apply it to a recently demonstrated experimental system where cold atoms are loaded in a hollow-core optical fiber. We show that when the interaction between photons is effectively repulsive, the system acts as a single-photon switch. In the case of attractive interaction, the system can exhibit either antibunching or bunching, associated with the resonant excitation of bound states of photons by the input field. These effects can be observed by probing statistics of photons transmitted through the nonlinear fiber.

  1. Electronic transport through a quantum ring coupled to ferromagnetic leads

    Institute of Scientific and Technical Information of China (English)

    Chi Feng; Sun Lian-Liang; Huang Ling; Zhao Jia

    2011-01-01

    We study the spin-dependent transport through a one-dimensional quantum ring with taking both the Rashba spin-orbit coupling (RSOC) and ferromagnetic leads into consideration. The linear conductance is obtained by the Green's function method. We find that due to the quantum interference effect arising from the RSOC-induced spin precession phase and the difference in travelling phase between the two arms of the ring, the conductance becomes spin-polarized even in the antiparallel magnetic configuration of the two leads, which is different from the case in single conduction channel system. The linear conductance, the spin polarization and the tunnel magnetoresistance are periodic functions of the two phases, and can be efficiently tuned by the structure parameters.

  2. Spin-dependent thermoelectric transport through double quantum dots

    Institute of Scientific and Technical Information of China (English)

    Wang Qiang; Xie Hai-Qing; Jiao Hu-Jun; Li Zhi-Jian; Nie Yi-Hang

    2012-01-01

    We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green's function in the linear response regime.It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling,the relative magnetic configurations,and the spin polarization of leads.In particular,the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantum dots and the leads for the spin-down electrons are small.Moreover,the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures.In the presence of the magnetic field,the spin accumulation in the leads strongly suppresses the thermoelectric efficiency,and a pure spin thermopower can be obtained.

  3. Using the Chebychev expansion in quantum transport calculations

    Energy Technology Data Exchange (ETDEWEB)

    Popescu, Bogdan; Rahman, Hasan; Kleinekathöfer, Ulrich, E-mail: u.kleinekathoefer@jacobs-university.de [Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen (Germany)

    2015-04-21

    Irradiation by laser pulses and a fluctuating surrounding liquid environment can, for example, lead to time-dependent effects in the transport through molecular junctions. From the theoretical point of view, time-dependent theories of quantum transport are still challenging. In one of these existing transport theories, the energy-dependent coupling between molecule and leads is decomposed into Lorentzian functions. This trick has successfully been combined with quantum master approaches, hierarchical formalisms, and non-equilibrium Green’s functions. The drawback of this approach is, however, its serious limitation to certain forms of the molecule-lead coupling and to higher temperatures. Tian and Chen [J. Chem. Phys. 137, 204114 (2012)] recently employed a Chebychev expansion to circumvent some of these latter problems. Here, we report on a similar approach also based on the Chebychev expansion but leading to a different set of coupled differential equations using the fact that a derivative of a zeroth-order Bessel function can again be given in terms of Bessel functions. Test calculations show the excellent numerical accuracy and stability of the presented formalism. The time span for which this Chebychev expansion scheme is valid without any restrictions on the form of the spectral density or temperature can be determined a priori.

  4. Quantum dot transport in soil, plants, and insects

    Energy Technology Data Exchange (ETDEWEB)

    Al-Salim, Najeh [Industrial Research Ltd, P.O. Box 31310, Lower Hutt 5040 (New Zealand); Barraclough, Emma; Burgess, Elisabeth [New Zealand Institute for Plant and Food Research Ltd, Private Bag 92169, Victoria Street West, Auckland 1142 (New Zealand); Clothier, Brent, E-mail: brent.clothier@plantandfood.co.nz [New Zealand Institute for Plant and Food Research Ltd, Private Bag 11600, Manawatu Mail Centre, Palmerston North 4442 (New Zealand); Deurer, Markus; Green, Steve [New Zealand Institute for Plant and Food Research Ltd, Private Bag 11600, Manawatu Mail Centre, Palmerston North 4442 (New Zealand); Malone, Louise [New Zealand Institute for Plant and Food Research Ltd, Private Bag 92169, Victoria Street West, Auckland 1142 (New Zealand); Weir, Graham [Industrial Research Ltd, P.O. Box 31310, Lower Hutt 5040 (New Zealand)

    2011-08-01

    Environmental risk assessment of nanomaterials requires information not only on their toxicity to non-target organisms, but also on their potential exposure pathways. Here we report on the transport and fate of quantum dots (QDs) in the total environment: from soils, through their uptake into plants, to their passage through insects following ingestion. Our QDs are nanoparticles with an average particle size of 6.5 nm. Breakthrough curves obtained with CdTe/mercaptopropionic acid QDs applied to columns of top soil from a New Zealand organic apple orchard, a Hastings silt loam, showed there to be preferential flow through the soil's macropores. Yet the effluent recovery of QDs was just 60%, even after several pore volumes, indicating that about 40% of the influent QDs were filtered and retained by the soil column via some unknown exchange/adsorption/sequestration mechanism. Glycine-, mercaptosuccinic acid-, cysteine-, and amine-conjugated CdSe/ZnS QDs were visibly transported to a limited extent in the vasculature of ryegrass (Lolium perenne), onion (Allium cepa) and chrysanthemum (Chrysanthemum sp.) plants when cut stems were placed in aqueous QD solutions. However, they were not seen to be taken up at all by rooted whole plants of ryegrass, onion, or Arabidopsis thaliana placed in these solutions. Leafroller (Lepidoptera: Tortricidae) larvae fed with these QDs for two or four days, showed fluorescence along the entire gut, in their frass (larval feces), and, at a lower intensity, in their haemolymph. Fluorescent QDs were also observed and elevated cadmium levels detected inside the bodies of adult moths that had been fed QDs as larvae. These results suggest that exposure scenarios for QDs in the total environment could be quite complex and variable in each environmental domain. - Research highlights: {yields} Quantum dots are transported rapidly through soil but half were retained. {yields} Intact roots of plants did not take up quantum dots. Excised plants

  5. Quantum dot transport in soil, plants, and insects

    International Nuclear Information System (INIS)

    Environmental risk assessment of nanomaterials requires information not only on their toxicity to non-target organisms, but also on their potential exposure pathways. Here we report on the transport and fate of quantum dots (QDs) in the total environment: from soils, through their uptake into plants, to their passage through insects following ingestion. Our QDs are nanoparticles with an average particle size of 6.5 nm. Breakthrough curves obtained with CdTe/mercaptopropionic acid QDs applied to columns of top soil from a New Zealand organic apple orchard, a Hastings silt loam, showed there to be preferential flow through the soil's macropores. Yet the effluent recovery of QDs was just 60%, even after several pore volumes, indicating that about 40% of the influent QDs were filtered and retained by the soil column via some unknown exchange/adsorption/sequestration mechanism. Glycine-, mercaptosuccinic acid-, cysteine-, and amine-conjugated CdSe/ZnS QDs were visibly transported to a limited extent in the vasculature of ryegrass (Lolium perenne), onion (Allium cepa) and chrysanthemum (Chrysanthemum sp.) plants when cut stems were placed in aqueous QD solutions. However, they were not seen to be taken up at all by rooted whole plants of ryegrass, onion, or Arabidopsis thaliana placed in these solutions. Leafroller (Lepidoptera: Tortricidae) larvae fed with these QDs for two or four days, showed fluorescence along the entire gut, in their frass (larval feces), and, at a lower intensity, in their haemolymph. Fluorescent QDs were also observed and elevated cadmium levels detected inside the bodies of adult moths that had been fed QDs as larvae. These results suggest that exposure scenarios for QDs in the total environment could be quite complex and variable in each environmental domain. - Research highlights: → Quantum dots are transported rapidly through soil but half were retained. → Intact roots of plants did not take up quantum dots. Excised plants did slightly.

  6. Computation of electron quantum transport in graphene nanoribbons using GPU

    CERN Document Server

    Ihnatsenka, S

    2011-01-01

    The performance potential for simulating quantum electron transport on graphical processing units (GPUs) is studied. Using graphene ribbons of realistic sizes as an example it is shown that GPUs provide significant speed-ups in comparison to central processing units as the transverse dimension of the ribbon grows. The recursive Green's function algorithm is employed and implementation details on GPUs are discussed. Calculated conductances were found to accumulate significant numerical error due to single-precision floating-point arithmetic at energies close to the charge neutrality point of the graphene.

  7. Electron transport through a quantum dot assisted by cavity photons

    OpenAIRE

    Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei; Gudmundsson, Vidar

    2013-01-01

    We investigate transient transport of electrons through a single-quantum-dot controlled by a plunger gate. The dot is embedded in a finite wire that is weakly coupled to leads and strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron-photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger gate voltage to lift a many-body state...

  8. Statistical theory of designed quantum transport across disordered networks.

    Science.gov (United States)

    Walschaers, Mattia; Mulet, Roberto; Wellens, Thomas; Buchleitner, Andreas

    2015-04-01

    We explain how centrosymmetry, together with a dominant doublet of energy eigenstates in the local density of states, can guarantee interference-assisted, strongly enhanced, strictly coherent quantum excitation transport between two predefined sites of a random network of two-level systems. Starting from a generalization of the chaos-assisted tunnelling mechanism, we formulate a random matrix theoretical framework for the analytical prediction of the transfer time distribution, of lower bounds of the transfer efficiency, and of the scaling behavior of characteristic statistical properties with the size of the network. We show that these analytical predictions compare well to numerical simulations, using Hamiltonians sampled from the Gaussian orthogonal ensemble. PMID:25974468

  9. On Parallel Transport in Quantum Bundles over Robertson-Walker Spacetimes

    OpenAIRE

    Coleman, James

    1996-01-01

    A recently-developed theory of quantum general relativity provides a propagator for free-falling particles in curved spacetimes. These propagators are constructed by parallel-transporting quantum states within a quantum bundle associated to the Poincare frame bundle. We consider such parallel transport in the case that the spacetime is a classical Robertson-Walker universe. An explicit integral formula is developed which expresses the propagators for parallel transport between any two points ...

  10. Directed transport in classical and quantum chaotic billiards

    Energy Technology Data Exchange (ETDEWEB)

    Acevedo, W; Dittrich, T [Departamento de Fisica, Universidad Nacional de Colombia, and CeiBA, Complejidad, Bogota DC (Colombia)], E-mail: jwacevedov@unal.edu.co, E-mail: tdittrich@unal.edu.co

    2009-01-30

    We construct an autonomous chaotic Hamiltonian ratchet as a channel billiard subdivided by equidistant walls attached perpendicularly to one side of the channel, leaving an opening on the opposite side. A static homogeneous magnetic field penetrating the billiard breaks time-reversal invariance and renders the classical motion partially chaotic. We show that the classical dynamics exhibits directed transport, owing to the asymmetric distribution of regular regions in phase space. The billiard is quantized by a numerical method based on a finite-element algorithm combined with the Landau gauge and the Bloch formalism for periodic potentials. We discuss features of the billiard eigenstates such as node lines and vortices in the probability flow. Evidence for directed quantum transport, inherited from the corresponding features of the classical dynamics, is presented in terms of level-velocity statistics.

  11. Fate and transport of some selected PhACs in a river receiving a high load of treated sewage

    Science.gov (United States)

    Bendz, D.; Ginn, T. R.; Paxeus, N.

    2003-04-01

    Pharmaceutical active compounds (PhACs) have lately been acknowledged to constitute a risk for humans and for the terrestrial and aquatic environment. Human and veterinary applications are the main sources of PhACs in the environment and the major pathway are excretion and discharge to the environment. Sewage treatment plants (STPs) play a crucial role for the introduction of the human PhACs in the environment through its removal efficiency and by separating these compounds into two exposure pathways associated with the aquatic and the solid (sludge) phase, respectively. Actually, STPs are recognized as being the main point discharge sources of human PhACs to the aquatic environment. In this study the fate and transport of a selected human PhACs belonging to different therapeutic classes (NSAIDs- non-steroidal antiinflamatory drugs, lipid regulators, antiepileptics, antibiotics and &beta-blockers) are investigated in a small river in the very south of Sweden receiving a high load of treated wastewater. In addition to the PhACs, triclosan (commonly used biocide) was included in this study. Water samples were taken of incoming and outgoing wastewater from the treatment plant, at the effluent in the river, and along the river up to 8 kilometers downstream were the river flows into the sea. After enrichment by solid-phase extraction the compounds were analyzed using GC-MS (methylated derivatives) or LC-MS/MS. In addition to the target compounds a screening analysis of the extracts revealed the presence of other wastewater related pollutants (caffeine, flame retardants, antioxidants). Several of the investigated substances demonstrate a surprising persistence in the aquatic environment. This emphasizes the need for a broader view on the concept of persistence by taking into account the recharge/loading rate in addition to removal mechanisms; transformation, volatility and physical sequestration by solids and the influence of different environmental media (Soil organic

  12. Scattering approach to quantum transport and many body effects

    Science.gov (United States)

    Pichard, Jean-Louis; Freyn, Axel

    2010-12-01

    We review a series of works discussing how the scattering approach to quantum transport developed by Landauer and Buttiker for one body elastic scatterers can be extended to the case where electron-electron interactions act inside the scattering region and give rise to many body scattering. Firstly, we give an exact numerical result showing that at zero temperature a many body scatterer behaves as an effective one body scatterer, with an interaction dependent transmission. Secondly, we underline that this effective scatterer depends on the presence of external scatterers put in its vicinity. The implications of this non local scattering are illustrated studying the conductance of a quantum point contact where electrons interact with a scanning gate microscope. Thirdly, using the numerical renormalization group developed by Wilson for the Kondo problem, we study a double dot spinless model with an inter-dot interaction U and inter-dot hopping td, coupled to leads by hopping terms tc. We show that the quantum conductance as a function of td is given by a universal function, independently of the values of U and tc, if one measures td in units of a characteristic scale τ(U,tc). Mapping the double dot system without spin onto a single dot Anderson model with spin and magnetic field, we show that τ(U,tc) = 2TK, where TK is the Kondo temperature of the Anderson model.

  13. Opto-electronic and quantum transport properties of semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Sabathil, M.

    2005-01-01

    In this work a novel and efficient method for the calculation of the ballistic transport properties of open semiconductor nanostructures connected to external reservoirs is presented. It is based on the Green's function formalism and reduces the effort to obtain the transmission and the carrier density to a single solution of a hermitian eigenvalue problem with dimensions proportional to the size of the decoupled device and the multiple inversion of a small matrix with dimensions proportional to the size of the contacts to the leads. Using this method, the 4-band GaAs hole transport through a 2-dimensional three-terminal T-junction device, and the resonant tunneling current through a 3-dimensional InAs quantum dot molecule embedded into an InP heterostructure have been calculated. The further extension of the method into a charge self-consistent scheme enables the efficient prediction of the IV-characteristics of highly doped nanoscale field effect transistors in the ballistic regime, including the influence of quasi bound states and the exchange-correlation interaction. Buettiker probes are used to emulate the effect of inelastic scattering on the current for simple 1D devices, systematically analyzing the dependence of the density of states and the resulting self-consistent potential on the scattering strength. The second major topic of this work is the modeling of the optical response of quantum confined neutral and charged excitons in single and coupled self-assembled InGaAs quantum dots. For this purpose the existing device simulator nextnano{sup 3} has been extended to incorporate particle-particle interactions within the means of density functional theory in local density approximation. In this way the exciton transition energies for neutral and charged excitons as a function of an externally applied electric field have been calculated, revealing a systematic reduction of the intrinsic dipole with the addition of extra holes to the exciton, a finding

  14. Electron transport through a quantum dot assisted by cavity photons

    International Nuclear Information System (INIS)

    We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron–photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source–drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry. (paper)

  15. Electron transport through a quantum dot assisted by cavity photons

    Science.gov (United States)

    Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei; Gudmundsson, Vidar

    2013-11-01

    We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron-photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source-drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry.

  16. Topological phases and transport properties of screened interacting quantum wires

    Science.gov (United States)

    Xu, Hengyi; Xiong, Ye; Wang, Jun

    2016-10-01

    We study theoretically the effects of long-range and on-site Coulomb interactions on the topological phases and transport properties of spin-orbit-coupled quasi-one-dimensional quantum wires imposed on a s-wave superconductor. The distributions of the electrostatic potential and charge density are calculated self-consistently within the Hartree approximation. Due to the finite width of the wires and charge repulsion, the potential and density distribute inhomogeneously in the transverse direction and tend to accumulate along the lateral edges where the hard-wall confinement is assumed. This result has profound effects on the topological phases and the differential conductance of the interacting quantum wires and their hybrid junctions with superconductors. Coulomb interactions renormalize the gate voltage and alter the topological phases strongly by enhancing the topological regimes and producing jagged boundaries. Moreover, the multicritical points connecting different topological phases are modified remarkably in striking contrast to the predictions of the two-band model. We further suggest the possible non-magnetic topological phase transitions manipulated externally with the aid of long-range interactions. Finally, the transport properties of normal-superconductor junctions are further examined, in particular, the impacts of Coulomb interactions on the zero-bias peaks related to the Majorana fermions and near zero-energy peaks.

  17. Kinesin-2 KIF3AC and KIF3AB Can Drive Long-Range Transport along Microtubules.

    Science.gov (United States)

    Guzik-Lendrum, Stephanie; Rank, Katherine C; Bensel, Brandon M; Taylor, Keenan C; Rayment, Ivan; Gilbert, Susan P

    2015-10-01

    Mammalian KIF3AC is classified as a heterotrimeric kinesin-2 that is best known for organelle transport in neurons, yet in vitro studies to characterize its single molecule behavior are lacking. The results presented show that a KIF3AC motor that includes the native helix α7 sequence for coiled-coil formation is highly processive with run lengths of ∼1.23 μm and matching those exhibited by conventional kinesin-1. This result was unexpected because KIF3AC exhibits the canonical kinesin-2 neck-linker sequence that has been reported to be responsible for shorter run lengths observed for another heterotrimeric kinesin-2, KIF3AB. However, KIF3AB with its native neck linker and helix α7 is also highly processive with run lengths of ∼1.62 μm and exceeding those of KIF3AC and kinesin-1. Loop L11, a component of the microtubule-motor interface and implicated in activating ADP release upon microtubule collision, is significantly extended in KIF3C as compared with other kinesins. A KIF3AC encoding a truncation in KIF3C loop L11 (KIF3ACΔL11) exhibited longer run lengths at ∼1.55 μm than wild-type KIF3AC and were more similar to KIF3AB run lengths, suggesting that L11 also contributes to tuning motor processivity. The steady-state ATPase results show that shortening L11 does not alter kcat, consistent with the observation that single molecule velocities are not affected by this truncation. However, shortening loop L11 of KIF3C significantly increases the microtubule affinity of KIF3ACΔL11, revealing another structural and mechanistic property that can modulate processivity. The results presented provide new, to our knowledge, insights to understand structure-function relationships governing processivity and a better understanding of the potential of KIF3AC for long-distance transport in neurons. PMID:26445448

  18. Quantum transport through a Coulomb blockaded quantum emitter coupled to a plasmonic dimer.

    Science.gov (United States)

    Goker, A; Aksu, H

    2016-01-21

    We study the electron transmission through a Coulomb blockaded quantum emitter coupled to metal nanoparticles possessing plasmon resonances by employing the time-dependent non-crossing approximation. We find that the coupling of the nanoparticle plasmons with the excitons results in a significant enhancement of the conductance through the discrete state with higher energy beyond the unitarity limit while the other discrete state with lower energy remains Coulomb blockaded. We show that boosting the plasmon-exciton coupling well below the Kondo temperature increases the enhancement adding another quantum of counductance upon saturation. Finite bias and increasing emitter resonance energy tend to reduce this enhancement. We attribute these observations to the opening of an additional transport channel via the plasmon-exciton coupling. PMID:26686761

  19. New quantum oscillations in magneto transport of a high-mobility two-dimensional electron system

    Science.gov (United States)

    Yang, Changli

    Quantum transport in two-dimensional electron systems (2DES) has been one of the major topics in condensed matter physics for many years. Although extensive studies have been performed in the regime of the quantum Hall effect (QHE) where a high magnetic field (typical B ˜ 10 T) is required, much less attention has been paid to the lower magnetic field regime where the Landau quantization of the 2DES is important but the QHE are absent (typical B ≲ 0.5 T). The 2D transport at the lower B regime was thought to be well understood and no surprise was expected. Contrary to this belief, three new classes of quantum oscillations have been discovered recently by our group (Quantum transport group of the University of Utah, led by Prof. RuiRui Du) in high-mobility 2DES at low magnetic fields. These new quantum oscillations are (1) the magneto-acoustic-phonon resonance (MAPR) involving acoustic phonons (in contrast with the well-known magneto-phonon resonance involving optical phonons), (2) the magneto-Zener-tunneling resonance (MZTR), induced by a relatively large dc current, and (3) the microwave-induced photo-conductivity resonance (MIPCR). In ultra-high-mobility samples, the minima of the MIPCR oscillations further develop into the so-called "zero resistance state" (ZRS). All these phenomena are manifested in magnetoresistance by periodic (in 1/B) oscillations. It is now clear that an important selection rule in 2D transport, namely q = 2 kF in momentum space or DeltaY = 2 Rc in real space, is underlying the MAPR and the MZTR, where q is the electron momentum transferred to a scatterer, kF is the Fermi wavevector of the 2DES, Delta Y is the guiding center shift of a scattered electron, and Rc is the cyclotron radius. This selection rule is not directly related to a conservation law but due to the very sharp cutoff at Delta Y = 2Rc for the overlap integral between displaced Landau orbits in the vicinity of the Fermi level. On the other hand, the origin of the MIPCR

  20. Quantum transport in nanowire-based hybrid devices

    Energy Technology Data Exchange (ETDEWEB)

    Guenel, Haci Yusuf

    2013-05-08

    We have studied the low-temperature transport properties of nanowires contacted by a normal metal as well as by superconducting electrodes. As a consequence of quantum coherence, we have demonstrated the electron interference effect in different aspects. The mesoscopic phase coherent transport properties were studied by contacting the semiconductor InAs and InSb nanowires with normal metal electrodes. Moreover, we explored the interaction of the microscopic quantum coherence of the nanowires with the macroscopic quantum coherence of the superconductors. In superconducting Nb contacted InAs nanowire junctions, we have investigated the effect of temperature, magnetic field and electric field on the supercurrent. Owing to relatively high critical temperature of superconducting Nb (T{sub c} ∝ 9 K), we have observed the supercurrent up to 4 K for highly doped nanowire-based junctions, while for low doped nanowire-based junctions a full control of the supercurrent was achieved. Due to low transversal dimension of the nanowires, we have found a monotonous decay of the critical current in magnetic field dependent measurements. The experimental results were analyzed within narrow junction model which has been developed recently. At high bias voltages, we have observed subharmonic energy gap structures as a consequence of multiple Andreev reflection. Some of the nanowires were etched, such that the superconducting Nb electrodes are connected to both ends of the nanowire rather than covering the surface of the nanowire. As a result of well defined nanowire-superconductor interfaces, we have examined quasiparticle interference effect in magnetotransport measurements. Furthermore, we have developed a new junction geometry, such that one of the superconducting Nb electrodes is replaced by a superconducting Al. Owing to the smaller critical magnetic field of superconducting Al (B{sub c} ∝ 15-50,mT), compared to superconducting Nb (B{sub c} ∝ 3 T), we were able to studied

  1. Quantum Transport through a Triple Quantum Dot System in the Presence of Majorana Bound States

    Science.gov (United States)

    Jiang, Zhao-Tan; Cao, Zhi-Yuan; Zhong, Cheng-Cheng

    2016-05-01

    We study the electron transport through a special quantum-dot (QD) structure composed of three QDs and two Majorana bound states (MBSs) using the nonequilibrium Green's function technique. This QD-MBS ring structure includes two channels with the two coupled MBSs being Channel 1 and one QD being Channel 2, and three types of transport processes such as the electron transmission (ET), the Andreev reflection (AR), and the crossed Andreev reflection (CAR). By comparing the ET, AR, and CAR processes through Channels 1 and 2, we make a systematic study on the transport properties of the QD-MBS ring. It is shown that there appear two kinds of characteristic transport patterns for Channels 1 and 2, as well as the interplay between the two patterns. Of particular interest is that there exists an AR-assisted ET process in Channel 2, which is different from that in Channel 1. Thus a clear “X” pattern due to the ET and AR processes appears in the ET, AR, and CAR transmission coefficients. Moreover, we study how Channel 2 affects the three transport processes when Channel 1 is tuned in the ET and CAR regimes. It is shown that the transport properties of the ET, AR and CAR processes can be adjusted by tuning the energy level of the QD embedded in Channel 2. We believe this research should be a helpful reference for understanding the transport properties in the QD-MBS coupled systems. Supported by National Natural Science Foundation of China under Grant No. 11274040, and by the Program for New Century Excellent Talents in University under Grant No. NCET-08-0044

  2. Spin transport properties in double quantum rings connected in series*

    Institute of Scientific and Technical Information of China (English)

    Du Jian; Wang Suxin; Pan Jianghong

    2011-01-01

    A new model of metal/semiconductor/metal double-quantum-ring connected in series is proposed and the transport properties in this model are theoretically studied. The results imply that the transmission coefficient shows periodic variations with increasing semiconductor ring size. The effects of the magnetic field and Rashba spin-orbit interaction on the transmission coefficient for two kinds of spin state electrons are different. The number of the transmission coefficient peaks is related to the length ratio between the upper ann and the half circumference of the ring. In addition, the transmission coefficient shows oscillation behavior with enhanced external magnetic field, and the corresponding average value is related to the two leads' relative position.

  3. Spin Quantum Kinetics in Relaxation and Transport of Semiconductors

    Science.gov (United States)

    Lee, Han-Chieh; Mou, Chung-Yu; Lyon, Stephen A.

    2007-03-01

    Generalized Kadanoff-Baym Equation (GKBE) with spin degree of freedom is firstly presented and its theoretical framework of applications, which aims to semiconductor quantum kinetics in femtosecond and nanometer scales, demonstrated. The GKBE was constructed by Green functions thermally averaging Pauli equation of motion with using Langreth theorem. As applied for relaxation, Kadanoff-Baym ansatz was made and carrier-carrier scattering (CCS) with random-phase approximation considered. The derivation can simulate an evolution of excited carriers spreading via CCS, buildup of magnetic field by Rashba effect and formation of spin relaxation, where energy non-conserving event and memory effect are figured out. For transport, retarded Green functions were retrieved from spin Dyson equation as an input for GKBE with the presence of electron-phonon (impurity) interaction. The part is useful for spin Hall effect in precisely estimating spin current and accumulation in nanostructures or ballistic regime.

  4. Riccati equation for simulation of leads in quantum transport

    Science.gov (United States)

    Bravi, M.; Farchioni, R.; Grosso, G.; Pastori Parravicini, G.

    2014-10-01

    We present a theoretical procedure with numerical demonstration of a workable and efficient method to evaluate the surface Green's function of semi-infinite leads connected to a device. Such a problem always occurs in quantum transport calculations but also in the study of surfaces and heterojunctions. We show here that these semi-infinite leads can be properly described by real-energy Green's functions obtained analytically by a smart solution of the Riccati matrix equation. The performance of our method is demonstrated in the case of a multichain two-dimensional electron-gas system, composed of a central ribbon connected to two semi-infinite leads, pierced by two opposite magnetic fields.

  5. Selective spin transport through a quantum heterostructure: Transfer matrix method

    Science.gov (United States)

    Dey, Moumita; Maiti, Santanu K.

    2016-09-01

    In the present work, we propose that a one-dimensional quantum heterostructure composed of magnetic and non-magnetic (NM) atomic sites can be utilized as a spin filter for a wide range of applied bias voltage. A simple tight-binding framework is given to describe the conducting junction where the heterostructure is coupled to two semi-infinite one-dimensional NM electrodes. Based on transfer matrix method, all the calculations are performed numerically which describe two-terminal spin-dependent transmission probability along with junction current through the wire. Our detailed analysis may provide fundamental aspects of selective spin transport phenomena in one-dimensional heterostructures at nanoscale level.

  6. Efficient wave-function matching approach for quantum transport calculations

    DEFF Research Database (Denmark)

    Sørensen, Hans Henrik Brandenborg; Hansen, Per Christian; Petersen, Dan Erik;

    2009-01-01

    the propagating and evanescent bulk modes of the left and right electrodes in order to obtain the correct coupling between device and electrode regions. In this paper we will describe a modified WFM approach that allows for the exclusion of the vast majority of the evanescent modes in all parts of the calculation......The wave-function matching (WFM) technique has recently been developed for the calculation of electronic transport in quantum two-probe systems. In terms of efficiency it is comparable to the widely used Green's function approach. The WFM formalism presented so far requires the evaluation of all....... This approach makes it feasible to apply iterative techniques to efficiently determine the few required bulk modes, which allows for a significant reduction of the computational expense of the WFM method. We illustrate the efficiency of the method on a carbon nanotube field-effect-transistor device displaying...

  7. Spin-dependent quantum transport effects in Cu nanowires

    International Nuclear Information System (INIS)

    In this work we investigate quantum transport in Cu nanowires created by bringing macroscopic Cu wires into and out of contact under an applied magnetic field in air. Here we show that a 70% magneto-conductance effect can be seen in a Cu nanowire in a field of 2 mT at room temperature. We propose that this phenomenon is a consequence of spin filtering due to the adsorption of atmospheric oxygen modifying the electronic band structure and introducing spin-split conduction channels. This is a remarkable result since bulk Cu is not magnetic and it may provide a new perspective in the quest for spintronic devices. (letter to the editor)

  8. The transport mechanism of the integer quantum Hall effect

    CERN Document Server

    LiMing, W

    2016-01-01

    The integer quantum Hall effect is analysed using a transport mechanism with a semi-classic wave packages of electrons in this paper. A strong magnetic field perpendicular to a slab separates the electron current into two branches with opposite wave vectors $({\\it k})$ and locating at the two edges of the slab, respectively, along the current. In this case back scattering of electrons ($k\\rightarrow -k$) is prohibited by the separation of electron currents. Thus the slab exhibits zero longitudinal resistance and plateaus of Hall resistance. When the Fermi level is scanning over a Landau level when the magnetic field increases, however, the electron waves locate around the central axis of the slab and overlap each other thus back scattering of electrons takes place frequently. Then longitudinal resistance appears and the Hall resistance goes up from one plateau to a new plateau.

  9. Enhancement of transport properties of a Brownian particle due to quantum effects: Smoluchowski limit

    International Nuclear Information System (INIS)

    Graphical abstract: By invoking physically motivated coordinate transformation into quantum Smoluchowski equation, we have presented a transparent treatment for the determination of the effective diffusion coefficient and current of a quantum Brownian particle. Substantial enhancement in the efficiency of the diffusive transport is envisaged due to the quantum correction effects. Highlights:: ► Transport of a quantum Brownian particle in a periodic potential has been addressed. ► Governing quantum Smoluchowski equation (QSE) includes state dependent diffusion. ► A coordinate transformation is used to recast QSE with constant diffusion. ► Transport properties increases in comparison to the corresponding classical result. ► This enhancement is purely a quantum effect. - Abstract: The transport property of a quantum Brownian particle that interacts strongly with a bath (in which a typical damping constant by far exceeds a characteristic frequency of the isolated system) under the influence of a tilted periodic potential has been studied by solving quantum Smoluchowski equation (QSE). By invoking physically motivated coordinate transformation into QSE, we have presented a transparent treatment for the determination of the effective diffusion coefficient of a quantum Brownian particle and the current (the average stationary velocity). Substantial enhancement in the efficiency of the diffusive transport is envisaged due to the quantum correction effects only if the bath temperature hovers around an appropriate range of intermediate values. Our findings also confirm the results obtained in the classical cases.

  10. Enhancement of transport properties of a Brownian particle due to quantum effects: Smoluchowski limit

    Energy Technology Data Exchange (ETDEWEB)

    Shit, Anindita [Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103 (India); Chattopadhyay, Sudip, E-mail: sudip_chattopadhyay@rediffmail.com [Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103 (India); Chaudhuri, Jyotipratim Ray, E-mail: jprc_8@yahoo.com [Department of Physics, Katwa College, Katwa, Burdwan 713130 (India)

    2012-03-13

    Graphical abstract: By invoking physically motivated coordinate transformation into quantum Smoluchowski equation, we have presented a transparent treatment for the determination of the effective diffusion coefficient and current of a quantum Brownian particle. Substantial enhancement in the efficiency of the diffusive transport is envisaged due to the quantum correction effects. Highlights:: Black-Right-Pointing-Pointer Transport of a quantum Brownian particle in a periodic potential has been addressed. Black-Right-Pointing-Pointer Governing quantum Smoluchowski equation (QSE) includes state dependent diffusion. Black-Right-Pointing-Pointer A coordinate transformation is used to recast QSE with constant diffusion. Black-Right-Pointing-Pointer Transport properties increases in comparison to the corresponding classical result. Black-Right-Pointing-Pointer This enhancement is purely a quantum effect. - Abstract: The transport property of a quantum Brownian particle that interacts strongly with a bath (in which a typical damping constant by far exceeds a characteristic frequency of the isolated system) under the influence of a tilted periodic potential has been studied by solving quantum Smoluchowski equation (QSE). By invoking physically motivated coordinate transformation into QSE, we have presented a transparent treatment for the determination of the effective diffusion coefficient of a quantum Brownian particle and the current (the average stationary velocity). Substantial enhancement in the efficiency of the diffusive transport is envisaged due to the quantum correction effects only if the bath temperature hovers around an appropriate range of intermediate values. Our findings also confirm the results obtained in the classical cases.

  11. Two-band electron transport in a double quantum well

    Science.gov (United States)

    Fletcher, R.; Tsaousidou, M.; Smith, T.; Coleridge, P. T.; Wasilewski, Z. R.; Feng, Y.

    2005-04-01

    The carrier densities and mobilities have been measured for the first two populated subbands in a GaAs double quantum well (DQW) as a function of the top gate voltage Vg . The densities and quantum mobilities ( μiq , i=1,2 ) were obtained from the de Haas-Shubnikov oscillations. The transport mobilities (μit) were determined from the semiclassical low-field magnetoresistance with intersubband scattering taken into account. At 0.32K the experimental data on both μit and μiq , as a function of Vg , lie on two curves which cross at the resonance point as expected from theoretical considerations. At 1.09K and 4.2K the μit curves no longer cross at resonance, but show a gap. The reason for this is not known. The mobilities have been calculated in the low-temperature limit within the Boltzmann framework by assuming that they are limited by scattering due to ionized impurities located at the outside interfaces. The assumption of short-range scattering is justified by the relatively small value of the ratio μit/μiq that is measured in the present system. The theoretical values obtained for μit and μiq are in reasonable agreement with the experiment for all values of Vg examined. We have also calculated the resistivity and intersubband scattering rates of the DQW as a function of Vg and again find good agreement with measured values.

  12. Transport through Intrinsic Quantum Dots in Interacting Carbon Nanotubes

    Science.gov (United States)

    Thorwart, Michael; Egger, Reinhold; Grifoni, Milena

    Single-wall carbon nanotubes (SWNTs) constitute molecular wires with remarkable electronic properties. Due to the special nature of their electronic bands, SWNTs have been predicted to exhibit Tomonaga-Luttinger liquid (TLL) rather than Fermi liquid behavior at low energy scales. We focus on the effects of electronic correlations, treated within a TLL model, in a SWNT containing two impurities defining a small island for electrons (i.e., a quantum dot). We present analytical and numerical results for the linear conductance, obtained from a master equation approach and dynamical quantum Monte Carlo simulations, respectively. The one dimensional character of transport is reflected in unconventional Coulomb blockade features for temperatures smaller than the level spacing in the dot. In this regime, TLL correlations among tunneling events require a generalization of the standard uncorrelated sequential tunneling picture for intermediate barrier transmission and strong interactions. In that case, correlated sequential tunneling processes can dominate, which lead to a different temperature dependence of the peak conductance. For sufficiently low temperatures, the simulations reveal a universal coherent resonant tunneling regime for arbitrary barrier transmission.

  13. Quantum system driven by incoherent a.c fields: Multi-crossing Landau Zener dynamics

    Science.gov (United States)

    Jipdi, M. N.; Fai, L. C.; Tchoffo, M.

    2016-10-01

    The paper investigates the multi-crossing dynamics of a Landau-Zener (LZ) system driven by two sinusoidal a.c fields applying the Dynamic Matrix approach (DMA). The system is shown to follow one-crossing and multi-crossing dynamics for low and high frequency regime respectively. It is shown that in low frequency regime, the resonance phenomenon occurs and leads to the decoupling of basis states; the effective gap vanishes and then the complete blockage of the system. For high frequency, the system achieves multi-crossing dynamics with two fictitious crossings; the system models a Landau-Zener-Stückelberg (LZS) interferometer with critical parameters that tailor probabilities. The system is then shown to depend only on the phase that permits the easiest control with possible application in implementing logic gates.

  14. Experimental control of transport resonances in a coherent quantum rocking ratchet.

    Science.gov (United States)

    Grossert, Christopher; Leder, Martin; Denisov, Sergey; Hänggi, Peter; Weitz, Martin

    2016-01-01

    The ratchet phenomenon is a means to get directed transport without net forces. Originally conceived to rectify stochastic motion and describe operational principles of biological motors, the ratchet effect can be used to achieve controllable coherent quantum transport. This transport is an ingredient of several perspective quantum devices including atomic chips. Here we examine coherent transport of ultra-cold atoms in a rocking quantum ratchet. This is realized by loading a rubidium atomic Bose-Einstein condensate into a periodic optical potential subjected to a biharmonic temporal drive. The achieved long-time coherence allows us to resolve resonance enhancement of the atom transport induced by avoided crossings in the Floquet spectrum of the system. By tuning the strength of the temporal modulations, we observe a bifurcation of a single resonance into a doublet. Our measurements reveal the role of interactions among Floquet eigenstates for quantum ratchet transport. PMID:26852803

  15. Mixed quantum-classical dynamics for charge transport in organics.

    Science.gov (United States)

    Wang, Linjun; Prezhdo, Oleg V; Beljonne, David

    2015-05-21

    Charge transport plays a crucial role in the working principle of most opto-electronic and energy devices. This is especially true for organic materials where the first theoretical models date back to the 1950s and have continuously evolved ever since. Most of these descriptions rely on perturbation theory to treat small interactions in the Hamiltonian. In particular, applying a perturbative treatment to the electron-phonon and electron-electron coupling results in the band and hopping models, respectively, the signature of which is conveyed by a characteristic temperature dependence of mobility. This perspective describes recent progress of studying charge transport in organics using mixed quantum-classical dynamics techniques, including mean field and surface hopping theories. The studies go beyond the perturbation treatments and represent the processes explicitly in the time-domain, as they occur in real life. The challenges, advantages, and disadvantages of both approaches are systematically discussed. Special focus is dedicated to the temperature dependence of mobility, the role of local and nonlocal electron-phonon couplings, as well as the interplay between electronic and electron-phonon interactions.

  16. Transport through a Strongly Correlated Quantum-Dot with Fano Interference

    OpenAIRE

    Wu, B. H.; J. C. Cao; Ahn, Kang-Hun

    2005-01-01

    We present the transport properties of a strongly correlated quantum dot attached to two leads with a side coupled non-interacting quantum dot. Transport properties are analyzed using the slave boson mean field theory which is reliable in the zero temperature and low bias regime. It is found that the transport properties are determined by the interplay of two fundamental physical phenomena,i.e. the Kondo effects and the Fano interference. The linear conductance will depart from the unitary li...

  17. Quench behavior of Sr0.6K0.4Fe2As2/Ag tapes with AC and DC transport currents at different temperature

    Science.gov (United States)

    Liu, Qi; Zhang, Guomin; Yang, Hua; Li, Zhenming; Liu, Wei; Jing, Liwei; Yu, Hui; Liu, Guole

    2016-09-01

    In applications, superconducting wires may carry AC or DC transport current. Thus, it is important to understand the behavior of normal zone propagation in conductors and magnets under different current conditions in order to develop an effective quench protection system. In this paper, quench behavior of Ag sheathed Sr0.6K0.4Fe2As2 (Sr-122 in the family of iron-based superconductor) tapes with AC and DC transport current is reported. The measurements are performed as a function of different temperature (20 K-30 K), varying transport current and operating frequency (50 Hz-250 Hz). The focus of the research is the minimum quench energy (MQE), the normal zone propagation velocity (NZPV) and the comparison of the related results with AC and DC transport current.

  18. Real-time transport in open quantum systems from PT-symmetric quantum mechanics

    Science.gov (United States)

    Elenewski, Justin E.; Chen, Hanning

    2014-08-01

    Nanoscale electronic transport is of intense technological interest, with applications ranging from semiconducting devices and molecular junctions to charge migration in biological systems. Most explicit theoretical approaches treat transport using a combination of density functional theory (DFT) and nonequilibrium Green's functions. This is a static formalism, with dynamic response properties accommodated only through complicated extensions. To circumvent this limitation, the carrier density may be propagated using real-time time-dependent DFT (RT-TDDFT), with boundary conditions corresponding to an open quantum system. Complex absorbing potentials can emulate outgoing particles at the simulation boundary, although these do not account for introduction of charge density. It is demonstrated that the desired positive particle flux is afforded by a class of PT-symmetric generating potentials that are characterized by anisotropic transmission resonances. These potentials add density every time a particle traverses the cell boundary, and may be used to engineer a continuous pulse train for incident packets. This is a first step toward developing a complete transport formalism unique to RT-TDDFT.

  19. Quantum noise in ac-driven resonant-tunneling double barrier structures: Photon-assisted tunneling vs. electron anti-bunching

    OpenAIRE

    Hammer, Jan; Belzig, Wolfgang

    2011-01-01

    We study the quantum noise of the electronic current in a double-barrier system with a single resonant level. In the framework of the Landauer formalism, we treat the double barrier as a quantum coherent scattering region that can exchange photons with a coupled electric field, e.g., a laser beam or a periodic ac bias voltage. As a consequence of the manifold parameters that are involved in this system, a complicated steplike structure arises in the nonsymmetrized current-current autocorrelat...

  20. Tunable Hybrid Quantum Electrodynamics from Non-Linear Electron Transport

    OpenAIRE

    Schiró, Marco; Hur, Karyn Le

    2013-01-01

    Recent advances in quantum electronics have allowed to engineer hybrid nano-devices comprising on chip a microwave electromagnetic resonator coupled to an artificial atom, a quantum dot. These systems realize novel platforms to explore non-equilibrium quantum impurity physics with light and matter. Coupling the quantum dot system to reservoir leads (source and drain) produces an electronic current as well as dissipation when applying a bias voltage across the system. Focusing on a standard mo...

  1. Quantum Transport Simulation of High-Power 4.6-μm Quantum Cascade Lasers

    Directory of Open Access Journals (Sweden)

    Olafur Jonasson

    2016-06-01

    Full Text Available We present a quantum transport simulation of a 4.6- μ m quantum cascade laser (QCL operating at high power near room temperature. The simulation is based on a rigorous density-matrix-based formalism, in which the evolution of the single-electron density matrix follows a Markovian master equation in the presence of applied electric field and relevant scattering mechanisms. We show that it is important to allow for both position-dependent effective mass and for effective lowering of very thin barriers in order to obtain the band structure and the current-field characteristics comparable to experiment. Our calculations agree well with experiments over a wide range of temperatures. We predict a room-temperature threshold field of 62 . 5 kV/cm and a characteristic temperature for threshold-current-density variation of T 0 = 199 K . We also calculate electronic in-plane distributions, which are far from thermal, and show that subband electron temperatures can be hundreds to thousands of degrees higher than the heat sink. Finally, we emphasize the role of coherent tunneling current by looking at the size of coherences, the off-diagonal elements of the density matrix. At the design lasing field, efficient injection manifests itself in a large injector/upper lasing level coherence, which underscores the insufficiency of semiclassical techniques to address injection in QCLs.

  2. Quantum dot transport in soil, plants, and insects.

    Science.gov (United States)

    Al-Salim, Najeh; Barraclough, Emma; Burgess, Elisabeth; Clothier, Brent; Deurer, Markus; Green, Steve; Malone, Louise; Weir, Graham

    2011-08-01

    Environmental risk assessment of nanomaterials requires information not only on their toxicity to non-target organisms, but also on their potential exposure pathways. Here we report on the transport and fate of quantum dots (QDs) in the total environment: from soils, through their uptake into plants, to their passage through insects following ingestion. Our QDs are nanoparticles with an average particle size of 6.5 nm. Breakthrough curves obtained with CdTe/mercaptopropionic acid QDs applied to columns of top soil from a New Zealand organic apple orchard, a Hastings silt loam, showed there to be preferential flow through the soil's macropores. Yet the effluent recovery of QDs was just 60%, even after several pore volumes, indicating that about 40% of the influent QDs were filtered and retained by the soil column via some unknown exchange/adsorption/sequestration mechanism. Glycine-, mercaptosuccinic acid-, cysteine-, and amine-conjugated CdSe/ZnS QDs were visibly transported to a limited extent in the vasculature of ryegrass (Lolium perenne), onion (Allium cepa) and chrysanthemum (Chrysanthemum sp.) plants when cut stems were placed in aqueous QD solutions. However, they were not seen to be taken up at all by rooted whole plants of ryegrass, onion, or Arabidopsis thaliana placed in these solutions. Leafroller (Lepidoptera: Tortricidae) larvae fed with these QDs for two or four days, showed fluorescence along the entire gut, in their frass (larval feces), and, at a lower intensity, in their haemolymph. Fluorescent QDs were also observed and elevated cadmium levels detected inside the bodies of adult moths that had been fed QDs as larvae. These results suggest that exposure scenarios for QDs in the total environment could be quite complex and variable in each environmental domain. PMID:21632093

  3. Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

    Science.gov (United States)

    Morrison, C.; Casteleiro, C.; Leadley, D. R.; Myronov, M.

    2016-09-01

    The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW.

  4. Edge-channel transport of dirac fermions in graphene quantum hall junctions

    International Nuclear Information System (INIS)

    Dirac fermions exhibit various characteristic transport phenomena in graphene. Particularly in high magnetic fields, the electronic states of Dirac fermions are quantized into Landau levels, and graphene shows a half-integer quantum Hall effect. Here, we discuss the edge-channel picture in graphene quantum Hall junctions and review experiments on the quantum Hall effect in graphene in-plane unipolar and bipolar junctions. (author)

  5. Quantum spin transport through Aharonov-Bohm ring with a tangent magnetic field

    Institute of Scientific and Technical Information of China (English)

    Li Zhi-Jian

    2005-01-01

    Quantum spin transport in a mesoscopic Aharonov-Bohm ring with two leads subject to a magnetic field with circular configuration is investigated by means of one-dimensional quantum waveguide theory. Within the framework magnetic flux or by the tangent magnetic field. In particular, the spin flips can be induced by hopping the AB magnetic flux or the tangent field.

  6. Coupled electron-phonon transport from molecular dynamics with quantum baths

    DEFF Research Database (Denmark)

    Lu, Jing Tao; Wang, J. S.

    2009-01-01

    Based on generalized quantum Langevin equations for the tight-binding wavefunction amplitudes and lattice displacements, electron and phonon quantum transport are obtained exactly using molecular dynamics (MD) in the ballistic regime. The electron-phonon interactions can be handled with a quasi-c...

  7. Phononless thermally activated transport through a disordered array of quantum wires

    OpenAIRE

    Chudnovskiy, A. L.

    2005-01-01

    Phononless plasmon assisted transport through a long disordered array of finite length quantum wires is investigated analytically. Two temperature regimes, the low- and the high-temperature ones, with qualitatively different temperature dependencies of thermally activated resistance are identified. The characteristics of plasmon assisted and phonon assisted transport mechanisms are compared. Generically strong electron-electron interaction in quantum wires results in a qualitative change of t...

  8. Transport phenomena in quantum wells and wires in presence of disorder and interactions

    OpenAIRE

    Vettchinkina, Valeria

    2012-01-01

    Present-day electronics employ circuits of smaller and smaller dimensions, and today the length scales are so small that the laws of physics which rule micro-cosmos, quantum mechanics, become directly important. This thesis reports on theoretical work on electron transport in different nanostructures. We have studied semiconductor quantum wells, layered materials where each layer can be only a few atomic layers thick, and transport in thin atomic wires. The layered materials have been stud...

  9. Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island

    OpenAIRE

    Martins, F.; Faniel, S.; Rosenow, B.; Pala, M. G.; Sellier, H.; S. Huant; L. Desplanque; Wallart, X; Bayot, V.; Hackens, B.

    2013-01-01

    We explore transport across an ultra-small Quantum Hall Island (QHI) formed by closed quan- tum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to first localize and then study a single QHI near a quantum point contact. The presence of Coulomb diamonds in the spectroscopy con- firms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias...

  10. An improved FEM model for computing transport AC loss in coils made of RABiTS YBCO coated conductors for electric machines

    DEFF Research Database (Denmark)

    Ainslie, Mark D; Rodriguez Zermeno, Victor Manuel; Hong, Zhiyong;

    2011-01-01

    AC loss can be a significant problem for any applications that utilize or produce an AC current or magnetic field, such as an electric machine. The authors investigate the electromagnetic properties of high temperature superconductors with a particular focus on the AC loss in superconducting coils...... and magnetic field-dependent critical current densities, and the inclusion and exclusion of a magnetic substrate, as found in RABiTS (rolling-assisted biaxially textured substrate) YBCO coated conductors. The coil model is based on the superconducting stator coils used in the University of Cambridge EPEC...... Superconductivity Group's all-superconducting permanent magnet synchronous motor design. To validate the modeling results, the transport AC loss of a stator coil is measured using an electrical method based on inductive compensation by means of a variable mutual inductance. Finally, the implications of the findings...

  11. Scanning gate spectroscopy of transport across a quantum Hall nano-island

    Science.gov (United States)

    Martins, F.; Faniel, S.; Rosenow, B.; Pala, M. G.; Sellier, H.; Huant, S.; Desplanque, L.; Wallart, X.; Bayot, V.; Hackens, B.

    2013-01-01

    We explore transport across an ultra-small quantum Hall island (QHI) formed by closed quantum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to first localize and then study a single QHI near a quantum point contact. The presence of Coulomb diamonds in the spectroscopy confirms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.

  12. Time-resolved electron transport in quantum-dot systems; Zeitaufgeloester Elektronentransport in Quantendotsystemen

    Energy Technology Data Exchange (ETDEWEB)

    Croy, Alexander

    2010-06-30

    In this thesis the time-resolved electron transport in quantum dot systems was studied. For this two different formalisms were presented: The nonequilibrium Green functions and the generalized quantum master equations. For both formalisms a propagation method for the numerical calculation of time-resolved expectation values, like the occupation and the electron current, was developed. For the demonstration of the propagation method two different question formulations were considered. On the one hand the stochastically driven resonant-level model was studied. On the other hand the pulse-induced transport through a double quantum dot was considered.

  13. Global coherence of quantum evolutions based on decoherent histories: Theory and application to photosynthetic quantum energy transport

    Science.gov (United States)

    Allegra, Michele; Giorda, Paolo; Lloyd, Seth

    2016-04-01

    Assessing the role of interference in natural and artificial quantum dynamical processes is a crucial task in quantum information theory. To this aim, an appropriate formalism is provided by the decoherent histories framework. While this approach has been deeply explored from different theoretical perspectives, it still lacks of a comprehensive set of tools able to concisely quantify the amount of coherence developed by a given dynamics. In this paper, we introduce and test different measures of the (average) coherence present in dissipative (Markovian) quantum evolutions, at various time scales and for different levels of environmentally induced decoherence. In order to show the effectiveness of the introduced tools, we apply them to a paradigmatic quantum process where the role of coherence is being hotly debated: exciton transport in photosynthetic complexes. To spot out the essential features that may determine the performance of the transport, we focus on a relevant trimeric subunit of the Fenna-Matthews-Olson complex and we use a simplified (Haken-Strobl) model for the system-bath interaction. Our analysis illustrates how the high efficiency of environmentally assisted transport can be traced back to a quantum recoil avoiding effect on the exciton dynamics, that preserves and sustains the benefits of the initial fast quantum delocalization of the exciton over the network. Indeed, for intermediate levels of decoherence, the bath is seen to selectively kill the negative interference between different exciton pathways, while retaining the initial positive one. The concepts and tools here developed show how the decoherent histories approach can be used to quantify the relation between coherence and efficiency in quantum dynamical processes.

  14. Anisotropic behavior of quantum transport in graphene superlattices

    DEFF Research Database (Denmark)

    Pedersen, Jesper Goor; Cummings, Aron W.; Roche, Stephan

    2014-01-01

    We report on the possibility to generate highly anisotropic quantum conductivity in disordered graphene-based superlattices. Our quantum simulations, based on an efficient real-space implementation of the Kubo-Greenwood formula, show that in disordered graphene superlattices the strength of multi...

  15. Cavity-photon-switched coherent transient transport in a double quantum waveguide

    Energy Technology Data Exchange (ETDEWEB)

    Abdullah, Nzar Rauf, E-mail: nra1@hi.is; Gudmundsson, Vidar, E-mail: vidar@raunvis.hi.is [Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik (Iceland); Tang, Chi-Shung [Department of Mechanical Engineering, National United University, 1, Lienda, 36003 Miaoli, Taiwan (China); Manolescu, Andrei [School of Science and Engineering, Reykjavik University, Menntavegur 1, IS-101 Reykjavik (Iceland)

    2014-12-21

    We study a cavity-photon-switched coherent electron transport in a symmetric double quantum waveguide. The waveguide system is weakly connected to two electron reservoirs, but strongly coupled to a single quantized photon cavity mode. A coupling window is placed between the waveguides to allow electron interference or inter-waveguide transport. The transient electron transport in the system is investigated using a quantum master equation. We present a cavity-photon tunable semiconductor quantum waveguide implementation of an inverter quantum gate, in which the output of the waveguide system may be selected via the selection of an appropriate photon number or “photon frequency” of the cavity. In addition, the importance of the photon polarization in the cavity, that is, either parallel or perpendicular to the direction of electron propagation in the waveguide system is demonstrated.

  16. Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot.

    Science.gov (United States)

    Rauf Abdullah, Nzar; Tang, Chi-Shung; Manolescu, Andrei; Gudmundsson, Vidar

    2016-09-21

    We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, [Formula: see text], between the photon energy, [Formula: see text], and the cyclotron energy, [Formula: see text]. Enhancement in the electron transport with increasing electron-photon coupling is observed when [Formula: see text]. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contact area for the leads. Suppression in the electron transport is found when [Formula: see text], as the external magnetic field causes circular confinement of the charge density around the dot. PMID:27420809

  17. Nonlinear transport in coupled quantum dots: A stationary probability approach

    Institute of Scientific and Technical Information of China (English)

    GONG JianPing; DUAN SuQing; YAN WeiXian; ZHAO XianGeng

    2009-01-01

    The stationary tunneling current and differential conductance of the coupled quantum dots system with split-gates are calculated by generalizing the Beenaker's linear response theory for the description of the Coulomb-blockade oscillations of the conductance in the single quantum dot. The calculation of the charging diagram in parallel through the double dot as function of the two side-gate voltages shows a remarkable agreement with the recent experimental results by Hatano et al. (Science, 2005, 309: 268-271)

  18. Quantum-based spectroscopy and efficient energy transport with biomolecules

    OpenAIRE

    León Montiel, Roberto de Jesús

    2014-01-01

    For many years, the fields of quantum optics and biology have rarely shared a common path. In quantum optics, most of the concepts and techniques developed over the years stand for systems where only a few degrees of freedom are considered and, more importantly, where the systems under study are assumed to be completely isolated from their surrounding environment. This situation is far from what we can find in nature. Biological complexes are, by definition, warm, wet and noisy systems subjec...

  19. Spin Selective Charge Transport through Cysteine Capped CdSe Quantum Dots.

    Science.gov (United States)

    Bloom, Brian P; Kiran, Vankayala; Varade, Vaibhav; Naaman, Ron; Waldeck, David H

    2016-07-13

    This work demonstrates that chiral imprinted CdSe quantum dots (QDs) can act as spin selective filters for charge transport. The spin filtering properties of chiral nanoparticles were investigated by magnetic conductive-probe atomic force microscopy (mCP-AFM) measurements and magnetoresistance measurements. The mCP-AFM measurements show that the chirality of the quantum dots and the magnetic orientation of the tip affect the current-voltage curves. Similarly, magnetoresistance measurements demonstrate that the electrical transport through films of chiral quantum dots correlates with the chiroptical properties of the QD. The spin filtering properties of chiral quantum dots may prove useful in future applications, for example, photovoltaics, spintronics, and other spin-driven devices. PMID:27336320

  20. Electron transport across a quantum wire embedding a saw-tooth superlattice

    Institute of Scientific and Technical Information of China (English)

    Chen Yuan-Ping; Yan Xiao-Hong; Lu Mao-Wang; Deng Yu-Xiang

    2004-01-01

    By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.

  1. Non-Hermitian Hamiltonian approach to quantum transport in disordered networks with sinks: validity and effectiveness

    OpenAIRE

    Giusteri, Giulio G.; Mattiotti, Francesco; Celardo, G. Luca

    2014-01-01

    We investigate the validity of the non-Hermitian Hamiltonian approach in describing quantum transport in disordered tight-binding networks connected to external environments, acting as sinks. Usually, non-Hermitian terms are added, on a phenomenological basis, to such networks to summarize the effects of the coupling to the sinks. Here we consider a paradigmatic model of open quantum network for which we derive a non-Hermitian effective model, discussing its limit of validity by a comparison ...

  2. Theoretical and experimental results of electronic transport of spin quantum cross structure devices

    OpenAIRE

    Kondo, Kenji; Kaiju, Hideo; Ishibashi, Akira

    2009-01-01

    Recently, we have proposed quantum cross structure (QCS) devices that consist of two metalthin films deposited on organic films with edge-to-edge configuration like crossed fins for switching devices. In this paper, we propose a spin quantum cross structure (SQCS) device, which is a QCS device consisting of two magnetic thin films. We show theoretical and experimental results of electronic transport characteristics regarding SQCS devices. The calculation of the I-Vcharacteristics has been per...

  3. Quantum Thermal Transport through Extremely Cold Dielectric Chains

    Institute of Scientific and Technical Information of China (English)

    LIU Hui-Ping; YI Lin

    2006-01-01

    In the framework of Green's function theory out of equilibrium, a Landauer-Buttiker (LB) formula for thermal conductance is derived. A simplified model for describing extremely cold dielectric chains is proposed for the first time. Fhrther we apply the present LB formula for studying thermal conductance at low-lying modes, emerging in dielectric atom chains. We find that quantum thermal conductance undergoes an anomalous transition due to new quasiparticle excitations, resulting from nonlinear atom-atom interactions. This theoretical prediction is in excellent agreement with a high-accuracy measurement to thermal conductance quantum.

  4. Typical pure nonequilibrium steady states and irreversibility for quantum transport

    Science.gov (United States)

    Monnai, Takaaki; Yuasa, Kazuya

    2016-07-01

    It is known that each single typical pure state in an energy shell of a large isolated quantum system well represents a thermal equilibrium state of the system. We show that such typicality holds also for nonequilibrium steady states (NESS's). We consider a small quantum system coupled to multiple infinite reservoirs. In the long run, the total system reaches a unique NESS. We identify a large Hilbert space from which pure states of the system are to be sampled randomly and show that the typical pure states well describe the NESS. We also point out that the irreversible relaxation to the unique NESS is important to the typicality of the pure NESS's.

  5. Transport through an impurity tunnel coupled to a Si/SiGe quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Foote, Ryan H., E-mail: rhfoote@wisc.edu; Ward, Daniel R.; Thorgrimsson, Brandur; Savage, D. E.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A., E-mail: maeriksson@wisc.edu [Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States); Prance, J. R. [Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB (United Kingdom); Gamble, John King; Nielsen, Erik [Center for Computing Research, Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States); Saraiva, A. L. [Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, 21941-972 Rio de Janeiro (Brazil)

    2015-09-07

    Achieving controllable coupling of dopants in silicon is crucial for operating donor-based qubit devices, but it is difficult because of the small size of donor-bound electron wavefunctions. Here, we report the characterization of a quantum dot coupled to a localized electronic state and present evidence of controllable coupling between the quantum dot and the localized state. A set of measurements of transport through the device enable the determination that the most likely location of the localized state is consistent with a location in the quantum well near the edge of the quantum dot. Our results are consistent with a gate-voltage controllable tunnel coupling, which is an important building block for hybrid donor and gate-defined quantum dot devices.

  6. Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot

    Directory of Open Access Journals (Sweden)

    Moskalenko ES

    2010-01-01

    Full Text Available Abstract Individual InAs/GaAs quantum dots are studied by micro-photoluminescence. By varying the strength of an applied external magnetic field and/or the temperature, it is demonstrated that the charge state of a single quantum dot can be tuned. This tuning effect is shown to be related to the in-plane electron and hole transport, prior to capture into the quantum dot, since the photo-excited carriers are primarily generated in the barrier.

  7. Consistent behaviour of AC susceptibility and transport properties in magnetic superconductor RuSr 2GdCu 2O 8

    Science.gov (United States)

    Očko, M.; Živkovic, I.; Prester, M.; Drobac, Dj.; Ariosa, D.; Berger, H.; Pavuna, D.

    2004-02-01

    We report on AC susceptibility, resistivity, thermopower and measurements of sintered magnetic superconductor RuSr 2GdCu 2O 8. The antiferromagnetic phase transition at 133 K is seen clearly in the AC susceptibility as well as in the derivative of resistivity and thermopower. Above the antiferromagnetic transition, we have found some new evidences of the similarity between HTC compounds and the magnetic superconductor. The onset of superconductivity is observed by both transport methods at 46 K, and an explanation why it is not seen in the AC susceptibility data is given. The end of the SC transition occurs at about 24 K in all measured properties. We discuss the steps of broad SC transition, especially the maximum in susceptibility at 34 K, and compare our results with related studies reported in the literature.

  8. Consistent behaviour of AC susceptibility and transport properties in magnetic superconductor RuSr{sub 2}GdCu{sub 2}O{sub 8}

    Energy Technology Data Exchange (ETDEWEB)

    Ocko, M. E-mail: ocko@ifs.hr; Zivkovic, I.; Prester, M.; Drobac, Dj.; Ariosa, D.; Berger, H.; Pavuna, D

    2004-02-01

    We report on AC susceptibility, resistivity, thermopower and measurements of sintered magnetic superconductor RuSr{sub 2}GdCu{sub 2}O{sub 8}. The antiferromagnetic phase transition at 133 K is seen clearly in the AC susceptibility as well as in the derivative of resistivity and thermopower. Above the antiferromagnetic transition, we have found some new evidences of the similarity between HTC compounds and the magnetic superconductor. The onset of superconductivity is observed by both transport methods at 46 K, and an explanation why it is not seen in the AC susceptibility data is given. The end of the SC transition occurs at about 24 K in all measured properties. We discuss the steps of broad SC transition, especially the maximum in susceptibility at 34 K, and compare our results with related studies reported in the literature.

  9. AC transport current loss analysis for anti-parallel current flow in face to face stacks of superconducting tapes

    Energy Technology Data Exchange (ETDEWEB)

    Yoo, Ja Eun; Han, Young Hee; Kim, Hey Rim; Park, Byung Jun; Yang, Seong Eun; Kim, Hee Sun; Yu, Seung Duck; Park, Ki Jun [Future Technology Laboratory, KEPCO Research Institute, Daejeon (Korea, Republic of)

    2014-06-15

    In this study we investigated ac transport current losses in the face to face stack for the anti-parallel current flow, and compared the electromagnetic properties with those of the single SC tape as well as those of the same stack for the parallel current path. The gap between the SC tapes in the stack varied in order to verify the electromagnetic influence of the neighbors when current flows in opposite direction, and the model was implemented in the finite element method program by the commercial software, COMSOL Multiphysics 4.2a. Conclusively speaking, the loss was remarkably decreased for the anti-parallel current case, which is attributed the magnetic flux compensation between the SC layers due to the opposite direction of the current flows. As the gap between SC tapes was increased, the loss mitigation became less effective. Besides, the current density distribution is very flat cross the sample width for the narrower gap case, which is believed to be benefit for the power electric system. These results are all in good agreement with those predicted theoretically for an infinite bifilar stack.

  10. Inter-dot coupling effects on transport through correlated parallel coupled quantum dots

    Indian Academy of Sciences (India)

    Shyam Chand; G Rajput; K C Sharma; P K Ahluwalia

    2009-05-01

    Transport through symmetric parallel coupled quantum dot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur–Buttiker like current formula are shown in terms of internal states of quantum dots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dot coupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.

  11. Coherent quantum transport in normal-metal/d-wave superconductor/normal-metal double tunnel junctions

    Institute of Scientific and Technical Information of China (English)

    DONG; Zhengchao; FU; Hao

    2004-01-01

    Taking into account the effects of quantum interference and interface scattering, combining the electron current with hole current contribution to tunnel current,we study the coherent quantum transport in normal-metal/d-wave superconductor/normal-metal (NM/d-wave SC/NM) double tunnel junctions by using extended Blonder-Tinkham-Klapwijk (BTK) approach. It is shown that all quasiparticle transport coefficients and conductance spectrum exhibit oscillating behavior with the energy, in which periodic vanishing of Andreev reflection (AR) above superconducting gap is found.In tunnel limit for the interface scattering strength taken very large, there are a series of bound states of quasiparticles formed in SC.

  12. Electron Transport Through a Quantum Wire with a Side-Coupled Quantum Dot:Fano Resonance

    Institute of Scientific and Technical Information of China (English)

    熊永建; 贺舟波

    2004-01-01

    The Fano resonance of a quantum wire (QW) with a side-coupled quantum dot (QD) is investigated. The QD has multilevel and is in the Coulomb blockade regime. We show that there are two aspects in contribution to asymmetric Fano dip line shape of conductance: (1) the quantum interference between the resonant level and non-resonant levels, (2) the asymmetric electron occupation of levels in the two sides of a resonant level in the QD. The smearing of the asymmetry of the dip structure with the increasing temperature is partially attributed to fluctuation of electron state in the QD.

  13. Quantum transport of bosonic cold atoms in double-well optical lattices

    International Nuclear Information System (INIS)

    We numerically investigate, using the time evolving block decimation algorithm, the quantum transport of ultracold bosonic atoms in a double-well optical lattice through slow and periodic modulation of the lattice parameters (intra- and inter-well tunneling, chemical potential, etc.). The transport of atoms does not depend on the rate of change of the parameters (as along as the change is slow) and can distribute atoms in optical lattices at the quantized level without involving external forces. The transport of atoms depends on the atom filling in each double well and the interaction between atoms. In the strongly interacting region, the bosonic atoms share the same transport properties as noninteracting fermions with quantized transport at the half filling and no atom transport at the integer filling. In the weakly interacting region, the number of the transported atoms is proportional to the atom filling. We show the signature of the quantum transport from the momentum distribution of atoms that can be measured in the time-of-flight image. A semiclassical transport model is developed to explain the numerically observed transport of bosonic atoms in the noninteracting and strongly interacting limits. The scheme may serve as an quantized battery for atomtronics applications.

  14. Modeling Quantum Transport in Nanoscale Vertical SOI nMOSFET

    Institute of Scientific and Technical Information of China (English)

    TONG Jian-nong; ZOU Xue-chang; SHEN Xu-bang

    2004-01-01

    The electron transports in micro-architecture semiconductor are simulated using vertical SOI nMOSFET with different models. Some details in transport can be presented by changing channel length, channel thickness and drain voltage. An interesting phenomenon similar to collimation effect in mesoscopic system is observed. This may suggest the quite intriguing possibility that scattering may open new channel in sufficiently narrow devices.

  15. Effects of guaifenesin, N-acetylcysteine, and ambroxol on MUC5AC and mucociliary transport in primary differentiated human tracheal-bronchial cells

    Directory of Open Access Journals (Sweden)

    Seagrave JeanClare

    2012-10-01

    Full Text Available Abstract Background Therapeutic intervention in the pathophysiology of airway mucus hypersecretion is clinically important. Several types of drugs are available with different possible modes of action. We examined the effects of guaifenesin (GGE, N-acetylcysteine (NAC and ambroxol (Amb on differentiated human airway epithelial cells stimulated with IL-13 to produce additional MUC5AC. Methods After IL-13 pre-treatment (3 days, the cultures were treated with GGE, NAC or Amb (10–300 μM in the continued presence of IL-13. Cellular and secreted MUC5AC, mucociliary transport rates (MTR, mucus rheology at several time points, and the antioxidant capacity of the drugs were assessed. Results IL-13 increased MUC5AC content (~25% and secretion (~2-fold and decreased MTR, but only slightly affected the G’ (elastic or G” (viscous moduli of the secretions. GGE significantly inhibited MUC5AC secretion and content in the IL-13-treated cells in a concentration-dependent manner (IC50s at 24 hr ~100 and 150 μM, respectively. NAC or Amb were less effective. All drugs increased MTR and decreased G’ and G” relative to IL-13 alone. Cell viability was not affected and only NAC exhibited antioxidant capacity. Conclusions Thus, GGE effectively reduces cellular content and secretion of MUC5AC, increases MTR, and alters mucus rheology, and may therefore be useful in treating airway mucus hypersecretion and mucostasis in airway diseases.

  16. Persistent quantum resonance transition in spin Hall transport

    Science.gov (United States)

    Chen, Kuo-Chin; Lee, Hsin-Han; Chang, Ching-Ray

    2016-01-01

    We propose an H-shaped two-dimensional topological insulator (2DTI) as a persistent quantum resonance device. The helical edge states of 2DTI are robust against a nonmagnetic field. However, the helical edge states interfere with bound states created by a nonmagnetic impurity. Transmissions between leads shows two kinds of quantum resonance in this device, the Breit-Wigner resonance and a Fano-like resonance. These resonances can be realized in the device through modulating the on-site impurity potential. Resonances in 2DTI are persistent because the helical state has no backscattering that is protected by time-reversal-symmetry conservation. The finite-size effect in 2DTI leads to the phase transition between the Fano and the Breit-Wigner resonances through modulating the thickness of the 2DTI leads.

  17. Charge transport model in nanodielectric composites based on quantum tunneling mechanism and dual-level traps

    Science.gov (United States)

    Li, Guochang; Chen, George; Li, Shengtao

    2016-08-01

    Charge transport properties in nanodielectrics present different tendencies for different loading concentrations. The exact mechanisms that are responsible for charge transport in nanodielectrics are not detailed, especially for high loading concentration. A charge transport model in nanodielectrics has been proposed based on quantum tunneling mechanism and dual-level traps. In the model, the thermally assisted hopping (TAH) process for the shallow traps and the tunnelling process for the deep traps are considered. For different loading concentrations, the dominant charge transport mechanisms are different. The quantum tunneling mechanism plays a major role in determining the charge conduction in nanodielectrics with high loading concentrations. While for low loading concentrations, the thermal hopping mechanism will dominate the charge conduction process. The model can explain the observed conductivity property in nanodielectrics with different loading concentrations.

  18. Curl flux, coherence, and population landscape of molecular systems: Nonequilibrium quantum steady state, energy (charge) transport, and thermodynamics

    CERN Document Server

    Zhang, Zhedong

    2015-01-01

    We established a theoretical framework in terms of the curl flux, population landscape, and coherence for non-equilibrium quantum systems at steady state, through exploring the energy and charge transport in molecular processes. The curl quantum flux plays the key role in determining transport properties and the system reaches equilibrium when flux vanishes. The novel curl quantum flux reflects the degree of non-equilibriumness and the time-irreversibility. We found an analytical expression for the quantum flux and its relationship to the environmental pumping (non-equilibriumness quantified by the voltage away from the equilibrium) and the quantum tunneling. Furthermore, we investigated another quantum signature, the coherence, quantitatively measured by the non-zero off diagonal element of the density matrix. Besides the environment-assistance which can give dramatic enhancement of coherence and quantum flux with high voltage at a fixed tunneling strength, the quantum flux is promoted by the coherence in th...

  19. Semiclassical matrix model for quantum chaotic transport with time-reversal symmetry

    Energy Technology Data Exchange (ETDEWEB)

    Novaes, Marcel, E-mail: marcel.novaes@gmail.com

    2015-10-15

    We show that the semiclassical approach to chaotic quantum transport in the presence of time-reversal symmetry can be described by a matrix model. In other words, we construct a matrix integral whose perturbative expansion satisfies the semiclassical diagrammatic rules for the calculation of transport statistics. One of the virtues of this approach is that it leads very naturally to the semiclassical derivation of universal predictions from random matrix theory.

  20. Coherent control of quantum transport: modulation-enhanced phase detection and band spectroscopy

    CERN Document Server

    Tarallo, Marco G; Wang, F Y; Tino, Guglielmo M

    2012-01-01

    Amplitude modulation of a tilted optical lattice can be used to steer the quantum transport of matter wave packets in a very flexible way. This allows the experimental study of the phase sensitivity in a multimode interferometer based on delocalization-enhanced Bloch oscillations and to probe the band structure modified by a constant force.

  1. Charge transport in bottom-up inorganic-organic and quantum-coherent nanostructures

    NARCIS (Netherlands)

    Makarenko, Ksenia Sergeevna

    2015-01-01

    This thesis is based on results obtained from experiments designed for a consistent study of charge transport in bottom-up inorganic-organic and quantum-coherent nanostructures. New unconventional ways to build elements of electrical circuits (like dielectrophoresis, wedging transfer and bottom-up f

  2. Introduction to quantum chromo transport theory for quark-gluon plasmas

    International Nuclear Information System (INIS)

    Upcoming heavy ion experiments at the AGS and SPS are aimed at producing and diagnosing a primordial form of matter, the quark-gluon plasma. In these lectures some recent developments on formulating a quantum transport theory for quark-gluon plasmas are introduced. 46 refs

  3. Quantum Hall transport as a probe of capacitance profile at graphene edges

    NARCIS (Netherlands)

    Vera-Marun, I. J.; Zomer, P. J.; Veligura, A.; Guimaraes, M. H. D.; Visser, L.; Tombros, N.; van Elferen, H. J.; Zeitler, U.; van Wees, B. J.

    2013-01-01

    The quantum Hall effect is a remarkable manifestation of quantized transport in a two-dimensional electron gas (2DEG). Given its technological relevance, it is important to understand its development in realistic nanoscale devices. In this work, we present how the appearance of different edge channe

  4. Simulation of interdigitated back-contact silicon heterojunction solar cells with quantum transport model

    Science.gov (United States)

    Kamioka, Takefumi; Hayashi, Yutaka; Nakamura, Kyotaro; Ohshita, Yoshio

    2015-08-01

    A simulation of interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells was performed using a quantum transport model to consider the quantum effect at the crystalline/amorphous (c/a) heterojunction interface. It was found that the impact of the quantum effect on the open-circuit voltage is comparable to that of the interface defect density at the c/a interface, indicating the importance of implementation of the quantum model. The optimal back-contact design was also discussed from the viewpoint of mass production, in which the design rule is relaxed. The degradation of the conversion efficiency by widening the gap between the p- and n-aSi:H layers can be compensated by improving passivation quality at the c/a interface.

  5. Nonlinear photon transport in a semiconductor waveguide-cavity system containing a single quantum dot

    CERN Document Server

    Hughes, S

    2011-01-01

    The input/output characteristics of coherent photon transport through a semiconductor cavity system containing a single quantum dot is presented. The nonlinear quantum optics formalism uses a master equation approach and focuses on a waveguide-cavity system containing a semiconductor quantum dot; our general technique also applies to studying coherent reflection from a micropillar cavity. We investigate the effects of light propagation and show the need for quantized multiphoton effects for various dot-cavity systems, including weakly-coupled, intermediately-coupled, and strongly-coupled regimes. We demonstrate that for mean photon numbers much less than 0.1, the commonly adopted weak excitation (single quantum) approximation breaks down---even in the weak coupling regime. As a measure of the photon correlations, we compute the Fano factor and the error associated with making a semiclassical approximation. We also investigate the role of electron--acoustic-phonon scattering and show that phonon-mediated scatt...

  6. Influence of shape on electron transport in ballistic quantum dots

    International Nuclear Information System (INIS)

    We have investigated the low-temperature (T=0.43--4.25 K) magnetotransport of quantum dots fabricated in the shape of an open circle and a circle having a central bar. The characteristic magnetic fields for both coherent backscattering and conductance fluctuations are strongly shape dependent: both are larger by a factor ≥3 in the device with the central bar. Comparison of large and small devices of nominally identical shape shows that characteristic trajectory areas are proportional to the device area

  7. Selective nanomanipulation of fluorescent polystyrene nano-beads and single quantum dots at gold nanostructures based on the AC-dielectrophoretic force

    Science.gov (United States)

    Kim, Jinsik; Hwang, Kyo Seon; Lee, Sangyoup; Park, Jung Ho; Shin, Hyun-Joon

    2015-11-01

    We introduced the selective manipulation of polystyrene (PS) nano-beads and single quantum dots (QDs) at a gold nanostructure by using the AC-dielectrophoretic (DEP) force. Manipulation in three degrees of freedom (end-facet, side, and position-selective manipulation) was accomplished in gold nanostructures between microelectrodes. A 10 μm gap between the microelectrodes, which has a 100 nm-wide nanowire and 200 nm-wide vortex nanostructures at the inside of the gap, was fabricated, and nanostructures were not connected with the electrodes. We also performed theoretical calculations to verify the selective manipulation through the floating AC-DEP force. A sufficiently high gradient of the square of the electric field (∇|E|2, ~1019 V2 m-3) was accomplished and controlled for achieving a strong attaching force of nanoparticles using the gap between microelectrodes and nanostructures as well as the rotation of structures. Fluorescent PS nano-beads and QDs were attached at the designed end facet, side, and position of nanostructures with high selectivity. A single QD attachment was also realized at gold nanostructures, and the attached QDs were verified as single using optical ``blinking'' measurements.We introduced the selective manipulation of polystyrene (PS) nano-beads and single quantum dots (QDs) at a gold nanostructure by using the AC-dielectrophoretic (DEP) force. Manipulation in three degrees of freedom (end-facet, side, and position-selective manipulation) was accomplished in gold nanostructures between microelectrodes. A 10 μm gap between the microelectrodes, which has a 100 nm-wide nanowire and 200 nm-wide vortex nanostructures at the inside of the gap, was fabricated, and nanostructures were not connected with the electrodes. We also performed theoretical calculations to verify the selective manipulation through the floating AC-DEP force. A sufficiently high gradient of the square of the electric field (∇|E|2, ~1019 V2 m-3) was accomplished and

  8. Tuning Fano resonances by magnetic forces for electron transport through a quantum wire side coupled to a quantum ring

    Science.gov (United States)

    Szafran, B.; Poniedziałek, M. R.

    2010-08-01

    We consider electron transport in a quantum wire with a side-coupled quantum ring in a two-dimensional model that accounts for a finite width of the channels. We use the finite difference technique to solve the scattering problem as well as to determine the ring-localized states of the energy continuum. The backscattering probability exhibits Fano peaks for magnetic fields for which a ring-localized states appear at the Fermi level. We find that the width of the Fano resonances changes at high magnetic field. The width is increased (decreased) for resonant states with current circulation that produce the magnetic dipole moment that is parallel (antiparallel) to the external magnetic field. We indicate that the opposite behavior of Fano resonances due to localized states with clockwise and counterclockwise currents results from the magnetic forces which change the strength of their coupling to the channel and modify the lifetime of localized states.

  9. The evolvement of the transport mechanism with the ensemble density of Si quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Balberg, Isaac [The Racah Institue of Physics, The Hebrew University, Jerusalen 91904 (Israel)

    2014-03-31

    In this review I will try to suggest a comprehensive understanding of the transport mechanisms in three dimensional systems of Si quantum dots (QDs) from the single QD to the very dense ensembles. This understanding is based on our systematic microscopic and macroscopic electrical measurements as a function of the density of Si nanocrystallites. In particular, the role of quantum confinement and Coulomb blockade effects in the transport will be discussed and the concept of QDs' 'touching' will be applied. This consideration will enable to reveal the presence of two transitions, a local carrier deconfinement transition and a percolation transition at which these effects are reminiscent of those found in the single QD. It is hoped that our discussion of the evolvement of the transport with the density of the QDs will provide guidance for the understanding of ensembles of semiconductor QDs in general and ensembles of Si QDs in particular.

  10. Quantum Mechanical Study on Tunnelling and Ballistic Transport of Nanometer Si MOSFETs

    International Nuclear Information System (INIS)

    Using self-consistent calculations of million-atom Schrödinger-Poisson equations, we investigate the I–V characteristics of tunnelling and ballistic transport of nanometer metal oxide semiconductor held effect transistors (MOSFET) based on a full 3-D quantum mechanical simulation under nonequilibtium condition. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. We find that the ballistic transport dominates the I–V characteristics, whereas the effects of tunnelling cannot be neglected with the maximal value up to 0.8 mA/μm when the channel length of MOSFET scales down to 25 nm. The effects of tunnelling transport lower the threshold voltage Vt. The ballistic current based on fully 3-D quantum mechanical simulation is relatively large and has small on-off ratio compared with results derived from the calculation methods of Luo et al

  11. Dynamical signatures of molecular symmetries in nonequilibrium quantum transport.

    Science.gov (United States)

    Thingna, Juzar; Manzano, Daniel; Cao, Jianshu

    2016-01-01

    Symmetries play a crucial role in ubiquitous systems found in Nature. In this work, we propose an elegant approach to detect symmetries by measuring quantum currents. Our detection scheme relies on initiating the system in an anti-symmetric initial condition, with respect to the symmetric sites, and using a probe that acts like a local noise. Depending on the position of the probe the currents exhibit unique signatures such as a quasi-stationary plateau indicating the presence of metastability and multi-exponential decays in case of multiple symmetries. The signatures are sensitive to the characteristics of the probe and vanish completely when the timescale of the coherent system dynamics is much longer than the timescale of the probe. These results are demonstrated using a 4-site model and an archetypal example of the para-benzene ring and are shown to be robust under a weak disorder. PMID:27311717

  12. Dynamical signatures of molecular symmetries in nonequilibrium quantum transport

    Science.gov (United States)

    Thingna, Juzar; Manzano, Daniel; Cao, Jianshu

    2016-06-01

    Symmetries play a crucial role in ubiquitous systems found in Nature. In this work, we propose an elegant approach to detect symmetries by measuring quantum currents. Our detection scheme relies on initiating the system in an anti-symmetric initial condition, with respect to the symmetric sites, and using a probe that acts like a local noise. Depending on the position of the probe the currents exhibit unique signatures such as a quasi-stationary plateau indicating the presence of metastability and multi-exponential decays in case of multiple symmetries. The signatures are sensitive to the characteristics of the probe and vanish completely when the timescale of the coherent system dynamics is much longer than the timescale of the probe. These results are demonstrated using a 4-site model and an archetypal example of the para-benzene ring and are shown to be robust under a weak disorder.

  13. Quantum Transport of Disordered Weyl Semimetals at the Nodal Point

    Science.gov (United States)

    Sbierski, Björn; Pohl, Gregor; Bergholtz, Emil J.; Brouwer, Piet W.

    2014-07-01

    Weyl semimetals are paradigmatic topological gapless phases in three dimensions. We here address the effect of disorder on charge transport in Weyl semimetals. For a single Weyl node with energy at the degeneracy point and without interactions, theory predicts the existence of a critical disorder strength beyond which the density of states takes on a nonzero value. Predictions for the conductivity are divergent, however. In this work, we present a numerical study of transport properties for a disordered Weyl cone at zero energy. For weak disorder, our results are consistent with a renormalization group flow towards an attractive pseudoballistic fixed point with zero conductivity and a scale-independent conductance; for stronger disorder, diffusive behavior is reached. We identify the Fano factor as a signature that discriminates between these two regimes.

  14. Relativistic quantum transport coefficients for second-order viscous hydrodynamics

    CERN Document Server

    Florkowski, Wojciech; Maksymiuk, Ewa; Ryblewski, Radoslaw; Strickland, Michael

    2015-01-01

    We express the transport coefficients appearing in the second-order evolution equations for bulk viscous pressure and shear stress tensor using Bose-Einstein, Boltzmann, and Fermi-Dirac statistics for the equilibrium distribution function and Grad's 14-moment approximation as well as the method of Chapman-Enskog expansion for the non-equilibrium part. Specializing to the case of boost-invariant and transversally homogeneous longitudinal expansion of the viscous medium, we compare the results obtained using the above methods with those obtained from the exact solution of massive 0+1d Boltzmann equation in the relaxation-time approximation. We show that compared to the 14-moment approximation, the hydrodynamic transport coefficients obtained using the Chapman-Enskog method result in better agreement with the exact solution of the Boltzmann equation in relaxation-time approximation.

  15. Quantum transport and geometric integration for molecular systems

    OpenAIRE

    Odell, Anders

    2010-01-01

    Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the measurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical d...

  16. Relativistic quantum transport coefficients for second-order viscous hydrodynamics

    OpenAIRE

    Florkowski, Wojciech; Jaiswal, Amaresh; Maksymiuk, Ewa; Ryblewski, Radoslaw; Strickland, Michael

    2015-01-01

    We express the transport coefficients appearing in the second-order evolution equations for bulk viscous pressure and shear stress tensor using Bose-Einstein, Boltzmann, and Fermi-Dirac statistics for the equilibrium distribution function and Grad's 14-moment approximation as well as the method of Chapman-Enskog expansion for the non-equilibrium part. Specializing to the case of transversally homogeneous and boost-invariant longitudinal expansion of the viscous medium, we compare the results ...

  17. Manifestation of quantum confinement in transport properties of ultrathin metallic films

    International Nuclear Information System (INIS)

    The influence of quantum size effect on the transport properties of ultrathin Cu and Fe films in the thickness range between 1 and 32 monolayers is studied by solving the linearized Boltzmann equation in the relaxation-time approximation using ab initio calculations within the framework of spin density functional theory. A strong manifestation of quantum confinement is found in the density of states. However, the plasma frequency shows a smooth or oscillating behavior for different film orientations. The results are in qualitative agreement to infrared-absorption experiments at Cu(111), Fe(001), and Fe(111) films

  18. Vibrational effects in charge transport through a molecular double quantum dot

    CERN Document Server

    Sowa, Jakub K; Briggs, G Andrew D; Gauger, Erik M

    2016-01-01

    Recent progress in the field of molecular electronics has revealed the fundamental importance of the coupling between the electronic degrees of freedom and specific vibrational modes. Considering the examples of a molecular dimer and a carbon nanotube double quantum dot, we here theoretically investigate transport through a two-site system that is strongly coupled to a single vibrational mode. Using a quantum master equation approach, we demonstrate that, depending on the relative positions of the two dots, electron-phonon interactions can lead to negative differential conductance and suppression of the current through the system. We also discuss the experimental relevance of the presented results and possible implementations of the studied system.

  19. Asymmetric Quantum Transport in a Double-Stranded Kronig-Penney Model

    Science.gov (United States)

    Cheon, Taksu; Poghosyan, Sergey S.

    2015-06-01

    We introduce a double-stranded Kronig-Penney model and analyze its transport properties. Asymmetric fluxes between two strands with suddenly alternating localization patterns are found as the energy is varied. The zero-size limit of the internal lines connecting two strands is examined using quantum graph vertices with four edges. We also consider a two-dimensional Kronig-Penney lattice with two types of alternating layer with δ and δ' connections, and show the existence of energy bands in which the quantum flux can flow only in selected directions.

  20. Quantum transport through two series Aharonov-Bohm interferometers with zero total magnetic flux

    Institute of Scientific and Technical Information of China (English)

    Wang Jian-Ming; Wang Rui; Zhang Yong-Ping; Liang Jiu-Qing

    2007-01-01

    With the help of nonequilibrium Green's function technique, the electronic transport through series AharonovBohm (AB) interferometers is investigated. We obtain the AB interference pattern of the transition probability characterized by the algebraic sum φ and the difference θ of two magnetic fluxes, and particularly a general rule of AB oscillation period depending on the ratio of integer quantum numbers of the fluxes. A parity effect is observed, showing the asymmetric AB oscillations with respect to the even and odd quantum numbers of the total flux in antiparallel AB interferometers. It is also shown that the AB flux can shift the Fano resonance peaks of the transmission spectrum.

  1. Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport

    Science.gov (United States)

    Lee, Chee Kong; Moix, Jeremy; Cao, Jianshu

    2015-04-01

    Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover the dynamics described by the Fermi's Golden Rule and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature, the zero-point fluctuations of the bath lead to non-zero transport rates and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data.

  2. Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Chee Kong [Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Centre for Quantum Technologies, National University of Singapore, 117543 Singapore (Singapore); Moix, Jeremy; Cao, Jianshu, E-mail: jianshu@mit.edu [Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)

    2015-04-28

    Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover the dynamics described by the Fermi’s Golden Rule and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature, the zero-point fluctuations of the bath lead to non-zero transport rates and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data.

  3. Hysteresis losses in MgB{sub 2} superconductors exposed to combinations of low AC and high DC magnetic fields and transport currents

    Energy Technology Data Exchange (ETDEWEB)

    Magnusson, N., E-mail: niklas.magnusson@sintef.no [SINTEF Energy Research, NO-7465 Trondheim (Norway); Abrahamsen, A.B. [DTU Wind Energy, Technical University of Denmark, DK-4000 Roskilde (Denmark); Liu, D. [Electrical Power Processing Group, TU Delft, Mekelweg 4, NL-2628 CD Delft (Netherlands); Runde, M. [SINTEF Energy Research, NO-7465 Trondheim (Norway); Polinder, H. [Electrical Power Processing Group, TU Delft, Mekelweg 4, NL-2628 CD Delft (Netherlands)

    2014-11-15

    Highlights: • A method for calculating hysteresis losses in the low AC – high DC magnetic field and transport current range has been shown. • The method can be used in the design of wind turbine generators for calculating the losses in the generator DC rotor. • First estimates indicate tolerable current ripple in the 0.1% range for a 4 T DC MgB{sub 2} generator rotor coil. - Abstract: MgB{sub 2} superconductors are considered for generator field coils for direct drive wind turbine generators. In such coils, the losses generated by AC magnetic fields may generate excessive local heating and add to the thermal load, which must be removed by the cooling system. These losses must be evaluated in the design of the generator to ensure a sufficient overall efficiency. A major loss component is the hysteresis losses in the superconductor itself. In the high DC – low AC current and magnetic field region experimental results still lack for MgB{sub 2} conductors. In this article we reason towards a simplified theoretical treatment of the hysteresis losses based on available models in the literature with the aim of setting the basis for estimation of the allowable magnetic fields and current ripples in superconducting generator coils intended for large wind turbine direct drive generators. The resulting equations use the DC in-field critical current, the geometry of the superconductor and the magnitude of the AC magnetic field component as parameters. This simplified approach can be valuable in the design of MgB{sub 2} DC coils in the 1–4 T range with low AC magnetic field and current ripples.

  4. The quantum Hall effect under the influence of a top-gate and integrating AC lock-in measurements

    OpenAIRE

    Kramer, Tobias; Heller, Eric J.; Parrott, Robert E.

    2009-01-01

    Low frequency AC-measurements are commonly used to determine the voltage and currents through mesoscopic devices. We calculate the effect of the alternating Hall voltage on the recorded time-averaged voltage in the presence of a top-gate covering a large part of the device. The gate is kept on a constant voltage, while the Hall voltage is recorded using an integrating alternating-current lock-in technique. The resulting Hall curves show inflection points at the arithmetic mean between two int...

  5. Electronic structure and quantum transport in controlled impurity systems

    Science.gov (United States)

    Ryu, Hoon

    Due to a continuous device downscaling, a precise control of dopant placements has become a critical factor to determine device performances. Recent progresses in the Scanning Tunneling Microscope lithography control dopant positions within a few atomic layers and have led experimentalists to propose various prototypes of planar patterned densely phosphorous delta-doping silicon (Si:P) devices. Theoretical understanding of electronic properties in such systems based on a realistic modeling approach is critical for potential device designs. Si:P devices are studied with the atomistic tight-binding (TB) approach coupled to charge-potential self-consistent simulations. The dispersion of a 1/4 mono-layer (ML) doped Si:P doping plane is simulated and compared to the previous literatures to validate our methodology. Upon the methodological validation, dispersions of 1/4ML doped ultra-narrow nanowires (NWs) are studied to explain experimentally observed metallic properties. Predicted channel conductances agree well with measured values. Then, a single donor quantum dot device with Si:P NW leads, is modeled to confirm the experimentally realized device is indeed a single atom transistor. Predicted charging energy and gate-control over the channel ground state establish strong connections to the experimental results. Finally, the numerical practicality of the Contact Block Reduction (CBR) method in simulating electron resonance tunneling features, is examined using Si:P NWs as examples. Based on a proof of principles on small TB systems, we show the CBR method can be practical on supercomputing clusters, due a better scalability than the one observed from the Recursive Greens Function and Wavefunction algorithm.

  6. Fragility of Nonlocal Edge-Mode Transport in the Quantum Spin Hall State

    Science.gov (United States)

    Mani, Arjun; Benjamin, Colin

    2016-07-01

    Nonlocal currents and voltages are better at withstanding the deleterious effects of dephasing than local currents and voltages in nanoscale systems. This hypothesis is known to be true in quantum Hall setups. We test this hypothesis in a four-terminal quantum spin Hall setup wherein we compare the local resistance measurement with the nonlocal one. In addition to inelastic-scattering-induced dephasing, we also test the resilience of the resistance measurements in the aforesaid setups to disorder and spin-flip scattering. We find the axiom that nonlocal resistance is less affected by the detrimental effects of disorder and dephasing to be untrue, in general, for the quantum spin Hall case. This has important consequences since it is widely communicated that nonlocal transport through edge channels in topological insulators have potential applications in low-power information processing.

  7. Superexchange interaction enhancement of the quantum transport in a DNA-type molecule

    Institute of Scientific and Technical Information of China (English)

    Wang Rui; Zhang Cun-Xi; Zhou Yun-Qing; Kong Ling-Min

    2011-01-01

    We use the transfer matrix method and the Green function technique to theoretically study the quantum tunnelling through a DNA-type molecule.Ferromagnetic electrodes are used to produce the spin-polarized transmission probability and therefore the spin current.The distance-dependent crossover comes from the topological variation from the onedimensional to the two-dimensional model transform as we switch on the interstrand coupling; a new base pair will present N - 1 extrachannels for the charge and spin as N being the total base pairs.This will restrain the decay of the transmission and improve the stability of the quantum transport.The spin and charge transfer through the DNA-type molecule is consistent with the quantum tunneling barrier.

  8. Fano interferences in the transport properties of triple quantum dot T-shaped systems

    International Nuclear Information System (INIS)

    We consider the transport and the noise characteristic in the case of a triple quantum dots T-shaped system where two of the dots form a two-level system and the other works in a detector-like setup. Our theoretical results are obtained using the equation of motion method for the case of zero on-site Coulomb interaction in the detector dot. The transport trough the T-shaped system can be controlled by varying the coupling between the two-level system dots or the coupling between the detector dot and the exterior electrodes. The Fano dips in the system's conductance can be observe both for strong (fast detector) and weak coupling (slow detector) between the detector dot and the external electrodes. Due to stronger electronic correlations the noise in the case of a slow detector are much higher. This setup may be of interest for the practical realization of qubit states in quantum dots systems.

  9. Normal and Anomalous Diffusion: An Analytical Study Based on Quantum Collision Dynamics and Boltzmann Transport Theory.

    Science.gov (United States)

    Mahakrishnan, Sathiya; Chakraborty, Subrata; Vijay, Amrendra

    2016-09-15

    Diffusion, an emergent nonequilibrium transport phenomenon, is a nontrivial manifestation of the correlation between the microscopic dynamics of individual molecules and their statistical behavior observed in experiments. We present a thorough investigation of this viewpoint using the mathematical tools of quantum scattering, within the framework of Boltzmann transport theory. In particular, we ask: (a) How and when does a normal diffusive transport become anomalous? (b) What physical attribute of the system is conceptually useful to faithfully rationalize large variations in the coefficient of normal diffusion, observed particularly within the dynamical environment of biological cells? To characterize the diffusive transport, we introduce, analogous to continuous phase transitions, the curvature of the mean square displacement as an order parameter and use the notion of quantum scattering length, which measures the effective interactions between the diffusing molecules and the surrounding, to define a tuning variable, η. We show that the curvature signature conveniently differentiates the normal diffusion regime from the superdiffusion and subdiffusion regimes and the critical point, η = ηc, unambiguously determines the coefficient of normal diffusion. To solve the Boltzmann equation analytically, we use a quantum mechanical expression for the scattering amplitude in the Boltzmann collision term and obtain a general expression for the effective linear collision operator, useful for a variety of transport studies. We also demonstrate that the scattering length is a useful dynamical characteristic to rationalize experimental observations on diffusive transport in complex systems. We assess the numerical accuracy of the present work with representative experimental results on diffusion processes in biological systems. Furthermore, we advance the idea of temperature-dependent effective voltage (of the order of 1 μV or less in a biological environment, for example

  10. Normal and Anomalous Diffusion: An Analytical Study Based on Quantum Collision Dynamics and Boltzmann Transport Theory.

    Science.gov (United States)

    Mahakrishnan, Sathiya; Chakraborty, Subrata; Vijay, Amrendra

    2016-09-15

    Diffusion, an emergent nonequilibrium transport phenomenon, is a nontrivial manifestation of the correlation between the microscopic dynamics of individual molecules and their statistical behavior observed in experiments. We present a thorough investigation of this viewpoint using the mathematical tools of quantum scattering, within the framework of Boltzmann transport theory. In particular, we ask: (a) How and when does a normal diffusive transport become anomalous? (b) What physical attribute of the system is conceptually useful to faithfully rationalize large variations in the coefficient of normal diffusion, observed particularly within the dynamical environment of biological cells? To characterize the diffusive transport, we introduce, analogous to continuous phase transitions, the curvature of the mean square displacement as an order parameter and use the notion of quantum scattering length, which measures the effective interactions between the diffusing molecules and the surrounding, to define a tuning variable, η. We show that the curvature signature conveniently differentiates the normal diffusion regime from the superdiffusion and subdiffusion regimes and the critical point, η = ηc, unambiguously determines the coefficient of normal diffusion. To solve the Boltzmann equation analytically, we use a quantum mechanical expression for the scattering amplitude in the Boltzmann collision term and obtain a general expression for the effective linear collision operator, useful for a variety of transport studies. We also demonstrate that the scattering length is a useful dynamical characteristic to rationalize experimental observations on diffusive transport in complex systems. We assess the numerical accuracy of the present work with representative experimental results on diffusion processes in biological systems. Furthermore, we advance the idea of temperature-dependent effective voltage (of the order of 1 μV or less in a biological environment, for example

  11. Contaminación acústica en el transporte sanitario urgente por carretera Acoustic contamination in urgent medical transportation by road

    Directory of Open Access Journals (Sweden)

    S. Ballesteros

    2012-12-01

    Full Text Available Fundamento. Determinar los niveles de exposición a ruido durante el transporte sanitario urgente por carretera y describir la percepción de deterioro de la función auditiva en los trabajadores del sector y el empleo de mecanismos de protección acústica. Metodología. Estudio observacional realizado en Bizkaia (España. Mediante dosimetrías sonométricas se registraron los valores instantáneos máximos de nivel sonoro (Lpico y niveles continuos equivalentes (Leq medidos en una ambulancia tipo de la red de emergencias. Paralelamente, se realizó una encuesta sobre una muestra representativa compuesta por 127 trabajadores en la que se exploró la percepción subjetiva de pérdida de audición y posibles factores asociados, además de otras cuestiones de interés desde la perspectiva de la prevención de riesgos laborales. Resultados. Se registraron Lpico oscilantes entre 80 y 109 dBA en función de la ubicación. Los niveles Leq medios evaluados durante 10 asistencias, desde la activación hasta la llegada al incidente y durante el traslado al hospital fueron de 79,5 dBA y 74,3 dBA (pBackground. To determine levels of exposure to noise during urgent medical transportation by road and to describe the perception of hearing function impairment in workers and the use of hearing protection devices. Material and methods. An observational study was performed in Bizkaia (Spain. The instantaneous maximum noise levels (Lpeak and its continuous equivalents (Leq were recorded through sonometric dosimetries, measured in a "sample ambulance" of the emergency network. In parallel, a survey was conducted on a representative sample of 127 workers in which self-reported hearing loss and associated factors were explored, in addition to other issues of concern from the perspective of prevention of occupational hazards. Results. Lpeak oscillating between 80 and 109 dBA were recorded depending on the location. The average Leq levels measured over 10 emergency

  12. Spin-dependent quantum transport through an Aharonov-Bohm structure spin splitter

    Institute of Scientific and Technical Information of China (English)

    Li Yu-Xian

    2008-01-01

    Using the tight-binding model approximation, this paper investigates theoretically spin-dependent quantum trans-port through an Aharonov-Bohm (AB) interferometer. An external magnetic field is applied to produce the spin-polarization and spin current. The AB interferometer, acting as a spin splitter, separates the opposite spin polarization current. By adjusting the energy and the direction of the magnetic field, large spin-polarized current can be obtained.

  13. Transport of Fluorescence through Highly Scattering Media: Corrections to the Determination of Quantum Yields

    OpenAIRE

    Seely, G. R.

    1987-01-01

    The equations for transport of fluorescence, generated within a highly scattering medium, are solved within the boundary conditions of the Kubelka and Munk treatment. Expressions are derived in closed form for fluorescence fluxes emanating from the front and back surfaces of a highly scattering infinite slab, whether or not fluorescence is absorbed within the sample. An “apparent” quantum yield, calculated from observed intensities of fluorescence and of back-scattered light from the front su...

  14. Type-controlled Nanodevices Based on Encapsulated Few-layer Black Phosphorus for Quantum Transport

    OpenAIRE

    Long, Gen; Xu, Shuigang; Shen, Junying; Hou, Jianqiang; Wu, Zefei; Han, Tianyi; Lin, Jiangxiazi; Wong, Wing Ki; Cai, Yuan; Lortz, Rolf; Wang, Ning

    2016-01-01

    We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are sui...

  15. Quantum pumping in closed systems, adiabatic transport, and the Kubo formula

    OpenAIRE

    Cohen, Doron

    2003-01-01

    Quantum pumping in closed systems is considered. We explain that the Kubo formula contains all the physically relevant ingredients for the calculation of the pumped charge ($Q$) within the framework of linear response theory. The relation to the common formulations of adiabatic transport and ``geometric magnetism" is clarified. We distinguish between adiabatic and dissipative contributions to $Q$. On the one hand we observe that adiabatic pumping does not have to be quantized. On the other ha...

  16. Interaction effects in the transport of particles in nanowire quantum dots

    OpenAIRE

    Kristinsdottir, Liney Halla

    2015-01-01

    Interactions between physical bodies constantly affect their properties. This thesis presents a theoretical study on the effects of interaction in few-body nanowire quantum dots. The focus is to a large extent on a phenomenon called Wigner localization, and how this, as well as other interaction effects, can be identified in an experiment by transport spectroscopy and by thermopower measurements. The physical systems considered are electrons in semiconductor nanowires and an ultracold gas...

  17. Room temperature ballistic transport in InSb quantum well nanodevices

    Science.gov (United States)

    Gilbertson, A. M.; Kormányos, A.; Buckle, P. D.; Fearn, M.; Ashley, T.; Lambert, C. J.; Solin, S. A.; Cohen, L. F.

    2011-12-01

    We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295 K and at current densities in excess of 106 A/cm2. This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at practical device dimensions.

  18. Charge transport and memristive properties of graphene quantum dots embedded in poly(3-hexylthiophene) matrix

    International Nuclear Information System (INIS)

    We show that graphene quantum dots (GQD) embedded in a semiconducting poly(3-hexylthiophene) polymeric matrix act as charge trapping nanomaterials. In plane current-voltage (I-V) measurements of thin films realized from this nanocomposite deposited on gold interdigitated electrodes revealed that the GQD enhanced dramatically the hole transport. I-V characteristics exhibited a strong nonlinear behavior and a pinched hysteresis loop, a signature of a memristive response. The transport properties of this nanocomposite were explained in terms of a trap controlled space charge limited current mechanism.

  19. Spin-Polarized Transport through Parallel Double Quantum Dots Coupled to Ferromagnetic Leads

    Institute of Scientific and Technical Information of China (English)

    HOU Tao; WU Shao-Quan; BI Ai-Hua; YANG Fu-Bin; SUN Wei-Li

    2008-01-01

    We theoretically study the spin-polarized transport phenomena of the parallel double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the equation-of-motion approach. We analyse the transmission probability of this system in both the equilibrium and nonequilibrium cases, and our results reveal that the transport properties show some noticeable characteristics depending upon both the spin-polarized strength p and the value of the magnetic flux Ф. Moreover, in the parallel configuration, the position of the Kondo peak shifts while it remains unchanged for the antiparallel configuration. These effects might have some potential applications in spintronics.

  20. 高温超导YBCO薄膜中的传输交流损耗%TRANSPORT AC LOSSES IN THIN FILM OF YBCO

    Institute of Scientific and Technical Information of China (English)

    赵玉峰; 何天虎; 冯旺军; 李世荣; 周又和

    2011-01-01

    基于Norris方程和Bean临界态模型,考虑薄膜超导体内的磁场和电流密度分布特性,通过解析求解的方法推导出薄膜超导体在传输外加电流时其内部的磁场和电流密度以及传输交流损耗的解析表达式,从定量的角度研究超导体截面几何形状对传输交流损耗的影响.结果表明薄膜超导体边缘处的剧烈变化的磁场和电流的分布以及无场区的电流承载能力是影响传输交流损耗的主要原因.%Based on Norris equations and Bean model, by considering the characteristics of magnetic field and current density in thin film, the magnetic field and current density in thin film are analytically derived, as well as the transport ac losses. The effect of the geometry of thin film on the transport ac losses is studied quantitatively. The results indicate that the dramatic change of magnetic field and current density at edge of thin film and the current capacity of the field-free region are the main reasons which affect the transport ac losses of thin film.

  1. Tunable Kondo Effect of a Three-Terminal Transport Quantum Dot Embedded in an Aharonov-Bohm Ring

    Institute of Scientific and Technical Information of China (English)

    CHEN Xiong-Wen; SHI Zhen-Gang; WU Shao-Quan; SONG Ke-Hui

    2006-01-01

    @@ We theoretically investigate the Kondo effect of a three-terminal transport quantum dot (QD) embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian.

  2. Microscopic observation of carrier-transport dynamics in quantum-structure solar cells using a time-of-flight technique

    Energy Technology Data Exchange (ETDEWEB)

    Toprasertpong, Kasidit; Fujii, Hiromasa; Sugiyama, Masakazu; Nakano, Yoshiaki [School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032 (Japan); Kasamatsu, Naofumi; Kada, Tomoyuki; Asahi, Shigeo; Kita, Takashi [Graduate School of Engineering, Kobe University, Nada-ku, Kobe 657-8501 (Japan); Wang, Yunpeng; Watanabe, Kentaroh [Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo 153-8904 (Japan)

    2015-07-27

    In this study, we propose a carrier time-of-flight technique to evaluate the carrier transport time across a quantum structure in an active region of solar cells. By observing the time-resolved photoluminescence signal with a quantum-well probe inserted under the quantum structure at forward bias, the carrier transport time can be efficiently determined at room temperature. The averaged drift velocity shows linear dependence on the internal field, allowing us to estimate the quantum structure as a quasi-bulk material with low effective mobility containing the information of carrier dynamics. We show that this direct and real-time observation is more sensitive to carrier transport than other conventional techniques, providing better insights into microscopic carrier transport dynamics to overcome a device design difficulty.

  3. Quantum Thermodynamics in Strong Coupling: Heat Transport and Refrigeration

    Directory of Open Access Journals (Sweden)

    Gil Katz

    2016-05-01

    Full Text Available The performance characteristics of a heat rectifier and a heat pump are studied in a non-Markovian framework. The device is constructed from a molecule connected to a hot and cold reservoir. The heat baths are modelled using the stochastic surrogate Hamiltonian method. The molecule is modelled by an asymmetric double-well potential. Each well is semi-locally connected to a heat bath composed of spins. The dynamics are driven by a combined system–bath Hamiltonian. The temperature of the baths is regulated by a secondary spin bath composed of identical spins in thermal equilibrium. A random swap operation exchange spins between the primary and secondary baths. The combined system is studied in various system–bath coupling strengths. In all cases, the average heat current always flows from the hot towards the cold bath in accordance with the second law of thermodynamics. The asymmetry of the double well generates a rectifying effect, meaning that when the left and right baths are exchanged the heat current follows the hot-to-cold direction. The heat current is larger when the high frequency is coupled to the hot bath. Adding an external driving field can reverse the transport direction. Such a refrigeration effect is modelled by a periodic driving field in resonance with the frequency difference of the two potential wells. A minimal driving amplitude is required to overcome the heat leak effect. In the strong driving regime the cooling power is non-monotonic with the system–bath coupling.

  4. On the application of quantum transport theory to electron sources

    Energy Technology Data Exchange (ETDEWEB)

    Jensen, K.L

    2003-05-15

    Electron sources (e.g., field emitter arrays, wide band-gap (WBG) semiconductor materials and coatings, carbon nanotubes, etc.) seek to exploit ballistic transport within the vacuum after emission from microfabricated structures. Regardless of kind, all sources strive to minimize the barrier to electron emission by engineering material properties (work function/electron affinity) or physical geometry (field enhancement) of the cathode. The unique capabilities of cold cathodes, such as instant ON/OFF performance, high brightness, high current density, large transconductance to capacitance ratio, cold emission, small size and/or low voltage operation characteristics, commend their use in several advanced devices when physical size, weight, power consumption, beam current, and pulse repletion frequency are important, e.g., RF power amplifier such as traveling wave tubes (TWTs) for radar and communications, electrodynamic tethers for satellite deboost/reboost, and electric propulsion systems such as Hall thrusters for small satellites. The theoretical program described herein is directed towards models to evaluate emission current from electron sources (in particular, emission from WBG and Spindt-type field emitter) in order to assess their utility, capabilities and performance characteristics. Modeling efforts particularly include: band bending, non-linear and resonant (Poole-Frenkel) potentials, the extension of one-dimensional theory to multi-dimensional structures, and emission site statistics due to variations in geometry and the presence of adsorbates. Two particular methodologies, namely, the modified Airy approach and metal-semiconductor statistical hyperbolic/ellipsoidal model, are described in detail in their present stage of development.

  5. Transport properties of silicon complementary-metal-oxide semiconductor quantum well field-effect transistors

    Science.gov (United States)

    Naquin, Clint Alan

    Introducing explicit quantum transport into silicon (Si) transistors in a manner compatible with industrial fabrication has proven challenging, yet has the potential to transform the performance horizons of large scale integrated Si devices and circuits. Explicit quantum transport as evidenced by negative differential transconductances (NDTCs) has been observed in a set of quantum well (QW) n-channel metal-oxide-semiconductor (NMOS) transistors fabricated using industrial silicon complementary MOS processing. The QW potential was formed via lateral ion implantation doping on a commercial 45 nm technology node process line, and measurements of the transfer characteristics show NDTCs up to room temperature. Detailed gate length and temperature dependence characteristics of the NDTCs in these devices have been measured. Gate length dependence of NDTCs shows a correlation of the interface channel length with the number of NDTCs formed as well as with the gate voltage (VG) spacing between NDTCs. The VG spacing between multiple NDTCs suggests a quasi-parabolic QW potential profile. The temperature dependence is consistent with partial freeze-out of carrier concentration against a degenerately doped background. A folding amplifier frequency multiplier circuit using a single QW NMOS transistor to generate a folded current-voltage transfer function via a NDTC was demonstrated. Time domain data shows frequency doubling in the kHz range at room temperature, and Fourier analysis confirms that the output is dominated by the second harmonic of the input. De-embedding the circuit response characteristics from parasitic cable and contact impedances suggests that in the absence of parasitics the doubling bandwidth could be as high as 10 GHz in a monolithic integrated circuit, limited by the transresistance magnitude of the QW NMOS. This is the first example of a QW device fabricated by mainstream Si CMOS technology being used in a circuit application and establishes the feasibility

  6. Büttiker probes for dissipative phonon quantum transport in semiconductor nanostructures

    Science.gov (United States)

    Miao, K.; Sadasivam, S.; Charles, J.; Klimeck, G.; Fisher, T. S.; Kubis, T.

    2016-03-01

    Theoretical prediction of phonon transport in modern semiconductor nanodevices requires atomic resolution of device features and quantum transport models covering coherent and incoherent effects. The nonequilibrium Green's function method is known to serve this purpose well but is numerically expensive in simulating incoherent scattering processes. This work extends the efficient Büttiker probe approach widely used in electron transport to phonons and considers salient implications of the method. Different scattering mechanisms such as impurity, boundary, and Umklapp scattering are included, and the method is shown to reproduce the experimental thermal conductivity of bulk Si and Ge over a wide temperature range. Temperature jumps at the lead/device interface are captured in the quasi-ballistic transport regime consistent with results from the Boltzmann transport equation. Results of this method in Si/Ge heterojunctions illustrate the impact of atomic relaxation on the thermal interface conductance and the importance of inelastic scattering to activate high-energy channels for phonon transport. The resultant phonon transport model is capable of predicting the thermal performance in the heterostructure efficiently.

  7. Role of inter-tube coupling and quantum interference on electrical transport in carbon nanotube junctions

    Science.gov (United States)

    Tripathy, Srijeet; Bhattacharyya, Tarun Kanti

    2016-09-01

    Due to excellent transport properties, Carbon nanotubes (CNTs) show a lot of promise in sensor and interconnect technology. However, recent studies indicate that the conductance in CNT/CNT junctions are strongly affected by the morphology and orientation between the tubes. For proper utilization of such junctions in the development of CNT based technology, it is essential to study the electronic properties of such junctions. This work presents a theoretical study of the electrical transport properties of metallic Carbon nanotube homo-junctions. The study focuses on discerning the role of inter-tube interactions, quantum interference and scattering on the transport properties on junctions between identical tubes. The electronic structure and transport calculations are conducted with an Extended Hückel Theory-Non Equilibrium Green's Function based model. The calculations indicate conductance to be varying with a changing crossing angle, with maximum conductance corresponding to lattice registry, i.e. parallel configuration between the two tubes. Further calculations for such parallel configurations indicate onset of short and long range oscillations in conductance with respect to changing overlap length. These oscillations are attributed to inter-tube coupling effects owing to changing π orbital overlap, carrier scattering and quantum interference of the incident, transmitted and reflected waves at the inter-tube junction.

  8. Scale-estimation of quantum coherent energy transport in multiple-minima systems.

    Science.gov (United States)

    Farrow, Tristan; Vedral, Vlatko

    2014-01-01

    A generic and intuitive model for coherent energy transport in multiple minima systems coupled to a quantum mechanical bath is shown. Using a simple spin-boson system, we illustrate how a generic donor-acceptor system can be brought into resonance using a narrow band of vibrational modes, such that the transfer efficiency of an electron-hole pair (exciton) is made arbitrarily high. Coherent transport phenomena in nature are of renewed interest since the discovery that a photon captured by the light-harvesting complex (LHC) in photosynthetic organisms can be conveyed to a chemical reaction centre with near-perfect efficiency. Classical explanations of the transfer use stochastic diffusion to model the hopping motion of a photo-excited exciton. This accounts inadequately for the speed and efficiency of the energy transfer measured in a series of recent landmark experiments. Taking a quantum mechanical perspective can help capture the salient features of the efficient part of that transfer. To show the versatility of the model, we extend it to a multiple minima system comprising seven-sites, reminiscent of the widely studied Fenna-Matthews-Olson (FMO) light-harvesting complex. We show that an idealised transport model for multiple minima coupled to a narrow-band phonon can transport energy with arbitrarily high efficiency. PMID:24980547

  9. Transport anomalies and quantum criticality in electron-doped cuprate superconductors

    Science.gov (United States)

    Zhang, Xu; Yu, Heshan; He, Ge; Hu, Wei; Yuan, Jie; Zhu, Beiyi; Jin, Kui

    2016-06-01

    Superconductivity research is like running a marathon. Three decades after the discovery of high-Tc cuprates, there have been mass data generated from transport measurements, which bring fruitful information. In this review, we give a brief summary of the intriguing phenomena reported in electron-doped cuprates from the aspect of electrical transport as well as the complementary thermal transport. We attempt to sort out common features of the electron-doped family, e.g. the strange metal, negative magnetoresistance, multiple sign reversals of Hall in mixed state, abnormal Nernst signal, complex quantum criticality. Most of them have been challenging the existing theories, nevertheless, a unified diagram certainly helps to approach the nature of electron-doped cuprates.

  10. Shot noise of the edge transport in the inverted band HgTe quantum wells

    Science.gov (United States)

    Tikhonov, E. S.; Shovkun, D. V.; Khrapai, V. S.; Kvon, Z. D.; Mikhailov, N. N.; Dvoretsky, S. A.

    2015-05-01

    We investigate the current noise in HgTe-based quantum wells with an inverted band structure in the regime of disordered edge transport. Consistent with previous experiments, the edge resistance strongly exceeds h/ e 2 and weakly depends on the temperature. The shot noise is well below the Poissonian value and characterized by the Fano factor with gate voltage and sample-to-sample variations in the range 0.1 < F < 0.3. Given the fact that our devices are shorter than the most pessimistic estimate of the ballistic dephasing length, these observations exclude the possibility of one-dimensional helical edge transport. Instead, we suggest that a disordered multi-mode conduction is responsible for the edge transport in our experiment.

  11. The role of ligand density and size in mediating quantum dot nuclear transport.

    Science.gov (United States)

    Tang, Peter S; Sathiamoorthy, Sarmitha; Lustig, Lindsay C; Ponzielli, Romina; Inamoto, Ichiro; Penn, Linda Z; Shin, Jumi A; Chan, Warren C W

    2014-10-29

    Studying the effects of the physicochemical properties of nanomaterials on cellular uptake, toxicity, and exocytosis can provide the foundation for designing safer and more effective nanoparticles for clinical applications. However, an understanding of the effects of these properties on subcellular transport, accumulation, and distribution remains limited. The present study investigates the effects of surface density and particle size of semiconductor quantum dots on cellular uptake as well as nuclear transport kinetics, retention, and accumulation. The current work illustrates that cellular uptake and nuclear accumulation of nanoparticles depend on surface density of the nuclear localization signal (NLS) peptides with nuclear transport reaching a plateau at 20% surface NLS density in as little as 30 min. These intracellular nanoparticles have no effects on cell viability up to 72 h post treatment. These findings will set a foundation for engineering more sophisticated nanoparticle systems for imaging and manipulating genetic targets in the nucleus.

  12. Dirac fermions in strong electric field and quantum transport in graphene

    CERN Document Server

    Gavrilov, S P; Yokomizo, N

    2012-01-01

    Our previous results on the nonperturbative calculations of the mean current and of the energy-momentum tensor in QED with the T-constant electric field are generalized to arbitrary dimensions. The renormalized mean values are found; the vacuum polarization and particle creation contributions to these mean values are isolated in the large T-limit, the vacuum polarization contributions being related to the one-loop effective Euler-Heisenberg Lagrangian. Peculiarities in odd dimensions are considered in detail. We adapt general results obtained in 2+1 dimensions to the conditions which are realized in the Dirac model for graphene. We study the quantum electronic and energy transport in the graphene at low carrier density and low temperatures when quantum interference effects are important. Our description of the quantum transport in the graphene is based on the so-called generalized Furry picture in QED where the strong external field is taken into account nonperturbatively; this approach is not restricted to a...

  13. Transport in constricted quantum Hall systems: beyond the Kane-Fisher paradigm

    International Nuclear Information System (INIS)

    A simple model of edge transport in a constricted quantum Hall system with a lowered local fi lling factor is studied. The current backscattered from the constriction is explained from a matching of the properties of the edge-current excitations in the constriction (ν2) and bulk (ν1) regions. We develop a hydrodynamic theory for bosonic edge modes inspired by this model, stressing the importance of boundary conditions in elucidating the nature of current transport. By invoking a generalised quasiparticle-quasihole symmetry of the quantum Hall circuit system, we fi nd that a competition between two tunneling process determines the fate of the low-bias transmission conductance. A novel generalisation of the Kane-Fisher quantum impurity model is found, describing transitions from a weak-coupling theory at partial transmission to strong- coupling theories for perfect transmission and reflection as well as a new symmetry dictated fixed point. These results provide satisfactory explanations for recent experimental results at fi lling-factors of 1/3 and 1. (author)

  14. Nanoelectronics. Spin transport in the NEGF method and quantum spin Hall effect by «bottom-up» approach

    OpenAIRE

    Yu.A. Kruglyak; P.A. Kondratenko; Yu.М. Lopatkin

    2015-01-01

    Spin transport with the NEGF method in the spinor representation, in particular, spin valve, rotating magnetic contacts, spin precession and rotating spins, Zeeman and Rashba spin Hamiltonians, quantum spin Hall effect, calculation the spin potential, and four-component description of transport are discussed in the frame of the «bottom – up» approach of modern nanoelectronics.

  15. Nanoelectronics. Spin transport in the NEGF method and quantum spin Hall effect by «bottom-up» approach

    Directory of Open Access Journals (Sweden)

    Yu.A. Kruglyak

    2015-12-01

    Full Text Available Spin transport with the NEGF method in the spinor representation, in particular, spin valve, rotating magnetic contacts, spin precession and rotating spins, Zeeman and Rashba spin Hamiltonians, quantum spin Hall effect, calculation the spin potential, and four-component description of transport are discussed in the frame of the «bottom – up» approach of modern nanoelectronics.

  16. A study of transport suppression in an undoped AlGaAs/GaAs quantum dot single-electron transistor

    DEFF Research Database (Denmark)

    See, A. M.; Klochan, O.; Micolich, P.;

    2013-01-01

    We report a study of transport blockade features in a quantum dot single-electron transistor, based on an undoped AlGaAs/GaAs heterostructure. We observe suppression of transport through the ground state of the dot, as well as negative differential conductance at finite source-drain bias. The...

  17. Type-controlled nanodevices based on encapsulated few-layer black phosphorus for quantum transport

    Science.gov (United States)

    Long, Gen; Xu, Shuigang; Shen, Junying; Hou, Jianqiang; Wu, Zefei; Han, Tianyi; Lin, Jiangxiazi; Wong, Wing Ki; Cai, Yuan; Lortz, Rolf; Wang, Ning

    2016-09-01

    We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are suitable metal electrodes for BP channels to control the transition from a p-type unipolar property to ambipolar characteristic because of different work functions. Record-high mobilities of 6000 cm2 V‑1 s‑1 and 8400 cm2 V‑1 s‑1 are respectively obtained for electrons and holes at cryogenic temperatures. High-mobility BP devices enable the investigation of quantum oscillations with an indistinguishable Zeeman effect in laboratory magnetic field.

  18. Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul [School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)

    2006-07-01

    A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  19. Electronic Transport for a Quantum Wire Partly Irradiated under THz Electromagnetic Wave

    Institute of Scientific and Technical Information of China (English)

    杨谋; 周光辉

    2003-01-01

    We study the electronic transport of a quantum wire partly irradiated under an external terahertz (THz) electromagnetic field. Using the free-electron model and scattering matrix approach we demonstrate that although the electrons in a ballistic quantum wire only suffer from lateral collision with photons, the reflection of electrons also takes place. More interestingly there is a sharp step-structure in the transmission probability as the total energy of electron increases to a threshold value when the frequency of electromagnetic field is resonant with the separation of lateral levels of the wire. The interference structure of transmission for the system apparently appears when the field only irradiates the middle part of the wire.

  20. Quantum transport through a multi-quantum-dot-pair chain side-coupled with Majorana bound states

    Science.gov (United States)

    Zhao-Tan, Jiang; Cheng-Cheng, Zhong

    2016-06-01

    We investigate the quantum transport properties through a special kind of quantum dot (QD) system composed of a serially coupled multi-QD-pair (multi-QDP) chain and side-coupled Majorana bound states (MBSs) by using the Green functions method, where the conductance can be classified into two kinds: the electron tunneling (ET) conductance and the Andreev reflection (AR) one. First we find that for the nonzero MBS-QDP coupling a sharp AR-induced zero-bias conductance peak with the height of e 2/h is present (or absent) when the MBS is coupled to the far left (or the other) QDP. Moreover, the MBS-QDP coupling can suppress the ET conductance and strengthen the AR one, and further split into two sub-peaks each of the total conductance peaks of the isolated multi-QDPs, indicating that the MBS will make obvious influences on the competition between the ET and AR processes. Then we find that the tunneling rate Γ L is able to affect the conductances of leads L and R in different ways, demonstrating that there exists a Γ L-related competition between the AR and ET processes. Finally we consider the effect of the inter-MBS coupling on the conductances of the multi-QDP chains and it is shown that the inter-MBS coupling will split the zero-bias conductance peak with the height of e 2/h into two sub-peaks. As the inter-MBS coupling becomes stronger, the two sub-peaks are pushed away from each other and simultaneously become lower, which is opposite to that of the single QDP chain where the two sub-peaks with the height of about e 2/2h become higher. Also, the decay of the conductance sub-peaks with the increase of the MBS-QDP coupling becomes slower as the number of the QDPs becomes larger. This research should be an important extension in studying the transport properties in the kind of QD systems coupled with the side MBSs, which is helpful for understanding the nature of the MBSs, as well as the MBS-related QD transport properties. Project supported by the National Natural

  1. Spin-polarized current in double quantum dots

    Institute of Scientific and Technical Information of China (English)

    Li Ai-Xian; Duan Su-Qing

    2012-01-01

    We analyze the transport through asymmetric double quantum dots with an inhomogeneous Zeeman splitting in the presence of crossed dc and ac magnetic fields.A strong spin-polarized current can be obtained by changing the dc magnetic field.It is mainly due to the resonant tunnelling.But for the ferromagnetic right electrode,the electron spin resonance also plays an important role in transport.We show that the double quantum dots with three-level mixing under crossed dc and ac magnetic fields can act not only as a bipolar spin filter but also as a spin inverter under suitable conditions.

  2. Theory of quantum transport in disordered systems driven by voltage pulse

    Science.gov (United States)

    Zhou, Chenyi; Chen, Xiaobin; Guo, Hong

    2016-08-01

    Predicting time-dependent quantum transport in the transient regime is important for understanding the intrinsic dynamic response of a nanodevice and for predicting the limit of how such a device can switch on or off a current. Theoretically, this problem becomes quite difficult to solve when the nanodevice contains disorder because the calculated transient current must be averaged over many disorder configurations. In this work, we present a theoretical formalism to calculate the configuration averaged time-dependent current flowing through a phase coherent device containing disorder sites where the transient current is driven by sharply turning on and off the external bias voltage. Our theory is based on the Keldysh nonequilibrium Green's function (NEGF) formalism and is applicable in the far from equilibrium nonlinear response quantum transport regime. The effects of disorder scattering are dealt with by the coherent potential approximation (CPA) extended in the time domain. We show that after approximations such as CPA and vertex corrections for calculating the multiple impurity scattering in the transient regime, the derived NEGFs perfectly satisfy a Ward identity. The theory is quantitatively verified by comparing its predictions to the exact solution for a tight-binding model of a disordered two-probe transport junction.

  3. Full counting statistics of renormalized dynamics in open quantum transport system

    Energy Technology Data Exchange (ETDEWEB)

    Luo, JunYan, E-mail: jyluo@zust.edu.cn [School of Science, Zhejiang University of Science and Technology, Hangzhou, 310023 (China); Shen, Yu; He, Xiao-Ling [School of Science, Zhejiang University of Science and Technology, Hangzhou, 310023 (China); Li, Xin-Qi [Department of Chemistry, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR (China); State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China); Department of Physics, Beijing Normal University, Beijing 100875 (China); Yan, YiJing [Department of Chemistry, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR (China)

    2011-11-28

    The internal dynamics of a double quantum dot system is renormalized due to coupling respectively with transport electrodes and a dissipative heat bath. Their essential differences are identified unambiguously in the context of full counting statistics. The electrode coupling caused level detuning renormalization gives rise to a fast-to-slow transport mechanism, which is not resolved at all in the average current, but revealed uniquely by pronounced super-Poissonian shot noise and skewness. The heat bath coupling introduces an interdot coupling renormalization, which results in asymmetric Fano factor and an intriguing change of line shape in the skewness. -- Highlights: ► We study full counting statistics of electron transport through double quantum dots. ► Essential differences due to coupling to the electrodes and heat bath are identified. ► Level detuning induced by electrodes results in strongly enhanced shot noise and skewness. ► Interdot coupling renormalization due to heat bath leads to asymmetric noise and intriguing skewness.

  4. Critical comparison of electrode models in density functional theory based quantum transport calculations

    Science.gov (United States)

    Jacob, D.; Palacios, J. J.

    2011-01-01

    We study the performance of two different electrode models in quantum transport calculations based on density functional theory: parametrized Bethe lattices and quasi-one-dimensional wires or nanowires. A detailed account of implementation details in both the cases is given. From the systematic study of nanocontacts made of representative metallic elements, we can conclude that the parametrized electrode models represent an excellent compromise between computational cost and electronic structure definition as long as the aim is to compare with experiments where the precise atomic structure of the electrodes is not relevant or defined with precision. The results obtained using parametrized Bethe lattices are essentially similar to the ones obtained with quasi-one-dimensional electrodes for large enough cross-sections of these, adding a natural smearing to the transmission curves that mimics the true nature of polycrystalline electrodes. The latter are more demanding from the computational point of view, but present the advantage of expanding the range of applicability of transport calculations to situations where the electrodes have a well-defined atomic structure, as is the case for carbon nanotubes, graphene nanoribbons, or semiconducting nanowires. All the analysis is done with the help of codes developed by the authors which can be found in the quantum transport toolbox ALACANT and are publicly available.

  5. Quantum transport with long-range steps on Watts-Strogatz networks

    Science.gov (United States)

    Wang, Yan; Xu, Xin-Jian

    2016-07-01

    We study transport dynamics of quantum systems with long-range steps on the Watts-Strogatz network (WSN) which is generated by rewiring links of the regular ring. First, we probe physical systems modeled by the discrete nonlinear schrödinger (DNLS) equation. Using the localized initial condition, we compute the time-averaged occupation probability of the initial site, which is related to the nonlinearity, the long-range steps and rewiring links. Self-trapping transitions occur at large (small) nonlinear parameters for coupling ɛ=-1 (1), as long-range interactions are intensified. The structure disorder induced by random rewiring, however, has dual effects for ɛ=-1 and inhibits the self-trapping behavior for ɛ=1. Second, we investigate continuous-time quantum walks (CTQW) on the regular ring ruled by the discrete linear schrödinger (DLS) equation. It is found that only the presence of the long-range steps does not affect the efficiency of the coherent exciton transport, while only the allowance of random rewiring enhances the partial localization. If both factors are considered simultaneously, localization is greatly strengthened, and the transport becomes worse.

  6. The Wigner Monte-Carlo method for nanoelectronic devices a particle description of quantum transport and decoherence

    CERN Document Server

    Querlioz, Damien

    2013-01-01

    This book gives an overview of the quantum transport approaches for nanodevices and focuses on the Wigner formalism. It details the implementation of a particle-based Monte Carlo solution of the Wigner transport equation and how the technique is applied to typical devices exhibiting quantum phenomena, such as the resonant tunnelling diode, the ultra-short silicon MOSFET and the carbon nanotube transistor. In the final part, decoherence theory is used to explain the emergence of the semi-classical transport in nanodevices.

  7. Topics in quantum transport of charge and heat in solid state systems

    Science.gov (United States)

    Choi, Yunjin

    In the thesis, we present a series of investigations for quantum transport of charge and heat in solid state systems. The first topic of the thesis focuses on the fundamental quantum problems which can be studied with electron transport along with the correlations of detectors to measure physical properties. We theoretically describe a generalized ``which-path'' measurement using a pair of coupled electronic Mach-Zehnder Interferometers. In the second topic of thesis, we investigate an operational approach to measure the tunneling time based on the Larmor clock. To handle the cases of indirect measurement from the first and second topics, we introduce the contextual values formalism. The form of the contextual values provides direct physical insight into the measurement being performed, providing information about the correlation strength between system and detector, the measurement inefficiency, the proper background removal, and the conditioned average value of the system operator. Additionally, the weak interaction limit of these conditioned averages produces weak values of the system operator and an additional detector dependent disturbance term for both cases. In our treatment of the third topic of the thesis, we propose a three terminal heat engine based on semiconductor superlattices for energy harvesting. The periodicity of the superlattice structure creates an energy miniband, giving an energy window to allow electron transport. We find that this device delivers a large amount of power, nearly twice that produced by the heat engine based on quantum wells, with a small reduction of efficiency. This engine also works as a refrigerator in a different regime of the system's parameters. The thermoelectric performance of the refrigerator is analyzed, including the cooling power and coefficient of performance in the optimized condition. We also calculate phonon heat current through the system and explore the reduction of phonon heat current compared to the bulk

  8. An Approximate Framework for Quantum Transport Calculation with Model Order Reduction

    CERN Document Server

    Chen, Quan; Yam, Chiyung; Zhang, Yu; Wong, Ngai; Chen, Guanhua

    2014-01-01

    A new approximate computational framework is proposed for computing the non-equilibrium charge density in the context of the non-equilibrium Green's function (NEGF) method for quantum mechanical transport problems. The framework consists of a new formulation, called the X-formulation, for single-energy density calculation based on the solution of sparse linear systems, and a projection-based nonlinear model order reduction (MOR) approach to address the large number of energy points required for large applied biases. The advantages of the new methods are confirmed by numerical experiments.

  9. QUANTUM-MECHANICAL PROPERTIES OF PROTON TRANSPORT IN THE HYDROGEN-BONDED MOLECULAR SYSTEMS

    Institute of Scientific and Technical Information of China (English)

    PANG XIAO-FENG; LI PING

    2000-01-01

    The dynamic equations of the proton transport along the hydrogen bonded molecular systems have been obtainedby using completely quantum-mechanical method to be based on new Hamiltonian and model we proposed. Somequantum-mechanical features of the proton-solitons have also been given in such a case. The alternate motion of twodefects resulting from proton transfer occurred in the systems can be explained by the results. The results obtainedshow that the proton-soliton has corpuscle feature and obey classical equations of motion, while the free soliton movesin uniform velocity along the hydrogen bonded chains.

  10. Spin-polarized transport through the T-shaped double quantum dots

    Institute of Scientific and Technical Information of China (English)

    Yang Fu-Bin; Wu Shao-Quan; Yan Cong-Hua; Huang Rui; Hou Tao; Bi Ai-Hua

    2008-01-01

    Using the Keldysh nonequilibrium Green function and equation-of-motion technique, this paper investigates the spin-polarized transport properties of the T-shaped double quantum dots (DQD) coupled to two ferromagnetic leads.There are both Fano effect and Kondo effect in the system, and due to their mutual interaction, the density of states,the current, and the differential conductance of the system depend sensitively on the spin-polarized strength. Thus the obtained results show that this system is provided with excellent spin filtering property, which indicates that this system may be a candidate for spin valve transistors in the spintronics.

  11. Quantum transport in the surface states of epitaxial Bi(111) thin films

    Science.gov (United States)

    Zhu, Kai; Wu, Lin; Gong, Xinxin; Xiao, Shunhao; Jin, Xiaofeng

    2016-09-01

    Although bulk Bi is a prototypical semimetal with a topologically trivial electronic band structure, we show by various quantum transport measurements that epitaxial Bi(111) thin films have unexpected and nontrivial properties. Not only the top and the bottom but also the side surfaces of epitaxial Bi(111) thin films are always robustly metallic while the interior has already become insulating. We identify the coupling between the top and the bottom surface states that drives the two originally independent surface conducting channels into a single connected one. The properties of Bi(111) thin films realized could lead to promising applications in spintronics.

  12. Electrons and holes in Si quantum well: a room-temperature transport and drag resistance study

    OpenAIRE

    Prunnila, M.; Laakso, S. J.; Kivioja, J. M.; Ahopelto, J.

    2008-01-01

    We investigate carrier transport in a single 22 nm-thick double-gated Si quantum well device, which has independent contacts to electrons and holes. Conductance, Hall density and Hall mobility are mapped in a broad double-gate voltage window. When the gate voltage asymmetry is not too large only either electrons or holes occupy the Si well and the Hall mobility shows the fingerprints of volume inversion/accumulation. At strongly asymmetric double-gate voltage an electric field induced electro...

  13. Quantum transport in Dirac materials: Signatures of tilted and anisotropic Dirac and Weyl cones

    Science.gov (United States)

    Trescher, Maximilian; Sbierski, Björn; Brouwer, Piet W.; Bergholtz, Emil J.

    2015-03-01

    We calculate conductance and noise for quantum transport at the nodal point for arbitrarily tilted and anisotropic Dirac or Weyl cones. Tilted and anisotropic dispersions are generic in the absence of certain discrete symmetries, such as particle-hole and lattice point group symmetries. Whereas anisotropy affects the conductance g , but leaves the Fano factor F (the ratio of shot noise power and current) unchanged, a tilt affects both g and F . Since F is a universal number in many other situations, this finding is remarkable. We apply our general considerations to specific lattice models of strained graphene and a pyrochlore Weyl semimetal.

  14. Quantum transport through complex networks - from light-harvesting proteins to semiconductor devices

    Energy Technology Data Exchange (ETDEWEB)

    Kreisbeck, Christoph

    2012-06-18

    Electron transport through small systems in semiconductor devices plays an essential role for many applications in micro-electronics. One focus of current research lies on establishing conceptually new devices based on ballistic transport in high mobility AlGaAs/AlGa samples. In the ballistic regime, the transport characteristics are determined by coherent interference effects. In order to guide experimentalists to an improved device design, the characterization and understanding of intrinsic device properties is crucial. We develop a time-dependent approach that allows us to simulate experimentally fabricated, complex devicegeometries with an extension of up to a few micrometers. Particularly, we explore the physical origin of unexpected effects that have been detected in recent experiments on transport through Aharonov-Bohm waveguide-interferometers. Such interferometers can be configured as detectors for transfer properties of embedded quantum systems. We demonstrate that a four-terminal waveguide-ring is a suitable setup for measuring the transmission phase of a harmonic quantum dot. Quantum effects are not restricted exclusively to artificial devices but have been found in biological systems as well. Pioneering experiments reveal quantum effects in light-harvesting complexes, the building blocks of photosynthesis. We discuss the Fenna-Matthews-Olson complex, which is a network of coupled bacteriochlorophylls. It acts as an energy wire in the photosynthetic apparatus of green sulfur bacteria. Recent experimental findings suggest that energy transfer takes place in the form of coherent wave-like motion, rather than through classical hopping from one bacteriochlorophyll to the next. However, the question of why and how coherent transfer emerges in light-harvesting complexes is still open. The challenge is to merge seemingly contradictory features that are observed in experiments on two-dimensional spectroscopy into a consistent theory. Here, we provide such a

  15. Adsorbate-induced quantum Hall system probed by scanning tunneling spectroscopy combined with transport measurements

    Energy Technology Data Exchange (ETDEWEB)

    Masutomi, Ryuichi, E-mail: masutomi@phys.s.u-tokyo.ac.jp; Okamoto, Tohru [Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)

    2015-06-22

    An adsorbate-induced quantum Hall system at the cleaved InSb surfaces is investigated in magnetic fields up to 14 T using low-temperature scanning tunneling microscopy and spectroscopy combined with transport measurements. We show that an enhanced Zeeman splitting in the Shubnikov-de Haas oscillations is explained by an exchange enhancement of spin splitting and potential disorder, both of which are obtained from the spatially averaged density of states (DOS). Moreover, the Altshuler–Aronov correlation gap is observed in the spatially averaged DOS at 0 T.

  16. Quantum transport measurement of few-layer WTe2 field effect devices

    Science.gov (United States)

    Chen, Jianhao; Liu, Xin; Tian, Shibing; Zhang, Chenglong; Jia, Shuang

    2015-03-01

    We have performed systematic quantum transport measurement on field effect devices fabricated from few-layer WTe2 single crystals. We found that the magnetoresistance of few-layer WTe2 could be very different from that of bulk samples, which may arise from the imbalance of electron and hole carriers in the samples. We shall discuss our findings in more details in light of recent progress in our experiment. This work is supported by National Natural Science Foundation of China (11374021 and 11327406); by China Ministry of Science and Technology under Contract # 2014CB920900 and 2013CB921900; and by the Young 1000-Talent Program of China.

  17. `Tight Binding' methods in quantum transport through molecules and small devices: From the coherent to the decoherent description

    OpenAIRE

    Pastawski, Horacio M.; Medina, Ernesto

    2001-01-01

    We discuss the steady-state electronic transport in solid-state and molecular devices in the quantum regime. The decimation technique allows a comprehensive description of the electronic structure. Such a method is used, in conjunction with the generalizations of Landauer's tunneling formalism, to describe a wide range of transport regimes. We analize mesoscopic and semiclassical metallic transport, the metal-insulator transition, and the resonant tunneling regime. The effects of decoherence ...

  18. Tunable spin selective transport and quantum phase transition in parallel double dot system

    Science.gov (United States)

    Xiong, Yong-Chen; Wang, Wei-Zhong; Luo, Shi-Jun; Yang, Jun-Tao

    2016-02-01

    We study theoretically the spin selective transport and the quantum phase transition (QPT) in a double dot device by means of the numerical renormalization group technique. When the gate voltage ε is in the Kondo regime and the interdot hopping t is large enough, a first order QPT between local spin singlet and Sz=1 of the triplet is observed as the magnetic field B increases. Beyond the Kondo regime, the QPTs depend closely on ε and t, and perfect spin filter is found, where the effect of spin filtering could easily be manipulated by tuning external parameters. We show that the interplay between the Zeeman effect and the antiferromagnetic interdot hopping, and occupancy switching are responsible for the QPT and the spin selective transport.

  19. Simulations of quantum transport in nanoscale systems: application to atomic gold and silver wires

    DEFF Research Database (Denmark)

    Mozos, J.L.; Ordejon, P.; Brandbyge, Mads;

    2002-01-01

    We present a first-principles method for studying the electronic transport through nanoscale atomic systems under non-equilibrium conditions. The method is based on density functional theory, and allows the calculation of the response of the system to an applied finite potential difference....... The potential drop profile and induced electronic current (and therefore the conductance) are obtained from first principles. The method takes into account the atomic structure of both the nanoscale structure and the semi-infinite electrodes through which the potential is applied. Non-equilibrium Green......'s function techniques are used to calculate the quantum conductance. Here we apply the method to the study of the electronic transport in wires of gold and silver with atomic thickness. We show the results of our calculations, and compare with some of the abundant experimental data on these systems....

  20. Andreev transport in a correlated ferromagnet-quantum-dot-superconductor device

    Science.gov (United States)

    Weymann, I.; Wójcik, K. P.

    2015-12-01

    The spin-resolved Andreev reflection processes in a hybrid ferromagnet-quantum-dot-superconductor device are theoretically studied. In particular, the transport coefficients, such as the Andreev transmission as well as the linear-response Andreev conductance, are calculated by means of the numerical renormalization group method. It is shown that, generally, transport properties are conditioned by the interplay of correlations leading to the Kondo effect, superconducting proximity effect, and ferromagnetic-contact-induced exchange field. The exchange field is shown to greatly affect the low-energy behavior of the Andreev transmission by splitting the Kondo resonance. Moreover, it leads to a nonmonotonic dependence of the Andreev conductance on the dot level position. At low temperatures, the conductance has a peak at the particle-hole symmetry point, which however becomes quickly suppressed with increasing the temperature. The mechanisms responsible for those effects are thoroughly discussed.

  1. Basic concepts of quantum interference and electron transport in single-molecule electronics.

    Science.gov (United States)

    Lambert, C J

    2015-02-21

    This tutorial outlines the basic theoretical concepts and tools which underpin the fundamentals of phase-coherent electron transport through single molecules. The key quantity of interest is the transmission coefficient T(E), which yields the electrical conductance, current-voltage relations, the thermopower S and the thermoelectric figure of merit ZT of single-molecule devices. Since T(E) is strongly affected by quantum interference (QI), three manifestations of QI in single-molecules are discussed, namely Mach-Zehnder interferometry, Breit-Wigner resonances and Fano resonances. A simple MATLAB code is provided, which allows the novice reader to explore QI in multi-branched structures described by a tight-binding (Hückel) Hamiltonian. More generally, the strengths and limitations of materials-specific transport modelling based on density functional theory are discussed. PMID:25255961

  2. Acoustically induced spin transport in (110)GaAs quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Couto, Odilon D.D. Jr.

    2008-09-29

    In this work, we employ surface acoustic waves (SAWs) to transport and manipulate optically generated spin ensembles in (110) GaAs quantum wells (QWs). The strong carrier confinement into the SAW piezoelectric potential allows for the transport of spin-polarized carrier packets along well-defined channels with the propagation velocity of the acoustic wave. In this way, spin transport over distances exceeding 60 m is achieved, corresponding to spin lifetimes longer than 20 ns. The demonstration of such extremely long spin lifetimes is enabled by three main factors: (i) Suppression of the D'yakonov-Perel' spin relaxation mechanism for z-oriented spins in (110) IIIV QWs; (ii) Suppression of the Bir-Aronov-Pikus spin relaxation mechanism caused by the type-II SAW piezoelectric potential; (iii) Suppression of spin relaxation induced by the mesoscopic carrier confinement into narrow stripes along the SAW wave front direction. A spin transport anisotropy under external magnetic fields (B{sub ext}) is demonstrated for the first time. Employing the well-defined average carrier momentum impinged by the SAW, we analyze the spin dephasing dynamics during transport along the [001] and [1 anti 10] in-plane directions. For transport along [001], fluctuations of the internal magnetic field (B{sub int}), which arises from the spin-orbit interaction associated with the bulk inversion asymmetry of the crystal, lead to decoherence within 2 ns as the spins precess around B{sub ext}. In contrast, for transport along the [1 anti 10] direction, the z-component of the spin polarization is maintained for times one order of magnitude longer due to the non-zero average value of B{sub int}. The dephasing anisotropy between the two directions is fully understood in terms of the dependence of the spin-orbit coupling on carrier momentum direction, as predicted by the D'yakonov-Perel' mechanism for the (110) system. (orig.)

  3. Nonequilibrium spin-polarized thermal transport in ferromagnetic-quantum dot-metal system

    Science.gov (United States)

    Xu, Li; Li, Zhi-Jian; Niu, Pengbin; Nie, Yi-Hang

    2016-10-01

    We use nonequilibrium Green function to analyze the nonequilibrium spin-polarized thermal transport through the ferromagnetic-quantum dot-metal system, in which a quantum dot (QD) is coupled to the ferromagnetic and metal electrodes with the voltage bias and the temperature shift. The differential thermoelectric conductance L (θ) is always zero and has no relation with the temperature shift when ε is equal to the Fermi level. The positive and negative values of L (θ) manifest the thermoelectric characteristic of electron-like (or hole-like) carrier when the temperature shift is nonzero. The electrostatic potential U becomes spin-dependent, and makes the dot level renormalization when the ferromagnetic-quantum dot-metal system is driven by the voltage bias and the temperature shift. We define that the spin polarization of the currents between the spin current Is and the electric current Ic is denoted as Is /Ic. The spin polarization Is /Ic shows novel and unique physical phenomenon when the voltage bias and the temperature shift are changed in the nonequilibrium state. Another interesting phenomenon is that we can obtain the pure spin current and a zero point of the thermocurrent Ith by adjusting the voltage bias and the temperature shift.

  4. Stochastic feedback control of quantum transport to realize a dynamical ensemble of two nonorthogonal pure states

    Science.gov (United States)

    Daryanoosh, Shakib; Wiseman, Howard M.; Brandes, Tobias

    2016-02-01

    A Markovian open quantum system which relaxes to a unique steady state ρss of finite rank can be decomposed into a finite physically realizable ensemble (PRE) of pure states. That is, as shown by R. I. Karasik and H. M. Wiseman [Phys. Rev. Lett. 106, 020406 (2011), 10.1103/PhysRevLett.106.020406], in principle there is a way to monitor the environment so that in the long-time limit the conditional state jumps between a finite number of possible pure states. In this paper we show how to apply this idea to the dynamics of a double quantum dot arising from the feedback control of quantum transport, as previously considered by C. Pöltl, C. Emary, and T. Brandes [Phys. Rev. B 84, 085302 (2011), 10.1103/PhysRevB.84.085302]. Specifically, we consider the limit where the system can be described as a qubit, and show that while the control scheme can always realize a two-state PRE, in the incoherent-tunneling regime there are infinitely many PREs compatible with the dynamics that cannot be so realized. For the two-state PREs that are realized, we calculate the counting statistics and see a clear distinction between the coherent and incoherent regimes.

  5. Enhancing light absorption within the carrier transport length in quantum junction solar cells.

    Science.gov (United States)

    Fu, Yulan; Hara, Yukihiro; Miller, Christopher W; Lopez, Rene

    2015-09-10

    Colloidal quantum dot (CQD) solar cells have attracted tremendous attention because of their tunable absorption spectrum window and potentially low processing cost. Recently reported quantum junction solar cells represent a promising approach to building a rectifying photovoltaic device that employs CQD layers on each side of the p-n junction. However, the ultimate efficiency of CQD solar cells is still highly limited by their high trap state density in both p- and n-type CQDs. By modeling photonic structures to enhance the light absorption within the carrier transport length and by ensuring that the carrier generation and collection efficiencies were both augmented, our work shows that overall device current density could be improved. We utilized a two-dimensional numerical model to calculate the characteristics of patterned CQD solar cells based on a simple grating structure. Our calculation predicts a short circuit current density as high as 31  mA/cm2, a value nearly 1.5 times larger than that of the conventional flat design, showing the great potential value of patterned quantum junction solar cells. PMID:26368966

  6. Scaling law and general expression for transport ac loss of a rectangular thin strip with power-law E(J) relation

    Science.gov (United States)

    Chen, Du-Xing; Li, Shuo; Fang, Jin

    2015-12-01

    Transport ac loss Q of a superconducting rectangular thin strip obeying a power-law relation E∝Jn as a function of current amplitude Im may be, following Norris, expressed by normalized quantities as q(im). A scaling law is deduced that if Icf, Ic and f being the critical current and frequency, is multiplied by a positive constant C, then im and qm are multiplied by C 1 /(n - 1)and C 2 /(n - 1) , respectively. Based on this scaling law and the well-known Norris formula, the general function of q(im, n, f) is obtained graphically or analytically for any practical purpose, after accurate numerical computations on a set of q(im) at several values of n and a fixed value of f.

  7. Quantum dot single molecule tracking reveals a wide range of diffusive motions of membrane transport proteins

    Science.gov (United States)

    Crane, Jonathan M.; Haggie, Peter M.; Verkman, A. S.

    2009-02-01

    Single particle tracking (SPT) provides information about the microscopic motions of individual particles in live cells. We applied SPT to study the diffusion of membrane transport proteins in cell plasma membranes in which individual proteins are labeled with quantum dots at engineered extracellular epitopes. Software was created to deduce particle diffusive modes from quantum dot trajectories. SPT of aquaporin (AQP) water channels and cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels revealed several types of diffusion. AQP1 was freely mobile in cell membranes, showing rapid, Brownian-type diffusion. The full-length (M1) isoform of AQP4 also diffused rapidly, though the diffusion of a shorter (M23) isoform of AQP4 was highly restricted due to its supermolecular assembly in raft-like orthogonal arrays. CFTR mobility was also highly restricted, in a spring-like potential, due to its tethering to the actin cytoskeleton through PDZ-domain C-terminus interactions. The biological significance of regulated diffusion of membrane transport proteins is a subject of active investigation.

  8. Scale-estimation of quantum coherent energy transport in multiple-minima systems

    CERN Document Server

    Farrow, Tristan

    2014-01-01

    A generic and intuitive model for coherent energy transport in multiple minima systems coupled to a quantum mechanical bath is shown. Using a simple spin-boson system, we illustrate how a generic donor-acceptor system can be brought into resonance using a narrow band of vibrational modes, such that the transfer efficiency of an electron-hole pair (exciton) is made arbitrarily high. Coherent transport phenomena in nature are of renewed interest since the discovery that a photon captured by the light-harvesting complex (LHC) in photosynthetic organisms can be conveyed to a chemical reaction centre with near-perfect efficiency. Classical explanations of the transfer use stochastic diffusion to model the hopping motion of a photo-excited exciton. This accounts inadequately for the speed and efficiency of the energy transfer measured in a series of recent landmark experiments. Taking a quantum mechanical perspective can help capture the salient features of the efficient part of that transfer. To show the versatili...

  9. Direct Imaging of Long-Range Exciton Transport in Quantum Dot Superlattices by Ultrafast Microscopy.

    Science.gov (United States)

    Yoon, Seog Joon; Guo, Zhi; Dos Santos Claro, Paula C; Shevchenko, Elena V; Huang, Libai

    2016-07-26

    Long-range charge and exciton transport in quantum dot (QD) solids is a crucial challenge in utilizing QDs for optoelectronic applications. Here, we present a direct visualization of exciton diffusion in highly ordered CdSe QDs superlattices by mapping exciton population using ultrafast transient absorption microscopy. A temporal resolution of ∼200 fs and a spatial precision of ∼50 nm of this technique provide a direct assessment of the upper limit for exciton transport in QD solids. An exciton diffusion length of ∼125 nm has been visualized in the 3 ns experimental time window and an exciton diffusion coefficient of (2.5 ± 0.2) × 10(-2) cm(2) s(-1) has been measured for superlattices constructed from 3.6 nm CdSe QDs with center-to-center distance of 6.7 nm. The measured exciton diffusion constant is in good agreement with Förster resonance energy transfer theory. We have found that exciton diffusion is greatly enhanced in the superlattices over the disordered films with an order of magnitude higher diffusion coefficient, pointing toward the role of disorder in limiting transport. This study provides important understandings on energy transport mechanisms in both the spatial and temporal domains in QD solids. PMID:27387010

  10. Spin-resolved Andreev transport through double-quantum-dot Cooper pair splitters

    Science.gov (United States)

    Trocha, Piotr; Weymann, Ireneusz

    2015-06-01

    We investigate the Andreev transport through double-quantum-dot Cooper pair splitters with ferromagnetic leads. The analysis is performed with the aid of the real-time diagrammatic technique in the sequential tunneling regime. We study the dependence of the Andreev current, the differential conductance, and the tunnel magnetoresistance on various parameters of the model in both the linear and nonlinear response regimes. In particular, we analyze the spin-resolved transport in the crossed Andreev reflection regime, where a blockade of the current occurs due to enhanced occupation of the triplet state. We show that in the triplet blockade, finite intradot correlations can lead to considerable leakage current due to direct Andreev reflection processes. Furthermore, we find additional regimes of current suppression resulting from enhanced occupation of singlet states, which decreases the rate of crossed Andreev reflection. We also study how the splitting of Andreev bound states, triggered by either dot level detuning, finite hopping between the dots, or finite magnetic field, affects the Andreev current. While in the first two cases the number of Andreev bound states is doubled, whereas transport properties are qualitatively similar, in the case of finite magnetic field further level splitting occurs, leading to a nontrivial behavior of spin-resolved transport characteristics, and especially that of tunneling magnetoresistance. Finally, we discuss the entanglement fidelity between split Cooper pair electrons and show that by tuning the device parameters, fidelity can reach unity.

  11. Open Quantum System Dynamics from a Measurement Perspective: Applications to Coherent Particle Transport and to Quantum~Brownian Motion

    CERN Document Server

    Kamleitner, Ingo

    2010-01-01

    We employ the theoretical framework of positive operator valued measures, to study Markovian open quantum systems. In particular, we discuss how a quantum system influences its environment. Using the theory of indirect measurements, we then draw conclusions about the information we could hypothetically obtain about the system by observing the environment. Although the environment is not actually observed, we can use these results to describe the change of the quantum system due to its interaction with the environment. We apply this technique to two different problems. In the first part, we study the coherently driven dynamics of a particle on a rail of quantum dots. This tunnelling between adjacent quantum dots can be controlled externally. We employ an adiabatic scheme similar to stimulated Raman adiabatic passage, to transfer the particle between different quantum dots. We compare two fundamentally different sources of decoherence. In the second part, we study the dynamics of a free quantum particle, which ...

  12. Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling

    Institute of Scientific and Technical Information of China (English)

    Tang Han-Zhao; Zhai Li-Xue; Liu Jian-Jun

    2012-01-01

    Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling (SOC)are investigated analytically using quantum networks and the transport matrix method.The results show that conductances remain at exactly the same values when the output leads are located at axisymmetric positions.However,for the nonaxisymmetrical case,there is a phase difference between the upper and lower arm,which leads to zero conductances appearing periodically.An isotropy of the conductance is destroyed by the Rashba SOC effect in the axisymmetric case.In addition,the position of zero conductance is regulated with the strength of the Rashba SOC.

  13. Engineering of Doping and Transport for Enhanced Colloidal Quantum Dot Photovoltaics

    Science.gov (United States)

    Zhitomirsky, David

    Colloidal Quantum Dots (CQDs) are nanoscale quantum-tuned semiconductor particles suspended in solution. When deployed as optoelectronic materials, CQDs are closely packed together into thin films enabling charge transport. This also enables the formation of semiconductor junctions and contacts with other bulk semiconductor materials and metals, respectively. However, limited attention has been given to understanding the fundamental electronic behavior of these materials as bulk-like films, gaining fine control over their semiconducting properties, and then leveraging these insights to make better semiconductor devices. In this thesis, I explore two of the most fundamental concepts to CQD semiconductor device physics: charge carrier doping density and charge carrier transport. With the aid of optoelectronic simulation, I show that these are the most important paths to pursue in order to improve the photovoltaic device performance. I develop a doping density theory that I then rigorously test for the PbS CQD materials system; this theory is also applicable to other types of CQD materials. I demonstrate doping densities on the order of 10 16 cm-3 to 1018 cm-3 for both p- and n-type films. My work enables a previously unavailable p-n homojunction within one CQD materials system, and furthermore allows to grade the doping within the active absorber layer to reach power conversion efficiencies (PCEs) exceeding 7%. I then study CQD size polydispersity, and use it to investigate the details of charge transport in the rough energetic landscapes inherent to these materials. Here, I find that midgap trap elimination is the most important concept in rapidly obtaining dramatic photovoltaic performance gains. By directly measuring the diffusion length in highly coupled CQD films, and combined with optoelectronic modeling, I was able to develop a new passivation strategy achieving a record 8.5% PCE. My research serves as a roadmap for future performance improvements in CQD

  14. Investigations of ion transport through nanoscale polymer membranes by fluorescence quenching of CdSe/CdS quantum dot/quantum rods.

    Science.gov (United States)

    Merkl, Jan-Philip; Wolter, Christopher; Flessau, Sandra; Schmidtke, Christian; Ostermann, Johannes; Feld, Artur; Mews, Alf; Weller, Horst

    2016-04-14

    Detailed steady-state and time-resolved fluorescence quenching measurements give deep insight into ion transport through nanometer thick diblock copolymer membranes, which were assembled as biocompatible shell material around CdSe/CdS quantum dot in quantum rods. We discuss the role of polymer chain length, intermolecular cross-linking and nanopore formation by analysing electron transfer processes from the photoexcited QDQRs to Cu(II) ions, which accumulate in the polymer membrane. Fluorescence investigations on single particle level additionally allow identifying ensemble inhomogeneities.

  15. Phase-coherent quantum transport in silicon nanowires based on Wigner transport equation: Comparison with the nonequilibrium-Green-function formalism

    Science.gov (United States)

    Barraud, Sylvain

    2009-09-01

    Various theoretical formulations are proposed for investigating the carrier transport in nanoscale electronic devices. In this paper, a discrete formulation of the Wigner transport equation (WTE) for the self-consistent simulation of phase-coherent quantum transport in silicon nanowire metal-oxide-semiconductor field-effect transistor (MOSFET) devices is presented. The device is simulated using an effective-mass Hamiltonian within the mode-space approximation. The numerical scheme proposed in this work solves self-consistently three dimensional Poisson's equation, two dimensional Schrödinger's equation in each cross-sectional plane of the nanowire, and the steady-state one dimensional WTE for each conduction mode to handle the quantum transport along the channel. Details on numerical implementation of the Wigner function method are given, and the results are compared with those of the nonequilibrium Green's function (NEGF) method in the ballistic limit. The calculations of current-voltage electrical characteristics of surround-gated silicon nanowires are performed using both NEGF and WTE formulations. The good agreement observed between these approaches means that a direct solution of the WTE is an accurate simulation method for modeling the ballistic quantum transport in silicon nanowire MOSFETs.

  16. Current flow paths in deformed graphene: from quantum transport to classical trajectories in curved space

    Science.gov (United States)

    Stegmann, Thomas; Szpak, Nikodem

    2016-05-01

    In this work we compare two fundamentally different approaches to the electronic transport in deformed graphene: (a) the condensed matter approach in which current flow paths are obtained by applying the non-equilibrium Green’s function (NEGF) method to the tight-binding model with local strain, (b) the general relativistic approach in which classical trajectories of relativistic point particles moving in a curved surface with a pseudo-magnetic field are calculated. The connection between the two is established in the long-wave limit via an effective Dirac Hamiltonian in curved space. Geometrical optics approximation, applied to focused current beams, allows us to directly compare the wave and the particle pictures. We obtain very good numerical agreement between the quantum and the classical approaches for a fairly wide set of parameters, improving with the increasing size of the system. The presented method offers an enormous reduction of complexity from irregular tight-binding Hamiltonians defined on large lattices to geometric language for curved continuous surfaces. It facilitates a comfortable and efficient tool for predicting electronic transport properties in graphene nanostructures with complicated geometries. Combination of the curvature and the pseudo-magnetic field paves the way to new interesting transport phenomena such as bending or focusing (lensing) of currents depending on the shape of the deformation. It can be applied in designing ultrasensitive sensors or in nanoelectronics.

  17. Spin effects in transport through triangular quantum dot molecule in different geometrical configurations

    Science.gov (United States)

    Wrześniewski, Kacper; Weymann, Ireneusz

    2015-07-01

    We analyze the spin-resolved transport properties of a triangular quantum dot molecule weakly coupled to external ferromagnetic leads. The calculations are performed by using the real-time diagrammatic technique up to the second-order of perturbation theory, which enables a description of both the sequential and cotunneling processes. We study the behavior of the current and differential conductance in the parallel and antiparallel magnetic configurations, as well as the tunnel magnetoresistance (TMR) and the Fano factor in both the linear and nonlinear response regimes. It is shown that the transport characteristics depend greatly on how the system is connected to external leads. Two specific geometrical configurations of the device are considered—the mirror one, which possesses the reflection symmetry with respect to the current flow direction and the fork one, in which this symmetry is broken. In the case of first configuration we show that, depending on the bias and gate voltages, the system exhibits both enhanced TMR and super-Poissonian shot noise. On the other hand, when the system is in the second configuration, we predict a negative TMR and a negative differential conductance in certain transport regimes. The mechanisms leading to those effects are thoroughly discussed.

  18. Transport phenomena in correlated quantum liquids: Ultracold Fermi gases and F/N junctions

    Science.gov (United States)

    Li, Hua

    Landau Fermi-liquid theory was first introduced by L. D. Landau in the effort of understanding the normal state of Fermi systems, where the application of the concept of elementary excitations to the Fermi systems has proved very fruitful in clarifying the physics of strongly correlated quantum systems at low temperatures. In this thesis, I use Landau Fermi-liquid theory to study the transport phenomena of two different correlated quantum liquids: the strongly interacting ultracold Fermi gases and the ferromagnet/normal-metal (F/N) junctions. The detailed work is presented in chapter II and chapter III of this thesis, respectively. Chapter I holds the introductory part and the background knowledge of this thesis. In chapter II, I study the transport properties of a Fermi gas with strong attractive interactions close to the unitary limit. In particular, I compute the transport lifetimes of the Fermi gas due to superfluid fluctuations above the BCS transition temperature Tc. To calculate the transport lifetimes I need the scattering amplitudes. The scattering amplitudes are dominated by the superfluid fluctuations at temperatures just above Tc. The normal scattering amplitudes are calculated from the Landau parameters. These Landau parameters are obtained from the local version of the induced interaction model for computing Landau parameters. I also calculate the leading order finite temperature corrections to the various transport lifetimes. A calculation of the spin diffusion coefficient is presented in comparison to the experimental findings. Upon choosing a proper value of F0a, I am able to present a good match between the theoretical result and the experimental measurement, which indicates the presence of the superfluid fluctuations near Tc. Calculations of the viscosity, the viscosity/entropy ratio and the thermal conductivity are also shown in support of the appearance of the superfluid fluctuations. In chapter III, I study the spin transport in the low

  19. Two-Dimensional Dirac Fermions in a Topological Insulator: Transport in the Quantum Limit

    Energy Technology Data Exchange (ETDEWEB)

    Analytis, J.G.; /SIMES, Stanford /SLAC /Stanford U., Geballe Lab /Stanford U., Appl. Phys. Dept.; McDonald, R.D.; /Los Alamos; Riggs, S.C.; /Natl. High Mag. Field Lab.; Chu, J.-H.; /SIMES, Stanford /SLAC /Stanford U., Geballe Lab /Stanford U., Appl. Phys. Dept.; Boebinger, G.S.; /Natl. High Mag. Field Lab.; Fisher, I.R.; /SIMES, Stanford /SLAC /Stanford U., Geballe Lab /Stanford U., Appl. Phys. Dept.

    2011-08-12

    Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi{sub 2}Se{sub 3} in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9 x 10{sup 16} cm{sup -3}, the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the {nu} = 1 Landau level attained by a field of {approx} 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.

  20. Reprint of : Regular and singular Fermi liquid in triple quantum dots: Coherent transport studies

    Science.gov (United States)

    Tooski, S. B.; Ramšak, A.; Bułka, B. R.

    2016-08-01

    A system of three coupled quantum dots in a triangular geometry (TQD) with electron-electron interaction and symmetrically coupled to two leads is analyzed with respect to the electron transport by means of the numerical renormalization group. Varying gate potentials this system exhibits extremely rich range of regimes with different many-electron states with various local spin orderings. It is demonstrated how the Luttinger phase changes in a controlled manner which then via the Friedel sum rule formula exactly reproduces the conductance through the TQD system. The analysis of the uncoupled TQD molecule from the leads gives a reliable qualitative understanding of various relevant regimes and an insight into the phase diagram with the regular Fermi liquid and singular-Fermi liquid phases.

  1. Quantum transport in III-V-semiconductor nanocolumns; Quantentransport in III-V-Halbleiternanosaeulen

    Energy Technology Data Exchange (ETDEWEB)

    Wensorra, Jakob

    2009-03-20

    The goal of this work has been to investigate und understand the electronic transport properties of vertical GaAs/AlAs nanocolumn resonant tunneling diodes (RTDs) and field effect transistors (RTTs) as well as of vertical InAs nanocolumn phase interference diodes. Besides the fabrication and electrical characterization of the devices, numerical calculations, simulations and quantum transport models represent the second important part of the work. GaAs/AlAs and InAs nanocolumns with lateral dimensions down to 30 nm have been processed by top-down approach. Room temperature DC electrical measurements on the nano-RTDs show a distinct negative differential resistance in the I-V characteristics for devices down to 30 nm lateral dimension. The miniaturization of the RTDs leads to the degradation of the transport properties, especially of the peak to valley current ratio (PVR), due to the increased surface scattering. Apart from the main current peak, new substructures can be observed in the I-V characteristics. These are shoulder like features for columns with diameters between 80 nm and 100 nm but become clear peaks when the column diameters are in the 55-75 nm range. For sub-65 nm column lateral dimensions, a strong increase of the PVR and a sharp single peak is observed. A local maximum of the PVR of 3 is reached for columns with 50 nm diameter. The sub-40 nm devices show only space charge limited currents in the I-V characteristics. This behavior can be shifted to smaller or larger diameters by increasing or reduction of the channel doping. For the smallest nanocolumns the lateral quantum confinement, caused by the low dimensionality of the system, leads to the formation of a 3D quantum-point-contact (QPC) in front of the DBQW structure. The quantization in this QPC depends on the column diameter and for a 50 nm column it exceeds the room temperature thermal broadening of the Fermi distribution function of about 25 meV. The measurements of the nano-RTTs indicate a

  2. Electron Transport of an Impurity Quantum Wire Under THz Electromagnetic Field Illumination

    Institute of Scientific and Technical Information of China (English)

    LI Yuan; ZHOU Guang-Hui; XIAO Xian-Bo; CHENG Fang; LIAO Wen-Hu

    2004-01-01

    @@ We theoretically investigate the effect of a single finite-size attractive impurity on the electron transport of a semiconductor quantum wire under the influence of a terahertz electromagnetic field illumination. In the freeparticle framework, the time-dependent electronic states are obtained by introducing an unitary transformation,and the electronic transmission of the system is obtained by using the scattering matrix approach. In the case of the field frequency resonant with the lateral energy spacing of the two lowest levels, a step-like structure for the transmission probability versus the total electron energy is predicted. Furthermore, due to the interplay between the single impurity and the applied field, the transmission probability curve in the non-resonant case shows a structure of a resonance dip on the interference pattern background with certain parameters of the impurity.

  3. Magnetoelectronic transport of the two-dimensional electron gas in CdSe single quantum wells

    Indian Academy of Sciences (India)

    P K Ghosh; A Ghosal; D Chattopadhyay

    2009-02-01

    Hall mobility and magnetoresistance coefficient for the two-dimensional (2D) electron transport parallel to the heterojunction interfaces in a single quantum well of CdSe are calculated with a numerical iterative technique in the framework of Fermi–Dirac statistics. Lattice scatterings due to polar-mode longitudinal optic (LO) phonons, and acoustic phonons via deformation potential and piezoelectric couplings, are considered together with background and remote ionized impurity interactions. The parallel mode of piezoelectric scattering is found to contribute more than the perpendicular mode. We observe that the Hall mobility decreases with increasing temperature but increases with increasing channel width. The magnetoresistance coefficient is found to decrease with increasing temperature and increase with increasing magnetic field in the classical region.

  4. Coherent plasmon transport using a quantum dot coupled plasmonic nanocavity system

    International Nuclear Information System (INIS)

    The transport property of surface plasmon polariton in an Ag nanowire-polymethylmethacrylate (PMMA)-Ag film waveguide structure coupled with a quantum dot (QD) is investigated theoretically. By placing a two-level QD into the plasmonic Fabry–Perot (FP) nanocavity, which is formed by two distributed Bragg reflectors (DBRs) patterned in the PMMA layer, a sharp asymmetric Fano line shape can be obtained in the transmission spectrum. We analyze this phenomenon theoretically using the transfer matrix method. The line shape is very sharp, and the required frequency shift from the minimum to maximum transmission is quite small. This will have great potential applications in single-photon modulation and switching in integrated nanophotonic circuitry. (paper)

  5. Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.

    Science.gov (United States)

    Zhang, Nanlin; Neo, Darren C J; Tazawa, Yujiro; Li, Xiuting; Assender, Hazel E; Compton, Richard G; Watt, Andrew A R

    2016-08-24

    The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

  6. Calculation of electrical transport properties and electron entanglement in inhomogeneous quantum wires

    Directory of Open Access Journals (Sweden)

    A A Shokri

    2013-10-01

    Full Text Available In this paper, we have investigated the spin-dependent transport properties and electron entanglement in a mesoscopic system, which consists of two semi-infinite leads (as source and drain separated by a typical quantum wire with a given potential. The properties studied include current-voltage characteristic, electrical conductivity, Fano factor and shot noise, and concurrence. The calculations are based on the transfer matrix method within the effective mass approximation. Using the Landauer formalism and transmission coefficient, the dependence of the considered quantities on type of potential well, length and width of potential well, energy of transmitted electron, temperature and the voltage have been theoretically studied. Also, the effect of the above-mentioned factors has been investigated in the nanostructure. The application of the present results may be useful in designing spintronice devices.

  7. Quantum Domains for Macroscopic Transport Effects in Nanostructures with Control Topology: Optics and e-Conductivity

    Directory of Open Access Journals (Sweden)

    Antipov A.

    2015-01-01

    Full Text Available The nanostructures with different morphology have been obtained by us by methods of both direct laser modification (from cw to fs laser radiation of the target surface/thin films and laser evaporation of the target substance in liquid to produce the colloid systems, and then – to deposite substance on substrate from colloid, and also – by a single drop deposition technique. The analysis of induced nanostructures has been carried out by absorption spectroscopy, scanning electron microscopy and transmission electron microscopy. The island conductivity is dominant for the nanocluster semiconductor systems induced by laser ablation technique, and electroresistance can dramatically decrease due to spontaneous selected multichannel/parallel electron transportation trajectories. A tunneling quantum coherent effect takes place for electron conductivity for the case.

  8. Quantum transport of strongly interacting photons in a one-dimensional nonlinear waveguide

    CERN Document Server

    Hafezi, Mohammad; Gritsev, Vladimir; Demler, Eugene; Lukin, Mikhail

    2009-01-01

    We present a theoretical technique for solving the quantum transport problem of a few photons through a one-dimensional, strongly nonlinear waveguide. We specifically consider the situation where the evolution of the optical field is governed by the quantum nonlinear Schr\\"odinger equation (NLSE). Although this kind of nonlinearity is quite general, we focus on a realistic implementation involving cold atoms loaded in a hollow-core optical fiber, where the atomic system provides a tunable nonlinearity that can be large even at a single-photon level. In particular, we show that when the interaction between photons is effectively repulsive, the transmission of multi-photon components of the field is suppressed. This leads to anti-bunching of the transmitted light and indicates that the system acts as a single-photon switch. On the other hand, in the case of attractive interaction, the system can exhibit either anti-bunching or bunching, which is in stark contrast to semiclassical calculations. We show that the ...

  9. Quantum transport of strongly interacting photons in a one-dimensional nonlinear waveguide

    Science.gov (United States)

    Hafezi, Mohammad; Chang, Darrick E.; Gritsev, Vladimir; Demler, Eugene; Lukin, Mikhail D.

    2012-01-01

    We present a theoretical technique for solving the quantum transport problem of a few photons through a one-dimensional, strongly nonlinear waveguide. We specifically consider the situation where the evolution of the optical field is governed by the quantum nonlinear Schrödinger equation. Although this kind of nonlinearity is quite general, we focus on a realistic implementation involving cold atoms loaded in a hollow-core optical fiber, where the atomic system provides a tunable nonlinearity that can be large even at a single-photon level. In particular, we show that when the interaction between photons is effectively repulsive, the transmission of multiphoton components of the field is suppressed. This leads to antibunching of the transmitted light and indicates that the system acts as a single-photon switch. On the other hand, in the case of attractive interaction, the system can exhibit either antibunching or bunching, which is in stark contrast to semiclassical calculations. We show that the bunching behavior is related to the resonant excitation of bound states of photons inside the system.

  10. Evidence for coherent transport in GaAs hole open quantum dots.

    Science.gov (United States)

    Faniel, S.; Hackens, B.; Vlad, A.; Gustin, C.; Moldovan, L.; Melinte, S.; Bayot, V.; Shayegan, M.

    2006-03-01

    We report magnetotransport measurements in GaAs hole open quantum dots. Our samples were fabricated from a p-type GaAs quantum well with a density of 2.2 x10^15 m-2 and a mobility of 35 m^2/Vs. Two different dots were patterned using e-beam lithography and wet etching. A top gate was added in order to control the dots openings and the hole density. The measurements were performed down to 30 mK with the magnetic field applied perpendicular to the plane of the two- dimensional system. We observed large, reproducible conductance fluctuations associated with the coherent transport of holes inside the dots at lowest temperatures which vanish above 500 mK. From the variance of these fluctuations and from the Random Matrix Theory, we extracted the hole dephasing time τ. The temperature dependence of the calculated τ lies between a T-1 and T-2 behavior and exhibits a saturation at very low temperature which is similar with τ measured in 2D electron systems. B. Hackens et al., Phys. Rev. Lett. 94, 146802 (2005).

  11. The permeability and transport mechanism of graphene quantum dots (GQDs) across the biological barrier

    Science.gov (United States)

    Wang, Xin-Yi; Lei, Rong; Huang, Hong-Duang; Wang, Na; Yuan, Lan; Xiao, Ru-Yue; Bai, Li-Dan; Li, Xue; Li, Li-Mei; Yang, Xiao-Da

    2015-01-01

    As an emerging nanomaterial, graphene quantum dots (GQDs) have shown enormous potential in theranostic applications. However, many aspects of the biological properties of GQDs require further clarification. In the present work, we prepared two sizes of GQDs and for the first time investigated their membrane permeabilities, one of the key factors of all biomedical applications, and transport mechanisms on a Madin Darby Canine Kidney (MDCK) cell monolayer. The experimental results revealed that under ~300 mg L-1, GQDs were innoxious to MDCK and did not affect the morphology and integrity of the cell monolayer. The Papp values were determined to be 1-3 × 10-6 cm s-1 for the 12 nm GQDs and 0.5-1.5 × 10-5 cm s-1 for the 3 nm GQDs, indicating that the 3 nm GQDs are well-transported species while the 12 nm GQDs have a moderate membrane permeability. The transport and uptake of GQDs by MDCK cells were both time and concentration-dependent. Moreover, the incubation of cells with GQDs enhanced the formation of lipid rafts, while inhibition of lipid rafts with methyl-β-cyclodextrin almost eliminated the membrane transport of GQDs. Overall, the experimental results suggested that GQDs cross the MDCK cell monolayer mainly through a lipid raft-mediated transcytosis. The present work has indicated that GQDs are a novel, low-toxic, highly-efficient general carrier for drugs and/or diagnostic agents in biomedical applications.As an emerging nanomaterial, graphene quantum dots (GQDs) have shown enormous potential in theranostic applications. However, many aspects of the biological properties of GQDs require further clarification. In the present work, we prepared two sizes of GQDs and for the first time investigated their membrane permeabilities, one of the key factors of all biomedical applications, and transport mechanisms on a Madin Darby Canine Kidney (MDCK) cell monolayer. The experimental results revealed that under ~300 mg L-1, GQDs were innoxious to MDCK and did not affect

  12. Cylindric quantum wires in a threading magnetic field: A proposal of characterization based on zero bias electron transport

    Science.gov (United States)

    Onorato, P.

    2012-12-01

    We study the quantum transport properties of cylindrical shaped wires, with submicrometric diameters and large aspect ratio. The zero bias conductance as a function of temperature, magnetic field and disorder is calculated for different kinds of nano cylinders, from semiconductor quantum wires to carbon nanotubes. A comparison between our findings and the experimental results allows the understanding of the charge carriers' localization, in the external surface or in the core of the wires, by highlighting the basic mechanism of charge transport. We discuss how we can infer that in InAs quantum wires the carriers move in the core. We examine the Aharonov-Bohm oscillations and the quenching that should be observed in the measured magneto conductivity of InAs nano cylinders and carbon nanotubes emphasizing the role of the angle between field and tube.

  13. Quantum Transport Detected by Strong Proximity Interaction at a Graphene-WS2 van der Waals Interface.

    Science.gov (United States)

    O'Farrell, E C T; Avsar, A; Tan, J Y; Eda, G; Özyilmaz, B

    2015-09-01

    Magnetotransport measurements demonstrate that graphene in a van der Waals heterostructure is a sensitive probe of quantum transport in an adjacent WS2 layer via strong Coulomb interactions. We observe a large low-field magnetoresistance (≫ e(2)/h) and a -ln T temperature dependence of the resistance. In-plane magnetic field resistance indicates the origin is orbital and nonclassical. We demonstrate a strong electron-hole asymmetry in the mobility and coherence length of graphene demonstrating the presence of localized Coulomb interactions with ionized donors in the WS2 substrate, which ultimately leads to screening as the Fermi level of graphene is tuned toward the conduction band of WS2. This leads us to conclude that graphene couples to quantum localization processes in WS2 via the Coulomb interaction and results in the observed signatures of quantum transport. Our results show that theoretical descriptions of the van der Waals interface should not ignore localized strong correlations. PMID:26258760

  14. Quantum transport in one-dimensional systems via a master equation approach: Numerics and an exact solution

    Indian Academy of Sciences (India)

    Marko Žnidarič

    2011-11-01

    We discuss recent findings about properties of quantum nonequilibrium steady states. In particular we focus on transport properties. It is shown that the time-dependent density matrix renormalization method can be used successfully to find a stationary solution of Lindblad master equation. Furthermore, for a specific model an exact solution is presented.

  15. Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the Vadose Zone

    Science.gov (United States)

    To investigate the coupled effects of solution chemistry and vadose zone processes on the mobility of quantum dot (QD) nanoparticles, laboratory scale transport experiments were performed. The complex coupled effects of ionic strength, size of QD aggregates, surface tension, contact angle, infiltrat...

  16. Generalized nonequilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

    Science.gov (United States)

    Yan, Jiawei; Ke, Youqi

    2016-07-01

    Electron transport properties of nanoelectronics can be significantly influenced by the inevitable and randomly distributed impurities/defects. For theoretical simulation of disordered nanoscale electronics, one is interested in both the configurationally averaged transport property and its statistical fluctuation that tells device-to-device variability induced by disorder. However, due to the lack of an effective method to do disorder averaging under the nonequilibrium condition, the important effects of disorders on electron transport remain largely unexplored or poorly understood. In this work, we report a general formalism of Green's function based nonequilibrium effective medium theory to calculate the disordered nanoelectronics. In this method, based on a generalized coherent potential approximation for the Keldysh nonequilibrium Green's function, we developed a generalized nonequilibrium vertex correction method to calculate the average of a two-Keldysh-Green's-function correlator. We obtain nine nonequilibrium vertex correction terms, as a complete family, to express the average of any two-Green's-function correlator and find they can be solved by a set of linear equations. As an important result, the averaged nonequilibrium density matrix, averaged current, disorder-induced current fluctuation, and averaged shot noise, which involve different two-Green's-function correlators, can all be derived and computed in an effective and unified way. To test the general applicability of this method, we applied it to compute the transmission coefficient and its fluctuation with a square-lattice tight-binding model and compared with the exact results and other previously proposed approximations. Our results show very good agreement with the exact results for a wide range of disorder concentrations and energies. In addition, to incorporate with density functional theory to realize first-principles quantum transport simulation, we have also derived a general form of

  17. Curl flux, coherence, and population landscape of molecular systems: Nonequilibrium quantum steady state, energy (charge) transport, and thermodynamics

    International Nuclear Information System (INIS)

    We established a theoretical framework in terms of the curl flux, population landscape, and coherence for non-equilibrium quantum systems at steady state, through exploring the energy and charge transport in molecular processes. The curl quantum flux plays the key role in determining transport properties and the system reaches equilibrium when flux vanishes. The novel curl quantum flux reflects the degree of non-equilibriumness and the time-irreversibility. We found an analytical expression for the quantum flux and its relationship to the environmental pumping (non-equilibriumness quantified by the voltage away from the equilibrium) and the quantum tunneling. Furthermore, we investigated another quantum signature, the coherence, quantitatively measured by the non-zero off diagonal element of the density matrix. Populations of states give the probabilities of individual states and therefore quantify the population landscape. Both curl flux and coherence depend on steady state population landscape. Besides the environment-assistance which can give dramatic enhancement of coherence and quantum flux with high voltage at a fixed tunneling strength, the quantum flux is promoted by the coherence in the regime of small tunneling while reduced by the coherence in the regime of large tunneling, due to the non-monotonic relationship between the coherence and tunneling. This is in contrast to the previously found linear relationship. For the systems coupled to bosonic (photonic and phononic) reservoirs the flux is significantly promoted at large voltage while for fermionic (electronic) reservoirs the flux reaches a saturation after a significant enhancement at large voltage due to the Pauli exclusion principle. In view of the system as a quantum heat engine, we studied the non-equilibrium thermodynamics and established the analytical connections of curl quantum flux to the transport quantities such as energy (charge) transfer efficiency, chemical reaction efficiency, energy

  18. Electrical transport in transverse direction through silicon carbon alloy multilayers containing regular size silicon quantum dots

    Science.gov (United States)

    Mandal, Aparajita; Kole, Arindam; Dasgupta, Arup; Chaudhuri, Partha

    2016-11-01

    Electrical transport in the transverse direction has been studied through a series of hydrogenated silicon carbon alloy multilayers (SiC-MLs) deposited by plasma enhanced chemical vapor deposition method. Each SiC-ML consists of 30 cycles of the alternating layers of a nearly amorphous silicon carbide (a-SiC:H) and a microcrystalline silicon carbide (μc-SiC:H) that contains high density of silicon quantum dots (Si-QDs). A detailed investigation by cross sectional TEM reveals preferential growth of densely packed Si-QDs of regular sizes ∼4.8 nm in diameter in a vertically aligned columnar structure within the SiC-ML. More than six orders of magnitude increase in transverse current through the SiC-ML structure were observed for decrease in the a-SiC:H layer thickness from 13 nm to 2 nm. The electrical transport mechanism was established to be a combination of grain boundary or band tail hopping and Frenkel-Poole (F-P) type conduction depending on the temperature and externally applied voltage ranges. Evaluation of trap concentration within the multilayer structures from the fitted room temperature current voltage characteristics by F-P function shows reduction up-to two orders of magnitude indicating an improvement in the short range order in the a-SiC:H matrix for decrease in the thickness of a-SiC:H layer.

  19. Generalized non-equilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

    Science.gov (United States)

    Yan, Jiawei; Ke, Youqi

    In realistic nanoelectronics, disordered impurities/defects are inevitable and play important roles in electron transport. However, due to the lack of effective quantum transport method, the important effects of disorders remain poorly understood. Here, we report a generalized non-equilibrium vertex correction (NVC) method with coherent potential approximation to treat the disorder effects in quantum transport simulation. With this generalized NVC method, any averaged product of two single-particle Green's functions can be obtained by solving a set of simple linear equations. As a result, the averaged non-equilibrium density matrix and various important transport properties, including averaged current, disordered induced current fluctuation and the averaged shot noise, can all be efficiently computed in a unified scheme. Moreover, a generalized form of conditionally averaged non-equilibrium Green's function is derived to incorporate with density functional theory to enable first-principles simulation. We prove the non-equilibrium coherent potential equals the non-equilibrium vertex correction. Our approach provides a unified, efficient and self-consistent method for simulating non-equilibrium quantum transport through disorder nanoelectronics. Shanghaitech start-up fund.

  20. Fano antiresonance and Kondo resonance for electronic transport through a laterally coupled carbon-nanotube quantum-dot system

    Energy Technology Data Exchange (ETDEWEB)

    Huo, Dong-Ming [Sichuan Univ., Chengdu (China). Inst. of Atomic and Molecular Physics

    2015-07-01

    We present nonequilibrium Green function calculations for electronic transport through a laterally coupled carbon-nanotube quantum-dot system. In this system, a one-dimensional double carbon nanotube quantum dot attached to polarised electrodes forms a main channel for electronic tunnelling. Each carbon nanotube quantum dot in the main channel couples to a dangling carbon nanotube quantum dot. Then, the conductance spectrum is calculated. The insulating band and resonance peak in this spectrum, due to Fano antiresonance and Kondo resonance, are discussed. The intradot electron's Coulomb interaction effect on the insulating band is also investigated. By controlling the coupling coefficient between the quantum dots, we can realise mutual transformation between Kondo resonance and Fano antiresonance at the Fermi level. The spin-orbit coupling and magnetic field's influence on the Kondo resonance peak are discussed in detail. Finally, spin magnetic moment and orbital magnetic moment of electrons in the quantum dot by applying parallel magnetic field are also predicted.

  1. Theory of valley-dependent transport in graphene-based lateral quantum structures

    Science.gov (United States)

    Chen, Feng-Wu; Chou, Mei-Yin; Chen, Yiing-Rei; Wu, Yu-Shu

    2016-08-01

    Modulation of electronic states in two-dimensional materials can be achieved by using in-plane variations of the band gap or the average potential in lateral quantum structures. In the atomic configurations with hexagonal symmetry, this approach makes it possible to tailor the valleytronic properties for potential device applications. In this work, we present a multiband theory to calculate the valley-dependent electron transport in graphene-based lateral quantum structures. As an example, we consider the structures with a single interface that exhibits an energy gap or potential discontinuity. The theoretical formalism proceeds within the tight-binding description, by first deriving the local bulk complex band structures in the regions of a constant gap or potential and, next, joining the local wave functions across the interface via a cell-averaged current operator to ensure the current continuity. The theory is applied to the study of electron reflection off and transmission through an interface. Both reflection and transmission are found to exhibit valley-contrast behavior that can be used to generate valley-polarized electron sources. The results vary with the type of interfaces, as well as between monolayer and bilayer graphene-based structures. In the monolayer case, the valley contrast originates from the band warping and only becomes sizable for incident carriers of high energy, whereas in AB-stacked bilayer graphene, the vertical interlayer coupling emerges as an additional important cause for valley contrast, and the favorable carrier energy is also found to be drastically lower. Our numerical results clearly demonstrate the propitious valleytronic properties of bilayer graphene structures.

  2. NATO Advanced Study Institute on International Summer School on Chaotic Dynamics and Transport in Classical and Quantum Systems

    CERN Document Server

    Collet, P; Métens, S; Neishtadt, A; Zaslavsky, G; Chaotic Dynamics and Transport in Classical and Quantum Systems

    2005-01-01

    This book offers a modern updated review on the most important activities in today dynamical systems and statistical mechanics by some of the best experts in the domain. It gives a contemporary and pedagogical view on theories of classical and quantum chaos and complexity in hamiltonian and ergodic systems and their applications to anomalous transport in fluids, plasmas, oceans and atom-optic devices and to control of chaotic transport. The book is issued from lecture notes of the International Summer School on "Chaotic Dynamics and Transport in Classical and Quantum Systems" held in Cargèse (Corsica) 18th to the 30th August 2003. It reflects the spirit of the School to provide lectures at the post-doctoral level on basic concepts and tools. The first part concerns ergodicity and mixing, complexity and entropy functions, SRB measures, fractal dimensions and bifurcations in hamiltonian systems. Then, models of dynamical evolutions of transport processes in classical and quantum systems have been largely expla...

  3. Time-dependent quantum many-body systems. Linear response, electronic transport, and reduced density matrices

    Energy Technology Data Exchange (ETDEWEB)

    Appel, H.

    2007-05-15

    In part I of this work we present a double-pole approximation (DPA) to the response equations of time-dependent density functional theory (TDDFT). The double-pole approximation provides an exact description of systems with two strongly coupled excitations which are isolated from the rest of the spectrum. In contrast to the traditional single-pole approximation of TDDFT the DPA also yields corrections to the Kohn-Sham oscillator strengths. We also demonstrate how to invert the double-pole solution which allows us to predict matrix elements of the exchange-correlation kernel f{sub xc} from experimental input. We attempt some first steps towards a time-dependent generalization of reduced density matrix functional theory (RDMFT). In part II we derive equations of motion for natural orbitals and occupation numbers. Using the equation of motion for the occupation numbers we show that an adiabatic extension of presently known ground-state functionals of static RDMFT always leads to occupation numbers which are constant in time. From the stationary conditions of the equations of motion for the N-body correlations (correlated parts of the N-body matrices) we derive a new class of ground-state functionals which can be used in static RDMFT. Applications are presented for a one-dimensional model system where the time-dependent many-body Schroedinger equation can be propagated numerically. We use optimal control theory to find optimized laser pulses for transitions in a model for atomic Helium. From the numerically exact correlated wavefunction we extract the exact time evolution of natural orbitals and occupation numbers for (i) laser-driven Helium and (ii) electron-ion scattering. Part III of this work considers time-dependent quantum transport within TDDFT. We present an algorithm for the calculation of extended eigenstates of single-particle Hamiltonians which is especially tailored to a finite-difference discretization of the Schroedinger equation. We consider the

  4. Time-dependent quantum many-body systems. Linear response, electronic transport, and reduced density matrices

    International Nuclear Information System (INIS)

    In part I of this work we present a double-pole approximation (DPA) to the response equations of time-dependent density functional theory (TDDFT). The double-pole approximation provides an exact description of systems with two strongly coupled excitations which are isolated from the rest of the spectrum. In contrast to the traditional single-pole approximation of TDDFT the DPA also yields corrections to the Kohn-Sham oscillator strengths. We also demonstrate how to invert the double-pole solution which allows us to predict matrix elements of the exchange-correlation kernel fxc from experimental input. We attempt some first steps towards a time-dependent generalization of reduced density matrix functional theory (RDMFT). In part II we derive equations of motion for natural orbitals and occupation numbers. Using the equation of motion for the occupation numbers we show that an adiabatic extension of presently known ground-state functionals of static RDMFT always leads to occupation numbers which are constant in time. From the stationary conditions of the equations of motion for the N-body correlations (correlated parts of the N-body matrices) we derive a new class of ground-state functionals which can be used in static RDMFT. Applications are presented for a one-dimensional model system where the time-dependent many-body Schroedinger equation can be propagated numerically. We use optimal control theory to find optimized laser pulses for transitions in a model for atomic Helium. From the numerically exact correlated wavefunction we extract the exact time evolution of natural orbitals and occupation numbers for (i) laser-driven Helium and (ii) electron-ion scattering. Part III of this work considers time-dependent quantum transport within TDDFT. We present an algorithm for the calculation of extended eigenstates of single-particle Hamiltonians which is especially tailored to a finite-difference discretization of the Schroedinger equation. We consider the propagation of

  5. Quantum Stabilization by Coherent Population of Scarred Eigenstates: Quantum Suppression of Transport Classically Initiated by Separatrix Band Entry

    CERN Document Server

    Perotti, Luca C

    2009-01-01

    Comparisons of experimental data with numerical predictions of a classical model indicate that an excited hydrogen atom in a pulsed microwave electric field exhibits a nonclassical increase of stability over a relatively wide range of frequencies. I show here that this is due to selective population of long-lived "scarred" states that are associated with the chaotic separatrix band surrounding the principal classical resonance zone in phase space. A quantum explanation is given in terms of adiabatic evolution of Floquet states and the destabilizing effect of two-level quantum resonances is investigated. The role of neighbouring classical resonance zones in defining the frequency range of stabilization is revealed both by quasienergy curves and by Husimi functions for the instantaneous quantum states.

  6. Dynamic thermoelectric and heat transport in mesoscopic capacitors

    OpenAIRE

    Lim, Jong-Soo; López, Rosa; Sánchez, David

    2013-01-01

    We discuss the low-frequency response of charge and heat transport to oscillatory voltage and temperature shifts in mesoscopic capacitors. We obtain, within scattering theory, generic expressions for the quantum admittances up to second order in the ac frequencies in terms of electric, thermoelectric, and heat capacitances and relaxation resistances. Remarkably, we find that the thermocurrent can lead or lag the applied temperature depending on the gate voltage applied to a quantum RC circuit...

  7. Effective bias and potentials in steady-state quantum transport: A NEGF reverse-engineering study

    Science.gov (United States)

    Karlsson, Daniel; Verdozzi, Claudio

    2016-03-01

    Using non-equilibrium Green's functions combined with many-body perturbation theory, we have calculated steady-state densities and currents through short interacting chains subject to a finite electric bias. By using a steady-state reverse-engineering procedure, the effective potential and bias which reproduce such densities and currents in a non-interacting system have been determined. The role of the effective bias is characterised with the aid of the so-called exchange-correlation bias, recently introduced in a steady-state density-functional- theory formulation for partitioned systems. We find that the effective bias (or, equivalently, the exchange-correlation bias) depends strongly on the interaction strength and the length of the central (chain) region. Moreover, it is rather sensitive to the level of many-body approximation used. Our study shows the importance of the effective/exchange-correlation bias out of equilibrium, thereby offering hints on how to improve the description of density- functional-theory based approaches to quantum transport.

  8. Quantum ballistic transport by interacting two-electron states in quasi-one-dimensional channels

    International Nuclear Information System (INIS)

    For quantum ballistic transport of electrons through a short conduction channel, the role of Coulomb interaction may significantly modify the energy levels of two-electron states at low temperatures as the channel becomes wide. In this regime, the Coulomb effect on the two-electron states is calculated and found to lead to four split energy levels, including two anticrossing-level and two crossing-level states. Moreover, due to the interplay of anticrossing and crossing effects, our calculations reveal that the ground two-electron state will switch from one anticrossing state (strong confinement) to a crossing state (intermediate confinement) as the channel width gradually increases and then back to the original anticrossing state (weak confinement) as the channel width becomes larger than a threshold value. This switching behavior leaves a footprint in the ballistic conductance as well as in the diffusion thermoelectric power of electrons. Such a switching is related to the triple spin degeneracy as well as to the Coulomb repulsion in the central region of the channel, which separates two electrons away and pushes them to different channel edges. The conductance reoccurrence region expands from the weak to the intermediate confinement regime with increasing electron density

  9. Fingerprint of different spin-orbit terms for spin transport in HgTe quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Rothe, D G; Reinthaler, R W; Liu, C-X; Hankiewicz, E M [Institut fuer Theoretische Physik und Astrophysik, Universitaet Wuerzburg, 97074 Wuerzburg (Germany); Molenkamp, L W [Physikalisches Institut (EP3), Universitaet Wuerzburg, 97074 Wuerzburg (Germany); Zhang, S-C, E-mail: hankiewicz@physik.uni-wuerzburg.d [Department of Physics, McCullough Building, Stanford University, Stanford, CA 94305-4045 (United States)

    2010-06-15

    Using k{center_dot}p theory, we derive an effective four-band model describing the physics of the typical two-dimensional topological insulator (HgTe/CdTe quantum well (QW)) in the presence of an out-of-plane (in the z-direction) inversion breaking potential and an in-plane potential. We find that up to third order in perturbation theory, only the inversion breaking potential generates new elements to the four-band Hamiltonian that are off-diagonal in spin space. When this new effective Hamiltonian is folded into an effective two-band model for the conduction (electron) or valence (heavy hole) bands, two competing terms appear: (i) a Rashba spin-orbit interaction originating from inversion breaking potential in the z-direction and (ii) an in-plane Pauli term as a consequence of the in-plane potential. Spin transport in the conduction band is further analysed within the Landauer-Buettiker formalism. We find that for asymmetrically doped HgTe QWs, the behaviour of the spin-Hall conductance is dominated by the Rashba term.

  10. Universal low-temperature Ohmic contacts for quantum transport in transition metal dichalcogenides

    Science.gov (United States)

    Xu, Shuigang; Wu, Zefei; Lu, Huanhuan; Han, Yu; Long, Gen; Chen, Xiaolong; Han, Tianyi; Ye, Weiguang; Wu, Yingying; Lin, Jiangxiazi; Shen, Junying; Cai, Yuan; He, Yuheng; Zhang, Fan; Lortz, Rolf; Cheng, Chun; Wang, Ning

    2016-06-01

    Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layer TMDC devices studied show excellent performance with remarkably improved field-effect mobilities ranging from 2300 to 16 000 cm2 V‑1 s‑1, as verified by the high carrier mobilities extracted from Hall effect measurements. Moreover, both high-mobility n-type and p-type TMDC channels can be realized by simply using appropriate contact metals. Prominent Shubnikov–de Haas oscillations have been observed and investigated in these high-quality TMDC devices.

  11. Quantum ballistic transport by interacting two-electron states in quasi-one-dimensional channels

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Danhong [Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, New Mexico 87117 (United States); Center for High Technology Materials, University of New Mexico, 1313 Goddard St SE, Albuquerque, New Mexico 87106 (United States); Gumbs, Godfrey [Center for High Technology Materials, University of New Mexico, 1313 Goddard St SE, Albuquerque, New Mexico 87106 (United States); Abranyos, Yonatan [Department of Physics and Astronomy, Hunter College of the City University of New York, 695 Park Avenue, New York, New York 10065 (United States); Pepper, Michael; Kumar, Sanjeev [Department of Electronic and Electrical Engineering, University College London, London, WC1E 7JE (United Kingdom); London Centre for Nanotechnology, 17-19 Gordon Street, London, WC1H 0AH (United Kingdom)

    2015-11-15

    For quantum ballistic transport of electrons through a short conduction channel, the role of Coulomb interaction may significantly modify the energy levels of two-electron states at low temperatures as the channel becomes wide. In this regime, the Coulomb effect on the two-electron states is calculated and found to lead to four split energy levels, including two anticrossing-level and two crossing-level states. Moreover, due to the interplay of anticrossing and crossing effects, our calculations reveal that the ground two-electron state will switch from one anticrossing state (strong confinement) to a crossing state (intermediate confinement) as the channel width gradually increases and then back to the original anticrossing state (weak confinement) as the channel width becomes larger than a threshold value. This switching behavior leaves a footprint in the ballistic conductance as well as in the diffusion thermoelectric power of electrons. Such a switching is related to the triple spin degeneracy as well as to the Coulomb repulsion in the central region of the channel, which separates two electrons away and pushes them to different channel edges. The conductance reoccurrence region expands from the weak to the intermediate confinement regime with increasing electron density.

  12. Comparison of DC and AC Transport in 1.5-7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic Ga-In versus Conducting Probe Atomic Force Microscope Junctions.

    Science.gov (United States)

    Sangeeth, C S Suchand; Demissie, Abel T; Yuan, Li; Wang, Tao; Frisbie, C Daniel; Nijhuis, Christian A

    2016-06-15

    We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms.

  13. Comparison of DC and AC Transport in 1.5-7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic Ga-In versus Conducting Probe Atomic Force Microscope Junctions.

    Science.gov (United States)

    Sangeeth, C S Suchand; Demissie, Abel T; Yuan, Li; Wang, Tao; Frisbie, C Daniel; Nijhuis, Christian A

    2016-06-15

    We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms. PMID:27172452

  14. Quantum transport of two-species Dirac fermions in dual-gated three-dimensional topological insulators

    Science.gov (United States)

    Xu, Yang; Miotkowski, Ireneusz; Chen, Yong P.

    2016-05-01

    Topological insulators are a novel class of quantum matter with a gapped insulating bulk, yet gapless spin-helical Dirac fermion conducting surface states. Here, we report local and non-local electrical and magneto transport measurements in dual-gated BiSbTeSe2 thin film topological insulator devices, with conduction dominated by the spatially separated top and bottom surfaces, each hosting a single species of Dirac fermions with independent gate control over the carrier type and density. We observe many intriguing quantum transport phenomena in such a fully tunable two-species topological Dirac gas, including a zero-magnetic-field minimum conductivity close to twice the conductance quantum at the double Dirac point, a series of ambipolar two-component half-integer Dirac quantum Hall states and an electron-hole total filling factor zero state (with a zero-Hall plateau), exhibiting dissipationless (chiral) and dissipative (non-chiral) edge conduction, respectively. Such a system paves the way to explore rich physics, ranging from topological magnetoelectric effects to exciton condensation.

  15. Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes

    Science.gov (United States)

    H, Dakhlaoui; S, Almansour

    2016-06-01

    In this work, the electronic properties of resonant tunneling diodes (RTDs) based on GaN-Al x Ga(1-x)N double barriers are investigated by using the non-equilibrium Green functions formalism (NEG). These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field, which greatly affect the electronic transport properties. The electronic density, the transmission coefficient, and the current-voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations. The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness, Al x Ga(1-x)N width, and the aluminum concentration x Al. The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier; it exhibits a series of resonant peaks and valleys as the quantum well width increases. In addition, it is found that the negative differential resistance (NDR) in the current-voltage (I-V) characteristic strongly depends on aluminum concentration x Al. It is shown that the peak-to-valley ratio (PVR) increases with x Al value decreasing. These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors. Project supported by the Deanship of Scientific Research of University of Dammam (Grant No. 2014137).

  16. Simultaneous distribution of AC and DC power

    Science.gov (United States)

    Polese, Luigi Gentile

    2015-09-15

    A system and method for the transport and distribution of both AC (alternating current) power and DC (direct current) power over wiring infrastructure normally used for distributing AC power only, for example, residential and/or commercial buildings' electrical wires is disclosed and taught. The system and method permits the combining of AC and DC power sources and the simultaneous distribution of the resulting power over the same wiring. At the utilization site a complementary device permits the separation of the DC power from the AC power and their reconstruction, for use in conventional AC-only and DC-only devices.

  17. Theoretical investigation of the electronic structure and quantum transport in the graphene-C(111) diamond surface system.

    Science.gov (United States)

    Selli, Daniele; Baburin, Igor; Leoni, Stefano; Zhu, Zhen; Tománek, David; Seifert, Gotthard

    2013-10-30

    We investigate the interaction of a graphene monolayer with the C(111) diamond surface using ab initio density functional theory. To accommodate the lattice mismatch between graphene and diamond, the overlayer deforms into a wavy structure that binds strongly to the diamond substrate. The detached ridges of the wavy graphene overlayer behave electronically as free-standing polyacetylene chains with delocalized π electrons, separated by regions containing only sp(3) carbon atoms covalently bonded to the (111) diamond surface. We performed quantum transport calculations for different geometries of the system to study how the buckling of the graphene layer and the associated bonding to the diamond substrate affect the transport properties. The system displays high carrier mobility along the ridges and a wide transport gap in the direction normal to the ridges. These intriguing, strongly anisotropic transport properties qualify the hybrid graphene-diamond system as a viable candidate for electronic nanodevices.

  18. Structural fluctuations and quantum transport through DNA molecular wires: a combined molecular dynamics and model Hamiltonian approach

    OpenAIRE

    Gutierrez, R.; Caetano, R.; Woiczikowski, P. B.; Kubar, T.; Elstner, M; Cuniberti, G.

    2009-01-01

    Charge transport through a short DNA oligomer (Dickerson dodecamer) in presence of structural fluctuations is investigated using a hybrid computational methodology based on a combination of quantum mechanical electronic structure calculations and classical molecular dynamics simulations with a model Hamiltonian approach. Based on a fragment orbital description, the DNA electronic structure can be coarse-grained in a very efficient way. The influence of dynamical fluctuations arising either fr...

  19. Spin-Polarized Transport through the T-Shaped Double Quantum Dots with Fano-Kondo Interaction

    Institute of Scientific and Technical Information of China (English)

    YANG Fu-Bin; WU Shao-Quan; SUN Wei-Li

    2007-01-01

    We theoretically investigate the spin-polarized transport properties of the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. We calculate the density of states and the liner conductance in this system with both parallel and antiparallel lead-polarization alignments, and our results show that the transport properties of this system depend on both the tunnelling strength between the two dots and the spin-polarized strength p. This system is a possible candidate for spin valve transistors in the spintronics.

  20. Realizing one-dimensional quantum and high-frequency transport features in aligned single-walled carbon nanotube ropes

    Energy Technology Data Exchange (ETDEWEB)

    Ncube, Siphephile; Chimowa, George; Chiguvare, Zivayi; Bhattacharyya, Somnath, E-mail: Somnath.Bhattacharyya@wits.ac.za [Nano-Scale Transport Physics Laboratory, School of Physics and DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg (South Africa)

    2014-07-14

    The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ropes over SWNT mats is verified from low temperature and frequency-dependent transport. The overall change of resistance versus in nanotube mats shows that 3D variable range hopping is the dominant conduction mechanism within the 2–300 K range. The magneto-resistance (MR) is found to be predominantly negative with a parabolic nature, which can also be described by the hopping model. Although the positive upturn of the MR at low temperatures establishes the contribution from quantum interference, the inherent quantum transport in individual tubes is suppressed at elevated temperatures. Therefore, to minimize multi-channel effects from inter-tube interactions and other defects, two-terminal devices were fabricated from aligned SWNT (extracted from a mat) for low temperature transport as well as high-frequency measurements. In contrast to the mat, the aligned ropes exhibit step-like features in the differential conductance within the 80–300 K temperature range. The effects of plasmon propagation, unique to one dimension, were identified in electronic transport as a non-universal power-law dependence of the differential conductance on temperature and source-drain voltage. The complex impedance showed high power transmission capabilities up to 65 GHz as well as oscillations in the frequency range up to 30 GHz. The measurements suggest that aligned SWNT ropes have a realistic potential for high-speed device applications.

  1. Realizing one-dimensional quantum and high-frequency transport features in aligned single-walled carbon nanotube ropes

    International Nuclear Information System (INIS)

    The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ropes over SWNT mats is verified from low temperature and frequency-dependent transport. The overall change of resistance versus in nanotube mats shows that 3D variable range hopping is the dominant conduction mechanism within the 2–300 K range. The magneto-resistance (MR) is found to be predominantly negative with a parabolic nature, which can also be described by the hopping model. Although the positive upturn of the MR at low temperatures establishes the contribution from quantum interference, the inherent quantum transport in individual tubes is suppressed at elevated temperatures. Therefore, to minimize multi-channel effects from inter-tube interactions and other defects, two-terminal devices were fabricated from aligned SWNT (extracted from a mat) for low temperature transport as well as high-frequency measurements. In contrast to the mat, the aligned ropes exhibit step-like features in the differential conductance within the 80–300 K temperature range. The effects of plasmon propagation, unique to one dimension, were identified in electronic transport as a non-universal power-law dependence of the differential conductance on temperature and source-drain voltage. The complex impedance showed high power transmission capabilities up to 65 GHz as well as oscillations in the frequency range up to 30 GHz. The measurements suggest that aligned SWNT ropes have a realistic potential for high-speed device applications.

  2. Hysteresis losses in MgB2 superconductors exposed to combinations of low AC and high DC magnetic fields and transport currents

    DEFF Research Database (Denmark)

    Magnusson, N.; Abrahamsen, Asger Bech; Liu, Dawei;

    2014-01-01

    MgB2 superconductors are considered for generator field coils for direct drive wind turbine generators. In such coils, the losses generated by AC magnetic fields may generate excessive local heating and add to the thermal load, which must be removed by the cooling system. These losses must...... be evaluated in the design of the generator to ensure a sufficient overall efficiency. A major loss component is the hysteresis losses in the superconductor itself. In the high DC – low AC current and magnetic field region experimental results still lack for MgB2 conductors. In this article we reason towards...... equations use the DC in-field critical current, the geometry of the superconductor and the magnitude of the AC magnetic field component as parameters. This simplified approach can be valuable in the design of MgB2 DC coils in the 1–4T range with low AC magnetic field and current ripples....

  3. Quantum Statistical Mechanics

    Science.gov (United States)

    Schieve, William C.; Horwitz, Lawrence P.

    2009-04-01

    1. Foundations of quantum statistical mechanics; 2. Elementary examples; 3. Quantum statistical master equation; 4. Quantum kinetic equations; 5. Quantum irreversibility; 6. Entropy and dissipation: the microscopic theory; 7. Global equilibrium: thermostatics and the microcanonical ensemble; 8. Bose-Einstein ideal gas condensation; 9. Scaling, renormalization and the Ising model; 10. Relativistic covariant statistical mechanics of many particles; 11. Quantum optics and damping; 12. Entanglements; 13. Quantum measurement and irreversibility; 14. Quantum Langevin equation: quantum Brownian motion; 15. Linear response: fluctuation and dissipation theorems; 16. Time dependent quantum Green's functions; 17. Decay scattering; 18. Quantum statistical mechanics, extended; 19. Quantum transport with tunneling and reservoir ballistic transport; 20. Black hole thermodynamics; Appendix; Index.

  4. Comparative study of donor-induced quantum dots in Si nano-channels by single-electron transport characterization and Kelvin probe force microscopy

    International Nuclear Information System (INIS)

    We comparatively study donor-induced quantum dots in Si nanoscale-channel transistors for a wide range of doping concentration by analysis of single-electron tunneling transport and surface potential measured by Kelvin probe force microscopy (KPFM). By correlating KPFM observations of donor-induced potential landscapes with simulations based on Thomas-Fermi approximation, it is demonstrated that single-electron tunneling transport at lowest gate voltages (for smallest coverage of screening electrons) is governed most frequently by only one dominant quantum dot, regardless of doping concentration. Doping concentration, however, primarily affects the internal structure of the quantum dot. At low concentrations, individual donors form most of the quantum dots, i.e., “donor-atom” quantum dots. In contrast, at high concentrations above metal-insulator transition, closely placed donors instead of individual donors form more complex quantum dots, i.e., “donor-cluster” quantum dots. The potential depth of these “donor-cluster” quantum dots is significantly reduced by increasing gate voltage (increasing coverage of screening electrons), leading to the occurrence of multiple competing quantum dots

  5. Bound states in the continuum and Fano antiresonance in electronic transport through a four-quantum-dot system

    International Nuclear Information System (INIS)

    We study the electronic transport through a four-quantum-dot (FQD) structure with a diamond-like shape through nonequilibrium Green's function theory. It is observed that the bound state in the continuum (BIC) appears in this multiple QDs system, and the position of the BIC in the total density of states (TDOS) spectrum is tightly determined by the strength of the electronic hopping between the upper QD and the lower one. As the symmetry in the energy levels in these two QDs is broken, the BIC is suppressed to a general conductance peak with a finite width, and meanwhile a Fano-type antiresonance with a zero point appears in the conductance spectrum. These results will develop our understanding of the BICs and their spintronic device applications of spin filter and quantum computing.

  6. An entropic Quantum Drift-Diffusion model for electron transport in resonant tunneling diodes

    OpenAIRE

    Degond, Pierre; Gallego, Samy; Méhats, Florian

    2007-01-01

    International audience We present an entropic Quantum Drift Diffusion model (eQDD) and show how it can be derived on a bounded domain as the diffusive approximation of the Quantum Liouville equation with a quantum BGK operator. Some links between this model and other existing models are exhibited, especially with the Density Gradient (DG) model and the Schrödinger-Poisson Drift Diffusion model (SPDD). Then a finite difference scheme is proposed to discretize the eQDD model coupled to the P...

  7. Resonant electronic transport through a triple quantum-dot with Λ-type level structure under dual radiation fields

    International Nuclear Information System (INIS)

    Due to quantum interference, light can transmit through dense atomic media, a phenomenon known as electromagnetically induced transparency (EIT). We propose that EIT is not limited to light transmission and there is an electronic analog where resonant transparency in charge transport in an opaque structure can be induced by electromagnetic radiation. A triple-quantum-dots system with Λ-type level structure is generally opaque due to the level in the center dot being significantly higher and therefore hopping from the left dot to the center dot is almost forbidden. We demonstrate that an electromagnetically induced electron transparency (EIET) in charge of transport can indeed occur in the Λ-type system. The direct evidence of EIET is that an electron can travel from the left dot to the right dot, while the center dot apparently becomes invisible. We analyze EIET and the related shot noise in both the zero and strong Coulomb blockade regimes. It is found that the EIET (position, height, and symmetry) can be tuned by several controllable parameters of the radiation fields, such as the Rabi frequencies and detuning frequencies. The result offers a transparency/opaque tuning technique in charge transport using interfering radiation fields

  8. Detection and correction of blinking bias in image correlation transport measurements of quantum dot tagged macromolecules

    DEFF Research Database (Denmark)

    Durisic, Nela; Bachir, Alexia I; Kolin, David L;

    2007-01-01

    Semiconductor nanocrystals or quantum dots (QDs) are becoming widely used as fluorescent labels for biological applications. Here we demonstrate that fluorescence fluctuation analysis of their diffusional mobility using temporal image correlation spectroscopy is highly susceptible to systematic e...

  9. Nonlinear transport in coupled quantum dots:A stationary probability approach

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    The stationary tunneling current and differential conductance of the coupled quantum dots system with split-gates are calculated by generalizing the Beenaker’s linear response theory for the description of the Coulomb-blockade oscillations of the conductance in the single quantum dot.The calculation of the charging diagram in parallel through the double dot as function of the two side-gate voltages shows a remarkable agreement with the recent experimental results by Hatano et al.

  10. Carrier transport in III-V quantum-dot structures for solar cells or photodetectors

    Science.gov (United States)

    Wang, Wenqi; Wang, Lu; Jiang, Yang; Ma, Ziguang; Sun, Ling; Liu, Jie; Sun, Qingling; Zhao, Bin; Wang, Wenxin; Liu, Wuming; Jia, Haiqiang; Chen, Hong

    2016-09-01

    According to the well-established light-to-electricity conversion theory, resonant excited carriers in the quantum dots will relax to the ground states and cannot escape from the quantum dots to form photocurrent, which have been observed in quantum dots without a p-n junction at an external bias. Here, we experimentally observed more than 88% of the resonantly excited photo carriers escaping from InAs quantum dots embedded in a short-circuited p-n junction to form photocurrent. The phenomenon cannot be explained by thermionic emission, tunneling process, and intermediate-band theories. A new mechanism is suggested that the photo carriers escape directly from the quantum dots to form photocurrent rather than relax to the ground state of quantum dots induced by a p-n junction. The finding is important for understanding the low-dimensional semiconductor physics and applications in solar cells and photodiode detectors. Project supported by the National Natural Science Foundation of China (Grant Nos. 11574362, 61210014, 11374340, and 11474205) and the Innovative Clean-Energy Research and Application Program of Beijing Municipal Science and Technology Commission, China (Grant No. Z151100003515001).

  11. Finite-time full counting statistics and factorial cumulants for transport through a quantum dot with normal and superconducting leads

    International Nuclear Information System (INIS)

    We study the finite-time full counting statistics for subgap transport through a single-level quantum dot tunnel-coupled to one normal and one superconducting lead. In particular, we determine the factorial and the ordinary cumulants both for finite times and in the long-time limit. We find that the factorial cumulants violate the sign criterion, indicating a non-binomial distribution, even in absence of Coulomb repulsion due to the presence of superconducting correlations. At short times the cumulants exhibit oscillations which are a signature of the coherent transfer of Cooper pairs between the dot and the superconductor. (paper)

  12. Coherent Transport Through a Quantum Dot Embedded in a Double-Slit-Like Aharonov-Bohm Ring

    Institute of Scientific and Technical Information of China (English)

    黄丽; 游建强; 颜晓红; 韦世豪

    2002-01-01

    Coherent transport through a quantum dot embedded in one arm ora double-slit-like Aharonov-Bohm (AB) ringis studied using the Green's function approach. We obtain experimental observations such as continuous phaseshift along a single resonance peak and sharp inter-resonance phase drop. The AB oscillations of the differentialconductance of the whole device are calculated by using the nonequilibrium Keldysh formalism. It is shown thatthe oscillating conductance has a continuous bias-voltage-dependent phase shift and is asymmetric in both linearand nonlinear response regimes.

  13. Pseudopotential-based electron quantum transport: Theoretical formulation and application to nanometer-scale silicon nanowire transistors

    Science.gov (United States)

    Fang, Jingtian; Vandenberghe, William G.; Fu, Bo; Fischetti, Massimo V.

    2016-01-01

    We present a formalism to treat quantum electronic transport at the nanometer scale based on empirical pseudopotentials. This formalism offers explicit atomistic wavefunctions and an accurate band structure, enabling a detailed study of the characteristics of devices with a nanometer-scale channel and body. Assuming externally applied potentials that change slowly along the electron-transport direction, we invoke the envelope-wavefunction approximation to apply the open boundary conditions and to develop the transport equations. We construct the full-band open boundary conditions (self-energies of device contacts) from the complex band structure of the contacts. We solve the transport equations and present the expressions required to calculate the device characteristics, such as device current and charge density. We apply this formalism to study ballistic transport in a gate-all-around (GAA) silicon nanowire field-effect transistor with a body-size of 0.39 nm, a gate length of 6.52 nm, and an effective oxide thickness of 0.43 nm. Simulation results show that this device exhibits a subthreshold slope (SS) of ˜66 mV/decade and a drain-induced barrier-lowering of ˜2.5 mV/V. Our theoretical calculations predict that low-dimensionality channels in a 3D GAA architecture are able to meet the performance requirements of future devices in terms of SS swing and electrostatic control.

  14. Pseudopotential-based electron quantum transport: Theoretical formulation and application to nanometer-scale silicon nanowire transistors

    Energy Technology Data Exchange (ETDEWEB)

    Fang, Jingtian, E-mail: jingtian.fang@utdallas.edu; Vandenberghe, William G.; Fu, Bo; Fischetti, Massimo V. [Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080 (United States)

    2016-01-21

    We present a formalism to treat quantum electronic transport at the nanometer scale based on empirical pseudopotentials. This formalism offers explicit atomistic wavefunctions and an accurate band structure, enabling a detailed study of the characteristics of devices with a nanometer-scale channel and body. Assuming externally applied potentials that change slowly along the electron-transport direction, we invoke the envelope-wavefunction approximation to apply the open boundary conditions and to develop the transport equations. We construct the full-band open boundary conditions (self-energies of device contacts) from the complex band structure of the contacts. We solve the transport equations and present the expressions required to calculate the device characteristics, such as device current and charge density. We apply this formalism to study ballistic transport in a gate-all-around (GAA) silicon nanowire field-effect transistor with a body-size of 0.39 nm, a gate length of 6.52 nm, and an effective oxide thickness of 0.43 nm. Simulation results show that this device exhibits a subthreshold slope (SS) of ∼66 mV/decade and a drain-induced barrier-lowering of ∼2.5 mV/V. Our theoretical calculations predict that low-dimensionality channels in a 3D GAA architecture are able to meet the performance requirements of future devices in terms of SS swing and electrostatic control.

  15. Textured Nanoporous Mo:BiVO4 Photoanodes with High Charge Transport and Charge Transfer Quantum Efficiencies for Oxygen Evolution

    Energy Technology Data Exchange (ETDEWEB)

    Nair, Vineet; Perkins, Craig L.; Lin, Qiyin; Law, Matt

    2016-04-01

    We have developed a simple spin coating method to make high-quality nanoporous photoelectrodes of monoclinic BiVO4 and studied the ability of these electrodes to transport photogenerated carriers to oxidize sulfite and water. Samples containing molybdenum and featuring [001] out-of-plane crystallographic texture show a photocurrent and external quantum efficiency (EQE) for sulfite oxidation as high as 3.1 mA cm-2 and 60%, respectively, at 1.23 V versus reversible hydrogen electrode. By using an optical model of the electrode stack to accurately determine the fraction of electrode absorptance due to the BiVO4 active layer, we estimate that on average 70 +/- 5% of all photogenerated carriers escape recombination. A comparison of internal quantum efficiency as a function of film processing, illumination direction, and film thickness shows that electron transport is efficient and hole transport limits the photocurrent (hole diffusion length <40 nm). We find that Mo addition primarily improves electron transport and texturing mostly improves hole transport. Mo enhances electron transport by thinning the surface depletion layer or passivating traps and recombination centers at grain boundaries and interfaces, while improved hole transport in textured films may result from more efficient lateral hole extraction due to the texturing itself or the reduced density of deep gap states observed in photoemission measurements. Photoemission data also reveal that the films have bismuth-rich, vanadium- and oxygen-deficient surface layers, while ion scattering spectroscopy indicates a Bi-V-O surface termination. Without added catalysts, the plain BiVO4 electrodes oxidized water with an initial photocurrent and peak EQE of 1.7 mA cm-2 and 30%, respectively, which equates to a hole transfer efficiency to water of >64% at 1.23 V. The electrodes quickly photocorrode during water oxidation but show good stability during sulfite oxidation and indefinite stability in the dark. By improving

  16. Vibrational excitons in ionophores: Experimental probes for quantum coherence-assisted ion transport and selectivity in ion channels

    CERN Document Server

    Ganim, Ziad; Vaziri, Alipasha

    2011-01-01

    Despite a large body of work, the exact molecular details underlying ion-selectivity and transport in the potassium channel have not been fully laid to rest. One major reason has been the lack of experimental methods that can probe these mechanisms dynamically on their biologically relevant time scales. Recently it was suggested that quantum coherence and its interplay with thermal vibration might be involved in mediating ion-selectivity and transport. In this work we present an experimental strategy for using time resolved infrared spectroscopy to investigate these effects. We show the feasibility by demonstrating the IR absorption and Raman spectroscopic signatures of potassium binding model molecules that mimic the transient interactions of potassium with binding sites of the selectivity filter during ion conduction. In addition to guide our experiments on the real system we have performed molecular dynamic-based simulations of the FTIR and 2DIR spectra of the entire KcsA complex, which is the largest comp...

  17. Optimal and robust energy transport in light-harvesting complexes: (II) A quantum interplay of multichromophoric geometries and environmental interactions

    CERN Document Server

    Mohseni, Masoud; Lloyd, Seth; Rabitz, Herschel

    2011-01-01

    Today, the physical principles for the high efficiency of excitation energy transfer in light-harvesting complexes are still not fully understood. Notably, the degree of robustness of these systems for transporting energy is not known considering their realistic interactions with vibrational and radiative environments within the surrounding solvent and scaffold proteins. In this work, we employ an efficient technique to simulate ultrafast quantum dynamics of such complex excitonic systems in their non-equilibrium environment in the non-perturbative and non-Markovian regimes. We demonstrate that the natural dynamics of the FMO complex leads to optimum and stable energy transport due to a convergence of energy/time scales among important internal and external parameters. In particular, we show that the FMO energy transfer efficiency is optimal and robust with respect to all the relevant parameters of environmental interactions and Frenkel-exciton Hamiltonian including reorganization energy \\lambda, bath frequen...

  18. Spin interference and the Fano effect in electron transport through a mesoscopic ring side-coupled with a quantum dot

    International Nuclear Information System (INIS)

    We investigate the electron transport through a mesoscopic ring side-coupled with a quantum dot (QD) in the presence of Rashba spin-orbit (SO) interaction. It is shown that both the Fano resonance and the spin interference effects play important roles in the electron transport properties. As the QD level is around the Fermi energy, the total conductance shows a typical Fano resonance line shape. By applying an electrical gate voltage to the QD, the total transmission through the system can be strongly modulated. By threading the mesoscopic ring with a magnetic flux, the time-reversal symmetry of the system is broken, and a spin polarized current can be obtained even though the incident current is unpolarized.

  19. Magneto-transport of an electron bilayer system in an undoped Si/SiGe double-quantum-well heterostructure

    Energy Technology Data Exchange (ETDEWEB)

    Laroche, D.; Nielsen, E.; Lu, T. M., E-mail: tlu@sandia.gov [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States); Huang, S.-H.; Liu, C. W.; Li, J.-Y., E-mail: jiunyun@ntu.edu.tw [Department of Electrical Engineering and Graduate Institute of Electronic Engineering, National Taiwan University, Taipei 10617, Taiwan (China); National Nano Device Laboratories, Hsinchu 30077, Taiwan (China)

    2015-04-06

    We report the design, the fabrication, and the magneto-transport study of an electron bilayer system embedded in an undoped Si/SiGe double-quantum-well heterostructure. Combined Hall densities (n{sub Hall}) ranging from 2.6 × 10{sup 10} cm{sup −2} to 2.7 × 10{sup 11} cm{sup −2} were achieved, yielding a maximal combined Hall mobility (μ{sub Hall}) of 7.7 × 10{sup 5} cm{sup 2}/(V ⋅ s) at the highest density. Simultaneous electron population of both quantum wells is clearly observed through a Hall mobility drop as the Hall density is increased to n{sub Hall} > 3.3 × 10{sup 10} cm{sup −2}, consistent with Schrödinger-Poisson simulations. The integer and fractional quantum Hall effects are observed in the device, and single-layer behavior is observed when both layers have comparable densities, either due to spontaneous interlayer coherence or to the symmetric-antisymmetric gap.

  20. Topological quantum-phase coherence in full counting statistics of transport electrons with two-body interaction

    International Nuclear Information System (INIS)

    The full counting statistics of electron transport through two parallel quantum dots with antiparallel magnetic fluxes is investigated as a probe to detect the topological quantum-phase coherence (TQPC), which results in the characteristic oscillation of the zero-frequency cumulants including the shot noise and skewness. We show explicitly the phase transition of cumulant spectrum-patterns induced by the topology change of electron path-loops while the pattern period, which depends only on the topology (or Chern number), is robust against the variation of Coulomb interaction and interdot coupling strengths. Most importantly we report for the first time on a new type of TQPC, which is generated by the two-particle interaction and does not exist in the single-particle wave function interference. Moreover, the accurately quantized peaks of Fano-factor spectrum, which characterize the super- and sub-Poissonian shot noises, are of fundamental importance in technical applications similar to the superconducting quantum interference device. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

  1. An entropic quantum drift-diffusion model for electron transport in resonant tunneling diodes

    International Nuclear Information System (INIS)

    We present an entropic quantum drift-diffusion model (eQDD) and show how it can be derived on a bounded domain as the diffusive approximation of the Quantum Liouville equation with a quantum BGK operator. Some links between this model and other existing models are exhibited, especially with the density gradient (DG) model and the Schroedinger-Poisson drift-diffusion model (SPDD). Then a finite difference scheme is proposed to discretize the eQDD model coupled to the Poisson equation and we show how this scheme can be slightly modified to discretize the other models. Numerical results show that the properties listed for the eQDD model are checked, as well as the model captures important features concerning the modeling of a resonant tunneling diode. To finish, some comparisons between the models stated above are realized

  2. Electron transport through individual Ge self-assembled quantum dots on Si

    Science.gov (United States)

    Chung, Hung-Chin; Chu, Wen-Huei; Liu, Chuan-Pu

    2006-08-01

    Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here the authors report on the fabrication of Ge quantum dots (QDs) onto Si (100) by ultrahigh-vacuum ion beam sputtering and the electrical properties of individual QDs. Transmission electron microscopy images show that samples with completely incoherent or coherent semispherical islands can be produced under different ion energies. The current-voltage (I-V) characteristics with conductive atomic force microscopy at room temperature. exhibit linear behavior at low bias and nonlinear behavior at large bias from coherent islands, whereas the staircase structures are clearly observed in the I-V curve from incoherent islands, which are attributed to electron tunneling through the quantized energy levels of a single Ge QD.

  3. Role of thermal stress on the magnetophonon peak structure in the parallel transport of the GaAs/AlGaAs multiple quantum wells

    International Nuclear Information System (INIS)

    The magnetophonon resonance in parallel transport of two types multiple quantum wells was studied. The transverse magnetresonance was measured in pulsed magnetic fields up to 30 T (within temperature region from 77 to 340 K). A fine structure of magnetophonon resonance peaks which depends on temperature and does not depend on the type of multiple quantum wells, was observed. This effect could be attributed to two phenomena: contribution of barrier phonons and influence of thermostresses. (author)

  4. Nonequilibrium quantum dynamics and transport: from integrability to many-body localization

    Science.gov (United States)

    Vasseur, Romain; Moore, Joel E.

    2016-06-01

    We review the non-equilibrium dynamics of many-body quantum systems after a quantum quench with spatial inhomogeneities, either in the Hamiltonian or in the initial state. We focus on integrable and many-body localized systems that fail to self-thermalize in isolation and for which the standard hydrodynamical picture breaks down. The emphasis is on universal dynamics, non-equilibrium steady states and new dynamical phases of matter, and on phase transitions far from thermal equilibrium. We describe how the infinite number of conservation laws of integrable and many-body localized systems lead to complex non-equilibrium states beyond the traditional dogma of statistical mechanics.

  5. ABC transporters affect the elimination and toxicity of CdTe quantum dots in liver and kidney cells.

    Science.gov (United States)

    Chen, Mingli; Yin, Huancai; Bai, Pengli; Miao, Peng; Deng, Xudong; Xu, Yingxue; Hu, Jun; Yin, Jian

    2016-07-15

    This paper aimed to investigate the role of adenosine triphosphate-binding cassette (ABC) transporters on the efflux and the toxicity of nanoparticles in liver and kidney cells. In this study, we synthesized CdTe quantum dots (QDs) that were monodispersed and emitted green fluorescence (maximum peak at 530nm). Such QDs tended to accumulate in human hepatocellular carcinoma cells (HepG2), human kidney cells 2 (HK-2), and Madin-Darby canine kidney (MDCK) cells, and cause significant toxicity in all the three cell lines. Using specific inhibitors and inducers of P-glycoprotein (Pgp) and multidrug resistance associated proteins (Mrps), the cellular accumulation and subsequent toxicity of QDs in HepG2 and HK-2 cells were significantly affected, while only slight changes appeared in MDCK cells, corresponding well with the functional expressions of ABC transporters in cells. Moreover, treatment of QDs caused concentration- and time- dependent induction of ABC transporters in HepG2 and HK-2 cells, but such phenomenon was barely found in MDCK cells. Furthermore, the effects of CdTe QDs on ABC transporters were found to be greater than those of CdCl2 at equivalent concentrations of cadmium, indicating that the effects of QDs should be a combination of free Cd(2+) and specific properties of QDs. Overall, these results indicated a strong dependence between the functional expressions of ABC transporters and the efflux of QDs, which could be an important reason for the modulation of QDs toxicity by ABC transporters. PMID:27131644

  6. Transport anomalies and quantum criticality in electron-doped cuprate superconductors

    OpenAIRE

    Zhang, Xu; Yu, Heshan; He, Ge; Hu, Wei; Yuan, Jie; Zhu, Beiyi; Jin, Kui

    2016-01-01

    Superconductivity research is like running a marathon. Three decades after the discovery of high-Tc cuprates, there have been mass data generated from transport measurements, which bring fruitful information. In this review, we give a brief summary of the intriguing phenomena reported in electron-doped cuprates from the aspect of electrical transport as well as the complementary thermal transport. We attempt to sort out common features of the electron-doped family, e.g. the strange metal, neg...

  7. Edge State Transport of Separately Contacted Bilayer Systems in the Fractional Quantum Hall Regime

    OpenAIRE

    Yoshioka, Daijiro; Nomura, Kentaro

    1999-01-01

    Hall and diagonal resistances of bilayer fractional quantum Hall systems are discussed theoretically. The bilayers have electrodes attached separately to each layer. They are assumed to be coupled weakly by interlayer tunneling, while the interlayer Coulomb interaction is negligibly small. It is shown that source-drain voltage dependence of the resistances reflects the Luttinger liquid parameter of the edge state.

  8. Inelastic Quantum Transport in Superlattices: Success and Failure of the Boltzmann Equation

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka; Rott, Stephan;

    1999-01-01

    the whole held range from linear response to negative differential conductivity. The quantum results are compared with the respective results obtained from a Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the limits of validity for the semiclassical theory in a nonlinear...

  9. Single-electron transport driven by surface acoustic waves: Moving quantum dots versus short barriers

    DEFF Research Database (Denmark)

    Utko, Pawel; Hansen, Jørn Bindslev; Lindelof, Poul Erik;

    2007-01-01

    We have investigated the response of the acoustoelectric-current driven by a surface-acoustic wave through a quantum point contact in the closed-channel regime. Under proper conditions, the current develops plateaus at integer multiples of ef when the frequency f of the surface-acoustic wave or t...

  10. Self-Consistent Calculation on the Time-Dependent Electrons Transport Properties of a Quantum Wire

    Directory of Open Access Journals (Sweden)

    J. Chuen

    2015-01-01

    Full Text Available Responses of a quantum wire (QW connected with wide reservoirs to time-dependent external voltages are investigated in self-consistent manner. Distributions of the internal potential and the induced charge density, capacitance, and conductance are calculated. Results indicate that these physical quantities depend strongly on the Fermi energy of systems and the frequency of external voltages. With the increase of the Fermi energy, capacitance and conductance show some resonant peaks due to the open of the next higher quantum channels and the oscillations related to the longitudinal resonant electron states. Frequency-dependent conductance shows two different responses to the external voltages, inductive-like and capacitive-like; and the peaks structure of capacitance is related to the plasmon-like excitation in mesoscopic conductor.

  11. Quantum transport equations for Bose systems taking into account nonlinear hydrodynamic processes

    Directory of Open Access Journals (Sweden)

    P.A. Hlushak

    2014-06-01

    Full Text Available sing the method of nonequilibrium statistical operator by Zubarev, an approach is proposed for the description of kinetics which takes into account the nonlinear hydrodynamic fluctuations for a quantum Bose system. Non-equilibrium statistical operator is presented which consistently describes both the kinetic and nonlinear hydrodynamic processes. Both a kinetic equation for the nonequilibrium one-particle distribution function and a generalized Fokker-Planck equation for nonequilibrium distribution function of hydrodynamic variables (densities of momentum, energy and particle number are obtained. A structure function of hydrodynamic fluctuations in cumulant representation is calculated, which makes it possible to analyse the generalized Fokker-Planck equation in Gaussian and higher approximations of the dynamic correlations of hydrodynamic variables which is important in describing the quantum turbulent processes.

  12. Fabrication and carrier transport phenomena of one-dimensional quantum wires of p-type silicon

    International Nuclear Information System (INIS)

    Fabrication of p-type Si quantum wires in n-type substrates has been performed by selective implantation of focused ion beams of Ga. It has been confirmed that this method and associated device structure have the advantage of being able to control the diameter of quantum wires. This is accomplished by applying a reverse substrate bias to change the depletion width of the pn junctions. The conductance shows a distinctive temperature dependence of 1n(σ) ∝ T-1/2, which coincides with the characteristic of one-dimensional (1D) hopping conductance. At 4.2 K, the authors have observed electrical properties characteristic of 1D systems, such as conductance dips in the wire-conductance versus substrate-bias characteristics and a positive magnetoconductance in the magnetic field range up to 5 T. 32 refs., 7 figs

  13. A device adaptive inflow boundary condition for Wigner equations of quantum transport

    Energy Technology Data Exchange (ETDEWEB)

    Jiang, Haiyan [Department of Applied Mathematics, Beijing Institute of Technology, Beijing 100081 (China); Lu, Tiao [HEDPS and CAPT, LMAM and School of Mathematical Sciences, Peking University, Beijing 100871 (China); Cai, Wei, E-mail: wcai@uncc.edu [Department of Mathematics and Statistics, University of North Carolina at Charlotte, Charlotte, NC 28223-0001 (United States)

    2014-02-01

    In this paper, an improved inflow boundary condition is proposed for Wigner equations in simulating a resonant tunneling diode (RTD), which takes into consideration the band structure of the device. The original Frensley inflow boundary condition prescribes the Wigner distribution function at the device boundary to be the semi-classical Fermi–Dirac distribution for free electrons in the device contacts without considering the effect of the quantum interaction inside the quantum device. The proposed device adaptive inflow boundary condition includes this effect by assigning the Wigner distribution to the value obtained from the Wigner transform of wave functions inside the device at zero external bias voltage, thus including the dominant effect on the electron distribution in the contacts due to the device internal band energy profile. Numerical results on computing the electron density inside the RTD under various incident waves and non-zero bias conditions show much improvement by the new boundary condition over the traditional Frensley inflow boundary condition.

  14. Conductance peak splitting in hole transport through a SiGe double quantum dot

    International Nuclear Information System (INIS)

    We have observed the splitting of Coulomb oscillation peaks in coupled Si0.9Ge0.1 double quantum dots at 4.2 K. The quantum dots are formed by trench isolation, which means that the dots can be made much smaller than possible with the surface-gated approach. A dot diameter of 50 nm or less increases the charging energy and, therefore, the operating temperature of the device compared to previous approaches. A simulation of the results using parameters calculated from the lithographic dimensions of the device shows that a good fit to the experimental data can be achieved with a realistic interdot capacitance value. [copyright] 2001 American Institute of Physics

  15. Quantum-Mechanical Contributions to Numerical Simulations of Charged Particle Transport at the DNA Scale

    Science.gov (United States)

    Champion, Christophe; Galassi, Mariel E.; Weck, Philippe F.; Fojón, Omar; Hanssen, Jocelyn; Rivarola, Roberto D.

    Two quantum mechanical models (CB1 and CDW-EIS) are here presented to provide accurate multiple differential and total cross sections for describing the two most important ionizing processes, namely, ionization and capture induced by heavy charged particles in targets of biological interest. Water and DNA bases are then successively investigated by reporting in particular a detailed study of the influence of the target description on the cross section calculations.

  16. Electric transport through c-nanotubes and grapheme nanoribbons double quantum dots coupled by a superconductor

    OpenAIRE

    Guerra Castro, Juan Manuel

    2014-01-01

    Abstract. Graphene has become a promising material for technological applications and research in fundamental physics due to its rich physical properties. A detailed study of its hexagonal crystalline structure has been performed and has revealed its unusual electronic properties of great interest in nanotechnology and quantum electronics[1, 2, 3]. Charge carriers exitations with energies near the Fermi can be approximated by an effective Weyl-Dirac Hamiltonian thus implying re...

  17. Blocking transport resonances via Kondo many-body entanglement in quantum dots

    OpenAIRE

    Niklas, Michael; Smirnov, Sergey; Mantelli, Davide; Marganska, Magdalena; Nguyen, Ngoc-Viet; Wernsdorfer, Wolfgang; Cleuziou, Jean-Pierre; Grifoni, Milena

    2016-01-01

    Many-body entanglement is at the heart of the Kondo effect, which has its hallmark in quantum dots as a zero-bias conductance peak at low temperatures. It signals the emergence of a conducting singlet state formed by a localized dot degree of freedom and conduction electrons. Carbon nanotubes offer the possibility to study the emergence of the Kondo entanglement by tuning many-body correlations with a gate voltage. Here we show another side of Kondo correlations, which counterintuitively tend...

  18. A Fabry-Perot interferometer with quantum mirrors: nonlinear light transport and rectification

    CERN Document Server

    Fratini, F; Safari, L; Poizat, J-Ph; Valente, D; Auffèves, A; Gerace, D; Santos, M F

    2014-01-01

    Optical transport represents a natural route towards fast communications, and it is currently used in large scale data transfer. The progressive miniaturization of devices for information processing calls for the microscopic tailoring of light transport and confinement at length scales appropriate for the upcoming technologies. With this goal in mind, we present a theoretical analysis of a one-dimensional Fabry-Perot interferometer built with two highly saturable nonlinear mirrors: a pair of two-level systems. Our approach captures non-linear and non-reciprocal effects of light transport that were not reported previously. Remarkably, we show that such an elementary device can operate as a microscopic integrated optical rectifier.

  19. Efimov effect and anomalous transport properties of a quantum Lorentz gas

    International Nuclear Information System (INIS)

    The Lorentz gas is one of the simplest many-body systems which admits a detailed theoretical analysis still having a meaningful physical interpretation. The author studies a complication for the description of the equilibrium state of a gas due to the so called Efimov effect, being an anomaly of the quantum mechanical three-body system. In statistical mechanical studies concerning the thermodynamic properties of gases it is usually assumed that the constituents interact via short-range potentials, which excludes for instance the Coulomb gas. The author investigates the consequences of the effective long-range behaviour due to the Efimov effect in the three-body sub-systems for a binary gas mixture. In particular problems arise in the cluster expansion of the quantum mechanical partition function where it can readily be shown that the bound-state part of the three-particle contribution diverges logarithmically at the Efimov point. The question is whether this implies the divergence of the complete three particle contribution or not. Some aspects of the non-equilibrium behaviour of the quantum Lorentz gas are studied. (Auth.)

  20. Thermal and transport properties of a nonrelativistic quantum gas interacting through a delta-shell potential

    International Nuclear Information System (INIS)

    This work extends the seminal work of Gottfried on the two-body quantum physics of particles interacting through a delta-shell potential to many-body physics by studying a system of nonrelativistic particles when the thermal De-Broglie wavelength of a particle is larger than the range of the potential and the density is such that average distance between particles is larger than the above range. The ability of the delta-shell potential to reproduce some basic properties of the deuteron are examined. Relations for moments of bound-states are derived. The virial expansion is used to calculate the first quantum correction to the ideal gas pressure in the form of the second virial coefficient. Additionally, all thermodynamical functions are calculated up to the first-order quantum corrections. For small departures from equilibrium, the net flows of mass, energy and momentum, characterized by the coefficients of diffusion, thermal conductivity and shear viscosity, respectively, are calculated. Properties of the gas are examined for various values of physical parameters including the case of infinite scattering length when the unitary limit is achieved. (author)

  1. Quantum transport in ferromagnetic graphene super lattice in the presence of Rashba spin–orbit coupling

    Energy Technology Data Exchange (ETDEWEB)

    Hasanirokh, Kobra; Esmaelpour, Mohammad; Mohammadpour, Hakimeh; Phirouznia, Arash

    2014-05-01

    Using the transfer matrix method, we study the electron transport through a single-layer graphene superlattice with alternating layers of ferromagnetic and normal regions with Rashba spin–orbit coupling. We show that the transport properties of the system depend strongly on the superlattice parameters. As another result, Rashba spin–orbit coupling manifests to be of crucial importance in controlling the transmission probabilities and Giant Magneto Resistance (GMR).

  2. Modeling molecular conduction in DNA wires: Charge transfer theories and dissipative quantum transport

    OpenAIRE

    Bulla, R; Gutierrez, R.; Cuniberti, G.

    2006-01-01

    Measurements of electron transfer rates as well as of charge transport characteristics in DNA produced a number of seemingly contradictory results, ranging from insulating behaviour to the suggestion that DNA is an efficient medium for charge transport. Among other factors, environmental effects appear to play a crucial role in determining the effectivity of charge propagation along the double helix. This chapter gives an overview over charge transfer theories and their implication for addres...

  3. Numerical study of the effect of disorder and magnetic field on the quantum transport of two-dimensional nanostructures modeled by tight-binding approximation

    Directory of Open Access Journals (Sweden)

    E Taghizdehsiskht

    2013-09-01

    Full Text Available  In recent years, semiconductor nanostructures have become the model systems of choice for investigation of electrical conduction on short length scales. Quantum transport is studied in a two dimensional electron gas because of the combination of a large Fermi wavelength and large mean free path. In the present work, a numerical method is implemented in order to contribute to the understanding of quantum transport in narrow channels in different conditions of disorder and magnetic fields. We have used an approach that has proved to be very useful in describing mesoscopic transport. We have assumed zero temperature and phase coherent transport. By using the trick that a conductor connected to infinite leads can be replaced by a finite conductor with the effect of the leads incorporated through a 'self-energy' function, a convenient method was provided for evaluating the Green's function of the whole device numerically. Then, Fisher-Lee relations was used for calculating the transmission coefficients through coherent mesoscopic conductors. Our calculations were done in a model system with Hard-wall boundary conditions in the transverse direction, and the Anderson model of disorder was used in disordered samples. We have presented the results of quantum transport for different strengths of disorder and introduced magnetic fields. Our results confirmed the Landauer formalism for calculation of electronic transport. We observed that weak localization effect can be removed by application of a weak perpendicular magnetic field. Finally, we numerically showed the transition to the integral quantum Hall effect regime through the suppression of backscattering on a disordered model system by calculating the two­ terminal conductance of a quasi-one-dimensional quantum conductor as a strong magnetic field is applied. Our results showed that this regime is entered when there is a negligible overlap between electron edge states localized at opposite sides of

  4. Charge transport through exciton shelves in cadmium chalcogenide quantum dot-DNA nano-bioelectronic thin films

    Energy Technology Data Exchange (ETDEWEB)

    Goodman, Samuel M.; Singh, Vivek [Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Noh, Hyunwoo [Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Materials Science and Engineering Program and Department of Nanoengineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, California 92093 (United States); Cha, Jennifer N. [Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Materials Science and Engineering Program and Department of Nanoengineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, California 92093 (United States); Materials Science and Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Nagpal, Prashant, E-mail: pnagpal@colorado.edu [Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Materials Science and Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); BioFrontiers Institute, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303 (United States); Renewable and Sustainable Energy Institute, University of Colorado Boulder, 2445 Kittredge Loop, Boulder, Colorado 80309 (United States)

    2015-02-23

    Quantum dot (QD), or semiconductor nanocrystal, thin films are being explored for making solution-processable devices due to their size- and shape-tunable bandgap and discrete higher energy electronic states. While DNA has been extensively used for the self-assembly of nanocrystals, it has not been investigated for the simultaneous conduction of multiple energy charges or excitons via exciton shelves (ES) formed in QD-DNA nano-bioelectronic thin films. Here, we present studies on charge conduction through exciton shelves, which are formed via chemically coupled QDs and DNA, between electronic states of the QDs and the HOMO-LUMO levels in the complementary DNA nucleobases. While several challenges need to be addressed in optimizing the formation of devices using QD-DNA thin films, a higher charge collection efficiency for hot-carriers and our detailed investigations of charge transport mechanism in these thin films highlight their potential for applications in nano-bioelectronic devices and biological transducers.

  5. Ab initio energetics, kinetics, and quantum transport characteristics of graphene nanoribbons as nanosensors for detecting nitrogen dioxide

    Science.gov (United States)

    Paulla, Kirti K.; Hassan, Ahmed J.; Knick, Cory R.; Farajian, Amir A.

    2014-03-01

    Molecules adsorption on graphene nanoribbons (GNRs) can be used to engineer and make use of their properties for applications such as energy storage and sensors. We investigate adsorption characteristics by considering nitrogen dioxide as a sample molecule for assessing nanosensor functionality of GNRs. Using ab initio modeling, energetics of various adsorption possibilities are determined and their rate constants are calculated and compared. Nonbonding and weak sp3 adsorptions at the hydrogen-terminated edges are shown to be more feasible than center adsorptions. This shows increased reactivity compared to graphene. Calculated quantum transport responses upon molecules adsorption indicate possibility of sensing extremely low nitrogen dioxide concentrations. Possible approaches for improving gas nanosensor functionality of GNRs are discussed. Reference: RSC Advances, 2013, DOI: 10.1039/c3ra46372a. This research was supported by the National Science Foundation Grant ECCS-0925939.

  6. Electron transport in physically-defined double quantum dots on a highly doped silicon-on-insulator substrate

    Science.gov (United States)

    Yamaoka, Y.; Oda, S.; Kodera, T.

    2016-09-01

    We study electron transport in physically-defined silicon quantum dots (QDs) on a highly doped silicon-on-insulator (SOI) substrate. We show that the QDs can be obtained as designed without unintentional localized states caused by fluctuating dopant potentials even when a highly doped SOI substrate is used. We observe the single electron tunneling phenomena both in the single QDs (SQDs) and in the double QDs (DQDs). The charging energy in the SQDs is ˜18 meV as estimated from the Coulomb diamond. This enables us to further estimate that the diameter of the SQDs is ˜35 nm, which is consistent with the designed fabrication specifications if the voltage condition is taken into account. A change of the charged state in the DQDs is detected using the SQD as a charge sensor. A periodic honeycomb-like charge stability diagram is obtained, which indicates that we achieved the fabrication of DQDs without unintentional localized states.

  7. Charge transport through exciton shelves in cadmium chalcogenide quantum dot-DNA nano-bioelectronic thin films

    Science.gov (United States)

    Goodman, Samuel M.; Noh, Hyunwoo; Singh, Vivek; Cha, Jennifer N.; Nagpal, Prashant

    2015-02-01

    Quantum dot (QD), or semiconductor nanocrystal, thin films are being explored for making solution-processable devices due to their size- and shape-tunable bandgap and discrete higher energy electronic states. While DNA has been extensively used for the self-assembly of nanocrystals, it has not been investigated for the simultaneous conduction of multiple energy charges or excitons via exciton shelves (ES) formed in QD-DNA nano-bioelectronic thin films. Here, we present studies on charge conduction through exciton shelves, which are formed via chemically coupled QDs and DNA, between electronic states of the QDs and the HOMO-LUMO levels in the complementary DNA nucleobases. While several challenges need to be addressed in optimizing the formation of devices using QD-DNA thin films, a higher charge collection efficiency for hot-carriers and our detailed investigations of charge transport mechanism in these thin films highlight their potential for applications in nano-bioelectronic devices and biological transducers.

  8. Photogenerated carriers transport behaviors in L-cysteine capped ZnSe core-shell quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Shan, Qingsong; Li, Kuiying, E-mail: kuiyingli@ysu.edu.cn; Lin, Yingying; Yin, Hua; Zhu, Ruiping [State Key Laboratory of Metastable Materials Manufacture Technology and Science, Yanshan University, Qinhuangdao 066004 (China); Xue, Zhenjie [Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 (China)

    2016-02-07

    The photoexcited carrier transport behavior of zinc selenide (ZnSe) quantum dots (QDs) with core–shell structure is studied because of their unique photoelectronic characteristics. The surface photovoltaic (SPV) properties of self-assembled ZnSe/ZnS/L-Cys core–shell QDs were probed via electric field induced surface photovoltage and transient photovoltage (TPV) measurements supplemented by Fourier transform infrared, laser Raman, absorption, and photoluminescence spectroscopies. The ZnSe QDs displayed p-type SPV characteristics with a broader stronger SPV response over the whole ultraviolet-to-near-infrared range compared with those of other core–shell QDs in the same group. The relationship between the SPV phase value of the QDs and external bias was revealed in their SPV phase spectrum. The wide transient photovoltage response region from 3.3 × 10{sup −8} to 2 × 10{sup −3} s was closely related to the long diffusion distance of photoexcited free charge carriers in the interfacial space–charge region of the QDs. The strong SPV response corresponding to the ZnSe core mainly originated from an obvious quantum tunneling effect in the QDs.

  9. Quantum heat transport of a two-qubit system: Interplay between system-bath coherence and qubit-qubit coherence

    Energy Technology Data Exchange (ETDEWEB)

    Kato, Akihito, E-mail: kato@kuchem.kyoto-u.ac.jp; Tanimura, Yoshitaka, E-mail: tanimura@kuchem.kyoto-u.ac.jp [Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502 (Japan)

    2015-08-14

    We consider a system consisting of two interacting qubits that are individually coupled to separate heat baths at different temperatures. The quantum effects in heat transport are investigated in a numerically rigorous manner with a hierarchial equations of motion (HEOM) approach for non-perturbative and non-Markovian system-bath coupling cases under non-equilibrium steady-state conditions. For a weak interqubit interaction, the total system is regarded as two individually thermostatted systems, whereas for a strong interqubit interaction, the two-qubit system is regarded as a single system coupled to two baths. The roles of quantum coherence (or entanglement) between the two qubits (q-q coherence) and between the qubit and bath (q-b coherence) are studied through the heat current calculated for various strengths of the system-bath coupling and interqubit coupling for high and low temperatures. The same current is also studied using the time convolutionless (TCL) Redfield equation and using an expression derived from the Fermi golden rule (FGR). We find that the HEOM results exhibit turnover behavior of the heat current as a function of the system-bath coupling strength for all values of the interqubit coupling strength, while the results obtained with the TCL and FGR approaches do not exhibit such behavior, because they do not possess the capability of treating the q-b and q-q coherences. The maximum current is obtained in the case that the q-q coherence and q-b coherence are balanced in such a manner that coherence of the entire heat transport process is realized. We also find that the heat current does not follow Fourier’s law when the temperature difference is very large, due to the non-perturbative system-bath interactions.

  10. Quantum many-particle electron transport in time-dependent systems with Bohmian trajectories

    OpenAIRE

    Alarcón Pardo, Alfonso

    2011-01-01

    Es conocido que a escalas nanométricas se debe tratar con en el problema de muchas partículas a la hora de estudiar dispositivos electrónicos. Es estos escenarios, la ecuación de Schrödinger dependiente del tiempo para muchas partículas solo se puede resolver para unos pocos grados de libertad. En este sentido, diferentes formalismos han sido desarrollados en la literatura (tales como time-dependent Density Functional Theory, Green's functions técnicas o Quantum Monte Carlo técnicas) para tra...

  11. Spin transport properties of a quantum dot coupled to ferromagnetic leads with noncollinear magnetizations.

    Science.gov (United States)

    Zhang, Hao; Zhang, Guang-Ming; Yu, Lu

    2009-04-15

    A correct general formula for the spin current through an interacting quantum dot coupled to ferromagnetic leads with magnetization at an arbitrary angle θ is derived within the framework of the Keldysh formalism. Under asymmetric conditions, the spin current component J(z) may change sign for 0spin current and spin tunneling magnetoresistance exhibit different angle dependence in the free and Coulomb blockade regimes. In the latter case, the competition of the spin precession and the spin-valve effect could lead to an anomaly in the angle dependence of the spin current. PMID:21825366

  12. Lactose as a “Trojan Horse” for Quantum Dot Cell Transport**

    OpenAIRE

    Benito-Alifonso, David; Tremel, Shirley; Hou, Bo; Lockyear, Harriet; Mantell, Judith; Fermin, David J.; Verkade, Paul; Berry, Monica; Galan, M. Carmen

    2013-01-01

    A series of glycan-coated quantum dots were prepared to probe the effect of glycan presentation in intracellular localization in HeLa and SV40 epithelial cells. We show that glycan density mostly impacts on cell toxicity, whereas glycan type affects the cell uptake and intracellular localization. Moreover, we show that lactose can act as a “Trojan horse” on bi-functionalized QDs to help intracellular delivery of other non-internalizable glycan moieties and largely avoid the endosomal/lysosoma...

  13. Ab initio nonequilibrium quantum transport and forces with the real-space projector augmented wave method

    DEFF Research Database (Denmark)

    Chen, Jingzhe; Thygesen, Kristian S.; Jacobsen, Karsten W.

    2012-01-01

    We present an efficient implementation of a nonequilibrium Green's function method for self-consistent calculations of electron transport and forces in nanostructured materials. The electronic structure is described at the level of density functional theory using the projector augmented wave method...... over k points and real space makes the code highly efficient and applicable to systems containing several hundreds of atoms. The method is applied to a number of different systems, demonstrating the effects of bias and gate voltages, multiterminal setups, nonequilibrium forces, and spin transport....

  14. Peltier ac calorimeter

    OpenAIRE

    Jung, D. H.; Moon, I. K.; Jeong, Y. H.

    2001-01-01

    A new ac calorimeter, utilizing the Peltier effect of a thermocouple junction as an ac power source, is described. This Peltier ac calorimeter allows to measure the absolute value of heat capacity of small solid samples with sub-milligrams of mass. The calorimeter can also be used as a dynamic one with a dynamic range of several decades at low frequencies.

  15. Electronic Transport in Chemical Vapor Deposited Graphene Synthesized on Cu: Quantum Hall Effect and Weak Localization

    OpenAIRE

    Cao, H. L.; Yu, Q. K.; Jauregui, L. A.; Tian, J; Wu, W.; Z. Liu; Jalilian, R.; Benjamin, D. K.; Jiang, Z.; J. Bao; Pei, S S; Chen, Y P

    2009-01-01

    We report on electronic properties of graphene synthesized by chemical vapor deposition (CVD) on copper then transferred to SiO2/Si. Wafer-scale (up to 4 in.) graphene films have been synthesized, consisting dominantly of monolayer graphene as indicated by spectroscopic Raman mapping. Low temperature transport measurements are performed on microdevices fabricated from such CVD graphene, displaying ambipolar field ...

  16. Phonon-Magnon Interaction in Low Dimensional Quantum Magnets Observed by Dynamic Heat Transport Measurements

    NARCIS (Netherlands)

    Montagnese, Matteo; Otter, Marian; Zotos, Xenophon; Fishman, Dmitry A.; Hlubek, Nikolai; Mityashkin, Oleg; Hess, Christian; Saint-Martin, Romuald; Singh, Surjeet; Revcolevschi, Alexandre; van Loosdrecht, Paul H. M.

    2013-01-01

    Thirty-five years ago, Sanders and Walton [Phys. Rev. B 15, 1489 (1977)] proposed a method to measure the phonon-magnon interaction in antiferromagnets through thermal transport which so far has not been verified experimentally. We show that a dynamical variant of this approach allows direct extract

  17. Quantum transport in graphene in presence of strain-induced pseudo-Landau levels

    DEFF Research Database (Denmark)

    Settnes, Mikkel; Leconte, Nicolas; Barrios-Vargas, Jose E.;

    2016-01-01

    We report on mesoscopic transport fingerprints in disordered graphene caused by strain-field induced pseudomagnetic Landau levels (pLLs). Efficient numerical real space calculations of the Kubo formula are performed for an ordered network of nanobubbles in graphene, creating pseudomagnetic fields...

  18. Effects of effective attractive multi-body interaction on quantum phase and transport dynamics of a strongly correlated bosonic gas across the superfluid to Mott insulator transition

    International Nuclear Information System (INIS)

    We theoretically investigate quantum phases and transport dynamics of ultracold atoms trapped in an optical lattice in the presence of effective multi-body interaction. When a harmonic external potential is added, several interesting phenomena are revealed, such as the broadening and the emergence of a central insulator plateau and the phase transition between superfluid and Mott insulator phase. We also study the transport of the system which runs across the superfluid—insulator transition after ramping up the lattice, and predict a slower relaxation which is attributed to the influence of the multi-body interaction on the mass transport

  19. A multi-agent quantum Monte Carlo model for charge transport: Application to organic field-effect transistors

    Energy Technology Data Exchange (ETDEWEB)

    Bauer, Thilo; Jäger, Christof M. [Department of Chemistry and Pharmacy, Computer-Chemistry-Center and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Nägelsbachstrasse 25, 91052 Erlangen (Germany); Jordan, Meredith J. T. [School of Chemistry, University of Sydney, Sydney, NSW 2006 (Australia); Clark, Timothy, E-mail: tim.clark@fau.de [Department of Chemistry and Pharmacy, Computer-Chemistry-Center and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Nägelsbachstrasse 25, 91052 Erlangen (Germany); Centre for Molecular Design, University of Portsmouth, Portsmouth PO1 2DY (United Kingdom)

    2015-07-28

    We have developed a multi-agent quantum Monte Carlo model to describe the spatial dynamics of multiple majority charge carriers during conduction of electric current in the channel of organic field-effect transistors. The charge carriers are treated by a neglect of diatomic differential overlap Hamiltonian using a lattice of hydrogen-like basis functions. The local ionization energy and local electron affinity defined previously map the bulk structure of the transistor channel to external potentials for the simulations of electron- and hole-conduction, respectively. The model is designed without a specific charge-transport mechanism like hopping- or band-transport in mind and does not arbitrarily localize charge. An electrode model allows dynamic injection and depletion of charge carriers according to source-drain voltage. The field-effect is modeled by using the source-gate voltage in a Metropolis-like acceptance criterion. Although the current cannot be calculated because the simulations have no time axis, using the number of Monte Carlo moves as pseudo-time gives results that resemble experimental I/V curves.

  20. The effect of atomic-scale defects and dopants on phosphorene electronic structure and quantum transport properties.

    Energy Technology Data Exchange (ETDEWEB)

    Lopez-Bezanilla, Alejandro

    2016-01-20

    By means of a multi-scale first-principles approach, a description of the local electronic structure of 2D and narrow phosphorene sheets with various types of modifications is presented. Firtly, a rational argument based on the geometry of the pristine and modified P network, and supported by the Wannier functions formalism is introduced to describe a hybridization model of the P atomic orbitals. Ab initio calculations show that non-isoelectronic foreign atoms form quasi-bound states at varying energy levels and create different polarization states depending on the number of valence electrons between P and the doping atom. The quantum transport properties of modified phosphorene ribbons are further described with great accuracy. The distortions on the electronic bands induced by the external species lead to strong backscattering effects on the propagating charge carriers. Depending on the energy of the charge carrier and the type of doping, the conduction may range from the diffusive to the localized regime. Interstitial defects at vacant sites lead to homogeneous transport fingerprints across different types of doping atoms. We suggest that the relatively low values of charge mobility reported in experimental measurements may have its origin in the presence of defects.

  1. A multi-agent quantum Monte Carlo model for charge transport: Application to organic field-effect transistors

    International Nuclear Information System (INIS)

    We have developed a multi-agent quantum Monte Carlo model to describe the spatial dynamics of multiple majority charge carriers during conduction of electric current in the channel of organic field-effect transistors. The charge carriers are treated by a neglect of diatomic differential overlap Hamiltonian using a lattice of hydrogen-like basis functions. The local ionization energy and local electron affinity defined previously map the bulk structure of the transistor channel to external potentials for the simulations of electron- and hole-conduction, respectively. The model is designed without a specific charge-transport mechanism like hopping- or band-transport in mind and does not arbitrarily localize charge. An electrode model allows dynamic injection and depletion of charge carriers according to source-drain voltage. The field-effect is modeled by using the source-gate voltage in a Metropolis-like acceptance criterion. Although the current cannot be calculated because the simulations have no time axis, using the number of Monte Carlo moves as pseudo-time gives results that resemble experimental I/V curves

  2. Quantum Transport and Nano Angle-resolved Photoemission Spectroscopy on the Topological Surface States of Single Sb2Te3 Nanowires

    Science.gov (United States)

    Arango, Yulieth C.; Huang, Liubing; Chen, Chaoyu; Avila, Jose; Asensio, Maria C.; Grützmacher, Detlev; Lüth, Hans; Lu, Jia Grace; Schäpers, Thomas

    2016-09-01

    We report on low-temperature transport and electronic band structure of p-type Sb2Te3 nanowires, grown by chemical vapor deposition. Magnetoresistance measurements unravel quantum interference phenomena, which depend on the cross-sectional dimensions of the nanowires. The observation of periodic Aharonov-Bohm-type oscillations is attributed to transport in topologically protected surface states in the Sb2Te3 nanowires. The study of universal conductance fluctuations demonstrates coherent transport along the Aharonov-Bohm paths encircling the rectangular cross-section of the nanowires. We use nanoscale angle-resolved photoemission spectroscopy on single nanowires (nano-ARPES) to provide direct experimental evidence on the nontrivial topological character of those surface states. The compiled study of the bandstructure and the magnetotransport response unambiguosly points out the presence of topologically protected surface states in the nanowires and their substantial contribution to the quantum transport effects, as well as the hole doping and Fermi velocity among other key issues. The results are consistent with the theoretical description of quantum transport in intrinsically doped quasi-one-dimensional topological insulator nanowires.

  3. Real-time visualization of prion transport in single live cells using quantum dots

    International Nuclear Information System (INIS)

    Prion diseases are fatal neurodegenerative disorders resulting from structural conversion of the cellular isoform of PrPC to the infectious scrapie isoform PrPSc. It is believed that such structural alteration may occur within the internalization pathway. However, there is no direct evidence to support this hypothesis. Employing quantum dots (QDs) as a probe, we have recorded a real-time movie demonstrating the process of prion internalization in a living cell for the first time. The entire internalization process can be divided into four discrete but connected stages. In addition, using methyl-beta-cyclodextrin to disrupt cell membrane cholesterol, we show that lipid rafts play an important role in locating cellular PrPC to the cell membrane and in initiating PrPC endocytosis.

  4. Real-time visualization of prion transport in single live cells using quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Luo, Kan [State Key Laboratory of Virology and Modern Virology Research Centre, College of Life Sciences, Wuhan University, Wuhan 430072 (China); Li, Shu [AIDS Research Centre, Institute of Pathogen Biology, Chinese Academy of Medical Science, Beijing 100730 (China); Xie, Min [College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072 (China); Wu, Di; Wang, WenXi; Chen, Rui; Huang, Liqin; Huang, Tao [State Key Laboratory of Virology and Modern Virology Research Centre, College of Life Sciences, Wuhan University, Wuhan 430072 (China); Pang, Daiwen, E-mail: dwpang@whu.edu.cn [College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072 (China); Xiao, Gengfu, E-mail: xiaogf@wh.iov.cn [State Key Laboratory of Virology and Modern Virology Research Centre, College of Life Sciences, Wuhan University, Wuhan 430072 (China); Wuhan Institute of Virology, Chinese Academy of Sciences, Wuhan 430071 (China)

    2010-04-09

    Prion diseases are fatal neurodegenerative disorders resulting from structural conversion of the cellular isoform of PrP{sup C} to the infectious scrapie isoform PrP{sup Sc}. It is believed that such structural alteration may occur within the internalization pathway. However, there is no direct evidence to support this hypothesis. Employing quantum dots (QDs) as a probe, we have recorded a real-time movie demonstrating the process of prion internalization in a living cell for the first time. The entire internalization process can be divided into four discrete but connected stages. In addition, using methyl-beta-cyclodextrin to disrupt cell membrane cholesterol, we show that lipid rafts play an important role in locating cellular PrP{sup C} to the cell membrane and in initiating PrP{sup C} endocytosis.

  5. Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots

    Science.gov (United States)

    Okuyama, Rin; Eto, Mikio; Brandes, Tobias

    2014-03-01

    We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.[1] Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby.

  6. Blocking transport resonances via Kondo many-body entanglement in quantum dots

    Science.gov (United States)

    Niklas, Michael; Smirnov, Sergey; Mantelli, Davide; Margańska, Magdalena; Nguyen, Ngoc-Viet; Wernsdorfer, Wolfgang; Cleuziou, Jean-Pierre; Grifoni, Milena

    2016-08-01

    Many-body entanglement is at the heart of the Kondo effect, which has its hallmark in quantum dots as a zero-bias conductance peak at low temperatures. It signals the emergence of a conducting singlet state formed by a localized dot degree of freedom and conduction electrons. Carbon nanotubes offer the possibility to study the emergence of the Kondo entanglement by tuning many-body correlations with a gate voltage. Here we show another side of Kondo correlations, which counterintuitively tend to block conduction channels: inelastic co-tunnelling lines in the magnetospectrum of a carbon nanotube strikingly disappear when tuning the gate voltage. Considering the global SU(2) \\xotime SU(2) symmetry of a nanotube coupled to leads, we find that only resonances involving flips of the Kramers pseudospins, associated to this symmetry, are observed at temperatures and voltages below the corresponding Kondo scale. Our results demonstrate the robust formation of entangled many-body states with no net pseudospin.

  7. Quantum transport in graphene in presence of strain-induced pseudo-Landau levels

    Science.gov (United States)

    Settnes, Mikkel; Leconte, Nicolas; Barrios-Vargas, Jose E.; Jauho, Antti-Pekka; Roche, Stephan

    2016-09-01

    We report on mesoscopic transport fingerprints in disordered graphene caused by strain-field induced pseudomagnetic Landau levels (pLLs). Efficient numerical real space calculations of the Kubo formula are performed for an ordered network of nanobubbles in graphene, creating pseudomagnetic fields up to several hundreds of Tesla, values inaccessible by real magnetic fields. Strain-induced pLLs yield enhanced scattering effects across the energy spectrum resulting in lower mean free path and enhanced localization effects. In the vicinity of the zeroth order pLL, we demonstrate an anomalous transport regime, where the mean free paths increases with disorder. We attribute this puzzling behavior to the low-energy sub-lattice polarization induced by the zeroth order pLL, which is unique to pseudomagnetic fields preserving time-reversal symmetry. These results, combined with the experimental feasibility of reversible deformation fields, open the way to tailor a metal-insulator transition driven by pseudomagnetic fields.

  8. A multiband envelope function model for quantum transport in a tunneling diode

    OpenAIRE

    Morandi, Omar; Modugno, Michele

    2006-01-01

    We present a simple model for electron transport in semiconductor devices that exhibit tunneling between the conduction and valence bands. The model is derived within the usual Bloch-Wannier formalism by a k-expansion, and is formulated in terms of a set of coupled equations for the electron envelope functions. Its connection with other models present in literature is discussed. As an application we consider the case of a Resonant Interband Tunneling Diode, demonstrating the ability of the mo...

  9. Spin-resolved polarized transport through a quasi one-dimensional mesoscopic quantum ring-shaped conductor with local Rashba spin-orbit interaction

    Energy Technology Data Exchange (ETDEWEB)

    Liu, P., E-mail: pliu@jit.edu.cn [Basic Department, Jinling Institute of Technology, Nanjing 211169 (China); Liu, Y. [Physics Department, Lehigh University, Bethlehem, PA 18015 (United States); Jiang, H.L.; Yang, Z.H. [Basic Department, Jinling Institute of Technology, Nanjing 211169 (China)

    2012-02-15

    We propose a quasi one-dimensional quantum ring-shaped model associated with Rashba spin-orbit (SO) interaction and Aharomov-Bohm flux to study a spin-dependent quantum transport. It is a possible candidate for spintronic current modulators. By tuning SO coupling strength and Fermi energy, we find there is a broad energy range of small vanishing spin transmission in the resonance and antiresonance interferences. More interestingly, the large on/off spin-resolved polarized conductance ratios are robust even in the presence of strong random on-site Anderson-type disorder in devices, which suggests a potential application in the real system.

  10. ACAC Converters for UPS

    Directory of Open Access Journals (Sweden)

    Rusalin Lucian R. Păun

    2008-05-01

    Full Text Available This paper propose a new control technique forsingle – phase ACAC converters used for a on-line UPSwith a good dynamic response, a reduced-partscomponents, a good output characteristic, a good powerfactorcorrection(PFC. This converter no needs anisolation transformer. A power factor correction rectifierand an inverter with the proposed control scheme has beendesigned and simulated using Caspoc2007, validating theconcept.

  11. Effect of ligand self-assembly on nanostructure and carrier transport behaviour in CdSe quantum dots

    International Nuclear Information System (INIS)

    Adjustment of the nanostructure and carrier behaviour of CdSe quantum dots (QDs) by varying the ligands used during QD synthesis enables the design of specific quantum devices via a self-assembly process of the QD core–shell structure without additional technologies. Surface photovoltaic (SPV) technology supplemented by X-ray diffractometry and infrared absorption spectroscopy were used to probe the characteristics of these QDs. Our study reveals that while CdSe QDs synthesized in the presence of and capped by thioglycolic acid, 3-mercaptopropionic acid, mercaptoethanol or α-thioglycerol ligands display zinc blende nanocrystalline structures, CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures, because different end groups in these ligands induce distinctive nucleation and growth mechanisms. Carboxyl end groups in the ligand served to increase the SPV response of the QDs, when illuminated by hν ≥ Eg,nano-CdSe. Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit photo-generated free charge carrier (FCC) transfer transitions of CdSe QDs illuminated by photon energy of 4.13 to 2.14 eV. The terminal hydroxyl group might better accommodate energy released in the non-radiative de-excitation process of photo-generated FCCs in the ligand's lowest unoccupied molecular orbital in the 300–580 nm wavelength region, when compared with other ligand end groups. - Highlights: • CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures. • Carboxyl end groups in the ligand serve to increase the SPV response of CdSe QDs. • Terminal hydroxyl group in the ligand might accommodate non-radiative de-excitation process in CdSe QDs. • Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit carriers transport of CdSe QDs

  12. Effect of ligand self-assembly on nanostructure and carrier transport behaviour in CdSe quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Li, Kuiying, E-mail: kuiyingli@ysu.edu.cn; Xue, Zhenjie

    2014-11-14

    Adjustment of the nanostructure and carrier behaviour of CdSe quantum dots (QDs) by varying the ligands used during QD synthesis enables the design of specific quantum devices via a self-assembly process of the QD core–shell structure without additional technologies. Surface photovoltaic (SPV) technology supplemented by X-ray diffractometry and infrared absorption spectroscopy were used to probe the characteristics of these QDs. Our study reveals that while CdSe QDs synthesized in the presence of and capped by thioglycolic acid, 3-mercaptopropionic acid, mercaptoethanol or α-thioglycerol ligands display zinc blende nanocrystalline structures, CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures, because different end groups in these ligands induce distinctive nucleation and growth mechanisms. Carboxyl end groups in the ligand served to increase the SPV response of the QDs, when illuminated by hν ≥ E{sub g,nano-CdSe}. Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit photo-generated free charge carrier (FCC) transfer transitions of CdSe QDs illuminated by photon energy of 4.13 to 2.14 eV. The terminal hydroxyl group might better accommodate energy released in the non-radiative de-excitation process of photo-generated FCCs in the ligand's lowest unoccupied molecular orbital in the 300–580 nm wavelength region, when compared with other ligand end groups. - Highlights: • CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures. • Carboxyl end groups in the ligand serve to increase the SPV response of CdSe QDs. • Terminal hydroxyl group in the ligand might accommodate non-radiative de-excitation process in CdSe QDs. • Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit carriers transport of CdSe QDs.

  13. Surface plasmon enhanced quantum transport in a hybrid metal nanoparticle array

    Science.gov (United States)

    Sun, Lin; Nan, Yali; Xu, Shang; Zhang, Sishi; Han, Min

    2014-07-01

    Hybrid Pd-Ag nanoparticle arrays composed of randomly distributed Pd nanoparticles in dense packing and a small number of dispersed Ag nanoparticles were fabricated with controlled coverage. Photo-enhanced conductance was observed in the nanoparticle arrays. Largest enhancement, which can be higher than 20 folds, was obtained with 450 nm light illumination. This wavelength was found to correlate with the surface plasmon resonance of the Ag nanoparticles. Electron transport measurements showed there were significant Coulomb blockade in the nanoparticle arrays and the blockade could be overcome with the surface plasmon enhanced local field of Ag nanoparticles induced by light illumination.

  14. Low-temperature quantum transport in CVD-grown single crystal graphene

    OpenAIRE

    Xiang, Shaohua; Miseikis, Vaidotas; Planat, Luca; Guiducci, Stefano; Roddaro, Stefano; Coletti, Camilla; Beltram, Fabio; Heun, Stefan

    2016-01-01

    Chemical vapor deposition (CVD) has been proposed for large-scale graphene synthesis for practical applications. However, the inferior electronic properties of CVD graphene are one of the key problems to be solved. In this study, we present a detailed study on the electronic properties of high-quality single crystal monolayer graphene. The graphene is grown by CVD on copper using a cold-wall reactor and then transferred to Si/SiO2. Our low-temperature magneto-transport data demonstrate that t...

  15. Quantum transport of Dirac fermions in graphene with a spatially varying Rashba spin-orbit coupling

    Science.gov (United States)

    Razzaghi, Leila; Hosseini, Mir Vahid

    2015-08-01

    We theoretically study electronic transport through a region with inhomogeneous Rashba spin-orbit (RSO) coupling placed between two normal regions in a monolayer graphene. The inhomogeneous RSO region is characterized by linearly varying RSO strength within its borders and constant RSO strength in the central region. We calculate the transmission properties within the transfer matrix approach. It is shown that the amplitude of conductance oscillations reduces and at the same time the magnitude of conductance increases with increasing border thickness. We also investigate how the Fano factor can be modified by the border thickness of RSO region.

  16. Quantum Chemical Investigations on Electron Transport Characteristics of Porphyrin and Metal-porphyrin

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    Recently, molecular electronics has become increasingly important. By applying the hybrid density functional theory coupled with the Green's function method, the current-voltage characteristics of the molecular junctions composed of gold-porphyrin-gold and gold-copper porphyrin-gold were investigated. The role of the metal coordination effect in organic molecular electron transport was highlighted. Although the thresholds of the bias voltage for both molecules were almost the same, approximately 0.9 V, the metal compound showed a larger increase in current because of the metal-coordination-enhanced molecule-electrode coupling in the frontier molecular orbitals.

  17. Quantum molecular dynamics simulations of transport properties in liquid and dense-plasma plutonium.

    Science.gov (United States)

    Kress, J D; Cohen, James S; Kilcrease, D P; Horner, D A; Collins, L A

    2011-02-01

    We have calculated the viscosity and self-diffusion coefficients of plutonium in the liquid phase using quantum molecular dynamics (QMD) and in the dense-plasma phase using orbital-free molecular dynamics (OFMD), as well as in the intermediate warm dense matter regime with both methods. Our liquid metal results for viscosity are about 40% lower than measured experimentally, whereas a previous calculation using an empirical interatomic potential (modified embedded-atom method) obtained results 3-4 times larger than the experiment. The QMD and OFMD results agree well at the intermediate temperatures. The calculations in the dense-plasma regime for temperatures from 50 to 5000 eV and densities about 1-5 times ambient are compared with the one-component plasma (OCP) model, using effective charges given by the average-atom code INFERNO. The INFERNO-OCP model results agree with the OFMD to within about a factor of 2, except for the viscosity at temperatures less than about 100 eV, where the disagreement is greater. A Stokes-Einstein relationship of the viscosities and diffusion coefficients is found to hold fairly well separately in both the liquid and dense-plasma regimes.

  18. Blocking transport resonances via Kondo many-body entanglement in quantum dots.

    Science.gov (United States)

    Niklas, Michael; Smirnov, Sergey; Mantelli, Davide; Margańska, Magdalena; Nguyen, Ngoc-Viet; Wernsdorfer, Wolfgang; Cleuziou, Jean-Pierre; Grifoni, Milena

    2016-01-01

    Many-body entanglement is at the heart of the Kondo effect, which has its hallmark in quantum dots as a zero-bias conductance peak at low temperatures. It signals the emergence of a conducting singlet state formed by a localized dot degree of freedom and conduction electrons. Carbon nanotubes offer the possibility to study the emergence of the Kondo entanglement by tuning many-body correlations with a gate voltage. Here we show another side of Kondo correlations, which counterintuitively tend to block conduction channels: inelastic co-tunnelling lines in the magnetospectrum of a carbon nanotube strikingly disappear when tuning the gate voltage. Considering the global SU(2) ⊗ SU(2) symmetry of a nanotube coupled to leads, we find that only resonances involving flips of the Kramers pseudospins, associated to this symmetry, are observed at temperatures and voltages below the corresponding Kondo scale. Our results demonstrate the robust formation of entangled many-body states with no net pseudospin. PMID:27526870

  19. Linear-scaling source-sink algorithm for simulating time-resolved quantum transport and superconductivity

    Science.gov (United States)

    Weston, Joseph; Waintal, Xavier

    2016-04-01

    We report on a "source-sink" algorithm which allows one to calculate time-resolved physical quantities from a general nanoelectronic quantum system (described by an arbitrary time-dependent quadratic Hamiltonian) connected to infinite electrodes. Although mathematically equivalent to the nonequilibrium Green's function formalism, the approach is based on the scattering wave functions of the system. It amounts to solving a set of generalized Schrödinger equations that include an additional "source" term (coming from the time-dependent perturbation) and an absorbing "sink" term (the electrodes). The algorithm execution time scales linearly with both system size and simulation time, allowing one to simulate large systems (currently around 106 degrees of freedom) and/or large times (currently around 105 times the smallest time scale of the system). As an application we calculate the current-voltage characteristics of a Josephson junction for both short and long junctions, and recover the multiple Andreev reflection physics. We also discuss two intrinsically time-dependent situations: the relaxation time of a Josephson junction after a quench of the voltage bias, and the propagation of voltage pulses through a Josephson junction. In the case of a ballistic, long Josephson junction, we predict that a fast voltage pulse creates an oscillatory current whose frequency is controlled by the Thouless energy of the normal part. A similar effect is found for short junctions; a voltage pulse produces an oscillating current which, in the absence of electromagnetic environment, does not relax.

  20. Equation of state and transport properties of warm dense helium via quantum molecular dynamics simulations

    Science.gov (United States)

    Li, Zhi-Guo; Cheng, Yan; Chen, Qi-Feng; Chen, Xiang-Rong

    2016-05-01

    The equation of state, self-diffusion, and viscosity coefficients of helium have been investigated by quantum molecular dynamics (QMD) simulations in the warm dense matter regime. Our simulations are validated through the comparison with the reliable experimental data. The calculated principal and reshock Hugoniots of liquid helium are in good agreement with the gas-gun data. On this basis, we revisit the issue for helium, i.e., the possibility of the instabilities predicted by chemical models at around 2000 GPa and 10 g/cm3 along the pressure isotherms of 6309, 15 849, and 31 623 K. Our calculations show no indications of instability in this pressure-temperature region, which reconfirm the predictions of previous QMD simulations. The self-diffusion and viscosity coefficients of warm dense helium have been systematically investigated by the QMD simulations. We carefully test the finite-size effects and convergences of statistics, and obtain numerically converged self-diffusion and viscosity coefficients by using the Kubo-Green formulas. The present results have been used to evaluate the existing one component plasma models. Finally, the validation of the Stokes-Einstein relationship for helium in the warm dense regime is discussed.

  1. Transport and Charge Manipulation in a Single Electron Silicon Double Quantum Dot

    Science.gov (United States)

    Wang, K.; Payette, C.; Dovzhenko, Y.; Petta, J. R.

    2013-03-01

    Silicon is one of the most promising candidates for ultra-coherent qubits due to its relatively early position in periodical table and the absence of nuclear spin in its naturally abundant isotope. Here we demonstrate a reliable recipe that enables us to reproducibly fabricate an accumulation mode few electron double quantum dot (DQD). We demonstrate tunable interdot tunnel coupling at single electron occupancy in the device. The charge state of the qubit is monitored by measuring the amplitude of the radio frequency signal that is reflected from a resonant circuit coupled to a charge sensor. By applying microwave radiation to the depletion gates, we probe the energy level structure of the DQD using photon assisted tunneling (PAT). We apply bursts of microwave radiation and monitor the dependence of the PAT peak height on the burst period to extract the charge relaxation time, T1. By experimentally tuning the charge qubit Hamiltonian, we measure the tunnel coupling and detuning dependence of T1. Supported by the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the U.S. Government.

  2. Blocking transport resonances via Kondo many-body entanglement in quantum dots

    Science.gov (United States)

    Niklas, Michael; Smirnov, Sergey; Mantelli, Davide; Margańska, Magdalena; Nguyen, Ngoc-Viet; Wernsdorfer, Wolfgang; Cleuziou, Jean-Pierre; Grifoni, Milena

    2016-01-01

    Many-body entanglement is at the heart of the Kondo effect, which has its hallmark in quantum dots as a zero-bias conductance peak at low temperatures. It signals the emergence of a conducting singlet state formed by a localized dot degree of freedom and conduction electrons. Carbon nanotubes offer the possibility to study the emergence of the Kondo entanglement by tuning many-body correlations with a gate voltage. Here we show another side of Kondo correlations, which counterintuitively tend to block conduction channels: inelastic co-tunnelling lines in the magnetospectrum of a carbon nanotube strikingly disappear when tuning the gate voltage. Considering the global SU(2) ⊗ SU(2) symmetry of a nanotube coupled to leads, we find that only resonances involving flips of the Kramers pseudospins, associated to this symmetry, are observed at temperatures and voltages below the corresponding Kondo scale. Our results demonstrate the robust formation of entangled many-body states with no net pseudospin. PMID:27526870

  3. Electrical transport through a quantum dot side-coupled to a topological superconductor.

    Science.gov (United States)

    Lee, Yu-Li

    2014-11-12

    We propose to measure the differential conductance G as a function of the bias V for a quantum dot side-coupled to a topological superconductor to detect the existence of the chiral Majorana edge states. It turns out that G for the spinless dot is an oscillatory (but not periodic) function of eV due to the coupling to the chiral Majorana edge states, where -e is the charge carried by the electron. The behaviour of G versus eV is distinguished from that of a multi-level dot in three respects. First of all, due to the coupling to the topological superconductor, the value of G will shift upon adding or removing a vortex in the topological superconductor. Next, for an off-resonance dot, the conductance peak in the present case takes a universal value e(2)/(2h) when the two leads are symmetrically coupled to the dot. Finally, for a symmetric setup and an on-resonance dot, the conductance peak will approach the same universal value e(2)/(2h) at a large bias.

  4. Effects of the thickness of NiO hole transport layer on the performance of all-inorganic quantum dot light emitting diode

    International Nuclear Information System (INIS)

    All-inorganic quantum dot light emitting diodes (QLEDs) have recently gained great attention owing to their high stability under oxygenic, humid environment and higher operating currents. In this work, we fabricated all-inorganic CdSe/ZnS core-shell QLEDs composed of ITO/NiO/QDs/ZnO/Al, in which NiO and ZnO thin film deposited via all-solution method were employed as hole and electron transport layer, respectively. To achieve high light emitting efficiency, the balance transport between electrons and holes play a key role. In this work, the effects of the thickness of NiO film on the performance of QLEDs were explored experimentally in details. NiO layers with various thicknesses were prepared with different rotation speeds. Experimental results showed that thinner NiO layer deposited at higher rotation speed had higher transmittance and larger band gap. Four typical NiO thickness based QLEDs were fabricated to optimize the hole transport layer. Thinner NiO layer based device performs bright emission with high current injection, which is ascribed to the reduced barrier height between hole transport layer and quantum dot. - Highlights: • All-inorganic quantum dot light emitting diodes (QLEDs) were fabricated. • Thinner NiO film can effectively enhance on–off properties of devices. • Improved performance of QLEDs is mainly attributed to energy barrier reduction

  5. Surface plasmon enhanced quantum transport in a hybrid metal nanoparticle array

    Energy Technology Data Exchange (ETDEWEB)

    Sun, Lin; Nan, Yali; Xu, Shang; Zhang, Sishi; Han, Min, E-mail: sjhanmin@nju.edu.cn

    2014-07-18

    Hybrid Pd–Ag nanoparticle arrays composed of randomly distributed Pd nanoparticles in dense packing and a small number of dispersed Ag nanoparticles were fabricated with controlled coverage. Photo-enhanced conductance was observed in the nanoparticle arrays. Largest enhancement, which can be higher than 20 folds, was obtained with 450 nm light illumination. This wavelength was found to correlate with the surface plasmon resonance of the Ag nanoparticles. Electron transport measurements showed there were significant Coulomb blockade in the nanoparticle arrays and the blockade could be overcome with the surface plasmon enhanced local field of Ag nanoparticles induced by light illumination. - Highlights: • We study photo-enhanced electron conductance of a hybrid Pd–Ag nanoparticle array. • The light-induced conductance enhancement is as high as 20 folds at 10 K. • The enhancement is correlate with the surface plasmon resonance of Ag nanoparticles. • Coulomb blockades is overcome with the surface plasmon enhanced local field.

  6. Full-Wave Techniques for the Analysis of Electrodynamics and Coherent Quantum Transport in Graphene Nanodevices.

    Directory of Open Access Journals (Sweden)

    Luca Pierantoni

    2012-11-01

    Full Text Available We report on full-wave techniques in the frequency (energy-domain and the time-domain, aimed at the investigation of the combined electromagnetic-coherent transport problem in carbon based nanostructured materials and devices viz. graphene nanoribbons. The frequency-domain approach is introduced in order to describe a Poisson-Schrödinger / Dirac system in a quasi static framework. Thetime-domain approach deals with the full-wave solution of the combined Maxwell-Schrödinger / Dirac system of equations. From the above theoretical platforms, home-made solvers are provided, aimed atdealing with challenging problems in realistic devices / systems environments, typical of the area of radio-frequency nanoelectronics.

  7. ACS: ALMA Common Software

    Science.gov (United States)

    Chiozzi, Gianluca; Šekoranja, Matej

    2013-02-01

    ALMA Common Software (ACS) provides a software infrastructure common to all ALMA partners and consists of a documented collection of common patterns and components which implement those patterns. The heart of ACS is based on a distributed Component-Container model, with ACS Components implemented as CORBA objects in any of the supported programming languages. ACS provides common CORBA-based services such as logging, error and alarm management, configuration database and lifecycle management. Although designed for ALMA, ACS can and is being used in other control systems and distributed software projects, since it implements proven design patterns using state of the art, reliable technology. It also allows, through the use of well-known standard constructs and components, that other team members whom are not authors of ACS easily understand the architecture of software modules, making maintenance affordable even on a very large project.

  8. A predictive model of the temperature dependence of AC transport losses in (Bi, Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes

    Energy Technology Data Exchange (ETDEWEB)

    Zhang Guomin; Lin Liangzhen; Xiao Liye; Yu Yunjia [Key Laboratory of Applied Superconductivity, Chinese Academy of Sciences, Beijing 100190 (China); Schwartz, Justin [Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 (United States); Pamidi, Sastry V [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States)

    2011-08-15

    Critical currents and AC losses of (Bi, Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} superconducting tapes were measured in self-field as a function of temperature. The experimental data of the temperature dependence of critical current were compared with calculated results. An approach to calculating AC losses as a function of temperature was developed and the calculated AC losses were compared with the measured data. The study shows that AC losses at any temperature can be estimated using the model from the critical parameters or from the measured AC loss factor at a certain temperature, such as 77 K.

  9. Analysis of AC loss in superconducting power devices calculated from short sample data

    OpenAIRE

    Rabbers, J.J.; Haken, ten, Bennie; Kate, ten, F.J.W.

    2003-01-01

    A method to calculate the AC loss of superconducting power devices from the measured AC loss of a short sample is developed. In coils and cables the magnetic field varies spatially. The position dependent field vector is calculated assuming a homogeneous current distribution. From this field profile and the transport current, the local AC loss is calculated. Integration over the conductor length yields the AC loss of the device. The total AC loss of the device is split up in different compone...

  10. All-optical production and transport of a large 6Li quantum gas in a crossed optical dipole trap

    Science.gov (United States)

    Gross, Ch.; Gan, H. C. J.; Dieckmann, K.

    2016-05-01

    We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line 2 S1 /2→3 P3 /2 laser cooling resulting in a high phase-space density of up to 3 ×10-4 . This allows utilizing a large-volume crossed optical dipole trap with a total power of 45 W , leading to high loading efficiency and 8 ×106 trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25 cm in 0.9 s , and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with 1.7 ×106 atoms at a temperature of 60 nK , corresponding to T /TF=0.16 (2 ) . Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of 3 ×105 lithium dimers.

  11. All-optical production and transport of a large $^6$Li quantum gas in a crossed optical dipole trap

    CERN Document Server

    Gross, Ch; Dieckmann, K

    2016-01-01

    We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line $ 2S_{1/2}\\rightarrow 3P_{3/2} $ laser cooling resulting in a high phase-space density of up to $3\\times10^{-4}$. This allows utilizing a large volume crossed optical dipole trap with a total power of $45\\,\\textrm{W}$, leading to high loading efficiency and $8\\times10^6$ trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of $25\\,\\textrm{cm}$ in $0.9\\,\\textrm{s}$, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with $1.7\\times 10^{6}$ atoms at a temperature of $60 \\, \\textrm{nK}$, corresponding to $T/T_{\\text{F}}=0.16\\left(2 \\right)$. Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of $3\\times10^5$ lithium dimers.

  12. An electrochemical one-step system for assaying methyltransferase activity based on transport of a quantum dot signaling tracer.

    Science.gov (United States)

    Baek, Songyi; Won, Byoung Yeon; Park, Ki Soo; Park, Hyun Gyu

    2013-11-15

    A one-step, electrochemical method for assaying methyltransferase (MTase) activity, based on the convective transport of a quantum dot (QD) signaling tracer, has been developed. The assay chip used in this system was prepared by modifying a gold matrix with CdSe/ZnS QD-tagged dsDNA, which contains a specific methylation site (5'-GATC-3') recognized by MTase. Treatment of the chip with DNA adenine methylation (Dam) MTase, generates a methylated sequence (5'-GAmTC-3') within the dsDNA. The methylated dsDNA is then subjected to a cleavage reaction, induced by DpnI, which leads to release from the gold matrix of a DNA fragment tethered to a QD. Detection of the released QD, using square wave anodic stripping voltammetry (SWASV) on a glassy carbon (GC) electrode, enables the reliable quantitation of the methylated DNA. Because it is accomplished in a simple and convenient one step and does not require any complicated secondary or tedious washing steps, the new assay method holds great promise for epigenetic analysis in facility-limited environments or point-of-care testing (POCT) applications. PMID:23777705

  13. Trap-Assisted Transport and Non-Uniform Charge Distribution in Sulfur-Rich PbS Colloidal Quantum Dot-based Solar Cells with Selective Contacts.

    Science.gov (United States)

    Malgras, Victor; Zhang, Guanran; Nattestad, Andrew; Clarke, Tracey M; Mozer, Attila J; Yamauchi, Yusuke; Kim, Jung Ho

    2015-12-01

    This study reports evidence of dispersive transport in planar PbS colloidal quantum dot heterojunction-based devices as well as the effect of incorporating a MoO3 hole selective layer on the charge extraction behavior. Steady state and transient characterization techniques are employed to determine the complex recombination processes involved in such devices. The addition of a selective contact drastically improves the device efficiency up to 3.15% (especially due to increased photocurrent and decreased series resistance) and extends the overall charge lifetime by suppressing the main first-order recombination pathway observed in device without MoO3. The lifetime and mobility calculated for our sulfur-rich PbS-based devices are similar to previously reported values in lead-rich quantum dots-based solar cells. Nevertheless, strong Shockley-Read-Hall mechanisms appear to keep restricting charge transport, as the equilibrium voltage takes more than 1 ms to be established. PMID:26541422

  14. Shape, orientation and magnitude of the curl quantum flux, the coherence and the statistical correlations in energy transport at nonequilibrium steady state

    CERN Document Server

    Zhang, Zhedong

    2015-01-01

    We provide a quantitative description of the nonequilibriumness based on the model of coupled oscillators interacting with multiple energy sources. This can be applied to the study of vibrational energy transport in molecules. The curl quantum flux quantifying the nonequilibriumness and time-irreversibility is quantified in the coherent representation and we find the geometric description of the shape and polarization of the flux which provides the connection between the microscopic description of quantum nonequilibriumness and the macroscopic observables, i.e., correlation function. We use the Wilson loop integral to quantify the magnitude of curl flux, which is shown to be correlated to the correlation function as well. Coherence contribution is explicitly demonstrated to be non-trivial and to considerably promote the heat transport quantified by heat current and efficiency. This comes from the fact that coherence effect is microscopically reflected by the geometric description of the flux. To uncover the e...

  15. Quantum and Transport Mobilities of a Two-Dimensional Electron Gas in the Presence of the Rashba Spin-Orbit Interaction

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    A systematic theoretical approach is developed to study the electronic and transport properties of a twodimensional electron gas (2DEG) in the presence of spin-orbit interactions induced by the Rashba effect. The standard random-phase approximation is employed to calculate the screening length caused by electron-electron interaction in different transition channels. The quantum and transport mobilities in different spin branches are evaluated using the momentum-balance equation derived from the Boltzmann equation,in which the electron interactions with both the remote and background impurities are taken into account in an InAlAs/InGaAs heterojunction at low-temperatures.

  16. Inter-island transport in CdSe/ZnSe quantum heterostructures

    CERN Document Server

    Wachter, S; Baldauf, M; Schmidt, M; Kurtz, E; Klingshirn, C F; Kalt, H; Litvinov, D A; Gerthsen, D

    2001-01-01

    We report on localization dynamics of excitons in ensembles of self- organized CdSe islands embedded in ZnSe. The experimental methods employed are temperature-dependent, spatially resolved photoluminescence ( mu -PL), spatially integrated PL (macro-PL), as well as time-resolved PL (TRPL). We observe the well known non- monotonous shift of the PL maximum with temperature caused by redistribution of the excitons amongst the islands. The measured shift is compared with the exact shift of the bandgap deduced from mu -PL measurements and found to depend strongly on island size and distribution. These transport processes are recovered in the temporal evolution of the PL. The decaytime of the spectrally integrated PL reaches its maximum at exactly the same temperature at which the redshift of the macro-PL turns into a blueshift. In TRPL the PL- spectrum consists of two contributions. We put the emission on the high energy side down to excited states in the islands. (17 refs).

  17. Effect of potential barrier height on the carrier transport in InGaAs/GaAsP multi-quantum wells and photoelectric properties of laser diode.

    Science.gov (United States)

    Dong, Hailiang; Sun, Jing; Ma, Shufang; Liang, Jian; Lu, Taiping; Jia, Zhigang; Liu, Xuguang; Xu, Bingshe

    2016-03-01

    The growth and strain-compensation behaviour of InGaAs/GaAsP multi-quantum wells, which were fabricated by metal-organic chemical vapor deposition, have been studied towards the application of these quantum wells in high-power laser diodes. The effect of the height of the potential barrier on the confined level of carrier transport was studied by incorporating different levels of phosphorus content into the GaAsP barrier. The crystal quality and interface roughness of the InGaAs/GaAsP multi-quantum wells with different phosphorus contents were evaluated by high resolution X-ray diffraction and in situ optical surface reflectivity measurements during the growth. The surface morphology and roughness were characterized by atomic force microscopy, which indicates the variation law of surface roughness, terrace width and uniformity with increasing phosphorus content, owing to strain accumulation. Moreover, the defect generation and structural disorder of the multi-quantum wells were investigated by Raman spectroscopy. The optical properties of the multi-quantum wells were characterized by photoluminescence, which shows that the spectral intensity increases as the phosphorus content increases. The results suggest that more electrons are well bound in InGaAs because of the high potential barrier. Finally, the mechanism of the effect of the height of the potential barrier on laser performance was proposed on the basis of simulation calculations and experimental results.

  18. Non-equilibrium quantum theory for nanodevices based on the Feynman-Vernon influence functional

    Science.gov (United States)

    Jin, Jinshuang; Wei-Yuan Tu, Matisse; Zhang, Wei-Min; Yan, YiJing

    2010-08-01

    In this paper, we present a non-equilibrium quantum theory for transient electron dynamics in nanodevices based on the Feynman-Vernon influence functional. Applying the exact master equation for nanodevices we recently developed to the more general case in which all the constituents of a device vary in time in response to time-dependent external voltages, we obtained non-perturbatively the transient quantum transport theory in terms of the reduced density matrix. The theory enables us to study transient quantum transport in nanostructures with back-reaction effects from the contacts, with non-Markovian dissipation and decoherence being fully taken into account. For a simple illustration, we apply the theory to a single-electron transistor subjected to ac bias voltages. The non-Markovian memory structure and the nonlinear response functions describing transient electron transport are obtained.

  19. Electronic linear AC transport in normal-metal contacted graphene nano-system%金属电极石墨烯纳米系统中的电子线性交流输运

    Institute of Scientific and Technical Information of China (English)

    叶恩钾; 韩裕

    2015-01-01

    利用紧束缚近似模型、递归格林函数方法和交流输运理论,研究金属电极石墨烯纳米系统中的电子交流输运性质。研究结果表明,金属电极石墨烯纳米系统中的界面散射导致总体的直流电导变小。在狄拉克点附近,中心锯齿型石墨烯条带部分的长度变化造成系统的共振和反共振效应,并对外加电压表现出类似电感(inductive-like)和电容(capacitive)的响应。电子局域态密度的分布表明,共振态电子在石墨烯纳米条带中呈边缘态分布,有利于系统的电子传导。反共振态电子的局域态密度小,系统不存在边缘态,从而抑制电子传导。%The properties of electronic AC transport of normal-metal contacted grpahene nano-system were investigated by employing the tight-binding approximation, Green’s function method and AC transport theory. The results show that the DC conductances are suppressed due to the interface scattering in the normal-metal contacted graphene nano-system. The variation of the length of center graphene nanoribbon gives rise to the resonance and anti-resonance effect near the Dirac point, and induces inductive-like and capacitive responses to applied voltage. According to the results of distribution of LDOS, the resonant electrons locate at the edge of the center graphene structure, which is benefit to the conductance. While there is no edge effect of the anti-resonant electrons, with a small value of LDOS and a suppression of conductance.

  20. Temperature dependence of critical currents and ac transport losses in (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} and YBa{sub 2}Cu{sub 3}O{sub y} tapes

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Guo Min [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States); Knoll, D C [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States); Nguyen, D N [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States); Sastry, P V P S S [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States); Schwartz, Justin [Center for Advanced Power Systems, Florida State University, Tallahassee, FL 32310 (United States)

    2007-06-15

    The critical currents and self-field ac losses of YBa{sub 2}Cu{sub 3}O{sub y} coated conductors and (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes were measured at several temperatures between 45 K and the respective critical temperature. The temperature dependence of ac losses was measured at 50 Hz using the lock-in method for a transport current. The frequency dependence of ac loss was measured at 55 K for transport current frequencies from 25 to 400 Hz. The results show that variation of ac transport loss as a function of normalized critical current is nearly the same at all temperatures in the measured temperature range for both YBa{sub 2}Cu{sub 3}O{sub y} and (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes. The temperature dependence of the loss factor, however, is different for (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} and YBa{sub 2}Cu{sub 3}O{sub y} tapes because of the ferromagnetic loss in the NiW-based coated conductor substrate. Similarities and differences in the temperature and frequency dependence of ac transport losses between (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} and YBa{sub 2}Cu{sub 3}O{sub y} tapes are discussed.

  1. AC power supply systems

    International Nuclear Information System (INIS)

    An ac power supply system includes a rectifier fed by a normal ac supply, and an inverter connected to the rectifier by a dc link, the inverter being effective to invert the dc output of the receiver at a required frequency to provide an ac output. A dc backup power supply of lower voltage than the normal dc output of the rectifier is connected across the dc link such that the ac output of the rectifier is derived from the backup supply if the voltage of the output of the inverter falls below that of the backup supply. The dc backup power may be derived from a backup ac supply. Use in pumping coolant in nuclear reactor is envisaged. (author)

  2. Coherent electron transport in one-dimensional GaAs/AlGaAs nanostructures and quantum rings; Kohaerenter Elektronentransport in eindimensionalen GaAs/AlGaAs-Nanostrukturen und Quantenringen

    Energy Technology Data Exchange (ETDEWEB)

    Buchholz, Sven Sebastian

    2011-07-01

    Phase-coherent charge-carrier transport uses the quantum-mechanical wave character of the charge carriers and is present in structures, the dimensions of which lie below the mean free path of the charge carriers. In transport experiments quantization effects and electron interference can become observable. Object of this thesis is the study of quantum phenomena in complex one-dimensional (1D) GaAs/AlGaAs structure. By electron-beam lithography 1D electron systems were prepared in form of both short bottle-necks (quantum-dot contacts) and several micrometer long electronic waveguides as well as quantum rings. It is shown that the transport in such structures is carried by energetically quantized 1D modes. By the integration of quantum-dot contacts as 1D-mode filters it has been succeeded to realize in complex multimode-waveguide structures transport on the lowest 1D mode. Electron interference in 1D quantum rings is analyzed via the Aharonov-Bohm effect. By an asymmetric four-point geometry it has been succeeded to remove the phase stiffness, which opens the possibility for phase-sensitive studies. By means of the electrostatic AB effect the electron-transmission phase was controlled and continuously detected. Furthermore quantum rings were used, in order to study decoherence in 1D electron systems by means of different parameters.

  3. Quantum inductance and high frequency oscillators in graphene nanoribbons.

    Science.gov (United States)

    Begliarbekov, Milan; Strauf, Stefan; Search, Christopher P

    2011-04-22

    Here we investigate high frequency AC transport through narrow graphene nanoribbons with top-gate potentials that form a localized quantum dot. We show that as a consequence of the finite dwell time of an electron inside the quantum dot (QD), the QD behaves like a classical inductor at sufficiently high frequencies ω ≥ GHz. When the geometric capacitance of the top-gate and the quantum capacitance of the nanoribbon are accounted for, the admittance of the device behaves like a classical serial RLC circuit with resonant frequencies ω ∼ 100-900 GHz and Q-factors greater than 10(6). These results indicate that graphene nanoribbons can serve as all-electronic ultra-high frequency oscillators and filters, thereby extending the reach of high frequency electronics into new domains.

  4. AC1 Wing

    Directory of Open Access Journals (Sweden)

    Adrian DOBRE

    2010-03-01

    Full Text Available The AC1 wing replaces the old wing of the wind tunnel model AEROTAXI, which has been made at scale 1:9. The new wing is part of CESAR program and improves the aerodynamic characteristics of the old one. The geometry of the whole wing was given by FOI Sweden and position of AC1 wing must coincide with the structure of the AEROTAXI model.

  5. AC1 Wing

    OpenAIRE

    Adrian DOBRE

    2010-01-01

    The AC1 wing replaces the old wing of the wind tunnel model AEROTAXI, which has been made at scale 1:9. The new wing is part of CESAR program and improves the aerodynamic characteristics of the old one. The geometry of the whole wing was given by FOI Sweden and position of AC1 wing must coincide with the structure of the AEROTAXI model.

  6. Transport in quantum wires

    Indian Academy of Sciences (India)

    Siddhartha Lal; Sumathi Rao; Diptiman Sen

    2002-02-01

    With a brief introduction to one-dimensional channels and conductance quantization in mesoscopic systems, we discuss some recent experimental puzzles in these systems, which include reduction of quantized conductances and an interesting odd–even effect in the presence of an in-plane magnetic field. We then discuss a recent non-homogeneous Luttinger liquid model proposed by us, which addresses and gives an explanation for the reduced conductances and the odd–even effect. We end with a brief summary and discussion of future projects.

  7. Vibrational Quantum Transport

    DEFF Research Database (Denmark)

    Engelund, Mads

    Denne afhandling beskriver teoretisk og numerisk arbejde, som undersøger indflydelsen af vibrationer på varmetransport og strøm i realistiske nanoskala systemer. Den vigtigste og mest tidskrævende del af dette arbejde har været at designe og implementere en praktisk metode til at beregne vibratio...

  8. Electron Interactions & Quantum Transport

    DEFF Research Database (Denmark)

    Rostgaard, Carsten

    middelfelt. For systemer karakteriseret ved høj elatisk transmission er dette ofte tilstrækkeligt til at opnå overensstemmelse mellem beregninger og eksperimenter. Mere generelt, og især for den teknologisk vigtige klasse af svagt bundne organiske molekyler, har der imidlertid vist sig en række problemer med...

  9. Vibrational Quantum Transport

    OpenAIRE

    Engelund, Mads; Jauho, Antti-Pekka; Brandbyge, Mads

    2010-01-01

    Denne afhandling beskriver teoretisk og numerisk arbejde, som undersøger indflydelsen af vibrationer på varmetransport og strøm i realistiske nanoskala systemer. Den vigtigste og mest tidskrævende del af dette arbejde har været at designe og implementere en praktisk metode til at beregne vibrationelle egenskaber for en nanostruktur koblet til makroskopiske elektroder, f. eks tilstandstætheden. Metoden indebærer at finde kraftkonstant-matricen for en nanostruktur og elektroder ved at udføre se...

  10. Quantum transport in graphene

    NARCIS (Netherlands)

    Oostinga, J.B.

    2010-01-01

    After the experimental discovery of graphene -a single atomic layer of graphite- a scientific rush started to explore graphene’s electronic behaviour. Graphene is a fascinating two-dimensional electronic system, because its electrons behave as relativistic particles. Moreover, it is a promising mate

  11. Application of solution-processed metal oxide layers as charge transport layers for CdSe/ZnS quantum-dot LEDs

    Science.gov (United States)

    Nguyen, Huu Tuan; Dinh Nguyen, Nang; Lee, Soonil

    2013-03-01

    We fabricated and characterized quantum-dot light emitting devices (QLEDs) that consisted of a CdSe/ZnS quantum-dot (QD) emitting layer, a hole-transporting nickel oxide (NiO) layer and/or an electron-transporting zinc oxide (ZnO) layer. Both the p-type NiO and n-type ZnO layers were formed by using sol-gel processes. All the fabricated CdSe/ZnS QLEDs showed similar electroluminescence spectra that originated from the green CdSe/ZnS QDs. However, different combinations of hole- and electron-transporting layers resulted in efficiency variations. In addition to the control of the respective concentrations of holes and electrons within a multilayer device structure, which determines the luminance and efficiency of QLEDs, the use of metal oxide layers is advantageous for long-term stability of QLEDs because they are air stable and can block the permeation of water vapor and oxygen in ambient air to a QD emitting layer. Moreover, the wet chemistry processing for their formation makes metal oxide layers attractive for low cost and/or large area manufacture of QLEDs.

  12. Isolation of sequences flanking Ac insertion sites by Ac casting.

    Science.gov (United States)

    Wang, Dafang; Peterson, Thomas

    2013-01-01

    Localizing Ac insertions is a fundamental task in studying Ac-induced mutation and chromosomal rearrangements involving Ac elements. Researchers may sometimes be faced with the situation in which the sequence flanking one side of an Ac/Ds element is known, but the other flank is unknown. Or, a researcher may have a small sequence surrounding the Ac/Ds insertion site and needs to obtain additional flanking genomic sequences. One way to rapidly clone unknown Ac/Ds flanking sequences is via a PCR-based method termed Ac casting. This approach utilizes the somatic transposition activity of Ac during plant development, and provides an efficient means for short-range genome walking. Here we describe the principle of Ac casting, and show how it can be applied to isolate Ac macrotransposon insertion sites.

  13. Electron transport in unipolar InGaN/GaN multiple quantum well structures grown by NH3 molecular beam epitaxy

    KAUST Repository

    Browne, David A.

    2015-05-14

    © 2015 AIP Publishing LLC. Unipolar-light emitting diode like structures were grown by NH3 molecular beam epitaxy on c plane (0001) GaN on sapphire templates. Studies were performed to experimentally examine the effect of random alloy fluctuations on electron transport through quantum well active regions. These unipolar structures served as a test vehicle to test our 2D model of the effect of compositional fluctuations on polarization-induced barriers. Variables that were systematically studied included varying quantum well number from 0 to 5, well thickness of 1.5 nm, 3 nm, and 4.5 nm, and well compositions of In0.14Ga0.86N and In0.19Ga0.81N. Diode-like current voltage behavior was clearly observed due to the polarization-induced conduction band barrier in the quantum well region. Increasing quantum well width and number were shown to have a significant impact on increasing the turn-on voltage of each device. Temperature dependent IV measurements clearly revealed the dominant effect of thermionic behavior for temperatures from room temperature and above. Atom probe tomography was used to directly analyze parameters of the alloy fluctuations in the quantum wells including amplitude and length scale of compositional variation. A drift diffusion Schrödinger Poisson method accounting for two dimensional indium fluctuations (both in the growth direction and within the wells) was used to correctly model the turn-on voltages of the devices as compared to traditional 1D simulation models.

  14. Long dephasing time and high temperature ballistic transport in an InGaAs open quantum dot

    OpenAIRE

    Hackens, B.; Faniel, S.; Delfosse, F.; Gustin, C.; Boutry, H.; Huynen, I.; Wallaert, X.; Bollaert, S.; Cappy, A.; Bayot, V.

    2002-01-01

    We report on measurements of the magnetoconductance of an open circular InGaAs quantum dot between 1.3K and 204K. We observe two types of magnetoconductance fluctuations: universal conductance fluctuations (UCFs), and 'focusing' fluctuations related to ballistic trajectories between openings. The electron phase coherence time extracted from UCFs amplitude is larger than in GaAs/AlGaAs quantum dots and follows a similar temperature dependence (between T^-1 and T^-2). Below 150K, the characteri...

  15. Quantum dissipative effects on non-equilibrium transport through a single-molecular transistor: The Anderson-Holstein-Caldeira-Leggett model

    Science.gov (United States)

    Raju, Ch. Narasimha; Chatterjee, Ashok

    2016-01-01

    The Anderson-Holstein model with Caldeira-Leggett coupling with environment is considered to describe the damping effect in a single molecular transistor (SMT) which comprises a molecular quantum dot (with electron-phonon interaction) mounted on a substrate (environment) and coupled to metallic electrodes. The electron-phonon interaction is first eliminated using the Lang-Firsov transformation and the spectral density function, charge current and differential conductance are then calculated using the non-equilibrium Keldysh Green function technique. The effects of damping rate, and electron-electron and electron-phonon interactions on the transport properties of SMT are studied at zero temperature.

  16. (4,4')-Bipyridine in Vacuo and in Solvents: A Quantum Chemical Study of a Prototypical Floppy Molecule From a Molecular Transport Perspective

    CERN Document Server

    Bâldea, Ioan; Wenzel, Wonfgang

    2015-01-01

    We report results of quantum chemical calculations for the neutral and anionic species of (4,4{'})-bipyridine (44BPY), a prototypical molecule with a floppy degree of freedom, placed in vacuo and in solvents. In addition to equilibrium geometries and vibrational frequencies and spectra, we present adiabatic energy curves for the vibrational modes with significant intramolecular reorganization upon charge transfer. Special attention is paid to the floppy strongly anharmonic degree of freedom of 44BPY, which is related to the most salient structural feature, namely the twist angle $\\theta$ between the two pyridine rings. The relevance of the present results for molecular transport will be emphasized. We show that the solvent acts as an effective gate electrode and propose a scissor operator to account for solvent effects on molecular transport. Our result on the conductance $G$ vs. $\\cos^2\\theta$ is consistent with a significant transmission in perpendicular conformation indicated by previous microscopic analys...

  17. Transport and Retention of CdSe/ZnS Quantum Dots in Saturated Sand: Effects of Organic Ligands, pH and Ionic Strength

    Science.gov (United States)

    Li, Chunyan; Snee, Preston; Darnault, Christophe

    2016-04-01

    The presence of nanomaterials in soil, water, and air systems following their life cycle or accidents and their effects on the environment and public health are inevitable. Ability to forecast the public health and ecological impacts of these nanomaterials encountered in the environment is limited. Therefore, it is critical to be able to predict the fate and transport on nanomaterials in the environment, in particular the subsurface, in order to conduct risk assessments. To assess the transport and retention of nanomaterials in the subsurface environment, we selected quantum dots (QDs). QDs are metal and semiconductor based nanomaterials that are essential to nanoscience and nanotechnology. Understanding the parameters that effect the transport and retention of QDs in the soil water environment is critical. Natural organic ligands are commonly found in soils and impact the soil physico-chemical processes through multifaceted reactions with metal ions present in soil solution and ligand exchange reactions on soil surfaces. Therefore, ligands may modify the surface properties of QDs and effect their stability, transport and retention in the subsurface environment. In this research, size, surface charge, and stability of CdSe/ZnS QDs in water solutions are monitored in batch experiments. The influence of organic ligands (acetate, oxalate, and citrate) on the stability of QDs at different pHs (1.5, 3.5, 5, 7 and 9) and ionic strengths (0.05 and 0.1 M) conditions were examined. The stability and aggregation phenomena of QDs were studied using UV-vis and DLS methods. Parameters from batch studies were selected to establish chemical conditions to be used in transport experiments to produce breakthrough curves and retention profiles in order to characterize the fate and transport of QDs in saturated sand. These transport experiments are essential to understand the mobility and retention processes in porous media where QD interactions with surfaces of heterogeneous

  18. AC-3 audio coder

    Science.gov (United States)

    Todd, Craig

    1995-12-01

    AC-3 is a system for coding up to 5.1 channels of audio into a low bit-rate data stream. High quality may be obtained with compression ratios approaching 12-1 for multichannel audio programs. The high compression ratio is achieved by methods which do not increase decoder memory, and thus cost. The methods employed include: the transmission of a high frequency resolution spectral envelope; and a novel forward/backward adaptive bit allocation algorithm. In order to satisfy practical requirements of an emissions coder, the AC-3 syntax includes a number of features useful to broadcasters and consumers. These features include: loudness uniformity between programs; dynamic range control; and broadcaster control of downmix coefficients. The AC-3 coder has been formally selected for inclusion of the U.S. HDTV broadcast standard, and has been informally selected for several additional applications.

  19. Low driving voltage in an organic light-emitting diode using MoO3/NPB multiple quantum well structure in a hole transport layer

    Institute of Scientific and Technical Information of China (English)

    Mu Xue; Wu Xiao-Ming; Hua Yu-Lin; Jiao Zhi-Qiang; Shen Li-Ying; Su Yue-Ju; Bai Juan-Juan

    2013-01-01

    The driving voltage of an organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO3)/N,N'-bis(naphthalene-l-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in the hole transport layer.For the device with double quantum well (DQW) structure of ITO/ [MoO3 (2.5 nm)/NPB (20 nm)]2/Alq3(50 nm)/LiF (0.8 nm)/Al (120 nm)],the turn-on voltage is reduced to 2.8 V,which is lowered by 0.4 V compared with that of the control device (without MQW structures),and the driving voltage is 5.6 V,which is reduced by l V compared with that of the control device at the 1000 cd/m2.In this work,the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure,which is attributed not only to the reduced energy barrier between ITO and NPB,but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.

  20. Low driving voltage in an organic light-emitting diode using MoO3/NPB multiple quantum well structure in a hole transport layer

    Science.gov (United States)

    Mu, Xue; Wu, Xiao-Ming; Hua, Yu-Lin; Jiao, Zhi-Qiang; Shen, Li-Ying; Su, Yue-Ju; Bai, Juan-Juan; Bi, Wen-Tao; Yin, Shou-Gen; Zheng, Jia-Jin

    2013-02-01

    The driving voltage of an organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO3)/N, N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in the hole transport layer. For the device with double quantum well (DQW) structure of ITO/ [MoO3 (2.5 nm)/NPB (20 nm)]2/Alq3(50 nm)/LiF (0.8 nm)/Al (120 nm)], the turn-on voltage is reduced to 2.8 V, which is lowered by 0.4 V compared with that of the control device (without MQW structures), and the driving voltage is 5.6 V, which is reduced by 1 V compared with that of the control device at the 1000 cd/m2. In this work, the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure, which is attributed not only to the reduced energy barrier between ITO and NPB, but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.