First-principles real-space tight-binding LMTO calculation of electronic structures for atomic clusters

International Nuclear Information System (INIS)

Xie, Z.L.; Dy, K.S.; Wu, S.Y.

1997-01-01

A real-space scheme has been developed for a first-principles calculation of electronic structures and total energies of atomic clusters. The scheme is based on the combination of the tight-binding linear muffin-tin orbital (TBLMTO) method and the method of real-space Green close-quote s function. With this approach, the local electronic density of states can be conveniently determined from the real-space Green close-quote s function. Furthermore, the full electron density of a cluster can be directly calculated in real space. The scheme has been shown to be very efficient due to the incorporation of the method of real-space Green close-quote s function and Delley close-quote s method of evaluating multicenter integrals. copyright 1996 The American Physical Society

Hofstadter's butterfly in a two-dimensional lattice consisting of two sublattices

Energy Technology Data Exchange (ETDEWEB)

Vugalter, G A; Pastukhov, A S [Department of Physics, Nizhny Novgorod State University, 23 Gagarin Avenue, Nizhny Novgorod 603950 (Russian Federation)

2004-06-04

Harper's equations for simple and complex two-dimensional lattices subject to a magnetic field have been derived in the tight-binding approximation. In our derivation we do not neglect the influence of the magnetic field on the electron eigenfunctions and eigenvalues in isolated atoms. Using a variational procedure for finding eigenfunctions and eigenvalues, we have self-consistently obtained Hofstadter's butterflies. Even for a simple square lattice Hofstadter's butterfly differs from the butterfly obtained in the case in which the influence of the magnetic field on the electron eigenvalues and eigenfunctions in isolated atoms is not taken into account.

Perspectives from ab-initio and tight-binding: Applications to transition metal compounds and superlattices

Science.gov (United States)

Venkataraman, Vijay Shankar

The experimental and theoretical study of transition metal compounds have occupied condensed matter physicists for the best part of the last century. The rich variety of physical behaviour exhibited by these compounds owes its origin to the subtle balance of the energy scales at play for the d orbitals. In this thesis, we study three different systems comprised of transition metal atoms from the third, the fourth, and the fifth group of the periodic table using a combination of ab-initio density functional theory (DFT) computations and effective tight-binding models for the electronic properties. We first consider the electronic properties of artificially fabricated perovskite superlattices of the form [(SrIrO3)m / SrTiO3] with integer m denoting the number of layers of SrIrO3. After discussing the results of experiments undertaken by our collaborators, we present the results of our DFT calculations and build tight-binding models for the m = 1 and m = 2 superlattices. The active ingredient is found to be the 5d orbitals with significant spin-orbit coupling. We then study the energies of magnetic ground states within DFT and compare and contrast our results with those obtained for the bulk Ruddlesden-Popper iridates. Together with experimental measurements, our results suggest that these superlattices are an exciting venue to probe the magnetism and metal-insulator transitions that occur from the intricate balance of the spin-orbit coupling and electron interactions, as has been reported for their bulk counterparts. Next, we consider alpha-RuCl3, a honeycomb lattice compound. We first show using DFT calculations in conjunction with experiments performed by our collaborators, how spin-orbit coupling in the 4d orbitals of Ru is essential to understand the insulating state realized in this compound. Then, in the latter half of the chapter, we study the magnetic ground states of a two-dimensional analogue of alpha-RuCl3 in weak and strong-coupling regimes obtained from

Quasiparticle dynamics and spin-orbital texture of the SrTiO3 two-dimensional electron gas.

Science.gov (United States)

King, P D C; McKeown Walker, S; Tamai, A; de la Torre, A; Eknapakul, T; Buaphet, P; Mo, S-K; Meevasana, W; Bahramy, M S; Baumberger, F

2014-02-27

Two-dimensional electron gases (2DEGs) in SrTiO3 have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the d-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO3-based 2DEGs, and yield new microscopic insights on their functional properties.

Extended Lagrangian formulation of charge-constrained tight-binding molecular dynamics.

Science.gov (United States)

Cawkwell, M J; Coe, J D; Yadav, S K; Liu, X-Y; Niklasson, A M N

2015-06-09

The extended Lagrangian Born-Oppenheimer molecular dynamics formalism [Niklasson, Phys. Rev. Lett., 2008, 100, 123004] has been applied to a tight-binding model under the constraint of local charge neutrality to yield microcanonical trajectories with both precise, long-term energy conservation and a reduced number of self-consistent field optimizations at each time step. The extended Lagrangian molecular dynamics formalism restores time reversal symmetry in the propagation of the electronic degrees of freedom, and it enables the efficient and accurate self-consistent optimization of the chemical potential and atomwise potential energy shifts in the on-site elements of the tight-binding Hamiltonian that are required when enforcing local charge neutrality. These capabilities are illustrated with microcanonical molecular dynamics simulations of a small metallic cluster using an sd-valent tight-binding model for titanium. The effects of weak dissipation on the propagation of the auxiliary degrees of freedom for the chemical potential and on-site Hamiltonian matrix elements that is used to counteract the accumulation of numerical noise during trajectories was also investigated.

Analytic and numeric Green's functions for a two-dimensional electron gas in an orthogonal magnetic field

International Nuclear Information System (INIS)

Cresti, Alessandro; Grosso, Giuseppe; Parravicini, Giuseppe Pastori

2006-01-01

We have derived closed analytic expressions for the Green's function of an electron in a two-dimensional electron gas threaded by a uniform perpendicular magnetic field, also in the presence of a uniform electric field and of a parabolic spatial confinement. A workable and powerful numerical procedure for the calculation of the Green's functions for a large infinitely extended quantum wire is considered exploiting a lattice model for the wire, the tight-binding representation for the corresponding matrix Green's function, and the Peierls phase factor in the Hamiltonian hopping matrix element to account for the magnetic field. The numerical evaluation of the Green's function has been performed by means of the decimation-renormalization method, and quite satisfactorily compared with the analytic results worked out in this paper. As an example of the versatility of the numerical and analytic tools here presented, the peculiar semilocal character of the magnetic Green's function is studied in detail because of its basic importance in determining magneto-transport properties in mesoscopic systems

Time-dependent density-functional tight-binding method with the third-order expansion of electron density.

Science.gov (United States)

Nishimoto, Yoshio

2015-09-07

We develop a formalism for the calculation of excitation energies and excited state gradients for the self-consistent-charge density-functional tight-binding method with the third-order contributions of a Taylor series of the density functional theory energy with respect to the fluctuation of electron density (time-dependent density-functional tight-binding (TD-DFTB3)). The formulation of the excitation energy is based on the existing time-dependent density functional theory and the older TD-DFTB2 formulae. The analytical gradient is computed by solving Z-vector equations, and it requires one to calculate the third-order derivative of the total energy with respect to density matrix elements due to the inclusion of the third-order contributions. The comparison of adiabatic excitation energies for selected small and medium-size molecules using the TD-DFTB2 and TD-DFTB3 methods shows that the inclusion of the third-order contributions does not affect excitation energies significantly. A different set of parameters, which are optimized for DFTB3, slightly improves the prediction of adiabatic excitation energies statistically. The application of TD-DFTB for the prediction of absorption and fluorescence energies of cresyl violet demonstrates that TD-DFTB3 reproduced the experimental fluorescence energy quite well.

Dielectric response of molecules in empirical tight-binding theory

Science.gov (United States)

Boykin, Timothy B.; Vogl, P.

2002-01-01

In this paper we generalize our previous approach to electromagnetic interactions within empirical tight-binding theory to encompass molecular solids and isolated molecules. In order to guarantee physically meaningful results, we rederive the expressions for relevant observables using commutation relations appropriate to the finite tight-binding Hilbert space. In carrying out this generalization, we examine in detail the consequences of various prescriptions for the position and momentum operators in tight binding. We show that attempting to fit parameters of the momentum matrix directly generally results in a momentum operator which is incompatible with the underlying tight-binding model, while adding extra position parameters results in numerous difficulties, including the loss of gauge invariance. We have applied our scheme, which we term the Peierls-coupling tight-binding method, to the optical dielectric function of the molecular solid PPP, showing that this approach successfully predicts its known optical properties even in the limit of isolated molecules.

Electron density of states in a one-dimensional distorted system with impurities: Coherent potential approximation

International Nuclear Information System (INIS)

Bulka, B.R.

1982-04-01

A tight-binding one-dimensional distorted system with impurities is considered and the electron density of states is calculated in the coherent potential approximation. It is shown that two types of impurities, an impurity built in a chain and a domain wall (a soliton), play the essential role and a drastic reduction of the energy gap is observed for a few per cent of impurities. The experimental situation in polyacetylene is also discussed. (author)

Extended Lagrangian Density Functional Tight-Binding Molecular Dynamics for Molecules and Solids

International Nuclear Information System (INIS)

Aradi, Balint; Frauenheim, Thomas

2015-01-01

A computationally fast quantum mechanical molecular dynamics scheme using an extended Lagrangian density functional tight-binding formulation has been developed and implemented in the DFTB+ electronic structure program package for simulations of solids and molecular systems. The scheme combines the computational speed of self-consistent density functional tight-binding theory with the efficiency and long-term accuracy of extended Lagrangian Born-Oppenheimer molecular dynamics. Furthermore, for systems without self-consistent charge instabilities, only a single diagonalization or construction of the single-particle density matrix is required in each time step. The molecular dynamics simulation scheme can also be applied to a broad range of problems in materials science, chemistry, and biology

Extended Lagrangian Density Functional Tight-Binding Molecular Dynamics for Molecules and Solids.

Science.gov (United States)

Aradi, Bálint; Niklasson, Anders M N; Frauenheim, Thomas

2015-07-14

A computationally fast quantum mechanical molecular dynamics scheme using an extended Lagrangian density functional tight-binding formulation has been developed and implemented in the DFTB+ electronic structure program package for simulations of solids and molecular systems. The scheme combines the computational speed of self-consistent density functional tight-binding theory with the efficiency and long-term accuracy of extended Lagrangian Born-Oppenheimer molecular dynamics. For systems without self-consistent charge instabilities, only a single diagonalization or construction of the single-particle density matrix is required in each time step. The molecular dynamics simulation scheme can be applied to a broad range of problems in materials science, chemistry, and biology.

Surface Passivation in Empirical Tight Binding

OpenAIRE

He, Yu; Tan, Yaohua; Jiang, Zhengping; Povolotskyi, Michael; Klimeck, Gerhard; Kubis, Tillmann

2015-01-01

Empirical Tight Binding (TB) methods are widely used in atomistic device simulations. Existing TB methods to passivate dangling bonds fall into two categories: 1) Method that explicitly includes passivation atoms is limited to passivation with atoms and small molecules only. 2) Method that implicitly incorporates passivation does not distinguish passivation atom types. This work introduces an implicit passivation method that is applicable to any passivation scenario with appropriate parameter...

Improved Density Functional Tight Binding Potentials for Metalloid Aluminum Clusters

Science.gov (United States)

2016-06-01

unlimited IMPROVED DENSITY-FUNCTIONAL TIGHT BINDING POTENTIALS FOR METALLOID ALUMINUM CLUSTERS by Joon H. Kim June 2016 Thesis Advisor...DATES COVERED Master’s thesis 4. TITLE AND SUBTITLE IMPROVED DENSITY-FUNCTIONAL TIGHT BINDING POTENTIALS FOR METALLOID ALUMINUM CLUSTERS 5. FUNDING...repulsive potentials for use in density-functional tight binding (DFTB) simulations of low-valence aluminum metalloid clusters . These systems are under

Incipient localization and tight-binding superconductivity: Tsub(c) calculation

International Nuclear Information System (INIS)

Kolley, E.; Kolley, W.

1984-01-01

Localization effects on the superconducting transition temperature Tsub(c) are examined in strongly disordered three-dimensional systems. A tight-binding formulation of strong-coupling superconductivity is combined, after configuration averaging, with the self-consistent treatment of Anderson localization developed by Vollhardt and Woelfle. The Coulomb interaction becomes retarded via the joint local local density of states, giving rise to an enhancement of the pseudopotential. Numerical Tsub(c) results as a function of disorder are compared with another theoretical work and experimental values for some high-Tsub(c) materials. (orig.)

Binding energy of two-dimensional biexcitons

DEFF Research Database (Denmark)

Singh, Jai; Birkedal, Dan; Vadim, Lyssenko

1996-01-01

Using a model structure for a two-dimensional (2D) biexciton confined in a quantum well, it is shown that the form of the Hamiltonian of the 2D biexciton reduces into that of an exciton. The binding energies and Bohr radii of a 2D biexciton in its various internal energy states are derived...... analytically using the fractional dimension approach. The ratio of the binding energy of a 2D biexciton to that of a 2D exciton is found to be 0.228, which agrees very well with the recent experimental value. The results of our approach are compared with those of earlier theories....

Atomic and electronic structures of a-SiC:H from tight-binding molecular dynamics

CERN Document Server

Ivashchenko, V I; Shevchenko, V I; Ivashchenko, L A; Rusakov, G V

2003-01-01

The atomic and electronic properties of amorphous unhydrogenated (a-SiC) and hydrogenated (a-SiC:H) silicon carbides are studied using an sp sup 3 s sup * tight-binding force model with molecular dynamics simulations. The parameters of a repulsive pairwise potential are determined from ab initio pseudopotential calculations. Both carbides are generated from dilute vapours condensed from high temperature, with post-annealing at low temperature for a-SiC:H. A plausible model for the inter-atomic correlations and electronic states in a-SiC:H is suggested. According to this model, the formation of the amorphous network is weakly sensitive to the presence of hydrogen. Hydrogen passivates effectively only the weak bonds of threefold-coordinated atoms. Chemical ordering is very much affected by the cooling rate and the structure of the high-temperature vapour. The as-computed characteristics are in rather good agreement with the results for a-SiC and a-Si:H from ab initio calculations.

Tight-binding approximations to time-dependent density functional theory — A fast approach for the calculation of electronically excited states

Energy Technology Data Exchange (ETDEWEB)

Rüger, Robert, E-mail: rueger@scm.com [Scientific Computing & Modelling NV, De Boelelaan 1083, 1081 HV Amsterdam (Netherlands); Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam (Netherlands); Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Linnéstr. 2, 04103 Leipzig (Germany); Lenthe, Erik van [Scientific Computing & Modelling NV, De Boelelaan 1083, 1081 HV Amsterdam (Netherlands); Heine, Thomas [Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Linnéstr. 2, 04103 Leipzig (Germany); Visscher, Lucas [Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam (Netherlands)

2016-05-14

We propose a new method of calculating electronically excited states that combines a density functional theory based ground state calculation with a linear response treatment that employs approximations used in the time-dependent density functional based tight binding (TD-DFTB) approach. The new method termed time-dependent density functional theory TD-DFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive TD-DFT calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TD-DFTB.

Tight binding simulation study on zigzag single-walled carbon nanotubes

Science.gov (United States)

Sharma, Deepa; Jaggi, Neena; Gupta, Vishu

2018-01-01

Tight binding simulation studies using the density functional tight binding (DFTB) model have been performed on various zigzag single-walled carbon-nanotubes (SWCNTs) to investigate their electronic properties using DFTB module of the Material Studio Software version 7.0. Various combinations of different eigen-solvers and charge mixing schemes available in the DFTB Module have been tried to chalk out the electronic structure. The analytically deduced values of the bandgap of (9, 0) SWCNT were compared with the experimentally determined value reported in the literature. On comparison, it was found that the tight binding approximations tend to drastically underestimate the bandgap values. However, the combination of Anderson charge mixing method with standard eigensolver when implemented using the smart algorithm was found to produce fairly close results. These optimized model parameters were then used to determine the band structures of various zigzag SWCNTs. (9, 0) Single-walled Nanotube which is extensively being used for sensing NH3, CH4 and NO2 has been picked up as a reference material since its experimental bandgap value has been reported in the literature. It has been found to exhibit a finite energy bandgap in contrast to its expected metallic nature. The study is of utmost significance as it not only probes and validates the simulation route for predicting suitable properties of nanomaterials but also throws light on the comparative efficacy of the different approximation and rationalization quantum mechanical techniques used in simulation studies. Such simulation studies if used intelligently prove to be immensely useful to the material scientists as they not only save time and effort but also pave the way to new experiments by making valuable predictions.

Tight-binding tunneling amplitude of an optical lattice

Science.gov (United States)

Arzamasovs, Maksims; Liu, Bo

2017-11-01

The particle in a periodic potential is an important topic in an undergraduate quantum mechanics curriculum and a stepping stone on the way to more advanced topics, such as courses on interacting electrons in crystalline solids, and graduate-level research in solid-state and condensed matter physics. The interacting many-body phenomena are usually described in terms of the second quantized lattice Hamiltonians which treat single-particle physics on the level of tight-binding approximation and add interactions on top of it. The aim of this paper is to show how the tight-binding tunneling amplitude can be related to the strength of the periodic potential for the case of a cosine potential used in the burgeoning field of ultracold atoms. We show how to approach the problem of computing the tunneling amplitude of a deep lattice using the JWKB (Jeffreys-Wentzel-Kramers-Brillouin, also known as semiclassical) approximation. We also point out that care should be taken when applying the method of the linear combination of atomic orbitals (LCAO) in an optical lattice context. A summary of the exact solution in terms of Mathieu functions is also given.

Tight-binding tunneling amplitude of an optical lattice

International Nuclear Information System (INIS)

Arzamasovs, Maksims; Liu, Bo

2017-01-01

The particle in a periodic potential is an important topic in an undergraduate quantum mechanics curriculum and a stepping stone on the way to more advanced topics, such as courses on interacting electrons in crystalline solids, and graduate-level research in solid-state and condensed matter physics. The interacting many-body phenomena are usually described in terms of the second quantized lattice Hamiltonians which treat single-particle physics on the level of tight-binding approximation and add interactions on top of it. The aim of this paper is to show how the tight-binding tunneling amplitude can be related to the strength of the periodic potential for the case of a cosine potential used in the burgeoning field of ultracold atoms. We show how to approach the problem of computing the tunneling amplitude of a deep lattice using the JWKB (Jeffreys–Wentzel–Kramers–Brillouin, also known as semiclassical) approximation. We also point out that care should be taken when applying the method of the linear combination of atomic orbitals (LCAO) in an optical lattice context. A summary of the exact solution in terms of Mathieu functions is also given. (paper)

Superconductivity of the two-dimensional Penson-Kolb model

International Nuclear Information System (INIS)

Czart, W.R.; Robaszkiewicz, S.

2001-01-01

Two-dimensional (d = 2) Penson-Kolb model, i.e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction, is considered and the effects of phase fluctuations on the s-wave superconductivity of this system are discussed within Kosterlitz-Thouless scenario. The London penetration depth λ at T = 0, the Kosterlitz Thouless critical temperature T c , and the Hartree-Fock approximation critical temperature T p are determined as a function of particle concentration and interaction. The Uemura type plots (T c vs. λ -2 (0)) are derived. Beyond weak coupling and for low concentrations they show the existence of universal scaling: T c ∼ 1/λ 2 (0), as it previously found for the attractive Hubbard model and for the models intersite electron pairing. (author)

Tuning the electronic properties of gated multilayer phosphorene: A self-consistent tight-binding study

Science.gov (United States)

Li, L. L.; Partoens, B.; Peeters, F. M.

2018-04-01

By taking account of the electric-field-induced charge screening, a self-consistent calculation within the framework of the tight-binding approach is employed to obtain the electronic band structure of gated multilayer phosphorene and the charge densities on the different phosphorene layers. We find charge density and screening anomalies in single-gated multilayer phosphorene and electron-hole bilayers in dual-gated multilayer phosphorene. Due to the unique puckered lattice structure, both intralayer and interlayer charge screenings are important in gated multilayer phosphorene. We find that the electric-field tuning of the band structure of multilayer phosphorene is distinctively different in the presence and absence of charge screening. For instance, it is shown that the unscreened band gap of multilayer phosphorene decreases dramatically with increasing electric-field strength. However, in the presence of charge screening, the magnitude of this band-gap decrease is significantly reduced and the reduction depends strongly on the number of phosphorene layers. Our theoretical results of the band-gap tuning are compared with recent experiments and good agreement is found.

Tight-binding calculation of Ti-Rh--type phase diagram

International Nuclear Information System (INIS)

Sluiter, M.; Turchi, P.; Fu Zezhong; de Fontaine, D.

1988-01-01

Tight-binding electronic band-structure calculations were combined with a free-energy expression from a statistical mechanical method called the cluster-variation method. The effective pair interactions used in the cluster-variation calculation were evaluated by the generalized perturbation method. Only d orbitals were included and the numbers of d electrons per atom were taken to be three for the pure A element and eight for the pure B. A phase diagram was constructed incorporating, for the first time, both fcc and bcc lattices and their simple-ordered superstructures. The calculated diagram agreed reasonably well with those determined empirically for Ti-Rh or Ti-Ir

Development of tight-binding based GW algorithm and its computational implementation for graphene

Energy Technology Data Exchange (ETDEWEB)

Majidi, Muhammad Aziz [Departemen Fisika, FMIPA, Universitas Indonesia, Kampus UI Depok (Indonesia); NUSNNI-NanoCore, Department of Physics, National University of Singapore (NUS), Singapore 117576 (Singapore); Singapore Synchrotron Light Source (SSLS), National University of Singapore (NUS), 5 Research Link, Singapore 117603 (Singapore); Naradipa, Muhammad Avicenna, E-mail: muhammad.avicenna11@ui.ac.id; Phan, Wileam Yonatan; Syahroni, Ahmad [Departemen Fisika, FMIPA, Universitas Indonesia, Kampus UI Depok (Indonesia); Rusydi, Andrivo [NUSNNI-NanoCore, Department of Physics, National University of Singapore (NUS), Singapore 117576 (Singapore); Singapore Synchrotron Light Source (SSLS), National University of Singapore (NUS), 5 Research Link, Singapore 117603 (Singapore)

2016-04-19

Graphene has been a hot subject of research in the last decade as it holds a promise for various applications. One interesting issue is whether or not graphene should be classified into a strongly or weakly correlated system, as the optical properties may change upon several factors, such as the substrate, voltage bias, adatoms, etc. As the Coulomb repulsive interactions among electrons can generate the correlation effects that may modify the single-particle spectra (density of states) and the two-particle spectra (optical conductivity) of graphene, we aim to explore such interactions in this study. The understanding of such correlation effects is important because eventually they play an important role in inducing the effective attractive interactions between electrons and holes that bind them into excitons. We do this study theoretically by developing a GW method implemented on the basis of the tight-binding (TB) model Hamiltonian. Unlike the well-known GW method developed within density functional theory (DFT) framework, our TB-based GW implementation may serve as an alternative technique suitable for systems which Hamiltonian is to be constructed through a tight-binding based or similar models. This study includes theoretical formulation of the Green’s function G, the renormalized interaction function W from random phase approximation (RPA), and the corresponding self energy derived from Feynman diagrams, as well as the development of the algorithm to compute those quantities. As an evaluation of the method, we perform calculations of the density of states and the optical conductivity of graphene, and analyze the results.

Development of tight-binding based GW algorithm and its computational implementation for graphene

International Nuclear Information System (INIS)

Majidi, Muhammad Aziz; Naradipa, Muhammad Avicenna; Phan, Wileam Yonatan; Syahroni, Ahmad; Rusydi, Andrivo

2016-01-01

Graphene has been a hot subject of research in the last decade as it holds a promise for various applications. One interesting issue is whether or not graphene should be classified into a strongly or weakly correlated system, as the optical properties may change upon several factors, such as the substrate, voltage bias, adatoms, etc. As the Coulomb repulsive interactions among electrons can generate the correlation effects that may modify the single-particle spectra (density of states) and the two-particle spectra (optical conductivity) of graphene, we aim to explore such interactions in this study. The understanding of such correlation effects is important because eventually they play an important role in inducing the effective attractive interactions between electrons and holes that bind them into excitons. We do this study theoretically by developing a GW method implemented on the basis of the tight-binding (TB) model Hamiltonian. Unlike the well-known GW method developed within density functional theory (DFT) framework, our TB-based GW implementation may serve as an alternative technique suitable for systems which Hamiltonian is to be constructed through a tight-binding based or similar models. This study includes theoretical formulation of the Green’s function G, the renormalized interaction function W from random phase approximation (RPA), and the corresponding self energy derived from Feynman diagrams, as well as the development of the algorithm to compute those quantities. As an evaluation of the method, we perform calculations of the density of states and the optical conductivity of graphene, and analyze the results.

Coherent Electron Focussing in a Two-Dimensional Electron Gas.

NARCIS (Netherlands)

Houten, H. van; Wees, B.J. van; Mooij, J.E.; Beenakker, C.W.J.; Williamson, J.G.; Foxon, C.T.

1988-01-01

The first experimental realization of ballistic point contacts in a two-dimensional electron gas for the study of transverse electron focussing by a magnetic field is reported. Multiple peaks associated with skipping orbits of electrons reflected specularly by the channel boundary are observed. At

MANAGING TIGHT BINDING RECEPTORS FOR NEW SPEARATIONS TECHNOLOGIES

Energy Technology Data Exchange (ETDEWEB)

DARYLE H BUSCH RICHARD S GIVENS

2004-12-10

Much of the earth's pollution involves compounds of the metallic elements, including actinides, strontium, cesium, technetium, and RCRA metals. Metal ions bind to molecules called ligands, which are the molecular tools that can manipulate the metal ions under most conditions. This DOE-EMSP sponsored program strives (1) to provide the foundations for using the most powerful ligands in transformational separations technologies and (2) to produce seminal examples of their applications to separations appropriate to the DOE EM mission. These ultra tight-binding ligands can capture metal ions in the most competitive of circumstances (from mineralized sites, lesser ligands, and even extremely dilute solutions), but they react so slowly that they are useless in traditional separations methodologies. Two attacks on this problem are underway. The first accommodates to the challenging molecular lethargy by developing a seminal slow separations methodology termed the soil poultice. The second designs ligands that are only tight-binding while wrapped around the targeted metal ion, but can be put in place by switch-binding and removed by switch-release. We envision a kind of molecular switching process to accelerate the union between metal ion and tight-binding ligand. Molecular switching processes are suggested for overcoming the slow natural equilibration rate with which ultra tight-binding ligands combine with metal ions. Ligands that bind relatively weakly combine with metal ions rapidly, so the trick is to convert a ligand from a weak, rapidly binding species to a powerful, slow releasing ligand--during the binding of the ligand to the metal ion. Such switch-binding ligands must react with themselves, and the reaction must take place under the influence of the metal ion. For example, our generation 1 ligands showed that a well-designed linear ligand with ends that readily combine, forms a cyclic molecule when it wraps around a metal ion. Our generation 2 ligands are

Magnetic anisotropy of two-dimensional nanostructures: Transition-metal triangular stripes

International Nuclear Information System (INIS)

Dorantes-Davila, J.; Villasenor-Gonzalez, P.; Pastor, G.M.

2005-01-01

The magnetic anisotropy energy (MAE) of one-dimensional stripes having infinite length and triangular lateral structure are investigated in the framework of a self-consistent tight-binding method. One observes discontinuous changes in the easy magnetization direction along the crossover from one to two dimensions. The MAE oscillates as a function of stripe width and depends strongly on the considered transition metal (TM). The MAE of the two-leg ladder is strongly reduced as compared to that of the monoatomic chain and the convergence to the two-dimensional limit is rather slow

Binding Direction-Based Two-Dimensional Flattened Contact Area Computing Algorithm for Protein-Protein Interactions.

Science.gov (United States)

Kang, Beom Sik; Pugalendhi, GaneshKumar; Kim, Ku-Jin

2017-10-13

Interactions between protein molecules are essential for the assembly, function, and regulation of proteins. The contact region between two protein molecules in a protein complex is usually complementary in shape for both molecules and the area of the contact region can be used to estimate the binding strength between two molecules. Although the area is a value calculated from the three-dimensional surface, it cannot represent the three-dimensional shape of the surface. Therefore, we propose an original concept of two-dimensional contact area which provides further information such as the ruggedness of the contact region. We present a novel algorithm for calculating the binding direction between two molecules in a protein complex, and then suggest a method to compute the two-dimensional flattened area of the contact region between two molecules based on the binding direction.

Binding Direction-Based Two-Dimensional Flattened Contact Area Computing Algorithm for Protein–Protein Interactions

Directory of Open Access Journals (Sweden)

Beom Sik Kang

2017-10-01

Full Text Available Interactions between protein molecules are essential for the assembly, function, and regulation of proteins. The contact region between two protein molecules in a protein complex is usually complementary in shape for both molecules and the area of the contact region can be used to estimate the binding strength between two molecules. Although the area is a value calculated from the three-dimensional surface, it cannot represent the three-dimensional shape of the surface. Therefore, we propose an original concept of two-dimensional contact area which provides further information such as the ruggedness of the contact region. We present a novel algorithm for calculating the binding direction between two molecules in a protein complex, and then suggest a method to compute the two-dimensional flattened area of the contact region between two molecules based on the binding direction.

Nanoscale capacitance: A quantum tight-binding model

Science.gov (United States)

Zhai, Feng; Wu, Jian; Li, Yang; Lu, Jun-Qiang

2017-01-01

Landauer-Buttiker formalism with the assumption of semi-infinite electrodes as reservoirs has been the standard approach in modeling steady electron transport through nanoscale devices. However, modeling dynamic electron transport properties, especially nanoscale capacitance, is a challenging problem because of dynamic contributions from electrodes, which is neglectable in modeling macroscopic capacitance and mesoscopic conductance. We implement a self-consistent quantum tight-binding model to calculate capacitance of a nano-gap system consisting of an electrode capacitance C‧ and an effective capacitance Cd of the middle device. From the calculations on a nano-gap made of carbon nanotube with a buckyball therein, we show that when the electrode length increases, the electrode capacitance C‧ moves up while the effective capacitance Cd converges to a value which is much smaller than the electrode capacitance C‧. Our results reveal the importance of electrodes in modeling nanoscale ac circuits, and indicate that the concepts of semi-infinite electrodes and reservoirs well-accepted in the steady electron transport theory may be not applicable in modeling dynamic transport properties.

Some Effective Tight-Binding Models for Electrons in DNA Conduction: A Review

International Nuclear Information System (INIS)

Yamada, H.; Iguchi, K.

2010-01-01

Quantum transport for DNA conduction has been widely studied with interest in application as a candidate in making nanowires as well as interest in the scientific mechanism. In this paper, we review recent works concerning the electronic states and the conduction/transfer in DNA polymers. We have mainly investigated the energy-band structure and the correlation effects of localization property in the two- and three-chain systems (ladder model) with long-range correlation as a simple model for electronic property in a double strand of DNA by using the tight-bindingmodel. In addition, we investigated the localization properties of electronic states in several actual DNA sequences such as bacteriophages of Escherichia coli, human-chromosome 22, compared with those of the artificial disordered sequences with correlation. The charge-transfer properties for poly(dA)-poly(dT) and poly(dG)-poly(dC) DNA polymers are also presented in terms of localization lengths within the frameworks of the polaron models due to the coupling between the charge carriers and the lattice vibrations of the double strand of DNA

Rate concept and retarded master equations for dissipative tight-binding models

International Nuclear Information System (INIS)

Egger, R.; Mak, C.H.; Weiss, U.

1994-01-01

Employing a ''noninteracting-cluster approximation,'' the dynamics of multistate dissipative tight-binding models has been formulated in terms of a set of generalized retarded master equations. The rates for the various pathways are expressed as power series in the intersite couplings. We apply this to the superexchange mechanism, which is relevant for bacterial photosynthesis and bridged electron transfer systems. This approach provides a general and unified description of both incoherent and coherent transport

Transferable tight-binding model for strained group IV and III-V materials and heterostructures

Science.gov (United States)

Tan, Yaohua; Povolotskyi, Michael; Kubis, Tillmann; Boykin, Timothy B.; Klimeck, Gerhard

2016-07-01

It is critical to capture the effect due to strain and material interface for device level transistor modeling. We introduce a transferable s p3d5s* tight-binding model with nearest-neighbor interactions for arbitrarily strained group IV and III-V materials. The tight-binding model is parametrized with respect to hybrid functional (HSE06) calculations for varieties of strained systems. The tight-binding calculations of ultrasmall superlattices formed by group IV and group III-V materials show good agreement with the corresponding HSE06 calculations. The application of the tight-binding model to superlattices demonstrates that the transferable tight-binding model with nearest-neighbor interactions can be obtained for group IV and III-V materials.

Tightness of the Ising-Kac Model on the Two-Dimensional Torus

Science.gov (United States)

Hairer, Martin; Iberti, Massimo

2018-05-01

We consider the sequence of Gibbs measures of Ising models with Kac interaction defined on a periodic two-dimensional discrete torus near criticality. Using the convergence of the Glauber dynamic proven by Mourrat and Weber (Commun Pure Appl Math 70:717-812, 2017) and a method by Tsatsoulis and Weber employed in (arXiv:1609.08447 2016), we show tightness for the sequence of Gibbs measures of the Ising-Kac model near criticality and characterise the law of the limit as the Φ ^4_2 measure on the torus. Our result is very similar to the one obtained by Cassandro et al. (J Stat Phys 78(3):1131-1138, 1995) on Z^2, but our strategy takes advantage of the dynamic, instead of correlation inequalities. In particular, our result covers the whole critical regime and does not require the large temperature/large mass/small coupling assumption present in earlier results.

Electronic Properties of SiNTs Under External Electric and Magnetic Fields Using the Tight-Binding Method

Science.gov (United States)

Chegel, Raad; Behzad, Somayeh

2014-02-01

We investigated the electronic properties of silicon nanotubes (SiNTs) under external transverse electric fields and axial magnetic fields using the tight-binding approximation. It was found that, after switching on the electric and magnetic fields, band modifications such as distortion of degeneracy, change in energy dispersion and subband spacing, and bandgap size reduction occur. The bandgap of silicon gear-like nanotubes (Si g-NTs) decreases linearly with increasing electric field strength, but the bandgap for silicon hexagonal nanotubes (Si h-NTs) first increases and then decreases (metallic) or first remains constant and then decreases (semiconducting). Our results show that the bandgap of Si h-NTs is very sensitive to both electric and magnetic fields, unlike Si g-NTs, which are more sensitive to electric than magnetic fields.

From structure to spectra. Tight-binding theory of InGaAs quantum dots

International Nuclear Information System (INIS)

Goldmann, Elias

2014-01-01

Self-assembled semiconductor quantum dots have raised considerable interest in the last decades due to a multitude of possible applications ranging from carrier storage to light emitters, lasers and future quantum communication devices. Quantum dots offer unique electronic and photonic properties due to the three-dimensional confinement of charge carriers and the coupling to a quasi-continuum of wetting layer and barrier states. In this work we investigate the electronic structure of In x Ga 1-x As quantum dots embedded in GaAs, considering realistic quantum dot geometries and Indium concentrations. We utilize a next-neighbour sp 3 s * tight-binding model for the calculation of electronic single-particle energies and wave functions bound in the nanostructure and account for strain arising from lattice mismatch of the constituent materials atomistically. With the calculated single-particle wave functions we derive Coulomb matrix elements and include them into a configuration interaction treatment, yielding many-particle states and energies of the interacting many-carrier system. Also from the tight-binding single-particle wave functions we derive dipole transition strengths to obtain optical quantum dot emission and absorption spectra with Fermi's golden rule. Excitonic fine-structure splittings are obtained, which play an important role for future quantum cryptography and quantum communication devices for entanglement swapping or quantum repeating. For light emission suited for long-range quantum-crypted fiber communication InAs quantum dots are embedded in an In x Ga 1-x As strain-reducing layer, shifting the emission wavelength into telecom low-absorption windows. We investigate the influence of the strain-reducing layer Indium concentration on the excitonic finestructure splitting. The fine-structure splitting is found to saturate and, in some cases, even reduce with strain-reducing layer Indium concentration, a result being counterintuitively. Our result

From structure to spectra. Tight-binding theory of InGaAs quantum dots

Energy Technology Data Exchange (ETDEWEB)

Goldmann, Elias

2014-07-23

Self-assembled semiconductor quantum dots have raised considerable interest in the last decades due to a multitude of possible applications ranging from carrier storage to light emitters, lasers and future quantum communication devices. Quantum dots offer unique electronic and photonic properties due to the three-dimensional confinement of charge carriers and the coupling to a quasi-continuum of wetting layer and barrier states. In this work we investigate the electronic structure of In{sub x}Ga{sub 1-x}As quantum dots embedded in GaAs, considering realistic quantum dot geometries and Indium concentrations. We utilize a next-neighbour sp{sup 3}s{sup *} tight-binding model for the calculation of electronic single-particle energies and wave functions bound in the nanostructure and account for strain arising from lattice mismatch of the constituent materials atomistically. With the calculated single-particle wave functions we derive Coulomb matrix elements and include them into a configuration interaction treatment, yielding many-particle states and energies of the interacting many-carrier system. Also from the tight-binding single-particle wave functions we derive dipole transition strengths to obtain optical quantum dot emission and absorption spectra with Fermi's golden rule. Excitonic fine-structure splittings are obtained, which play an important role for future quantum cryptography and quantum communication devices for entanglement swapping or quantum repeating. For light emission suited for long-range quantum-crypted fiber communication InAs quantum dots are embedded in an In{sub x}Ga{sub 1-x}As strain-reducing layer, shifting the emission wavelength into telecom low-absorption windows. We investigate the influence of the strain-reducing layer Indium concentration on the excitonic finestructure splitting. The fine-structure splitting is found to saturate and, in some cases, even reduce with strain-reducing layer Indium concentration, a result being

Substrate-Triggered Exosite Binding: Synergistic Dendrimer/Folic Acid Action for Achieving Specific, Tight-Binding to Folate Binding Protein.

Science.gov (United States)

Chen, Junjie; van Dongen, Mallory A; Merzel, Rachel L; Dougherty, Casey A; Orr, Bradford G; Kanduluru, Ananda Kumar; Low, Philip S; Marsh, E Neil G; Banaszak Holl, Mark M

2016-03-14

Polymer-ligand conjugates are designed to bind proteins for applications as drugs, imaging agents, and transport scaffolds. In this work, we demonstrate a folic acid (FA)-triggered exosite binding of a generation five poly(amidoamine) (G5 PAMAM) dendrimer scaffold to bovine folate binding protein (bFBP). The protein exosite is a secondary binding site on the protein surface, separate from the FA binding pocket, to which the dendrimer binds. Exosite binding is required to achieve the greatly enhanced binding constants and protein structural change observed in this study. The G5Ac-COG-FA1.0 conjugate bound tightly to bFBP, was not displaced by a 28-fold excess of FA, and quenched roughly 80% of the initial fluorescence. Two-step binding kinetics were measured using the intrinsic fluorescence of the FBP tryptophan residues to give a KD in the low nanomolar range for formation of the initial G5Ac-COG-FA1.0/FBP* complex, and a slow conversion to the tight complex formed between the dendrimer and the FBP exosite. The extent of quenching was sensitive to the choice of FA-dendrimer linker chemistry. Direct amide conjugation of FA to G5-PAMAM resulted in roughly 50% fluorescence quenching of the FBP. The G5Ac-COG-FA, which has a longer linker containing a 1,2,3-triazole ring, exhibited an ∼80% fluorescence quenching. The binding of the G5Ac-COG-FA1.0 conjugate was compared to poly(ethylene glycol) (PEG) conjugates of FA (PEGn-FA). PEG2k-FA had a binding strength similar to that of FA, whereas other PEG conjugates with higher molecular weight showed weaker binding. However, no PEG conjugates gave an increased degree of total fluorescence quenching.

Time-efficient simulations of tight-binding electronic structures with Intel Xeon PhiTM many-core processors

Science.gov (United States)

Ryu, Hoon; Jeong, Yosang; Kang, Ji-Hoon; Cho, Kyu Nam

2016-12-01

Modelling of multi-million atomic semiconductor structures is important as it not only predicts properties of physically realizable novel materials, but can accelerate advanced device designs. This work elaborates a new Technology-Computer-Aided-Design (TCAD) tool for nanoelectronics modelling, which uses a sp3d5s∗ tight-binding approach to describe multi-million atomic structures, and simulate electronic structures with high performance computing (HPC), including atomic effects such as alloy and dopant disorders. Being named as Quantum simulation tool for Advanced Nanoscale Devices (Q-AND), the tool shows nice scalability on traditional multi-core HPC clusters implying the strong capability of large-scale electronic structure simulations, particularly with remarkable performance enhancement on latest clusters of Intel Xeon PhiTM coprocessors. A review of the recent modelling study conducted to understand an experimental work of highly phosphorus-doped silicon nanowires, is presented to demonstrate the utility of Q-AND. Having been developed via Intel Parallel Computing Center project, Q-AND will be open to public to establish a sound framework of nanoelectronics modelling with advanced HPC clusters of a many-core base. With details of the development methodology and exemplary study of dopant electronics, this work will present a practical guideline for TCAD development to researchers in the field of computational nanoelectronics.

One-dimensional versus two-dimensional electronic states in vicinal surfaces

International Nuclear Information System (INIS)

Ortega, J E; Ruiz-Oses, M; Cordon, J; Mugarza, A; Kuntze, J; Schiller, F

2005-01-01

Vicinal surfaces with periodic arrays of steps are among the simplest lateral nanostructures. In particular, noble metal surfaces vicinal to the (1 1 1) plane are excellent test systems to explore the basic electronic properties in one-dimensional superlattices by means of angular photoemission. These surfaces are characterized by strong emissions from free-electron-like surface states that scatter at step edges. Thereby, the two-dimensional surface state displays superlattice band folding and, depending on the step lattice constant d, it splits into one-dimensional quantum well levels. Here we use high-resolution, angle-resolved photoemission to analyse surface states in a variety of samples, in trying to illustrate the changes in surface state bands as a function of d

Inter-layer Cooper pairing of two-dimensional electrons

International Nuclear Information System (INIS)

Inoue, Masahiro; Takemori, Tadashi; Yoshizaki, Ryozo; Sakudo, Tunetaro; Ohtaka, Kazuo

1987-01-01

The authors point out the possibility that the high transition temperatures of the recently discovered oxide superconductors are dominantly caused by the inter-layer Cooper pairing of two-dimensional electrons that are coupled through the exchange of three-dimensional phonons. (author)

Theory of the one- and two-dimensional electron gas

International Nuclear Information System (INIS)

Emery, V.J.

1987-01-01

Two topics are discussed: (1) the competition between 2k/sub F/ and 4k/sub F/ charge state waves in a one-dimensional electron gas and (2) a two-dimensional model of high T/sub c/ superconductivity in the oxides

Noninteracting beams of ballistic two-dimensional electrons

International Nuclear Information System (INIS)

Spector, J.; Stormer, H.L.; Baldwin, K.W.; Pfeiffer, L.N.; West, K.W.

1991-01-01

We demonstrate that two beams of two-dimensional ballistic electrons in a GaAs-AlGaAs heterostructure can penetrate each other with negligible mutual interaction analogous to the penetration of two optical beams. This allows electrical signal channels to intersect in the same plane with negligible crosstalk between the channels

Electronic and Magnetic Properties of Transition-Metal Oxide Nanocomposites: A Tight-Binding Modeling at Mesoscale

Science.gov (United States)

Tai, Yuan-Yen; Zhu, Jian-Xin

Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. In this talk, I will present an overview on our recent efforts in theoretical understanding of the electronic and magnetic properties TMO nanocomposites. In particular, I will introduce our recently developed tight-binding modeling of these properties arising from the interplay of competing interactions at the interfaces of planar and pillar nanocomposites. Our theoretical tool package will provide a unique capability to address the emergent phenomena in TMO nanocomposites and their mesoscale response to such effects like strain and microstructures at the interfaces, and ultimately help establish design principles of new multifunctionality with TMOs. This work was carried out under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy at LANL under Contract No. DE-AC52-06NA25396, and was supported by the LANL LDRD Program.

Surface Passivation in Empirical Tight Binding

Science.gov (United States)

He, Yu; Tan, Yaohua; Jiang, Zhengping; Povolotskyi, Michael; Klimeck, Gerhard; Kubis, Tillmann

2016-03-01

Empirical Tight Binding (TB) methods are widely used in atomistic device simulations. Existing TB methods to passivate dangling bonds fall into two categories: 1) Method that explicitly includes passivation atoms is limited to passivation with atoms and small molecules only. 2) Method that implicitly incorporates passivation does not distinguish passivation atom types. This work introduces an implicit passivation method that is applicable to any passivation scenario with appropriate parameters. This method is applied to a Si quantum well and a Si ultra-thin body transistor oxidized with SiO2 in several oxidation configurations. Comparison with ab-initio results and experiments verifies the presented method. Oxidation configurations that severely hamper the transistor performance are identified. It is also shown that the commonly used implicit H atom passivation overestimates the transistor performance.

Atomistic Tight-Binding Theory of Electron-Hole Exchange Interaction in Morphological Evolution of CdSe/ZnS Core/Shell Nanodisk to CdSe/ZnS Core/Shell Nanorod

Directory of Open Access Journals (Sweden)

Worasak Sukkabot

2016-01-01

Full Text Available Based on the atomistic tight-binding theory (TB and a configuration interaction (CI description, the electron-hole exchange interaction in the morphological transformation of CdSe/ZnS core/shell nanodisk to CdSe/ZnS core/shell nanorod is described with the aim of understanding the impact of the structural shapes on the change of the electron-hole exchange interaction. Normally, the ground hole states confined in typical CdSe/ZnS core/shell nanocrystals are of heavy hole-like character. However, the atomistic tight-binding theory shows that a transition of the ground hole states from heavy hole-like to light hole-like contribution with the increasing aspect ratios of the CdSe/ZnS core/shell nanostructures is recognized. According to the change in the ground-state hole characters, the electron-hole exchange interaction is also significantly altered. To do so, optical band gaps, ground-state electron character, ground-state hole character, oscillation strengths, ground-state coulomb energies, ground-state exchange energies, and dark-bright (DB excitonic splitting (stoke shift are numerically demonstrated. These atomistic computations obviously show the sensitivity with the aspect ratios. Finally, the alteration in the hole character has a prominent effect on dark-bright (DB excitonic splitting.

DFTBaby: A software package for non-adiabatic molecular dynamics simulations based on long-range corrected tight-binding TD-DFT(B)

Science.gov (United States)

Humeniuk, Alexander; Mitrić, Roland

2017-12-01

A software package, called DFTBaby, is published, which provides the electronic structure needed for running non-adiabatic molecular dynamics simulations at the level of tight-binding DFT. A long-range correction is incorporated to avoid spurious charge transfer states. Excited state energies, their analytic gradients and scalar non-adiabatic couplings are computed using tight-binding TD-DFT. These quantities are fed into a molecular dynamics code, which integrates Newton's equations of motion for the nuclei together with the electronic Schrödinger equation. Non-adiabatic effects are included by surface hopping. As an example, the program is applied to the optimization of excited states and non-adiabatic dynamics of polyfluorene. The python and Fortran source code is available at http://www.dftbaby.chemie.uni-wuerzburg.de.

Binding of two-electron metastable states in semiconductor quantum dots under a magnetic field

Science.gov (United States)

Garagiola, Mariano; Pont, Federico M.; Osenda, Omar

2018-04-01

Applying a strong enough magnetic field results in the binding of few-electron resonant states. The mechanism was proposed many years ago but its verification in laboratory conditions is far more recent. In this work we study the binding of two-electron resonant states. The electrons are confined in a cylindrical quantum dot which is embedded in a semiconductor wire. The geometry considered is similar to the one used in actual experimental setups. The low-energy two-electron spectrum is calculated numerically from an effective-mass approximation Hamiltonian modelling the system. Methods for binding threshold calculations in systems with one and two electrons are thoroughly studied; in particular, we use quantum information quantities to assess when the strong lateral confinement approximation can be used to obtain reliable low-energy spectra. For simplicity, only cases without bound states in the absence of an external field are considered. Under these conditions, the binding threshold for the one-electron case is given by the lowest Landau energy level. Moreover, the energy of the one-electron bounded resonance can be used to obtain the two-electron binding threshold. It is shown that for realistic values of the two-electron model parameters it is feasible to bind resonances with field strengths of a few tens of tesla.

Self-consistent tight-binding model of B and N doping in graphene

DEFF Research Database (Denmark)

Pedersen, Thomas Garm; Pedersen, Jesper Goor

2013-01-01

. The impurity potential depends sensitively on the impurity occupancy, leading to a self-consistency requirement. We solve this problem using the impurity Green's function and determine the self-consistent local density of states at the impurity site and, thereby, identify acceptor and donor energy resonances.......Boron and nitrogen substitutional impurities in graphene are analyzed using a self-consistent tight-binding approach. An analytical result for the impurity Green's function is derived taking broken electron-hole symmetry into account and validated by comparison to numerical diagonalization...

Tight-binding study of the structural and magnetic properties of vanadium clusters

International Nuclear Information System (INIS)

Zhao Jijun; Lain, K.D.

1995-01-01

The structural and magnetic properties of small vanadium clusters are studied in the framework of tight-binding theory. According to parameters of the cluster dimer and bulk solid, we developed a tight-binding interatomic potential and calculated the bonding energies for the different possible structures to determine the ground state atomic configurations of the small vanadium clusters. The theoretical bonding energies for the vanadium clusters agree with the experiment much better than the simple droplet model. However, the calculated values for the clusters of odd atomic number are somewhat higher than the measured ones, corresponding to the pair occupation of delocalized 4s 1 electrons. Based on the optimized geometries, we study the magnetic properties of these clusters through a parametrized Hubbard Hamiltonian. We find the small V clusters of ground-state structures exhibit antiferromagnetic behavior while the alignment of local moments in the clusters with the unoptimized structures may show either ferromagnetic or antiferromagnetic characteristics. The average magnetic moments of the clusters decrease nonmonotonically as cluster size increases and the theoretical results are consistent with the upper limits obtained from a recent experiment. (orig.)

Low-order moment expansions to tight binding for interatomic potentials: Successes and failures

International Nuclear Information System (INIS)

Kress, J.D.; Voter, A.F.

1995-01-01

We discuss the use of moment-based approximations to tight binding. Using a maximum entropy form for the electronic density of states, we show that a general interatomic potential can be defined that is suitable for molecular-dynamics simulations and has several other desirable features. For covalent materials (C and Si), properties where the atoms are in equivalent environments are well converged at low-order moments. For defect environments, which offer a more critical (and relevant) test, the method is found to give less satisfactory results. For example, the vacancy formation energy for Si is too low by ∼2 eV at 10 moments relative to exact tight binding. Attempts to improve the accuracy were unsuccessful, leading to the conclusion that potentials based on this approach are inadequate for covalent materials. We speculate that this may be a deficiency of low-order moment methods in general. For metals, in contrast to the covalent systems, we find that the low-order moment approach is better behaved. This finding is consistent with the success of existing empirical fourth-moment potentials for metals

Two-dimensional electronic femtosecond stimulated Raman spectroscopy

Directory of Open Access Journals (Sweden)

Ogilvie J.P.

2013-03-01

Full Text Available We report two-dimensional electronic spectroscopy with a femtosecond stimulated Raman scattering probe. The method reveals correlations between excitation energy and excited state vibrational structure following photoexcitation. We demonstrate the method in rhodamine 6G.

A simple analytical model for electronic conductance in a one dimensional atomic chain across a defect

International Nuclear Information System (INIS)

Khater, Antoine; Szczesniak, Dominik

2011-01-01

An analytical model is presented for the electronic conductance in a one dimensional atomic chain across an isolated defect. The model system consists of two semi infinite lead atomic chains with the defect atom making the junction between the two leads. The calculation is based on a linear combination of atomic orbitals in the tight-binding approximation, with a single atomic one s-like orbital chosen in the present case. The matching method is used to derive analytical expressions for the scattering cross sections for the reflection and transmission processes across the defect, in the Landauer-Buttiker representation. These analytical results verify the known limits for an infinite atomic chain with no defects. The model can be applied numerically for one dimensional atomic systems supported by appropriate templates. It is also of interest since it would help establish efficient procedures for ensemble averages over a field of impurity configurations in real physical systems.

Two-particle correlations in the one-dimensional Hubbard model: a ground-state analytical solution

CERN Document Server

Vallejo, E; Espinosa, J E

2003-01-01

A solution to the extended Hubbard Hamiltonian for the case of two-particles in an infinite one-dimensional lattice is presented, using a real-space mapping method and the Green function technique. This Hamiltonian considers the on-site (U) and the nearest-neighbor (V) interactions. The method is based on mapping the correlated many-body problem onto an equivalent site-impurity tight-binding one in a higher dimensional space. In this new space we obtained the analytical solution for the ground state binding energy. Results are in agreement with the numerical solution obtained previously [1], and with those obtained in the reciprocal space [2]. (Author)

Tight-binding molecular dynamics simulation of charge state effects in semiconductors

CERN Document Server

Khakimov, Z M; Sulaymonov, N T; Kiv, A E; Levin, A A

2002-01-01

New model of Si-H bond dissociation has been proposed and tested in the cluster Si sub 1 sub 0 H sub 1 sub 6 by the simulation approach that combines classical molecular dynamics method and the self-consistent tight-binding electronic and total energy calculation one. It is shown that the monohydride Si-H bond is unstable with respect to formation of silicon dangling bond and bend bridge Si-H-Si bond when this cluster traps the single positive charge. In this case hydrogen atom migrates rather rotating around Si-Si bond than crossing the center of this bond (the bond-centered position). The model can be useful for understanding hydrogen related phenomena at surfaces, interfaces, internal voids of various hydrogenated silicon systems: electronic devices, silicon solar cells, and nanocrystalline and porous silicon. (author)

The tight binding model study of the role of band filling on the charge gap in graphene-on-substrate in paramagnetic state

Science.gov (United States)

Panda, Rudrashish; Sahu, Sivabrata; Rout, G. C.

2017-05-01

We communicate here a tight binding theoretical model study of the band filling effect on the charge gap in graphene-on-substrate. The Hamiltonian consists of nearest neighbor electron hopping and substrate induced gap. Besides this the Coulomb interaction is considered here within mean-field approximation in the paramagnetic limit. The electron occupancies at two sublattices are calculated by Green's function technique and are solved self consistently. Finally the charge gap i.e. Δ ¯=U [ - ] is calculated and computed numerically. The results are reported.

Two-dimensional optimization of free-electron-laser designs

Science.gov (United States)

Prosnitz, D.; Haas, R.A.

1982-05-04

Off-axis, two-dimensional designs for free electron lasers are described that maintain correspondence of a light beam with a synchronous electron at an optimal transverse radius r > 0 to achieve increased beam trapping efficiency and enhanced laser beam wavefront control so as to decrease optical beam diffraction and other deleterious effects.

Combinatorial computational chemistry approach of tight-binding quantum chemical molecular dynamics method to the design of the automotive catalysts

International Nuclear Information System (INIS)

Ito, Yuki; Jung, Changho; Luo, Yi; Koyama, Michihisa; Endou, Akira; Kubo, Momoji; Imamura, Akira; Miyamoto, Akira

2006-01-01

Recently, we have developed a new tight-binding quantum chemical molecular dynamics program 'Colors' for combinatorial computational chemistry approach. This methodology is based on our original tight-binding approximation and realized over 5000 times acceleration compared to the conventional first-principles molecular dynamics method. In the present study, we applied our new program to the simulations on various realistic large-scale models of the automotive three-way catalysts, ultrafine Pt particle/CeO 2 (111) support. Significant electron transfer from the Pt particle to the CeO 2 (111) surface was observed and it was found to strongly depend on the size of the Pt particle. Furthermore, our simulation results suggest that the reduction of the Ce atom due to the electron transfer from the Pt particle to the CeO 2 surface is a main reason for the strong interaction of the Pt particle and CeO 2 (111) support

Efficient self-consistency for magnetic tight binding

Science.gov (United States)

Soin, Preetma; Horsfield, A. P.; Nguyen-Manh, D.

2011-06-01

Tight binding can be extended to magnetic systems by including an exchange interaction on an atomic site that favours net spin polarisation. We have used a published model, extended to include long-ranged Coulomb interactions, to study defects in iron. We have found that achieving self-consistency using conventional techniques was either unstable or very slow. By formulating the problem of achieving charge and spin self-consistency as a search for stationary points of a Harris-Foulkes functional, extended to include spin, we have derived a much more efficient scheme based on a Newton-Raphson procedure. We demonstrate the capabilities of our method by looking at vacancies and self-interstitials in iron. Self-consistency can indeed be achieved in a more efficient and stable manner, but care needs to be taken to manage this. The algorithm is implemented in the code PLATO. Program summaryProgram title:PLATO Catalogue identifier: AEFC_v2_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEFC_v2_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 228 747 No. of bytes in distributed program, including test data, etc.: 1 880 369 Distribution format: tar.gz Programming language: C and PERL Computer: Apple Macintosh, PC, Unix machines Operating system: Unix, Linux, Mac OS X, Windows XP Has the code been vectorised or parallelised?: Yes. Up to 256 processors tested RAM: Up to 2 Gbytes per processor Classification: 7.3 External routines: LAPACK, BLAS and optionally ScaLAPACK, BLACS, PBLAS, FFTW Catalogue identifier of previous version: AEFC_v1_0 Journal reference of previous version: Comput. Phys. Comm. 180 (2009) 2616 Does the new version supersede the previous version?: Yes Nature of problem: Achieving charge and spin self-consistency in magnetic tight binding can be very

An efficient magnetic tight-binding method for transition metals and alloys

DEFF Research Database (Denmark)

Barreteau, Cyrille; Spanjaard, Daniel; Desjonquères, Marie-Catherine

2016-01-01

that does not necessitate any further fitting is proposed to deal with systems made of several chemical elements. This model is extended to spin (and orbital) polarized materials by adding Stoner-like and spin–orbit interactions. Collinear and non-collinear magnetism as well as spin-spirals are considered......An efficient parameterized self-consistent tight-binding model for transition metals using s, p and d valence atomic orbitals as a basis set is presented. The parameters of our tight-binding model for pure elements are determined from a fit to bulk ab-initio calculations. A very simple procedure...

Tunneling between parallel two-dimensional electron liquids

Czech Academy of Sciences Publication Activity Database

Jungwirth, Tomáš; MacDonald, A. H.

361/362, - (1996), s. 167-170 ISSN 0039-6028. [International Conference on the Electronic Properties of Two Dimensional Systems /11./. Nottingham, 07.08.1995-11.08.1995] R&D Projects: GA ČR GA202/94/1278 Grant - others:INT(XX) 9106888 Impact factor: 2.783, year: 1996

Model of two-dimensional electron gas formation at ferroelectric interfaces

Energy Technology Data Exchange (ETDEWEB)

Aguado-Puente, P.; Bristowe, N. C.; Yin, B.; Shirasawa, R.; Ghosez, Philippe; Littlewood, P. B.; Artacho, Emilio

2015-07-01

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here, we study under what circumstances similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms, a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO3/SrTiO3 heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first-principles simulations of free-standing slabs of PbTiO3. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a nonvolatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces.

Three-dimensional measurement of a tightly focused laser beam

Directory of Open Access Journals (Sweden)

Xiangsheng Xie

2013-02-01

Full Text Available The spatial structure of a tightly focused light field is measured with a double knife-edge scanning method. The measurement method is based on the use of a high-quality double knife-edge fabricated from a right-angled silicon fragment mounted on a photodetector. The reconstruction of the three-dimensional structures of tightly focused spots is carried out with both uniform and partially obstructed linearly polarized incident light beams. The optical field distribution is found to deviate substantially from the input beam profile in the tightly focused region, which is in good agreement with the results of numerical simulations.

A scaling analysis of electronic localization in two-dimensional random media

International Nuclear Information System (INIS)

Ye Zhen

2003-01-01

By an improved scaling analysis, we suggest that there may appear two possibilities concerning the electronic localization in two-dimensional random media. The first is that all electronic states are localized in two dimensions, as conjectured previously. The second possibility is that electronic behaviors in two- and three-dimensional random systems are similar, in agreement with a recent calculation based on a direct calculation of the conductance with the use of the Kubo formula. In this case, non-localized states are possible in two dimensions, and have some peculiar properties. A few predictions are proposed. Moreover, the present analysis accommodates results from the previous scaling analysis

Coherent electron focusing with quantum point contacts in a two-dimensional electron gas

NARCIS (Netherlands)

Houten, H. van; Beenakker, C.W.J.; Williamson, J.G.; Broekaart, M.E.I.; Loosdrecht, P.H.M. van; Wees, B.J. van; Mooij, J.E.; Foxon, C.T.; Harris, J.J.

1989-01-01

Transverse electron focusing in a two-dimensional electron gas is investigated experimentally and theoretically for the first time. A split Schottky gate on top of a GaAs-AlxGa1–xAs heterostructure defines two point contacts of variable width, which are used as injector and collector of ballistic

Tight-Binding simulations of InAs/GaAs quantum dots; Tight-Binding Simulationen von InAs/GaAs Quantenpunkten

Energy Technology Data Exchange (ETDEWEB)

Kleinsorge, Alexander

2008-06-23

For several years, the technological potential of self-organized grown quantum dots (QD) has been known. Their usage as an effective light source or memory requires the precise prediction of their electronic properties. Hence, this report will study InAs quantum dots at GaAs substrate. After relaxing the atomic positions with a many body potential of Abell-Tersoff type, I calculated the electronic structure using the Tight-Binding method which is reasonable for large systems. During the investigation of wavefunctions depend on the shape, size and temperature, the impact of strain showed up as the main reason for the p-splitting. Typically flat QDs (relative to lateral dimensions) are grown, therefore the energy of bound states depends mostly on their height. The crystal's orientation had a strong impact on the wavefunctions. Moreover, the understanding of STS experiments, which inspected the connection between shape and wavefunction, is better now. Because of the possible simultaneous occupation of semiconductor quantum dots with an electron and a hole, there is a dipole moment of the exciton (due to their different behaviour inside the QD). This is a further experimental access to inner details of the QD. I ascertained the interplay of composition profile and dipole moment. The force caused by additional potentials (piezoelectricity, outer homogeneous and inhomogeneous electrical fields) was also an subject of my inquiries. To conclude, I executed kMC simulations, to better apprehend the annealing experiments. I was able to explain the narrowing of the PL peak width better. Furthermore I showed a ramification of the strain field to the diffusion development (and the following electronic properties). (orig.)

A tight binding model study of tunneling conductance spectra of spin and orbitally ordered CMR manganites

Science.gov (United States)

Panda, Saswati; Sahoo, D. D.; Rout, G. C.

2018-04-01

We report here a tight binding model for colossal magnetoresistive (CMR) manganites to study the pseudo gap (PG) behavior near Fermi level. In the Kubo-Ohata type DE model, we consider first and second nearest neighbor interactions for transverse spin fluctuations in core band and hopping integrals in conduction band, in the presence of static band Jahn-Teller distortion. The model Hamiltonian is solved using Zubarev's Green's function technique. The electron density of states (DOS) is found out from the Green's functions. We observe clear PG near Fermi level in the electron DOS.

Kinetics of two-dimensional electron plasma, interacting with fluctuating potential

International Nuclear Information System (INIS)

Boiko, I.I.; Sirenko, Y.M.

1990-01-01

In this paper, from the first principles, after the fashion of Klimontovich, the authors derive quantum kinetic equation for electron gas, inhomogeneous in z-direction and homogeneous in XY-plane. Special attention is given to the systems with quasi-two-dimensional electron gas (2 DEG), which are widely explored now. Both interaction between the particles of 2 DEG (in general, of several sorts), and interaction with an external system (phonons, impurities, after change carries etc.) are considered. General theory is used to obtain energy and momentum balance equations and relaxation frequencies for 2 DEG in the basis of plane waves. The case of crossed electric and magnetic fields is also treated. As an illustration the problems of 2 DEG scattering on semibounded three-dimensional electron gas and on two-dimensional hole gas are considered; transverse conductivity of nondegenerate 2 DEG, scattered by impurities in ultraquantum magnetic field, is calculated

Proton transfer along water bridges in biological systems with density-functional tight-binding

Science.gov (United States)

Reiss, Krystle; Wise, Abigail; Mazzuca, James

2015-03-01

When examining the dynamics of charge transfer in high dimensional enzymatic systems, the cost of quantum mechanical treatment of electrons increases exponentially with the size of the system. As a semi-empirical method, density-functional tight-binding aids in shortening these calculation times, but can be inaccurate in the regime where bonds are being formed and broken. To address these inaccuracies with respect to proton transfer in an enzymatic system, DFTB is being used to calculate small model systems containing only a single amino acid residue donor, represented by an imidazole molecule, and a water acceptor. When DFTB calculations are compared to B3LYP geometry calculations of the donor molecule, we observe a bond angle error on the order of 1.2 degrees and a bond length error on the order of 0.011 Å. As we move forward with small donor-acceptor systems, comparisons between DFTB and B3LYP energy profiles will provide a better clue as to what extent improvements need to be made. To improve the accuracy of the DFTB calculations, the internuclear repulsion term may be altered. This would result in energy profiles that closely resemble those produced by higher-level theory. Alma College Provost's Office.

Electronic transport on the spatial structure of the protein: Three-dimensional lattice model

International Nuclear Information System (INIS)

Sarmento, R.G.; Frazão, N.F.; Macedo-Filho, A.

2017-01-01

Highlights: • The electronic transport on the structure of the three-dimensional lattice model of the protein is studied. • The signing of the current–voltage is directly affected by permutations of the weak bonds in the structure. • Semiconductor behave of the proteins suggest a potential application in the development of novel biosensors. - Abstract: We report a numerical analysis of the electronic transport in protein chain consisting of thirty-six standard amino acids. The protein chains studied have three-dimensional structure, which can present itself in three distinct conformations and the difference consist in the presence or absence of thirteen hydrogen-bondings. Our theoretical method uses an electronic tight-binding Hamiltonian model, appropriate to describe the protein segments modeled by the amino acid chain. We note that the presence and the permutations between weak bonds in the structure of proteins are directly related to the signing of the current–voltage. Furthermore, the electronic transport depends on the effect of temperature. In addition, we have found a semiconductor behave in the models investigated and it suggest a potential application in the development of novel biosensors for molecular diagnostics.

Electronic transport on the spatial structure of the protein: Three-dimensional lattice model

Energy Technology Data Exchange (ETDEWEB)

Sarmento, R.G. [Departamento de Ciências Biológicas, Universidade Federal do Piauí, 64800-000 Floriano, PI (Brazil); Frazão, N.F. [Centro de Educação e Saúde, Universidade Federal de Campina Grande, 581750-000 Cuité, PB (Brazil); Macedo-Filho, A., E-mail: amfilho@gmail.com [Campus Prof. Antonio Geovanne Alves de Sousa, Universidade Estadual do Piauí, 64260-000 Piripiri, PI (Brazil)

2017-01-30

Highlights: • The electronic transport on the structure of the three-dimensional lattice model of the protein is studied. • The signing of the current–voltage is directly affected by permutations of the weak bonds in the structure. • Semiconductor behave of the proteins suggest a potential application in the development of novel biosensors. - Abstract: We report a numerical analysis of the electronic transport in protein chain consisting of thirty-six standard amino acids. The protein chains studied have three-dimensional structure, which can present itself in three distinct conformations and the difference consist in the presence or absence of thirteen hydrogen-bondings. Our theoretical method uses an electronic tight-binding Hamiltonian model, appropriate to describe the protein segments modeled by the amino acid chain. We note that the presence and the permutations between weak bonds in the structure of proteins are directly related to the signing of the current–voltage. Furthermore, the electronic transport depends on the effect of temperature. In addition, we have found a semiconductor behave in the models investigated and it suggest a potential application in the development of novel biosensors for molecular diagnostics.

Nonequilibrium Transport and the Bernoulli Effect of Electrons in a Two-Dimensional Electron Gas

Science.gov (United States)

Kaya, Ismet I.

2013-02-01

Nonequilibrium transport of charged carriers in a two-dimensional electron gas is summarized from an experimental point of view. The transport regime in which the electron-electron interactions are enhanced at high bias leads to a range of striking effects in a two-dimensional electron gas. This regime of transport is quite different than the ballistic transport in which particles propagate coherently with no intercarrier energy transfer and the diffusive transport in which the momentum of the electron system is lost with the involvement of the phonons. Quite a few hydrodynamic phenomena observed in classical gasses have the electrical analogs in the current flow. When intercarrier scattering events dominate the transport, the momentum sharing via narrow angle scattering among the hot and cold electrons lead to negative resistance and electron pumping which can be viewed as the analog of the Bernoulli-Venturi effect observed classical gasses. The recent experimental findings and the background work in the field are reviewed.

Two-dimensional electron gas in monolayer InN quantum wells

International Nuclear Information System (INIS)

Pan, W.; Wang, G. T.; Dimakis, E.; Moustakas, T. D.; Tsui, D. C.

2014-01-01

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in a superlattice structure of 40 InN quantum wells consisting of one monolayer of InN embedded between 10 nm GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5 × 10 15  cm −2 (or 1.25 × 10 14  cm −2 per InN quantum well, assuming all the quantum wells are connected by diffused indium contacts) and 420 cm 2 /Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES

Determining the static electronic and vibrational energy correlations via two-dimensional electronic-vibrational spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Dong, Hui; Lewis, Nicholas H. C.; Oliver, Thomas A. A.; Fleming, Graham R., E-mail: grfleming@lbl.gov [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, Californial 94720 (United States); Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California 94720 (United States)

2015-05-07

Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions.

Electronic Transport in Two-Dimensional Materials

Science.gov (United States)

Sangwan, Vinod K.; Hersam, Mark C.

2018-04-01

Two-dimensional (2D) materials have captured the attention of the scientific community due to the wide range of unique properties at nanometer-scale thicknesses. While significant exploratory research in 2D materials has been achieved, the understanding of 2D electronic transport and carrier dynamics remains in a nascent stage. Furthermore, because prior review articles have provided general overviews of 2D materials or specifically focused on charge transport in graphene, here we instead highlight charge transport mechanisms in post-graphene 2D materials, with particular emphasis on transition metal dichalcogenides and black phosphorus. For these systems, we delineate the intricacies of electronic transport, including band structure control with thickness and external fields, valley polarization, scattering mechanisms, electrical contacts, and doping. In addition, electronic interactions between 2D materials are considered in the form of van der Waals heterojunctions and composite films. This review concludes with a perspective on the most promising future directions in this fast-evolving field.

Absolute Negative Resistance Induced by Directional Electron-Electron Scattering in a Two-Dimensional Electron Gas

Science.gov (United States)

Kaya, Ismet I.; Eberl, Karl

2007-05-01

A three-terminal device formed by two electrostatic barriers crossing an asymmetrically patterned two-dimensional electron gas displays an unusual potential depression at the middle contact, yielding absolute negative resistance. The device displays momentum and current transfer ratios that far exceed unity. The observed reversal of the current or potential in the middle terminal can be interpreted as the analog of Bernoulli’s effect in a Fermi liquid. The results are explained by directional scattering of electrons in two dimensions.

Optical-phonon-induced frictional drag in coupled two-dimensional electron gases

DEFF Research Database (Denmark)

Hu, Ben Yu-Kuang

1998-01-01

The role of optical phonons in frictional drag between two adjacent but electrically isolated two-dimensional electron gases is investigated. Since the optical phonons in III-V materials have a considerably larger coupling to electrons than acoustic phonons (which are the dominant drag mechanism ...

Phase separation and d-wave superconductivity induced by extended electron-exciton interaction

Energy Technology Data Exchange (ETDEWEB)

Cheng Ming [Department of Physics and Texas Center for Superconductivity, University of Houston, 4800 Calhoun Road, Houston, Texas 77204 (United States)], E-mail: cheng896@hotmail.com; Su Wupei [Department of Physics and Texas Center for Superconductivity, University of Houston, 4800 Calhoun Road, Houston, Texas 77204 (United States)

2008-12-15

Using an auxiliary-field quantum Monte Carlo (AFQMC) method, we have studied a two-dimensional tight-binding model in which the conduction electrons can polarize an adjacent layer of molecules through electron-electron repulsion. Calculated average conduction electron density as a function of chemical potential exhibits a clear break characteristic of phase separation. Compared to the noninteracting system, the d-wave pair-field correlation function shows significant enhancement. The simultaneous presence of phase separation and d-wave superconductivity suggests that an effective extended pairing force is induced by the electron-exciton coupling.

Phase separation and d-wave superconductivity induced by extended electron-exciton interaction

International Nuclear Information System (INIS)

Cheng Ming; Su Wupei

2008-01-01

Using an auxiliary-field quantum Monte Carlo (AFQMC) method, we have studied a two-dimensional tight-binding model in which the conduction electrons can polarize an adjacent layer of molecules through electron-electron repulsion. Calculated average conduction electron density as a function of chemical potential exhibits a clear break characteristic of phase separation. Compared to the noninteracting system, the d-wave pair-field correlation function shows significant enhancement. The simultaneous presence of phase separation and d-wave superconductivity suggests that an effective extended pairing force is induced by the electron-exciton coupling

Insights into the Structural Changes Occurring upon Photoconversion in the Orange Carotenoid Protein from Broadband Two-Dimensional Electronic Spectroscopy

Energy Technology Data Exchange (ETDEWEB)

De Re, Eleonora; Schlau-Cohen, Gabriela S.; Leverenz, Ryan L.; Huxter, Vanessa M.; Oliver, Thomas A. A.; Mathies, Richard A.; Fleming, Graham R.

2014-05-22

Carotenoids play an essential role in photoprotection, interacting with other pigments to safely dissipate excess absorbed energy as heat. In cyanobacteria, the short time scale photoprotective mechanisms involve the photoactive orange carotenoid protein (OCP), which binds a single carbonyl carotenoid. Blue-green light induces the photoswitching of OCP from its ground state form (OCPO) to a metastable photoproduct (OCPR). OCPR can bind to the phycobilisome antenna and induce fluorescence quenching. The photoswitching is accompanied by structural and functional changes at the level of the protein and of the bound carotenoid. In this study, we use broadband two-dimensional electronic spectroscopy to look at the differences in excited state dynamics of the carotenoid in the two forms of OCP. Our results provide insight into the origin of the pronounced vibrational lineshape and oscillatory dynamics observed in linear absorption and 2D electronic spectroscopy of OCPO and the large inhomogeneous broadening in OCPR, with consequences for the chemical function of the two forms.

Phononic crystals of spherical particles: A tight binding approach

Energy Technology Data Exchange (ETDEWEB)

Mattarelli, M., E-mail: maurizio.mattarelli@fisica.unipg.it [NiPS Laboratory, Dipartimento di Fisica, Università di Perugia, Via Pascoli, 06100 Perugia (Italy); Secchi, M. [CMM - Fondazione Bruno Kessler, Via Sommarive 18, 38123 Trento (Italy); Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123 Trento (Italy); Montagna, M. [Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123 Trento (Italy)

2013-11-07

The vibrational dynamics of a fcc phononic crystal of spheres is studied and compared with that of a single free sphere, modelled either by a continuous homogeneous medium or by a finite cluster of atoms. For weak interaction among the spheres, the vibrational dynamics of the phononic crystal is described by shallow bands, with low degree of dispersion, corresponding to the acoustic spheroidal and torsional modes of the single sphere. The phonon displacements are therefore related to the vibrations of a sphere, as the electron wave functions in a crystal are related to the atomic wave functions in a tight binding model. Important dispersion is found for the two lowest phonon bands, which correspond to zero frequency free translation and rotation of a free sphere. Brillouin scattering spectra are calculated at some values of the exchanged wavevectors of the light, and compared with those of a single sphere. With weak interaction between particles, given the high acoustic impedance mismatch in dry systems, the density of phonon states consist of sharp bands separated by large gaps, which can be well accounted for by a single particle model. Based on the width of the frequency gaps, tunable with the particle size, and on the small number of dispersive acoustic phonons, such systems may provide excellent materials for application as sound or heat filters.

Two-dimensional electron flow in pulsed power transmission lines and plasma opening switches

International Nuclear Information System (INIS)

Church, B.W.; Longcope, D.W.; Ng, C.K.; Sudan, R.N.

1991-01-01

The operation of magnetically insulated transmission lines (MITL) and the interruption of current in a plasma opening switch (POS) are determined by the physics of the electrons emitted by the cathode surface. A mathematical model describes the self-consistent two-dimensional flow of an electron fluid. A finite element code, FERUS, has been developed to solve the two equations which describe Poisson's and Ampere's law in two dimensions. The solutions from this code are obtained for parameters where the electron orbits are considerably modified by the self-magnetic field of the current. Next, the self-insulated electron flow in a MITL with a step change in cross-section is studied using a conventional two-dimensional fully electromagnetic particle-in-cell code, MASK. The equations governing two-dimensional quasi-static electron flow are solved numerically by a third technique which is suitable for predicting current interruption in a POS. The object of the study is to determine the critical load impedance, Z CL , required for current interruption for a given applied voltage, cathode voltage and plasma length. (author). 9 refs, 5 figs

Screened Raman response in two-dimensional d(x2-y2)-wave superconductors: Relative intensities in different symmetry channels

DEFF Research Database (Denmark)

Wenger, F.; Käll, M.

1997-01-01

We analyze the Raman-scattering response in a two-dimensional d(x2-y2)-wave superconductor and point out a strong suppression of relative intensity in the screened A(1g) channel compared to the B-1g channel for a generic tight-binding model. This is in contrast with the observed behavior in high...

Stable biexcitons in two-dimensional metal-halide perovskites with strong dynamic lattice disorder

Science.gov (United States)

Thouin, Félix; Neutzner, Stefanie; Cortecchia, Daniele; Dragomir, Vlad Alexandru; Soci, Cesare; Salim, Teddy; Lam, Yeng Ming; Leonelli, Richard; Petrozza, Annamaria; Kandada, Ajay Ram Srimath; Silva, Carlos

2018-03-01

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors—chemical, electronic, and structural—that govern strong multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA) 2PbI4 (PEA = phenylethylammonium). We determine the binding energy of biexcitons—correlated two-electron, two-hole quasiparticles—to be 44 ±5 meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by ˜25 % upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.

Terahertz Plasma Waves in Two Dimensional Quantum Electron Gas with Electron Scattering

International Nuclear Information System (INIS)

Zhang Liping

2015-01-01

We investigate the Terahertz (THz) plasma waves in a two-dimensional (2D) electron gas in a nanometer field effect transistor (FET) with quantum effects, the electron scattering, the thermal motion of electrons and electron exchange-correlation. We find that, while the electron scattering, the wave number along y direction and the electron exchange-correlation suppress the radiation power, but the thermal motion of electrons and the quantum effects can amplify the radiation power. The radiation frequency decreases with electron exchange-correlation contributions, but increases with quantum effects, the wave number along y direction and thermal motion of electrons. It is worth mentioning that the electron scattering has scarce influence on the radiation frequency. These properties could be of great help to the realization of practical THz plasma oscillations in nanometer FET. (paper)

Effect of increasing length on the electronic transport of an armchair graphene nano-ribbons

Directory of Open Access Journals (Sweden)

Sh Aghamiri Esfahani

2015-12-01

Full Text Available In this research, we have investigated the effect of increasing length on the electronic transport of an armchair graphene nano-ribbons with nitrogen atom impurity and without impurity. The semi-infinite, one-dimensional molecular systems are connected to two electrodes and the electron-electron interaction is ignored. The system is described by a simple tight binding model. All calculations are based on the Green's function and Landauer–Buttiker approach, and the electrodes are described in a wide band approximation.

Liquid structure and freezing of the two-dimensional classical electron fluid

International Nuclear Information System (INIS)

Ballone, P.; Pastore, G.; Rovere, M.; Tosi, M.P.

1984-11-01

Accurate theoretical results are reported for the pair correlation function of the classical two-dimensional electron liquid with r -1 interactions at strong coupling. The approach involves an evaluation of the bridge diagram corrections to the hypernetted-chain approximation, the role of low dimensionality being evident, relative to the case of the three-dimensional classical plasma, in an enhanced sensitivity to long range correlations. The liquid structure results are utilized in a density-wave theory of first-order freezing into the triangular lattice, the calculated coupling strength at freezing being in reasonable agreement with computer simulation results and with data on electron films on a liquid-He surface. The stability of the triangular electron lattice against deformation into a body-centered rectangular lattice is also discussed. (author)

Many electron variational ground state of the two dimensional Anderson lattice

International Nuclear Information System (INIS)

Zhou, Y.; Bowen, S.P.; Mancini, J.D.

1991-02-01

A variational upper bound of the ground state energy of two dimensional finite Anderson lattices is determined as a function of lattice size (up to 16 x 16). Two different sets of many-electron basis vectors are used to determine the ground state for all values of the coulomb integral U. This variational scheme has been successfully tested for one dimensional models and should give good estimates in two dimensions

Electron-phonon heat exchange in quasi-two-dimensional nanolayers

Science.gov (United States)

Anghel, Dragos-Victor; Cojocaru, Sergiu

2017-12-01

We study the heat power P transferred between electrons and phonons in thin metallic films deposited on free-standing dielectric membranes. The temperature range is typically below 1 K, such that the wavelengths of the excited phonon modes in the system is large enough so that the picture of a quasi-two-dimensional phonon gas is applicable. Moreover, due to the quantization of the components of the electron wavevectors perpendicular to the metal film's surface, the electrons spectrum forms also quasi two-dimensional sub-bands, as in a quantum well (QW). We describe in detail the contribution to the electron-phonon energy exchange of different electron scattering channels, as well as of different types of phonon modes. We find that heat flux oscillates strongly with thickness of the film d while having a much smoother variation with temperature (Te for the electrons temperature and Tph for the phonons temperature), so that one obtains a ridge-like landscape in the two coordinates, (d, Te) or (d, Tph), with crests and valleys aligned roughly parallel to the temperature axis. For the valley regions we find P ∝ Te3.5 - Tph3.5. From valley to crest, P increases by more than one order of magnitude and on the crests P cannot be represented by a simple power law. The strong dependence of P on d is indicative of the formation of the QW state and can be useful in controlling the heat transfer between electrons and crystal lattice in nano-electronic devices. Nevertheless, due to the small value of the Fermi wavelength in metals, the surface imperfections of the metallic films can reduce the magnitude of the oscillations of P vs. d, so this effect might be easier to observe experimentally in doped semiconductors.

Surface passivation for tight-binding calculations of covalent solids

International Nuclear Information System (INIS)

Bernstein, N

2007-01-01

Simulation of a cluster representing a finite portion of a larger covalently bonded system requires the passivation of the cluster surface. We compute the effects of an explicit hybrid orbital passivation (EHOP) on the atomic structure in a model bulk, three-dimensional, narrow gap semiconductor, which is very different from the wide gap, quasi-one-dimensional organic molecules where most passivation schemes have been studied in detail. The EHOP approach is directly applicable to minimal atomic orbital basis methods such as tight-binding. Each broken bond is passivated by a hybrid created from an explicitly expressed linear combination of basis orbitals, chosen to represent the contribution of the missing neighbour, e.g. a sp 3 hybrid for a single bond. The method is tested by computing the forces on atoms near a point defect as a function of cluster geometry. We show that, compared to alternatives such as pseudo-hydrogen passivation, the force on an atom converges to the correct bulk limit more quickly as a function of cluster radius, and that the force is more stable with respect to perturbations in the position of the cluster centre. The EHOP method also obviates the need for parameterizing the interactions between the system atoms and the passivating atoms. The method is useful for cluster calculations of non-periodic defects in large systems and for hybrid schemes that simulate large systems by treating finite regions with a quantum-mechanical model, coupled to an interatomic potential description of the rest of the system

Hall Conductivity in a Quasi-Two-Dimensional Disordered Electron System

Institute of Scientific and Technical Information of China (English)

YANG Yong-Hong; WANG Yong-Gang; LIU Mei

2002-01-01

By making use of the diagrammatic techniques in perturbation theory,we have investigated the Hall effect in a quasi-two-dimensional disordered electron system.In the weakly localized regime,the analytical expression for quantum correction to Hall conductivity has been obtained using the Kubo formalism and quasiclassical approximation.The relevant dimensional crossover behavior from three dimensions to two dimensions with decreasing the interlayer hopping energy is discussed.The quantum interference effect is shown to have a vanishing correction t,o the Hall coefficient.

Two-dimensional electron states bound to an off-plane donor in a magnetic field

International Nuclear Information System (INIS)

Bruno-Alfonso, A; Candido, L; Hai, G-Q

2010-01-01

The states of an electron confined in a two-dimensional (2D) plane and bound to an off-plane donor impurity center, in the presence of a magnetic field, are investigated. The energy levels of the ground state and the first three excited states are calculated variationally. The binding energy and the mean orbital radius of these states are obtained as a function of the donor center position and the magnetic field strength. The limiting cases are discussed for an in-plane donor impurity (i.e. a 2D hydrogen atom) as well as for the donor center far away from the 2D plane in strong magnetic fields, which corresponds to a 2D harmonic oscillator.

Effect of impurity doping on tunneling conductance in AB-stacked bi-layer graphene: A tight-binding study

Energy Technology Data Exchange (ETDEWEB)

Rout, G. C., E-mail: siva1987@iopb.res.in, E-mail: skp@iopb.res.in, E-mail: gcr@iopb.res.in [Physics Enclave, Plot No-664/4825, Lane-4A, Shree Vihar, Bhubaneswar-751031, Odisha (India); Sahu, Sivabrata [School of Applied Sciences (Physics), KIIT University, Bhubaneswar-751024, Odisha (India); Panda, S. K. [K.D. Science College, Pochilima, Hinjilicut,Pin-761101 Ganjam, Orissa (India)

2016-04-13

We report here a microscopic tight-binding model calculation for AB-stacked bilayer graphene in presence of biasing potential between the two layers and the impurity effects to study the evolution of the total density of states with special emphasis on opening of band gap near Dirac point. We have calculated the electron Green’s functions for both the A and B sub-lattices by Zubarev technique. The imaginary part of the Green’s function gives the partial and total density of states of electrons. The density of states are computed numerically for 1000 × 1000 grid points of the electron momentum. The evolution of the opening of band gap near van-Hove singularities as well as near Dirac point is investigated by varying the different interlayer hoppings and the biasing potentials. The inter layer hopping splits the density of states at van-Hove singularities and produces a V-shaped gap near Dirac point. Further the biasing potential introduces a U shaped gap near Dirac point with a density minimum at the applied potential(i.e. at V/2).

Effect of impurity doping on tunneling conductance in AB-stacked bi-layer graphene: A tight-binding study

International Nuclear Information System (INIS)

Rout, G. C.; Sahu, Sivabrata; Panda, S. K.

2016-01-01

We report here a microscopic tight-binding model calculation for AB-stacked bilayer graphene in presence of biasing potential between the two layers and the impurity effects to study the evolution of the total density of states with special emphasis on opening of band gap near Dirac point. We have calculated the electron Green’s functions for both the A and B sub-lattices by Zubarev technique. The imaginary part of the Green’s function gives the partial and total density of states of electrons. The density of states are computed numerically for 1000 × 1000 grid points of the electron momentum. The evolution of the opening of band gap near van-Hove singularities as well as near Dirac point is investigated by varying the different interlayer hoppings and the biasing potentials. The inter layer hopping splits the density of states at van-Hove singularities and produces a V-shaped gap near Dirac point. Further the biasing potential introduces a U shaped gap near Dirac point with a density minimum at the applied potential(i.e. at V/2).

Bulk and interface dielectric functions: New results within the tight-binding approximation

International Nuclear Information System (INIS)

Elvira, V.D.; Duran, J.C.

1991-01-01

A tight-binding approach is used to analyze the dielectric behaviour of bulk semiconductors and semiconductor interfaces. This time interactions between second nearest neighbours are taken into account and several electrostatic models are proposed for the induced charge density around the atoms. The bulk dielectric function of different semiconductors (Si, Ge, GaAs and AlAs) are obtained and compared with other theoretical and experimental results. Finally, the energy band offset for GaAs-AlAs(1,0,0) interface is obtained and related to bulk properties of both semiconductors. The results presented in this paper show how the use of very simple but more realistic electrostatic models improve the analysis of the screening properties in semiconductors, giving a new support to the consistent tight-binding method for studying characteristics related to those properties. (Author)

Investigation of gas molecules adsorption on carbon nano tubes electric properties in tight binding model

International Nuclear Information System (INIS)

Moradian, R.; Mohammadi, Y.

2007-01-01

Based on tight binding model we investigated effects of bi-atomic molecules gas(in the general form denoted by X 2 )on single-walled carbon nano tubes electronic properties. We found for some specified values of hopping integrals and random on-site energies, adsorbed molecules bound states located inside of the (10,0) single-walled carbon nano tubes energy gap, where it is similar to the reported experimental results for O 2 adsorption while for other values there is no bound states inside of energy gap. This is similar to the N 2 adsorption on semiconductor single-walled carbon nano tubes.

Strongly anisotropic spin-orbit splitting in a two-dimensional electron gas

DEFF Research Database (Denmark)

Michiardi, Matteo; Bianchi, Marco; Dendzik, Maciej

2015-01-01

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a...

Electronic properties in a two-dimensional disordered electron liquid: Spin-valley interplay

International Nuclear Information System (INIS)

Burmistrov, I. S.; Chtchelkatchev, N. M.

2008-01-01

We report a detailed study of the influence of the spin and valley splittings on such physical observables of the two-dimensional disordered electron liquid as resistivity and spin and valley susceptibilities. We explain qualitatively the nonmonotonic dependence of the resistivity on temperature in the presence of a parallel magnetic field. In the presence of either spin or valley splitting we predict a temperature dependence of the resistivity with two maximum points

Electronic Structure of Single- and Multiple-shell Carbon Fullerenes

OpenAIRE

Lin, Yeong-Lieh; Nori, Franco

1993-01-01

We study the electronic states of giant single-shell and the recently discovered nested multi-shell carbon fullerenes within the tight-binding approximation. We use two different approaches, one based on iterations and the other on symmetry, to obtain the $\\pi$-state energy spectra of large fullerene cages: $C_{240}$, $C_{540}$, $C_{960}$, $C_{1500}$, $C_{2160}$ and $C_{2940}$. Our iteration technique reduces the dimensionality of the problem by more than one order of magnitude (factors of $\\...

Exploring the interactions and binding sites between Cd and functional groups in soil using two-dimensional correlation spectroscopy and synchrotron radiation based spectromicroscopies

Energy Technology Data Exchange (ETDEWEB)

Sun, Fusheng [Jiangsu Provincial Key Lab for Organic Solid Waste Utilization and National Engineering Research Center for Organic-Based Fertilizers, College of Resources & Environmental Sciences, Nanjing Agricultural University, Nanjing 210095 (China); Department of Soil Science, North Carolina State University, Raleigh, NC 27695 (United States); Polizzotto, Matthew L. [Department of Soil Science, North Carolina State University, Raleigh, NC 27695 (United States); Guan, Dongxing [Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210026 (China); Wu, Jun [College of Environment, Zhejiang University of Technology, Hangzhou 310014 (China); Shen, Qirong; Ran, Wei [Jiangsu Provincial Key Lab for Organic Solid Waste Utilization and National Engineering Research Center for Organic-Based Fertilizers, College of Resources & Environmental Sciences, Nanjing Agricultural University, Nanjing 210095 (China); Wang, Boren [Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081 (China); Yu, Guanghui, E-mail: yuguanghui@njau.edu.cn [Jiangsu Provincial Key Lab for Organic Solid Waste Utilization and National Engineering Research Center for Organic-Based Fertilizers, College of Resources & Environmental Sciences, Nanjing Agricultural University, Nanjing 210095 (China)

2017-03-15

Highlights: • The interactions and binding between Cd and functional groups are essential for their fates. • Two-dimensional correlation spectroscopy can identify Cd binding to functional groups in soils. • Synchrotron radiation based spectromicroscopy shows the micro-scale distribution of Cd in soils. • Soil functional groups controlling Cd binding can be modified by fertilization treatments. - Abstract: Understanding how heavy metals bind and interact in soils is essential for predicting their distributions, reactions and fates in the environment. Here we propose a novel strategy, i.e., combining two-dimensional correlation spectroscopy (2D COS) and synchrotron radiation based spectromicroscopies, for identifying heavy metal binding to functional groups in soils. The results showed that although long-term (23 yrs) organic fertilization treatment caused the accumulation of Cd (over 3 times) in soils when compared to no fertilization and chemical fertilization treatments, it significantly (p < 0.05) reduced the Cd concentration in wheat grain. The 2D COS analyses demonstrated that soil functional groups controlling Cd binding were modified by fertilization treatments, providing implications for the reduced bioavailability of heavy metals in organic fertilized soils. Furthermore, correlative micro X-ray fluorescence spectromicroscopy, electron probe micro-analyzer mapping, and synchrotron-radiation-based FTIR spectromicroscopy analysis showed that Cd, minerals, and organic functional groups were heterogeneously distributed at the micro-scale in soil colloids. Only minerals, rather than organic groups, had a similar distribution pattern with Cd. Together, this strategy has a potential to explore the interactions and binding sites among heavy metals, minerals and organic components in soil.

Two-dimensional vibrational-electronic spectroscopy

Science.gov (United States)

Courtney, Trevor L.; Fox, Zachary W.; Slenkamp, Karla M.; Khalil, Munira

2015-10-01

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (νCN) and either a ligand-to-metal charge transfer transition ([FeIII(CN)6]3- dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN)5FeIICNRuIII(NH3)5]- dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific νCN modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems.

Two-dimensional vibrational-electronic spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Courtney, Trevor L.; Fox, Zachary W.; Slenkamp, Karla M.; Khalil, Munira, E-mail: mkhalil@uw.edu [Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195 (United States)

2015-10-21

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (ν{sub CN}) and either a ligand-to-metal charge transfer transition ([Fe{sup III}(CN){sub 6}]{sup 3−} dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN){sub 5}Fe{sup II}CNRu{sup III}(NH{sub 3}){sub 5}]{sup −} dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific ν{sub CN} modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a

Theory of the electronic structure of dilute bismide and bismide-nitride alloys of GaAs: Tight-binding and k.p models

International Nuclear Information System (INIS)

Usman, Muhammad; Broderick, Christopher A.; O'Reilly, Eoin P.

2013-01-01

The addition of dilute concentrations of bismuth (Bi) into GaAs to form GaBi x As 1−x alloys results in a large reduction of the band gap energy (E g ) accompanied by a significant increase of the spin-orbit-splitting energy (Δ SO ), leading to an E g SO regime for x ∼ 10% which is technologically relevant for the design of highly efficient photonic devices. The quaternary alloy GaBi x N y As 1−x−y offers further flexibility for band gap tuning, because both nitrogen and bismuth can independently induce band gap reduction. This work reports sp 3 s* tight binding and 14-band k⋅p models for the study of the electronic structure of GaBi x As 1−x and GaBi x N y As 1−x−y alloys. Our results are in good agreement with the available experimental data

Two-dimensional electronic spectroscopy with birefringent wedges

Energy Technology Data Exchange (ETDEWEB)

Réhault, Julien; Maiuri, Margherita; Oriana, Aurelio; Cerullo, Giulio [IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano (Italy)

2014-12-15

We present a simple experimental setup for performing two-dimensional (2D) electronic spectroscopy in the partially collinear pump-probe geometry. The setup uses a sequence of birefringent wedges to create and delay a pair of phase-locked, collinear pump pulses, with extremely high phase stability and reproducibility. Continuous delay scanning is possible without any active stabilization or position tracking, and allows to record rapidly and easily 2D spectra. The setup works over a broad spectral range from the ultraviolet to the near-IR, it is compatible with few-optical-cycle pulses and can be easily reconfigured to two-colour operation. A simple method for scattering suppression is also introduced. As a proof of principle, we present degenerate and two-color 2D spectra of the light-harvesting complex 1 of purple bacteria.

Thermoelectric power and topological transitions in quasi-two-dimensional electronic systems

International Nuclear Information System (INIS)

Blanter, Ya.M.; Pantsulaya, A.V.; Varlamov, A.A.

1991-05-01

Electron-impurity relaxation time and the thermoelectric power (TEP) of quasi-two-dimensional electron gas are calculated. Two cases are discussed: the isotropic spectrum and the electronic topological transition (ETT) of the ''neck-breaking'' type. Methods of thermal diagramatic technique are used for the calculation. It is found that the TEP in the vicinity of the ETT greatly exceeds its background value. The results of experimental investigations of the TEP in the metal-oxide-semiconductor structures are compared with the predictions of the proposed theory. (author). 17 refs, 5 figs

Calculation of elastic constants of BCC transition metals: tight-binding recursion method

International Nuclear Information System (INIS)

Masuda, K.; Hamada, N.; Terakura, K.

1984-01-01

The elastic constants of BCC transition metals (Fe, Nb, Mo and W) are calculated by using the tight-binding d band and the Born-Mayer repulsive potential. Introducing a small distortion characteristic to C 44 (or C') elastic deformation and calculating the energy change up to second order in the atomic displacement, the shear elastic constants C 44 and C' are determined. The elastic constants C 11 and C 12 are then calculated by using the relations B=1/3(C 11 + 2C 12 ) and C'=1/2(C 11 -C 12 ), where B is the bulk modulus. In general, the agreement between the present results and the experimental values is satisfactory. The characteristic elasticity behaviour, i.e. the strong Nsub(d) (number of d electrons) dependence of the observed anisotropy factor A=C 44 /C', will also be discussed. (author)

The tight binding model study of the role of anisotropic AFM spin ordering in the charge ordered CMR manganites

Science.gov (United States)

Kar, J. K.; Panda, Saswati; Rout, G. C.

2017-05-01

We propose here a tight binding model study of the interplay between charge and spin orderings in the CMR manganites taking anisotropic effect due to electron hoppings and spin exchanges. The Hamiltonian consists of the kinetic energies of eg and t2g electrons of manganese ion. It further includes double exchange and Heisenberg interactions. The charge density wave interaction (CDW) describes an extra mechanism for the insulating character of the system. The CDW gap and spin parameters are calculated using Zubarev's Green's function technique and computed self-consistently. The results are reported in this communication.

Magnetooscillations of the tunneling current between two-dimensional electron systems

International Nuclear Information System (INIS)

Raichev, O.E.; Vasko, F.T.

1995-08-01

We calculate electric current caused by electron tunnelling between two-dimensional layers in the magnetic field applied perpendicular to the layers. An elastic scattering of the electrons is taken into account. Analytical results are obtained for two regimes: i) small magnetic field, when the Landau quantization is suppressed by the scattering and the oscillatory part of the current shows nearly harmonic behaviour; ii) high magnetic field, when the Landau levels are well-defined and the conductivity shows series of sharp peaks corresponding to resonant magnetotunneling. In the last case, we used two alternative approaches: self-consistent Born approximation and path integral method, and compared obtained results. (author). 12 refs, 3 figs

Ultra-fast computation of electronic spectra for large systems by tight-binding based simplified Tamm-Dancoff approximation (sTDA-xTB)

International Nuclear Information System (INIS)

Grimme, Stefan; Bannwarth, Christoph

2016-01-01

The computational bottleneck of the extremely fast simplified Tamm-Dancoff approximated (sTDA) time-dependent density functional theory procedure [S. Grimme, J. Chem. Phys. 138, 244104 (2013)] for the computation of electronic spectra for large systems is the determination of the ground state Kohn-Sham orbitals and eigenvalues. This limits such treatments to single structures with a few hundred atoms and hence, e.g., sampling along molecular dynamics trajectories for flexible systems or the calculation of chromophore aggregates is often not possible. The aim of this work is to solve this problem by a specifically designed semi-empirical tight binding (TB) procedure similar to the well established self-consistent-charge density functional TB scheme. The new special purpose method provides orbitals and orbital energies of hybrid density functional character for a subsequent and basically unmodified sTDA procedure. Compared to many previous semi-empirical excited state methods, an advantage of the ansatz is that a general eigenvalue problem in a non-orthogonal, extended atomic orbital basis is solved and therefore correct occupied/virtual orbital energy splittings as well as Rydberg levels are obtained. A key idea for the success of the new model is that the determination of atomic charges (describing an effective electron-electron interaction) and the one-particle spectrum is decoupled and treated by two differently parametrized Hamiltonians/basis sets. The three-diagonalization-step composite procedure can routinely compute broad range electronic spectra (0-8 eV) within minutes of computation time for systems composed of 500-1000 atoms with an accuracy typical of standard time-dependent density functional theory (0.3-0.5 eV average error). An easily extendable parametrization based on coupled-cluster and density functional computed reference data for the elements H–Zn including transition metals is described. The accuracy of the method termed sTDA-xTB is first

Ultra-fast computation of electronic spectra for large systems by tight-binding based simplified Tamm-Dancoff approximation (sTDA-xTB)

Energy Technology Data Exchange (ETDEWEB)

Grimme, Stefan, E-mail: grimme@thch.uni-bonn.de; Bannwarth, Christoph [Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms Universität Bonn, Beringstraße 4, 53115 Bonn (Germany)

2016-08-07

The computational bottleneck of the extremely fast simplified Tamm-Dancoff approximated (sTDA) time-dependent density functional theory procedure [S. Grimme, J. Chem. Phys. 138, 244104 (2013)] for the computation of electronic spectra for large systems is the determination of the ground state Kohn-Sham orbitals and eigenvalues. This limits such treatments to single structures with a few hundred atoms and hence, e.g., sampling along molecular dynamics trajectories for flexible systems or the calculation of chromophore aggregates is often not possible. The aim of this work is to solve this problem by a specifically designed semi-empirical tight binding (TB) procedure similar to the well established self-consistent-charge density functional TB scheme. The new special purpose method provides orbitals and orbital energies of hybrid density functional character for a subsequent and basically unmodified sTDA procedure. Compared to many previous semi-empirical excited state methods, an advantage of the ansatz is that a general eigenvalue problem in a non-orthogonal, extended atomic orbital basis is solved and therefore correct occupied/virtual orbital energy splittings as well as Rydberg levels are obtained. A key idea for the success of the new model is that the determination of atomic charges (describing an effective electron-electron interaction) and the one-particle spectrum is decoupled and treated by two differently parametrized Hamiltonians/basis sets. The three-diagonalization-step composite procedure can routinely compute broad range electronic spectra (0-8 eV) within minutes of computation time for systems composed of 500-1000 atoms with an accuracy typical of standard time-dependent density functional theory (0.3-0.5 eV average error). An easily extendable parametrization based on coupled-cluster and density functional computed reference data for the elements H–Zn including transition metals is described. The accuracy of the method termed sTDA-xTB is first

Slater-Koster Tight-Binding parametrization of single and few-layer Black-Phosphorus from first-principles calculations

Science.gov (United States)

Menezes, Marcos; Capaz, Rodrigo

Black Phosphorus (BP) is a promising material for applications in electronics, especially due to the tuning of its band gap by increasing the number of layers. In single-layer BP, also called Phosphorene, the P atoms form two staggered chains bonded by sp3 hybridization, while neighboring layers are bonded by Van-der-Waals interactions. In this work, we present a Tight-Binding (TB) parametrization of the electronic structure of single and few-layer BP, based on the Slater-Koster model within the two-center approximation. Our model includes all 3s and 3p orbitals, which makes this problem more complex than that of graphene, where only 2pz orbitals are needed for most purposes. The TB parameters are obtained from a least-squares fit of DFT calculations carried on the SIESTA code. We compare the results for different basis-sets used to expand the ab-initio wavefunctions and discuss their applicability. Our model can fit a larger number of bands than previously reported calculations based on Wannier functions. Moreover, our parameters have a clear physical interpretation based on chemical bonding. As such, we expect our results to be useful in a further understanding of multilayer BP and other 2D-materials characterized by strong sp3 hybridization. CNPq, FAPERJ, INCT-Nanomateriais de Carbono.

Superconductivity in engineered two-dimensional electron gases

Science.gov (United States)

Chubukov, Andrey V.; Kivelson, Steven A.

2017-11-01

We consider Kohn-Luttinger mechanism for superconductivity in a two-dimensional electron gas confined to a narrow well between two grounded metallic planes with two occupied subbands with Fermi momenta kF L>kF S . On the basis of a perturbative analysis, we conclude that non-s -wave superconductivity emerges even when the bands are parabolic. We analyze the conditions that maximize Tc as a function of the distance to the metallic planes, the ratio kF L/kF S , and rs, which measures the strength of Coulomb correlations. The largest attraction is in p -wave and d -wave channels, of which p wave is typically the strongest. For rs=O (1 ) we estimate that the dimensionless coupling λ ≈10-1 , but it likely continues increasing for larger rs (where we lose theoretical control).

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. I. Model and validation

Energy Technology Data Exchange (ETDEWEB)

Hegde, Ganesh, E-mail: ghegde@purdue.edu; Povolotskyi, Michael; Kubis, Tillmann; Klimeck, Gerhard, E-mail: gekco@purdue.edu [Network for Computational Nanotechnology (NCN), Department of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907 (United States); Boykin, Timothy [Department of Electrical and Computer Engineering, University of Alabama, Huntsville, Alabama (United States)

2014-03-28

Semi-empirical Tight Binding (TB) is known to be a scalable and accurate atomistic representation for electron transport for realistically extended nano-scaled semiconductor devices that might contain millions of atoms. In this paper, an environment-aware and transferable TB model suitable for electronic structure and transport simulations in technologically relevant metals, metallic alloys, metal nanostructures, and metallic interface systems are described. Part I of this paper describes the development and validation of the new TB model. The new model incorporates intra-atomic diagonal and off-diagonal elements for implicit self-consistency and greater transferability across bonding environments. The dependence of the on-site energies on strain has been obtained by appealing to the Moments Theorem that links closed electron paths in the system to energy moments of angular momentum resolved local density of states obtained ab initio. The model matches self-consistent density functional theory electronic structure results for bulk face centered cubic metals with and without strain, metallic alloys, metallic interfaces, and metallic nanostructures with high accuracy and can be used in predictive electronic structure and transport problems in metallic systems at realistically extended length scales.

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. I. Model and validation

International Nuclear Information System (INIS)

Hegde, Ganesh; Povolotskyi, Michael; Kubis, Tillmann; Klimeck, Gerhard; Boykin, Timothy

2014-01-01

Semi-empirical Tight Binding (TB) is known to be a scalable and accurate atomistic representation for electron transport for realistically extended nano-scaled semiconductor devices that might contain millions of atoms. In this paper, an environment-aware and transferable TB model suitable for electronic structure and transport simulations in technologically relevant metals, metallic alloys, metal nanostructures, and metallic interface systems are described. Part I of this paper describes the development and validation of the new TB model. The new model incorporates intra-atomic diagonal and off-diagonal elements for implicit self-consistency and greater transferability across bonding environments. The dependence of the on-site energies on strain has been obtained by appealing to the Moments Theorem that links closed electron paths in the system to energy moments of angular momentum resolved local density of states obtained ab initio. The model matches self-consistent density functional theory electronic structure results for bulk face centered cubic metals with and without strain, metallic alloys, metallic interfaces, and metallic nanostructures with high accuracy and can be used in predictive electronic structure and transport problems in metallic systems at realistically extended length scales

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. I. Model and validation

Science.gov (United States)

Hegde, Ganesh; Povolotskyi, Michael; Kubis, Tillmann; Boykin, Timothy; Klimeck, Gerhard

2014-03-01

Semi-empirical Tight Binding (TB) is known to be a scalable and accurate atomistic representation for electron transport for realistically extended nano-scaled semiconductor devices that might contain millions of atoms. In this paper, an environment-aware and transferable TB model suitable for electronic structure and transport simulations in technologically relevant metals, metallic alloys, metal nanostructures, and metallic interface systems are described. Part I of this paper describes the development and validation of the new TB model. The new model incorporates intra-atomic diagonal and off-diagonal elements for implicit self-consistency and greater transferability across bonding environments. The dependence of the on-site energies on strain has been obtained by appealing to the Moments Theorem that links closed electron paths in the system to energy moments of angular momentum resolved local density of states obtained ab initio. The model matches self-consistent density functional theory electronic structure results for bulk face centered cubic metals with and without strain, metallic alloys, metallic interfaces, and metallic nanostructures with high accuracy and can be used in predictive electronic structure and transport problems in metallic systems at realistically extended length scales.

Properties and modification of two-dimensional electronic states on noble metals; Eigenschaften und Modifikation zweidimensionaler Elektronenzustaende auf Edelmetallen

Energy Technology Data Exchange (ETDEWEB)

Forster, F.

2007-07-06

In this thesis investigations on two-dimensional electronic structures of (111)-noble metal surfaces and the influence of various adsorbates upon them is presented. It chiefly focuses on the surface-localized Shockley states of Cu, Ag and Au and their band dispersion (binding energy, band mass, and spin-orbit splitting) which turns out to be a sensitive probe for surface modifications induced by adsorption processes. Angular resolved photoelectron spectroscopy enables the observation of even subtle changes in the electronic band structure of these two dimensional systems. Different mechanisms taking place at surfaces and the substrate/adsorbate interfaces influence the Shockley state in a different manner and will be analyzed using suitable adsorbate model systems. The experimental results are matched with appropriate theoretical models like the phase accumulation model and the nearly-free electron model and - if possible - with ab initio calculations based on density functional theory. This allows for the integration of the results into a stringent overall picture. The influence of sub-monolayer adsorption of Na upon the surface state regarding the significant change in surface work function is determined. A systematic study of the physisorption of noble gases shows the effect of the repulsive adsorbate-substrate interaction upon the electrons of the surface state. A step-by-step coverage of the Cu and Au(111) surfaces by monolayers of Ag creates a gradual change in the surface potential and causes the surface state to become increasingly Ag-like. For N=7 ML thick and layer-by-layer growing Ag films on Au(111), new two-dimensional electronic structures can be observed, which are attributed to the quantum well states of the Ag adsorbate. The question whether they are localized within the Ag-layer or substantially within the substrate is resolved by the investigation of their energetic and spatial evolution with increasing Ag-film thicknesses N. For this, beside the

NMRKIN: Simulating line shapes from two-dimensional spectra of proteins upon ligand binding

International Nuclear Information System (INIS)

Guenther, Ulrich L.; Schaffhausen, Brian

2002-01-01

The analysis of the shape of signals in NMR spectra is a powerful tool to study exchange and reaction kinetics. Line shapes in two-dimensional spectra of proteins recorded for titrations with ligands provide information about binding rates observed at individual residues. Here we describe a fast method to simulate a series of line shapes derived from two-dimensional spectra of a protein during a ligand titration. This procedure, which takes the mutual effects of two dimensions into account, has been implemented in MATLAB as an add-on to NMRLab (Guenther et al., 2000). In addition, more complex kinetic models, including sequential and parallel reactions, were simulated to demonstrate common features of more complex line shapes which could be encountered in protein-ligand interactions. As an example of this method, we describe its application to line shapes obtained for a titration of the p85 N-SH2 domain of PI3-kinase with a peptide derived from polyomavirus middle T antigen (MT)

Tight-Binding simulations of InAs/GaAs quantum dots

International Nuclear Information System (INIS)

Kleinsorge, Alexander

2008-01-01

For several years, the technological potential of self-organized grown quantum dots (QD) has been known. Their usage as an effective light source or memory requires the precise prediction of their electronic properties. Hence, this report will study InAs quantum dots at GaAs substrate. After relaxing the atomic positions with a many body potential of Abell-Tersoff type, I calculated the electronic structure using the Tight-Binding method which is reasonable for large systems. During the investigation of wavefunctions depend on the shape, size and temperature, the impact of strain showed up as the main reason for the p-splitting. Typically flat QDs (relative to lateral dimensions) are grown, therefore the energy of bound states depends mostly on their height. The crystal's orientation had a strong impact on the wavefunctions. Moreover, the understanding of STS experiments, which inspected the connection between shape and wavefunction, is better now. Because of the possible simultaneous occupation of semiconductor quantum dots with an electron and a hole, there is a dipole moment of the exciton (due to their different behaviour inside the QD). This is a further experimental access to inner details of the QD. I ascertained the interplay of composition profile and dipole moment. The force caused by additional potentials (piezoelectricity, outer homogeneous and inhomogeneous electrical fields) was also an subject of my inquiries. To conclude, I executed kMC simulations, to better apprehend the annealing experiments. I was able to explain the narrowing of the PL peak width better. Furthermore I showed a ramification of the strain field to the diffusion development (and the following electronic properties). (orig.)

Two dimensional electron systems for solid state quantum computation

Science.gov (United States)

Mondal, Sumit

Two dimensional electron systems based on GaAs/AlGaAs heterostructures are extremely useful in various scientific investigations of recent times including the search for quantum computational schemes. Although significant strides have been made over the past few years to realize solid state qubits on GaAs/AlGaAs 2DEGs, there are numerous factors limiting the progress. We attempt to identify factors that have material and design-specific origin and develop ways to overcome them. The thesis is divided in two broad segments. In the first segment we describe the realization of a new field-effect induced two dimensional electron system on GaAs/AlGaAs heterostructure where the novel device-design is expected to suppress the level of charge noise present in the device. Modulation-doped GaAs/AlGaAs heterostructures are utilized extensively in the study of quantum transport in nanostructures, but charge fluctuations associated with remote ionized dopants often produce deleterious effects. Electric field-induced carrier systems offer an attractive alternative if certain challenges can be overcome. We demonstrate a field-effect transistor in which the active channel is locally devoid of modulation-doping, but silicon dopant atoms are retained in the ohmic contact region to facilitate low-resistance contacts. A high quality two-dimensional electron gas is induced by a field-effect that is tunable over a density range of 6.5x10 10cm-2 to 2.6x1011cm-2 . Device design, fabrication, and low temperature (T=0.3K) characterization results are discussed. The demonstrated device-design overcomes several existing limitations in the fabrication of field-induced 2DEGs and might find utility in hosting nanostructures required for making spin qubits. The second broad segment describes our effort to correlate transport parameters measured at T=0.3K to the strength of the fractional quantum Hall state observed at nu=5/2 in the second Landau level of high-mobility GaAs/AlGaAs two dimensional

Observation of Zero-Dimensional States in a One-Dimensional Electron Interferometer

NARCIS (Netherlands)

Wees, B.J. van; Kouwenhoven, L.P.; Harmans, C.J.P.M.; Williamson, J.G.; Timmering, C.E.; Broekaart, M.E.I.; Foxon, C.T.; Harris, J.J.

1989-01-01

We have studied the electron transport in a one-dimensional electron interferometer. It consists of a disk-shaped two-dimensional electron gas, to which quantum point contacts are attached. Discrete zero-dimensional states are formed due to constructive interference of electron waves traveling along

Band Alignment Determination of Two-Dimensional Heterojunctions and Their Electronic Applications

KAUST Repository

Chiu, Ming-Hui

2018-01-01

Two-dimensional (2D) layered materials such as MoS2 have been recognized as high on-off ratio semiconductors which are promising candidates for electronic and optoelectronic devices. In addition to the use of individual 2D materials, the accelerated

Band structure and orbital character of monolayer MoS2 with eleven-band tight-binding model

Science.gov (United States)

Shahriari, Majid; Ghalambor Dezfuli, Abdolmohammad; Sabaeian, Mohammad

2018-02-01

In this paper, based on a tight-binding (TB) model, first we present the calculations of eigenvalues as band structure and then present the eigenvectors as probability amplitude for finding electron in atomic orbitals for monolayer MoS2 in the first Brillouin zone. In these calculations we are considering hopping processes between the nearest-neighbor Mo-S, the next nearest-neighbor in-plan Mo-Mo, and the next nearest-neighbor in-plan and out-of-plan S-S atoms in a three-atom based unit cell of two-dimensional rhombic MoS2. The hopping integrals have been solved in terms of Slater-Koster and crystal field parameters. These parameters are calculated by comparing TB model with the density function theory (DFT) in the high-symmetry k-points (i.e. the K- and Γ-points). In our TB model all the 4d Mo orbitals and the 3p S orbitals are considered and detailed analysis of the orbital character of each energy level at the main high-symmetry points of the Brillouin zone is described. In comparison with DFT calculations, our results of TB model show a very good agreement for bands near the Fermi level. However for other bands which are far from the Fermi level, some discrepancies between our TB model and DFT calculations are observed. Upon the accuracy of Slater-Koster and crystal field parameters, on the contrary of DFT, our model provide enough accuracy to calculate all allowed transitions between energy bands that are very crucial for investigating the linear and nonlinear optical properties of monolayer MoS2.

A study of the performance of tight-binding models for silicon and silicon-germanium alloys

Science.gov (United States)

Roberts, Amanda Killen

1998-11-01

An important challenge in achieving small-scale semiconductor devices is to confine dopants to small, well-defined regions because device performance depends on their accurate placement. However, semiconductor processing involves repeated annealing cycles which can cause dopants to diffuse away from their intended locations. For this reason, it is important to understand the basic physical processes of dopant diffusion on atomic length scales. Tight binding models offer the possibility of studying diffusion in larger systems and for longer time scales than is possible with current LDA methods. However, while a wide variety of tight binding models exist for silicon, these models are not necessarily suited for dynamical studies and they are rarely extended to elements which are dopants in silicon, or to multicomponent systems. This dissertation addresses these issues. We present the first systematic comparison of three parameterized, two-center, sp-based, tight binding models which, because of their simplicity, are suitable for dynamical studies. The models we considered are those by Goodwin et al. (GSP), Kwon et al., and Sawada. We evaluated these models for Si to determine their relative strengths and weaknesses in comparison to experimental and LDA results. Our results show that none of these models is outstanding over the others, and all give acceptable representations of the properties of Si which are of interest for dynamical studies. Having carefully investigated the fitting process to find simple ways to fit tight binding parameters, we have provided information as to the role of each of the GSP parameters in the fitting procedure. As a result, we have recorded a detailed prescription for fitting which can be followed by researchers wanting to extend the models to additional species. Based on our findings about the performance of the Si models, we extended the GSP model to second-nearest neighbors and produced new parameter sets for Si, Ge, and SiGe. This has

Quantum confined Stark effect in Gaussian quantum wells: A tight-binding study

International Nuclear Information System (INIS)

Ramírez-Morales, A.; Martínez-Orozco, J. C.; Rodríguez-Vargas, I.

2014-01-01

The main characteristics of the quantum confined Stark effect (QCSE) are studied theoretically in quantum wells of Gaussian profile. The semi-empirical tight-binding model and the Green function formalism are applied in the numerical calculations. A comparison of the QCSE in quantum wells with different kinds of confining potential is presented

Quantum confined Stark effect in Gaussian quantum wells: A tight-binding study

Energy Technology Data Exchange (ETDEWEB)

Ramírez-Morales, A.; Martínez-Orozco, J. C.; Rodríguez-Vargas, I. [Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad Esquina Con Paseo La Bufa S/N, 98060 Zacatecas, Zac. (Mexico)

2014-05-15

The main characteristics of the quantum confined Stark effect (QCSE) are studied theoretically in quantum wells of Gaussian profile. The semi-empirical tight-binding model and the Green function formalism are applied in the numerical calculations. A comparison of the QCSE in quantum wells with different kinds of confining potential is presented.

Two-dimensional collective electron magnetotransport, oscillations, and chaos in a semiconductor superlattice.

Science.gov (United States)

Bonilla, L L; Carretero, M; Segura, A

2017-12-01

When quantized, traces of classically chaotic single-particle systems include eigenvalue statistics and scars in eigenfuntions. Since 2001, many theoretical and experimental works have argued that classically chaotic single-electron dynamics influences and controls collective electron transport. For transport in semiconductor superlattices under tilted magnetic and electric fields, these theories rely on a reduction to a one-dimensional self-consistent drift model. A two-dimensional theory based on self-consistent Boltzmann transport does not support that single-electron chaos influences collective transport. This theory agrees with existing experimental evidence of current self-oscillations, predicts spontaneous collective chaos via a period doubling scenario, and could be tested unambiguously by measuring the electric potential inside the superlattice under a tilted magnetic field.

Two-dimensional collective electron magnetotransport, oscillations, and chaos in a semiconductor superlattice

Science.gov (United States)

Bonilla, L. L.; Carretero, M.; Segura, A.

2017-12-01

When quantized, traces of classically chaotic single-particle systems include eigenvalue statistics and scars in eigenfuntions. Since 2001, many theoretical and experimental works have argued that classically chaotic single-electron dynamics influences and controls collective electron transport. For transport in semiconductor superlattices under tilted magnetic and electric fields, these theories rely on a reduction to a one-dimensional self-consistent drift model. A two-dimensional theory based on self-consistent Boltzmann transport does not support that single-electron chaos influences collective transport. This theory agrees with existing experimental evidence of current self-oscillations, predicts spontaneous collective chaos via a period doubling scenario, and could be tested unambiguously by measuring the electric potential inside the superlattice under a tilted magnetic field.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

Science.gov (United States)

Sanders, Nocona; Bayerl, Dylan; Shi, Guangsha; Mengle, Kelsey A; Kioupakis, Emmanouil

2017-12-13

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

Nonlocality, Correlations, and Magnetotransport in a Spatially Modulated Two-Dimensional Electron Gas

Science.gov (United States)

Raichev, O. E.

2018-04-01

It is shown that the classical commensurability phenomena in weakly modulated two-dimensional electron systems is a manifestation of the intrinsic properties of the correlation functions describing a homogeneous electron gas in a magnetic field. The theory demonstrates the importance for consideration of nonlocal response and removes the gap between classical and quantum approaches to magnetotransport in such systems.

Vibrationally resolved UV/Vis spectroscopy with time-dependent density functional based tight binding

NARCIS (Netherlands)

Ruger, R.; Niehaus, T.; van Lenthe, E.; Heine, T.; Visscher, L.

2016-01-01

We report a time-dependent density functional based tight-binding (TD-DFTB) scheme for the calculation of UV/Vis spectra, explicitly taking into account the excitation of nuclear vibrations via the adiabatic Hessian Franck-Condon method with a harmonic approximation for the nu- clear wavefunction.

Spin-polarized transport in a two-dimensional electron gas with interdigital-ferromagnetic contacts

DEFF Research Database (Denmark)

Hu, C.-M.; Nitta, Junsaku; Jensen, Ane

2001-01-01

Ferromagnetic contacts on a high-mobility, two-dimensional electron gas (2DEG) in a narrow gap semiconductor with strong spin-orbit interaction are used to investigate spin-polarized electron transport. We demonstrate the use of magnetized contacts to preferentially inject and detect specific spi...

Scaling Universality between Band Gap and Exciton Binding Energy of Two-Dimensional Semiconductors

Science.gov (United States)

Jiang, Zeyu; Liu, Zhirong; Li, Yuanchang; Duan, Wenhui

2017-06-01

Using first-principles G W Bethe-Salpeter equation calculations and the k .p theory, we unambiguously show that for two-dimensional (2D) semiconductors, there exists a robust linear scaling law between the quasiparticle band gap (Eg) and the exciton binding energy (Eb), namely, Eb≈Eg/4 , regardless of their lattice configuration, bonding characteristic, as well as the topological property. Such a parameter-free universality is never observed in their three-dimensional counterparts. By deriving a simple expression for the 2D polarizability merely with respect to Eg, and adopting the screened hydrogen model for Eb, the linear scaling law can be deduced analytically. This work provides an opportunity to better understand the fantastic consequence of the 2D nature for materials, and thus offers valuable guidance for their property modulation and performance control.

Environment-dependent crystal-field tight-binding based on density-functional theory

International Nuclear Information System (INIS)

Urban, Alexander

2012-01-01

Electronic structure calculations based on Kohn-Sham density-functional theory (DFT) allow the accurate prediction of chemical bonding and materials properties. Due to the high computational demand DFT calculations are, however, restricted to structures containing at most several hundreds of atoms, i.e., to length scales of a few nanometers. Though, many processes of technological relevance, for example in the field of nanoelectronics, are governed by phenomena that occur on a slightly larger length scale of up to 100 nanometers, which corresponds to tens of thousands of atoms. The semiempirical Slater-Koster tight-binding (TB) method makes it feasible to calculate the electronic structure of such large systems. In contrast to first-principles-based DFT, which is universally applicable to almost all chemical species, the TB method is based on parametrized models that are usually specialized for a particular application or for one certain class of compounds. Usually the model parameters (Slater-Koster tables) are empirically adjusted to reproduce either experimental reference data (e.g., geometries, elastic constants) or data from first-principles methods such as DFT. The construction of a new TB model is therefore connected with a considerable effort that is often contrasted by a low transferability of the parametrization. In this thesis we develop a systematic methodology for the derivation of accurate and transferable TB models from DFT calculations. Our procedure exploits the formal relationship between the two methods, according to which the TB total energy can be understood as a direct approximation of the Kohn--Sham energy functional. The concept of our method is different to previous approaches such as the DFTB method, since it allows to extract TB parameters from converged DFT wave functions and Hamiltonians of arbitrary reference structures. In the following the different subjects of this thesis are briefly summarized. We introduce a new technique for the

Coupling of Hubbard fermions with phonons in La{sub 2} CuO{sub 4}: A combined study using density-functional theory and the generalized tight-binding method

Energy Technology Data Exchange (ETDEWEB)

Shneyder, E.I., E-mail: shneyder@iph.krasn.ru [Kirensky Institute of Physics SB RAS, Krasnoyarsk 660036 (Russian Federation); Reshetnev Siberian State Aerospace University, Krasnoyarsk 660014 (Russian Federation); Spitaler, J. [Materials Center Leoben Forschung GmbH, Rosegger-Straße 18, A-8700 Leoben (Austria); Kokorina, E.E.; Nekrasov, I.A. [Institute of Electrophysics UB RAS, Amundsena Str. 106, 620016 Yekaterinburg (Russian Federation); Gavrichkov, V.A. [Kirensky Institute of Physics SB RAS, Krasnoyarsk 660036 (Russian Federation); Draxl, C. [Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Großen Windkanal 6, 12489 Berlin (Germany); Ovchinnikov, S.G. [Kirensky Institute of Physics SB RAS, Krasnoyarsk 660036 (Russian Federation)

2015-11-05

We present results for the electron-phonon interaction of the Γ-point phonons in the tetragonal high-temperature phase of La{sub 2} CuO{sub 4} obtained from a hybrid scheme, combining density-functional theory (DFT) with the generalized tight-binding approach. As a starting point, eigenfrequencies and eigenvectors for the Γ-point phonons are determined from DFT within the frozen phonon approach utilizing the augmented plane wave + local orbitals method. The so obtained characteristics of electron-phonon coupling are converted into parameters of the generalized tight-binding method. This approach is a version of cluster perturbation theory and takes the strong on-site electron correlations into account. The obtained parameters describe the interaction of phonons with Hubbard fermions which form quasiparticle bands in strongly correlated electron systems. As a result, it is found that the Γ-point phonons with the strongest electron-phonon interaction are the A{sub 2u} modes (236 cm{sup −1}, 131 cm{sup −1} and 476 cm{sup −1}). Finally it is shown, that the single-electron spectral-weight redistribution between different Hubbard fermion quasiparticles results in a suppression of electron-phonon interaction which is strongest for the triplet Hubbard band with z oriented copper and oxygen electrons. - Highlights: • Electron-phonon interaction in strongly correlated electron systems is analyzed. • Interaction parameters between strongly correlated electrons and phonons are obtained. • The suppression of these parameters by strong electron correlations is demonstrated.

Dimensional reduction of a generalized flux problem

International Nuclear Information System (INIS)

Moroz, A.

1992-01-01

In this paper, a generalized flux problem with Abelian and non-Abelian fluxes is considered. In the Abelian case we shall show that the generalized flux problem for tight-binding models of noninteracting electrons on either 2n- or (2n + 1)-dimensional lattice can always be reduced to an n-dimensional hopping problem. A residual freedom in this reduction enables one to identify equivalence classes of hopping Hamiltonians which have the same spectrum. In the non-Abelian case, the reduction is not possible in general unless the flux tensor factorizes into an Abelian one times are element of the corresponding algebra

Orbital order and effective mass enhancement in t2 g two-dimensional electron gases

Science.gov (United States)

Tolsma, John; Principi, Alessandro; Polini, Marco; MacDonald, Allan

2015-03-01

It is now possible to prepare d-electron two-dimensional electron gas systems that are confined near oxide heterojunctions and contain t2 g electrons with a density much smaller than one electron per metal atom. I will discuss a generic model that captures all qualitative features of electron-electron interaction physics in t2 g two-dimensional electron gas systems, and the use of a GW approximation to explore t2 g quasiparticle properties in this new context. t2 g electron gases contain a high density isotropic light mass xy component and low-density xz and yz anisotropic components with light and heavy masses in orthogonal directions. The high density light mass band screens interactions within the heavy bands. As a result the wave vector dependence of the self-energy is reduced and the effective mass is increased. When the density in the heavy bands is low, the difference in anisotropy between the two heavy bands favors orbital order. When orbital order does not occur, interactions still reshape the heavy-band Fermi surfaces. I will discuss these results in the context of recently reported magnetotransport experiments.

Origin of Hund's multiplicity rule in quasi-two-dimensional two-electron quantum dots

International Nuclear Information System (INIS)

Sako, Tokuei; Paldus, Josef; Diercksen, Geerd H. F.

2010-01-01

The origin of Hund's multiplicity rules has been studied for a system of two electrons confined by a quasi-two-dimensional harmonic-oscillator potential by relying on a full configuration interaction wave function and Cartesian anisotropic Gaussian basis sets. In terms of appropriate normal-mode coordinates the wave function factors into a product of the center-of-mass and the internal components. The 1 Π u singlet state and the 3 Π u triplet state represent the energetically lowest pair of states to which Hund's multiplicity rule applies. They are shown to involve excitations into different degrees of freedom, namely, into the center-of-mass angular mode and the internal angular mode for the singlet and triplet states, respectively. The presence of an angular nodal line in the internal space allows then the triplet state to avoid the singularity in the electron-electron interaction potential, leading to the energy lowering of the triplet state relative to its counterpart singlet state.

Effect of impurities on the two-dimensional electron gas polarizability

International Nuclear Information System (INIS)

Nkoma, J.S.

1980-06-01

The polarizability for a two-dimensional electron gas is calculated in the presence of impurities by a Green function formalism. This leads to a system with finite mean free path due to electrons scattering off impurities. The calculated polarizability is found to be strongly dependent on the mean free path. The main feature is the suppression of the sharp corner at wave vector 2ksub(F) for finite mean free paths, and the pure metal result is recovered for the infinite mean free path. A possible application of the results to the transport properties of semiconductor inversion layers is discussed. (author)

Two-dimensional confinement of heavy fermions

International Nuclear Information System (INIS)

Shishido, Hiroaki; Shibauchi, Takasada; Matsuda, Yuji; Terashima, Takahito

2010-01-01

Metallic systems with the strongest electron correlations are realized in certain rare-earth and actinide compounds whose physics are dominated by f-electrons. These materials are known as heavy fermions, so called because the effective mass of the conduction electrons is enhanced via correlation effects up to as much as several hundreds times the free electron mass. To date the electronic structure of all heavy-fermion compounds is essentially three-dimensional. Here we report on the first realization of a two-dimensional heavy-fermion system, where the dimensionality is adjusted in a controllable fashion by fabricating heterostructures using molecular beam epitaxy. The two-dimensional heavy fermion system displays striking deviations from the standard Fermi liquid low-temperature electronic properties. (author)

Large-scale calculations of solid oxide fuel cell cermet anode by tight-binding quantum chemistry method

International Nuclear Information System (INIS)

Koyama, Michihisa; Kubo, Momoji; Miyamoto, Akira

2005-01-01

Improvement of anode characteristics of solid oxide fuel cells is important for the better cell performance and especially the direct use of hydrocarbons. A mixture of ceramics and metal is generally used as anode, and different combinations of ceramics and metals lead to different electrode characteristics. We performed large-scale calculations to investigate the characteristics of Ni/CeO 2 and Cu/CeO 2 anodes at the electronic level using our tight-binding quantum chemical molecular dynamics program. Charge distribution analysis clarified the electron transfer from metal to oxide in both anodes. The calculations of density of states clarified different contributions of Ni and Cu orbitals to the energy levels at around Fermi level in each cermet. Based on the obtained results, we made considerations to explain different characteristics of both cermet anodes. The effectiveness of our approach for the investigation of complex cermet system was proved

Spin-Dependent Scattering Effects and Dimensional Crossover in a Quasi-Two-Dimensional Disordered Electron System

Institute of Scientific and Technical Information of China (English)

YANG YongHong; WANG YongGang; LIU Mei; WANG Jin

2002-01-01

Two kinds of spin-depcndcnt scattering effects (magnetic-iinpurity and spin-orbit scatterings) axe investi-gated theoretically in a quasi-two-dimensional (quasi-2D) disordered electron system. By making use of the diagrammatictechniques in perturbation theory, we have calculated the dc conductivity and magnetoresistance due to weak-localizationeffects, the analytical expressions of them are obtained as functions of the interlayer hopping energy and the charac-teristic times: elastic, inelastic, magnetic and spin-orbit scattering times. The relevant dimensional crossover behaviorfrom 3D to 2D with decreasing the interlayer coupling is discussed, and the condition for the crossover is shown to bedependent on the aforementioned scattering times. At low temperature there exists a spin-dcpendent-scattering-induccddimensional crossover in this system.

Cyclotron resonance study of the two-dimensional electron layers and double layers in tilted magnetic fields

Czech Academy of Sciences Publication Activity Database

Goncharuk, Natalya; Smrčka, Ludvík; Kučera, Jan

2004-01-01

Roč. 22, - (2004), s. 590-593 ISSN 1386-9477. [International Conference on Electronic Properties of Two-Dimensional Systems /15./. Nara, 14.07.2003-18.07.2003] R&D Projects: GA ČR GA202/01/0754 Institutional research plan: CEZ:AV0Z1010914 Keywords : single layer * double layer * two-dimensional electron system * cyclotron resonance Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.898, year: 2004

A new series of two-dimensional silicon crystals with versatile electronic properties

Science.gov (United States)

Chae, Kisung; Kim, Duck Young; Son, Young-Woo

2018-04-01

Silicon (Si) is one of the most extensively studied materials owing to its significance to semiconductor science and technology. While efforts to find a new three-dimensional (3D) Si crystal with unusual properties have made some progress, its two-dimensional (2D) phases have not yet been explored as much. Here, based on a newly developed systematic ab initio materials searching strategy, we report a series of novel 2D Si crystals with unprecedented structural and electronic properties. The new structures exhibit perfectly planar outermost surface layers of a distorted hexagonal network with their thicknesses varying with the atomic arrangement inside. Dramatic changes in electronic properties ranging from semimetal to semiconducting with indirect energy gaps and even to one with direct energy gaps are realized by varying thickness as well as by surface oxidation. Our predicted 2D Si crystals with flat surfaces and tunable electronic properties will shed light on the development of silicon-based 2D electronics technology.

Tight-binding calculation of radiation loss in photonic crystal CROW.

Science.gov (United States)

Ma, Jing; Martínez, Luis Javier; Fan, Shanhui; Povinelli, Michelle L

2013-01-28

The tight binding approximation (TBA) is used to relate the intrinsic, radiation loss of a coupled resonator optical waveguide (CROW) to that of a single constituent resonator within a light cone picture. We verify the validity of the TBA via direct, full-field simulation of CROWs based on the L2 photonic crystal cavity. The TBA predicts that the quality factor of the CROW increases with that of the isolated cavity. Moreover, our results provide a method to design CROWs with low intrinsic loss across the entire waveguide band.

A two-dimensional laser-wire scanner for electron accelerators

Energy Technology Data Exchange (ETDEWEB)

Bosco, A. [Physics Department John Adams Institute for Accelerator Science at Royal Holloway, University of London, Egham, Surrey TW20 0EX (United Kingdom)], E-mail: alessio.bosco@rhul.ac.uk; Price, M.T.; Blair, G.A.; Boogert, S.T.; Boorman, G.; Malton, S.; Driouichi, C. [Physics Department John Adams Institute for Accelerator Science at Royal Holloway, University of London, Egham, Surrey TW20 0EX (United Kingdom); Kamps, T. [Berliner Elektronenspeicherring, Gesellschaft fur Synchrotronstrahlung, Albert Einstein-Str. 15, 12489 Berlin (Germany); Poirier, F.; Balewski, K.; Elsen, E.; Gharibyan, V.; Lewin, H.-C.; Schreiber, S.; Walker, N.; Wittenburg, K. [Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22607 Hamburg (Germany)

2008-07-21

A two-dimensional laser-wire scanner capable of measuring the transverse charge profiles of an electron (or positron) bunch has been constructed at the PETRA accelerator in DESY. The development of the system is explained in this paper, along with descriptions of its photon detector and laser system. Results of transverse profile scans are presented for both horizontal and vertical directions. The measurement error is 1.3% from a multi-scan measurement in the vertical direction, where single scans can be performed in less than 50 s.

Total-energy Assisted Tight-binding Method Based on Local Density Approximation of Density Functional Theory

Science.gov (United States)

Fujiwara, Takeo; Nishino, Shinya; Yamamoto, Susumu; Suzuki, Takashi; Ikeda, Minoru; Ohtani, Yasuaki

2018-06-01

A novel tight-binding method is developed, based on the extended Hückel approximation and charge self-consistency, with referring the band structure and the total energy of the local density approximation of the density functional theory. The parameters are so adjusted by computer that the result reproduces the band structure and the total energy, and the algorithm for determining parameters is established. The set of determined parameters is applicable to a variety of crystalline compounds and change of lattice constants, and, in other words, it is transferable. Examples are demonstrated for Si crystals of several crystalline structures varying lattice constants. Since the set of parameters is transferable, the present tight-binding method may be applicable also to molecular dynamics simulations of large-scale systems and long-time dynamical processes.

Phase-Dependent Resistance in a Superconductor—Two-Dimensional-Electron-Gas Quasiparticle Interferometer

NARCIS (Netherlands)

Dimoulas, A.; Heida, J.P.; Wees, B.J. v.; Klapwijk, T.M.; Graaf, W. v.d.; Borghs, G.

1995-01-01

We have investigated the interplay between Josephson coupling and quasiparticle interference effects in the resistance of a two-dimensional electron gas connected to superconducting electrodes with an interrupted ring geometry. By reducing the influence of the Josephson coupling strength at high dc

Chiral-like tunneling of electrons in two-dimensional semiconductors with Rashba spin-orbit coupling.

Science.gov (United States)

Ang, Yee Sin; Ma, Zhongshui; Zhang, C

2014-01-21

The unusual tunneling effects of massless chiral fermions (mCF) and massive chiral fermions (MCF) in a single layer graphene and bilayer graphene represent some of the most bizarre quantum transport phenomena in condensed matter system. Here we show that in a two-dimensional semiconductor with Rashba spin-orbit coupling (R2DEG), the real-spin chiral-like tunneling of electrons at normal incidence simultaneously exhibits features of mCF and MCF. The parabolic branch of opposite spin in R2DEG crosses at a Dirac-like point and has a band turning point. These features generate transport properties not found in usual two-dimensional electron gas. Albeit its π Berry phase, electron backscattering is present in R2DEG. An electron mimics mCF if its energy is in the vicinity of the subband crossing point or it mimics MCF if its energy is near the subband minima.

Dielectric constant of graphene-on-polarized substrate: A tight ...

Indian Academy of Sciences (India)

2017-06-24

Jun 24, 2017 ... We report here a microscopic tight-binding theoretical study of the dynamic dielectric response of graphene-on-polarizable substrate with impurity. The Hamiltonian consists of first, second and third nearest neighbour electron hopping interactions besides doping and substrate-induced effects on graphene.

Complexes of dipolar excitons in layered quasi-two-dimensional nanostructures

Science.gov (United States)

Bondarev, Igor V.; Vladimirova, Maria R.

2018-04-01

We discuss neutral and charged complexes (biexcitons and trions) formed by indirect excitons in layered quasi-two-dimensional semiconductor heterostructures. Indirect excitons—long-lived neutral Coulomb-bound pairs of electrons and holes of different layers—have been known for semiconductor coupled quantum wells and have recently been reported for van der Waals heterostructures such as double bilayer graphene and transition-metal dichalcogenides. Using the configuration space approach, we derive the analytical expressions for the trion and biexciton binding energies as a function of interlayer distance. The method captures essential kinematics of complex formation to reveal significant binding energies, up to a few tens of meV for typical interlayer distances ˜3 -5 Å , with the trion binding energy always being greater than that of the biexciton. Our results can contribute to the understanding of more complex many-body phenomena such as exciton Bose-Einstein condensation and Wigner-like electron-hole crystallization in layered semiconductor heterostructures.

Growth and electronic properties of two-dimensional systems on (110) oriented GaAs

Energy Technology Data Exchange (ETDEWEB)

Fischer, F.

2005-07-01

As the only non-polar plane the (110) surface has a unique role in GaAs. Together with Silicon as a dopant it is an important substrate orientation for the growth of n-type or p-type heterostructures. As a consequence, this thesis will concentrate on growth and research on that surface. In the course of this work we were able to realize two-dimensional electron systems with the highest mobilities reported so far on this orientation. Therefore, we review the necessary growth conditions and the accompanying molecular process. The two-dimensional electron systems allowed the study of a new, intriguing transport anisotropy not explained by current theory. Moreover, we were the first growing a two-dimensional hole gas on (110) GaAs with Si as dopant. For this purpose we invented a new growth modulation technique necessary to retrieve high mobility systems. In addition, we discovered and studied the metal-insulator transition in thin bulk p-type layers on (110) GaAs. Besides we investigated the activation process related to the conduction in the valence band and a parallelly conducting hopping band. The new two-dimensional hole gases revealed interesting physics. We studied the zero B-field spin splitting in these systems and compared it with the known theory. Furthermore, we investigated the anisotropy of the mobility. As opposed to the expectations we observed a strong persistent photoconductivity in our samples. Landau levels for two dimensional hole systems are non-linear and can show anticrossings. For the first time we were able to resolve anticrossings in a transport experiment and study the corresponding activation process. Finally, we compared these striking results with theoretical calculations. (orig.)

On the exact spectra of two electrons confined by two-dimensional quantum dots

International Nuclear Information System (INIS)

Soldatov, A.V.; Bogolubov Jr, N.N.

2005-12-01

Applicability of the method of intermediate problems to investigation of the energy spectrum and eigenstates of a two- electron two-dimensional quantum dot (QD) formed by a parabolic confining potential is discussed. It is argued that the method of intermediate problems, which provides convergent improvable lower bound estimates for eigenvalues of linear half-bound Hermitian operators in Hilbert space, can be fused with the classical Rayleigh-Ritz variational method and stochastic variational method thus providing an efficient tool of verification of the results obtained so far by various analytical and numerical methods being of current usage for studies of quantum dot models. (author)

Interplay between topology and disorder in a two-dimensional semi-Dirac material

Science.gov (United States)

Sriluckshmy, P. V.; Saha, Kush; Moessner, Roderich

2018-01-01

We investigate the role of disorder in a two-dimensional semi-Dirac material characterized by a linear dispersion in one direction and a parabolic dispersion in the orthogonal direction. Using the self-consistent Born approximation, we show that disorder can drive a topological Lifshitz transition from an insulator to a semimetal, as it generates a momentum-independent off-diagonal contribution to the self-energy. Breaking time-reversal symmetry enriches the topological phase diagram with three distinct regimes—single-node trivial, two-node trivial, and two-node Chern. We find that disorder can drive topological transitions from both the single- and two-node trivial to the two-node Chern regime. We further analyze these transitions in an appropriate tight-binding Hamiltonian of an anisotropic hexagonal lattice by calculating the real-space Chern number. Additionally, we compute the disorder-averaged entanglement entropy which signals both the topological Lifshitz and Chern transition as a function of the anisotropy of the hexagonal lattice. Finally, we discuss experimental aspects of our results.

Numerical prediction of pressure loss in tight-lattice rod bundle by use of 3-dimensional two-fluid model simulation code ACE-3D

International Nuclear Information System (INIS)

Yoshida, Hiroyuki; Takase, Kazuyuki; Suzuki, Takayuki

2009-01-01

Two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase flow simulation method such as interface tracking method or particle interaction method. Therefore, two-fluid model is useful for thermal hydraulic analysis in large-scale domain such as a rod bundle. Japan Atomic Energy Agency (JAEA) develops three dimensional two-fluid model analysis code ACE-3D that adopts boundary fitted coordinate system in order to simulate complex shape flow channel. In this paper, boiling two-phase flow analysis in a tight-lattice rod bundle was performed by the ACE-3D. In the results, the void fraction, which distributes in outermost region of rod bundle, is lower than that in center region of rod bundle. The tendency of void fraction distribution agreed with the measurement results by neutron radiography qualitatively. To evaluate effects of two-phase flow model used in the ACE-3D, numerical simulation of boiling two-phase in tight-lattice rod bundle with no lift force model was also performed. In the results, the lift force model has direct effects on void fraction concentration in gap region, and pressure distribution in horizontal plane induced by void fraction distribution cause of bubble movement from the gap region to the subchannel region. The predicted pressure loss in the section that includes no spacer accorded with experimental results with around 10% of differences. The predicted friction pressure loss was underestimated around 20% of measured values, and the effect of the turbulence model is considered as one of the causes of this underestimation. (author)

Finite temperature magnon spectra in yttrium iron garnet from a mean field approach in a tight-binding model

Science.gov (United States)

Shen, Ka

2018-04-01

We study magnon spectra at finite temperature in yttrium iron garnet using a tight-binding model with nearest-neighbor exchange interaction. The spin reduction due to thermal magnon excitation is taken into account via the mean field approximation to the local spin and is found to be different at two sets of iron atoms. The resulting temperature dependence of the spin wave gap shows good agreement with experiment. We find that only two magnon modes are relevant to the ferromagnetic resonance.

Computational Search for Two-Dimensional MX2 Semiconductors with Possible High Electron Mobility at Room Temperature

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Zhishuo Huang

2016-08-01

Full Text Available Neither of the two typical two-dimensional materials, graphene and single layer MoS 2 , are good enough for developing semiconductor logical devices. We calculated the electron mobility of 14 two-dimensional semiconductors with composition of MX 2 , where M (=Mo, W, Sn, Hf, Zr and Pt are transition metals, and Xs are S, Se and Te. We approximated the electron phonon scattering matrix by deformation potentials, within which long wave longitudinal acoustical and optical phonon scatterings were included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WS 2 , PtS 2 and PtSe 2 are promising for logical devices regarding the possible high electron mobility and finite band gap. Especially, the phonon limited electron mobility in PtSe 2 reaches about 4000 cm 2 ·V - 1 ·s - 1 at room temperature, which is the highest among the compounds with an indirect bandgap of about 1.25 eV under the local density approximation. Our results can be the first guide for experiments to synthesize better two-dimensional materials for future semiconductor devices.

Analysis of the tight-binding description of the structure of metallic 2D systems

International Nuclear Information System (INIS)

Baquero, R.

1990-12-01

Bidimensional metallic systems as interfaces, quantum wells and superlattices with sharp interfaces became recently available and their properties can now be experimentally studied in detail. To calculate the Local Density of States (LDOS) for surfaces, interfaces, quantum wells and superlattices we use empirical tight-binding Hamiltonians together with the Green function matching method (GFM). In this paper we show some examples of our results employing the method just outlined to describe metallic 2D systems. In particular, we refer briefly to the effect on the LDOS of the very recently established contraction of the first interatomic layer distance in the Ta(001) surface. We then discuss the Nb-V ideal (100) interface and conclude that under certain conditions the V-side of an interface can show magnetism as the V(001) surface does. As a last example, we present a calculation that relates the changes with gold coverage of the reaction rate of the catalytic reaction of cyclohexene into benzene on a Pt(001) surface to the changes on the LDOS of the outermost Pt atomic layer. We show that the behavior of the LDOS around the Fermi level is an important factor to the explanation of the behavior of this catalytic reaction. We conclude by stating that the empirical tight-binding method is a very simple and useful tool for the description of 2D metallic systems. The advantage is that the computational demands are low and all the ingredients to take full profit of this method are available (reliable tight-binding parameters and suitable methods for the calculation of the Green function). (author). 14 refs, 3 figs

Dielectric constant of graphene-on-polarized substrate: A tight ...

Indian Academy of Sciences (India)

Sivabrata Sahu

Corresponding author. E-mail: gcr@iopb.res.in. Published online 24 June 2017. Abstract. We report here a microscopic tight-binding theoretical study of the dynamic dielectric response of graphene-on-polarizable substrate with impurity. The Hamiltonian consists of first, second and third nearest- neighbour electron hopping ...

Topological states in a two-dimensional metal alloy in Si surface: BiAg/Si(111)-4 ×4 surface

Science.gov (United States)

Zhang, Xiaoming; Cui, Bin; Zhao, Mingwen; Liu, Feng

2018-02-01

A bridging topological state with a conventional semiconductor platform offers an attractive route towards future spintronics and quantum device applications. Here, based on first-principles and tight-binding calculations, we demonstrate the existence of topological states hosted by a two-dimensional (2D) metal alloy in a Si surface, the BiAg/Si(111)-4 ×4 surface, which has already been synthesized experimentally. It exhibits a topological insulating state with an energy gap of 71 meV (˜819 K ) above the Fermi level and a topological metallic state with quasiquantized conductance below the Fermi level. The underlying mechanism leading to the formation of such nontrivial states is revealed by analysis of the "charge-transfer" and "orbital-filtering" effect of the Si substrate. A minimal effective tight-binding model is employed to reveal the formation mechanism of the topological states. Our finding opens opportunities to detect topological states and measure its quantized conductance in a large family of 2D surface metal alloys, which have been or are to be grown on semiconductor substrates.

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry

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E. Kalesaki

2014-01-01

Full Text Available We study theoretically two-dimensional single-crystalline sheets of semiconductors that form a honeycomb lattice with a period below 10 nm. These systems could combine the usual semiconductor properties with Dirac bands. Using atomistic tight-binding calculations, we show that both the atomic lattice and the overall geometry influence the band structure, revealing materials with unusual electronic properties. In rocksalt Pb chalcogenides, the expected Dirac-type features are clouded by a complex band structure. However, in the case of zinc-blende Cd-chalcogenide semiconductors, the honeycomb nanogeometry leads to rich band structures, including, in the conduction band, Dirac cones at two distinct energies and nontrivial flat bands and, in the valence band, topological edge states. These edge states are present in several electronic gaps opened in the valence band by the spin-orbit coupling and the quantum confinement in the honeycomb geometry. The lowest Dirac conduction band has S-orbital character and is equivalent to the π-π^{⋆} band of graphene but with renormalized couplings. The conduction bands higher in energy have no counterpart in graphene; they combine a Dirac cone and flat bands because of their P-orbital character. We show that the width of the Dirac bands varies between tens and hundreds of meV. These systems emerge as remarkable platforms for studying complex electronic phases starting from conventional semiconductors. Recent advancements in colloidal chemistry indicate that these materials can be synthesized from semiconductor nanocrystals.

Anomalous electron doping independent two-dimensional superconductivity

Science.gov (United States)

Zhou, Wei; Xing, Xiangzhuo; Zhao, Haijun; Feng, Jiajia; Pan, Yongqiang; Zhou, Nan; Zhang, Yufeng; Qian, Bin; Shi, Zhixiang

2017-07-01

Transition metal (Co and Ni) co-doping effects are investigated on an underdoped Ca0.94La0.06Fe2As2 compound. It is discovered that electron doping from substituting Fe with transition metal (TM = Co, Ni) can trigger high-{T}{{c}} superconductivity around 35 K, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Remarkably, the critical temperature for the high-{T}{{c}} superconductivity remains constant against a wide range of TM doping levels. And the net electron doping density dependence of the superconducting {T}{{c}} based on the rigid band model can be nicely scaled into a single curve for Co and Ni substitutions, in stark contrast to the case of Ba(Fe1-x TM x )2As2. This carrier density independent superconductivity and the unusual scaling behavior are presumably resulted from the interface superconductivity based on the similarity with the interface superconductivity in a La2-x Sr x CuO4-La2CuO4 bilayer. Evidence of the two-dimensional character of the superfluid by angle-resolved magneto-resistance measurements can further strengthen the interface nature of the high-{T}{{c}} superconductivity.

Gate dielectric strength dependent performance of CNT MOSFET and CNT TFET: A tight binding study

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Md. Shamim Sarker

Full Text Available This paper presents a comparative study between CNT MOSFET and CNT TFET taking into account of different dielectric strength of gate oxide materials. Here we have studied the transfer characteristics, on/off current (ION/IOFF ratio and subthreshold slope of the device using Non Equilibrium Greens Function (NEGF formalism in tight binding frameworks. The results are obtained by solving the NEGF and Poisson’s equation self-consistently in NanoTCADViDES environment and found that the ON state performance of CNT MOSFET and CNT TFET have significant dependency on the dielectric strength of the gate oxide materials. The figure of merits of the devices also demonstrates that the CNT TFET is promising for high-speed and low-power logic applications. Keywords: CNT TFET, Subthreshold slop, Barrier width, Conduction band (C.B and Valance band (V.B, Oxide dielectric strength, Tight binding approach

Resonant spin Hall effect in two dimensional electron gas

Science.gov (United States)

Shen, Shun-Qing

2005-03-01

Remarkable phenomena have been observed in 2DEG over last two decades, most notably, the discovery of integer and fractional quantum Hall effect. The study of spin transport provides a good opportunity to explore spin physics in two-dimensional electron gas (2DEG) with spin-orbit coupling and other interaction. It is already known that the spin-orbit coupling leads to a zero-field spin splitting, and competes with the Zeeman spin splitting if the system is subjected to a magnetic field perpendicular to the plane of 2DEG. The result can be detected as beating of the Shubnikov-de Haas oscillation. Very recently the speaker and his collaborators studied transport properties of a two-dimensional electron system with Rashba spin-orbit coupling in a perpendicular magnetic field. The spin-orbit coupling competes with the Zeeman splitting to generate additional degeneracies between different Landau levels at certain magnetic fields. It is predicted theoretically that this degeneracy, if occurring at the Fermi level, gives rise to a resonant spin Hall conductance, whose height is divergent as 1/T and whose weight is divergent as -lnT at low temperatures. The charge Hall conductance changes by 2e^2/h instead of e^2/h as the magnetic field changes through the resonant point. The speaker will address the resonance condition, symmetries in the spin-orbit coupling, the singularity of magnetic susceptibility, nonlinear electric field effect, the edge effect and the disorder effect due to impurities. This work was supported by the Research Grants Council of Hong Kong under Grant No.: HKU 7088/01P. *S. Q. Shen, M. Ma, X. C. Xie, and F. C. Zhang, Phys. Rev. Lett. 92, 256603 (2004) *S. Q. Shen, Y. J. Bao, M. Ma, X. C. Xie, and F. C. Zhang, cond-mat/0410169

ProbeZT: Simulation of transport coefficients of molecular electronic junctions under environmental effects using Büttiker's probes

Science.gov (United States)

Korol, Roman; Kilgour, Michael; Segal, Dvira

2018-03-01

We present our in-house quantum transport package, ProbeZT. This program provides linear response coefficients: electrical and electronic thermal conductances, as well as the thermopower of molecular junctions in which electrons interact with the surrounding thermal environment. Calculations are performed based on the Büttiker probe method, which introduces decoherence, energy exchange and dissipation effects phenomenologically using virtual electrode terminals called probes. The program can realize different types of probes, each introducing various environmental effects, including elastic and inelastic scattering of electrons. The molecular system is described by an arbitrary tight-binding Hamiltonian, allowing the study of different geometries beyond simple one-dimensional wires. Applications of the program to study the thermoelectric performance of molecular junctions are illustrated. The program also has a built-in functionality to simulate electron transport in double-stranded DNA molecules based on a tight-binding (ladder) description of the junction.

High-Resolution Two-Dimensional Optical Spectroscopy of Electron Spins

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M. Salewski

2017-08-01

Full Text Available Multidimensional coherent optical spectroscopy is one of the most powerful tools for investigating complex quantum mechanical systems. While it was conceived decades ago in magnetic resonance spectroscopy using microwaves and radio waves, it has recently been extended into the visible and UV spectral range. However, resolving MHz energy splittings with ultrashort laser pulses still remains a challenge. Here, we analyze two-dimensional Fourier spectra for resonant optical excitation of resident electrons to localized trions or donor-bound excitons in semiconductor nanostructures subject to a transverse magnetic field. Particular attention is devoted to Raman coherence spectra, which allow one to accurately evaluate tiny splittings of the electron ground state and to determine the relaxation times in the electron spin ensemble. A stimulated steplike Raman process induced by a sequence of two laser pulses creates a coherent superposition of the ground-state doublet which can be retrieved only optically because of selective excitation of the same subensemble with a third pulse. This provides the unique opportunity to distinguish between different complexes that are closely spaced in energy in an ensemble. The related experimental demonstration is based on photon-echo measurements in an n-type CdTe/(Cd,MgTe quantum-well structure detected by a heterodyne technique. The difference in the sub-μeV range between the Zeeman splittings of donor-bound electrons and electrons localized at potential fluctuations can be resolved even though the homogeneous linewidth of the optical transitions is larger by 2 orders of magnitude.

Two-dimensional spatial structure of the dissipative trapped-electron mode

International Nuclear Information System (INIS)

Rewoldt, G.; Tang, W.M.; Frieman, E.A.

1976-09-01

This paper deals with the complete two-dimensional structure of the dissipative trapped-electron mode over its full width, which may extend over several mode-rational surfaces. The complete integro-differential equation is studied in the limit k/sub r/rho/sub i/ less than 1, where rho/sub i/ is the ion gyroradius, and k/sub r/, the radial wavenumber, is regarded as a differential operator. This is converted into a matrix equation which is then solved by standard numerical methods

Effects of ethanol and acetaldehyde on tight junction integrity: in vitro study in a three dimensional intestinal epithelial cell culture model.

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Elhaseen Elamin

Full Text Available BACKGROUND: Intestinal barrier dysfunction and translocation of endotoxins are involved in the pathogenesis of alcoholic liver disease. Exposure to ethanol and its metabolite, acetaldehyde at relatively high concentrations have been shown to disrupt intestinal epithelial tight junctions in the conventional two dimensional cell culture models. The present study investigated quantitatively and qualitatively the effects of ethanol at concentrations detected in the blood after moderate ethanol consumption, of its metabolite acetaldehyde and of the combination of both compounds on intestinal barrier function in a three-dimensional cell culture model. METHODS AND FINDINGS: Caco-2 cells were grown in a basement membrane matrix (Matrigel™ to induce spheroid formation and were then exposed to the compounds at the basolateral side. Morphological differentiation of the spheroids was assessed by immunocytochemistry and transmission electron microscopy. The barrier function was assessed by the flux of FITC-labeled dextran from the basal side into the spheroids' luminal compartment using confocal microscopy. Caco-2 cells grown on Matrigel assembled into fully differentiated and polarized spheroids with a central lumen, closely resembling enterocytes in vivo and provide an excellent model to study epithelial barrier functionality. Exposure to ethanol (10-40 mM or acetaldehyde (25-200 µM for 3 h, dose-dependently and additively increased the paracellular permeability and induced redistribution of ZO-1 and occludin without affecting cell viability or tight junction-encoding gene expression. Furthermore, ethanol and acetaldehyde induced lysine residue and microtubules hyperacetylation. CONCLUSIONS: These results indicate that ethanol at concentrations found in the blood after moderate drinking and acetaldehyde, alone and in combination, can increase the intestinal epithelial permeability. The data also point to the involvement of protein hyperacetylation in

The band gap of II-Vi ternary alloys in a tight-binding description

Energy Technology Data Exchange (ETDEWEB)

Olguin, Daniel; Blanquero, Rafael [Instituto Politecnico Nacional, Mexico, D.F (Mexico); De Coss, Romeo [Instituto Politecnico Nacional, Yucatan (Mexico)

2001-02-01

We present tight-binding calculations for the band gap of II-Vi pseudobinary ternary alloys. We use an sp{sup 3} s* tight-binding Hamiltonian which include spin-orbit coupling. The band gap composition dependence is calculated using a extended version of the virtual crystal approximation, which introduce an empirical correction factor that takes into account the non-linear dependence of the band gap with the composition. The results compare quite well with the experimental data, both for the ternary alloys with wide band gap and for the narrow band gap ones. [Spanish] Presentamos el calculo de la banda de energia prohibida de aleaciones ternarias de compuestos II-VI. El calculo, que incluye interaccion espin-orbita, se hace con el metodo de enlace fuerte, utilizando una base ortogonal de cinco orbitales atomicos por atomo (sp{sup 3} s*), en conjunto con la aproximacion del cristal virtual. En la aproximacion del cristal virtual, incluimos un factor de correccion que toma en cuenta la no linealidad de la banda de energia prohibida como funcion de la concentracion. Con esta correccion nuestros resultados reproducen aceptablemente los datos experimentales hallados en la literatura.

Two-dimensional electron gas in AlGaN/GaN heterostructures

International Nuclear Information System (INIS)

Li, J.Z.; Lin, J.Y.; Jiang, H.X.; Khan, M.A.; Chen, Q.

1997-01-01

The formation of a two-dimensional electron gas (2DEG) system by an AlGaN/GaN heterostructure has been further confirmed by measuring its electrical properties. The effect of persistent photoconductivity (PPC) has been observed and its unique features have been utilized to study the properties of 2DEG formed by the AlGaN/GaN heterointerface. Sharp electronic transitions from the first to the second subbands in the 2DEG channel have been observed by monitoring the 2DEG carrier mobility as a function of carrier concentration through the use of PPC. These results are expected to have significant implications on field-effect transistor and high electron mobility transistor applications based on the GaN system. copyright 1997 American Vacuum Society

Rapid long range intramolecular electron transfer within a steroid molecule with two electron binding groups

International Nuclear Information System (INIS)

Huddleston, R.K.; Miller, J.R.

1983-01-01

Intramolecular electron transfer has been observed to have occurred in less than 100 ns in a steroid molecule having two distinct electron binding groups separated by distances distributed from 7--11 A. Experiments were carried out in organic glasses at 77 K with pulse radiolysis techniques to create trapped electrons which were captured by a group on one end of the steroid molecule. Although one of the groups, benzoate, is held to the steroid spacer by a flexible linkage, the rigidity of the glassy matrices prevented movement to alter the initial distance. Interestingly, no effects of distance were seen: all ET processes appeared to have occurred much faster than our 100 ns time resolution, consistent with measurements of the rate of intermolecular electron transfer between the same functional groups in random solutions. Solvation energetics, on the other hand, had a remarkable influence on the extent and direction of electron transfer. A change in solvent polarity was observed to reverse the direction of electron transfer. Evidence was obtained for a distribution of solvation environments for ions in glasses which may be as broad as 0.15 eV

Stability enhancement and electronic tunability of two-dimensional SbIV compounds via surface functionalization

Science.gov (United States)

Zhou, Wenhan; Guo, Shiying; Liu, Xuhai; Cai, Bo; Song, Xiufeng; Zhu, Zhen; Zhang, Shengli

2018-01-01

We propose a family of hydrogenated- and halogenated-SbIV (SbIVX-2) materials that simultaneously have two-dimensional (2D) structures, high stability and appealing electronic properties. Based on first-principles total-energy and vibrational-spectra calculations, SbIVX-2 monolayers are found both thermally and dynamically stable. Varying IV and X elements can rationally tune the electronic properties of SbIVX-2 monolayers, effectively modulating the band gap from 0 to 3.42 eV. Regarding such superior stability and broad band-gap range, SbIVX-2 monolayers are expected to be synthesized in experiments and taken as promising candidates for low-dimensional electronic and optoelectronic devices, such as blue-to-ultraviolet light-emitting diodes (LED) and photodetectors.

Photoemission and electron-stimulated desorption studies of H on W(110): Single- versus two-binding-site models

International Nuclear Information System (INIS)

Weng, S.

1982-01-01

The chemisorption of H on W(110) at room temperature is studied with the use of angle-integrated photoemission and electron-stimulated desorption (ESD). The ESD cross sections of H + are found to be sol low that no significant H + signals with meaningful ion energy distributions are observed. The photoemission results show, however, two types of H adatoms, referred to as β 2 and β 1 states, for this chemisorptive system. Both states are found to appear simultaneously rather than sequentially as suggested by previous studies, and exhibit a simple 1-theta adsorption kinetics with different initial sticking coefficients. The β 2 state induces two binding energy levels at -2.0 and -6.0 eV, respectively, whereas the β 1 state induces a level at -3.8 eV. The work-function change (with a maximum value of -0.45 eV) is found to follow exactly with the intensity of the β 2 state. These results are found to be compatible with the two-binding-site model, inherently suggested by the reflection high-enery electron-diffraction data. However, the results can also be consistent with a single-binding-site model suggested by a recent angle-resolved photoemission and inelastic electron scattering study. A model based on the present results is proposed and critically compared with previous studies. Unresolved problems associated with both single- and two-binding-site models are also discussed

Magneto-spin Hall conductivity of a two-dimensional electron gas

OpenAIRE

Milletari', M.; Raimondi, R.; Schwab, P.

2008-01-01

It is shown that the interplay of long-range disorder and in-plane magnetic field gives rise to an out-of-plane spin polarization and a finite spin Hall conductivity of the two-dimensional electron gas in the presence of Rashba spin-orbit coupling. A key aspect is provided by the electric-field induced in-plane spin polarization. Our results are obtained first in the \\textit{clean} limit where the spin-orbit splitting is much larger than the disorder broadening of the energy levels via the di...

Self-Consistent Optimization of Excited States within Density-Functional Tight-Binding.

Science.gov (United States)

Kowalczyk, Tim; Le, Khoa; Irle, Stephan

2016-01-12

We present an implementation of energies and gradients for the ΔDFTB method, an analogue of Δ-self-consistent-field density functional theory (ΔSCF) within density-functional tight-binding, for the lowest singlet excited state of closed-shell molecules. Benchmarks of ΔDFTB excitation energies, optimized geometries, Stokes shifts, and vibrational frequencies reveal that ΔDFTB provides a qualitatively correct description of changes in molecular geometries and vibrational frequencies due to excited-state relaxation. The accuracy of ΔDFTB Stokes shifts is comparable to that of ΔSCF-DFT, and ΔDFTB performs similarly to ΔSCF with the PBE functional for vertical excitation energies of larger chromophores where the need for efficient excited-state methods is most urgent. We provide some justification for the use of an excited-state reference density in the DFTB expansion of the electronic energy and demonstrate that ΔDFTB preserves many of the properties of its parent ΔSCF approach. This implementation fills an important gap in the extended framework of DFTB, where access to excited states has been limited to the time-dependent linear-response approach, and affords access to rapid exploration of a valuable class of excited-state potential energy surfaces.

Metal-insulator transition in one-dimensional lattices with chaotic energy sequences

International Nuclear Information System (INIS)

Pinto, R.A.; Rodriguez, M.; Gonzalez, J.A.; Medina, E.

2005-01-01

We study electronic transport through a one-dimensional array of sites by using a tight binding Hamiltonian, whose site-energies are drawn from a chaotic sequence. The correlation degree between these energies is controlled by a parameter regulating the dynamic Lyapunov exponent measuring the degree of chaos. We observe the effect of chaotic sequences on the localization length, conductance, conductance distribution and wave function, finding evidence of a metal-insulator transition (MIT) at a critical degree of chaos. The one-dimensional metallic phase is characterized by a Gaussian conductance distribution and exhibits a peculiar non-selfaveraging

Metal-insulator transition in one-dimensional lattices with chaotic energy sequences

Energy Technology Data Exchange (ETDEWEB)

Pinto, R.A. [Laboratorio de Fisica Estadistica, Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020-A (Venezuela)]. E-mail: ripinto@ivic.ve; Rodriguez, M. [Laboratorio de Fisica Estadistica, Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020-A (Venezuela); Gonzalez, J.A. [Laboratorio de Fisica Computacional, Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020-A (Venezuela); Medina, E. [Laboratorio de Fisica Estadistica, Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020-A (Venezuela)

2005-06-20

We study electronic transport through a one-dimensional array of sites by using a tight binding Hamiltonian, whose site-energies are drawn from a chaotic sequence. The correlation degree between these energies is controlled by a parameter regulating the dynamic Lyapunov exponent measuring the degree of chaos. We observe the effect of chaotic sequences on the localization length, conductance, conductance distribution and wave function, finding evidence of a metal-insulator transition (MIT) at a critical degree of chaos. The one-dimensional metallic phase is characterized by a Gaussian conductance distribution and exhibits a peculiar non-selfaveraging.

Correlation induced electron-hole asymmetry in quasi- two-dimensional iridates.

Science.gov (United States)

Pärschke, Ekaterina M; Wohlfeld, Krzysztof; Foyevtsova, Kateryna; van den Brink, Jeroen

2017-09-25

The resemblance of crystallographic and magnetic structures of the quasi-two-dimensional iridates Ba 2 IrO 4 and Sr 2 IrO 4 to La 2 CuO 4 points at an analogy to cuprate high-Tc superconductors, even if spin-orbit coupling is very strong in iridates. Here we examine this analogy for the motion of a charge (hole or electron) added to the antiferromagnetic ground state. We show that correlation effects render the hole and electron case in iridates very different. An added electron forms a spin polaron, similar to the cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplet states of total angular momentum that strongly affect the hole motion. This not only has ramifications for the interpretation of (inverse-)photoemission experiments but also demonstrates that correlation physics renders electron- and hole-doped iridates fundamentally different.Some iridate compounds such as Sr 2 IrO 4 have electronic and atomic structures similar to quasi-2D copper oxides, raising the prospect of high temperature superconductivity. Here, the authors show that there is significant electron-hole asymmetry in iridates, contrary to expectations from the cuprates.

Energy transfer between two vacuum-gapped metal plates: Coulomb fluctuations and electron tunneling

Science.gov (United States)

Zhang, Zu-Quan; Lü, Jing-Tao; Wang, Jian-Sheng

2018-05-01

Recent experimental measurements for near-field radiative heat transfer between two bodies have been able to approach the gap distance within 2 nm , where the contributions of Coulomb fluctuation and electron tunneling are comparable. Using the nonequilibrium Green's function method in the G0W0 approximation, based on a tight-binding model, we obtain for the energy current a Caroli formula from the Meir-Wingreen formula in the local equilibrium approximation. Also, the Caroli formula is consistent with the evanescent part of the heat transfer from the theory of fluctuational electrodynamics. We go beyond the local equilibrium approximation to study the energy transfer in the crossover region from electron tunneling to Coulomb fluctuation based on a numerical calculation.

Spin-charge conversion in disordered two-dimensional electron gases lacking inversion symmetry

Science.gov (United States)

Huang, Chunli; Milletarı, Mirco; Cazalilla, Miguel A.

2017-11-01

We study the spin-charge conversion mechanisms in a two-dimensional gas of electrons moving in a smooth disorder potential by accounting for both Rashba-type and Mott's skew scattering contributions. We find that the quantum interference effects between spin-flip and skew scattering give rise to anisotropic spin precession scattering (ASP), a direct spin-charge conversion mechanism that was discovered in an earlier study of graphene decorated with adatoms [Huang et al., Phys. Rev. B 94, 085414 (2016), 10.1103/PhysRevB.94.085414]. Our findings suggest that, together with other spin-charge conversion mechanisms such as the inverse galvanic effect, ASP is a fairly universal phenomenon that should be present in disordered two-dimensional systems lacking inversion symmetry.

Magneto-transport studies on curved two-dimensional electron gases in InGaAs-microscrolls

International Nuclear Information System (INIS)

Schumacher, O.

2007-01-01

In this thesis magneto-resistance studies on evenly curved two-dimensional electron systems in cylindric geometry are presented and discussed. A principle first introduced by Prinz and co-workers in 1998 enables us to roll up thin semiconductor layer systems by taking advantage of internal elastic strain. The radius of such a semiconductor tube can be adjusted ranging from a few nanometers up to several micrometers. The tubes' shape and place on the substrate can be defined by lithographic methods which are presented in this work. Furthermore, we show rolled-up structures containing a two-dimensional electron system in the tube wall. With a special lithographic procedure we are able to structure, to contact and to roll up these 2D-electron-gases in Hall geometry. As a result, a cylindric two-dimensional electron system is produced, which experiences a modulation of the perpendicular magnetic field component. The radius of curvature of our structures is about 10 μm, the carrier mobility is optimized to values up to 125,000 cm 2 /Vs. In transport experiments on curved Hall bars containing two dimensional electron systems two Hall bar orientations, with respect to the curvature, may be distinguished. In this work both orientations, i.e. with a Hall bar along the tube curvature as well as a Hall bar along the tube axis, are presented and discussed. Measurements on Hall bars along the curvature show signatures in the longitudinal resistance, which can be understood with the help of the Landauer-Buettiker-formalism and the model of magnetic barriers. For Hall bars oriented along the tube axis the perpendicular magnetic field component averaged over the width of the bar defines the minimum position of the Shubnikov-de Haas-oscillations as well as the slope of the Hall resistance. Furthermore, measurements on so-called van the Pauw-lamellas are presented. In this geometry the magneto-resistance shows a slope which refers to highly mobile conditions at the zero crossing of

Coulomb engineering of the bandgap and excitons in two-dimensional materials

Science.gov (United States)

Raja, Archana; Chaves, Andrey; Yu, Jaeeun; Arefe, Ghidewon; Hill, Heather M.; Rigosi, Albert F.; Berkelbach, Timothy C.; Nagler, Philipp; Schüller, Christian; Korn, Tobias; Nuckolls, Colin; Hone, James; Brus, Louis E.; Heinz, Tony F.; Reichman, David R.; Chernikov, Alexey

2017-01-01

The ability to control the size of the electronic bandgap is an integral part of solid-state technology. Atomically thin two-dimensional crystals offer a new approach for tuning the energies of the electronic states based on the unusual strength of the Coulomb interaction in these materials and its environmental sensitivity. Here, we show that by engineering the surrounding dielectric environment, one can tune the electronic bandgap and the exciton binding energy in monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behaviour to present an in-plane dielectric heterostructure with a spatially dependent bandgap, as an initial step towards the creation of diverse lateral junctions with nanoscale resolution. PMID:28469178

Two-dimensional electron density characterisation of arc interruption phenomenon in current-zero phase

Science.gov (United States)

Inada, Yuki; Kamiya, Tomoki; Matsuoka, Shigeyasu; Kumada, Akiko; Ikeda, Hisatoshi; Hidaka, Kunihiko

2018-01-01

Two-dimensional electron density imaging over free burning SF6 arcs and SF6 gas-blast arcs was conducted at current zero using highly sensitive Shack-Hartmann type laser wavefront sensors in order to experimentally characterise electron density distributions for the success and failure of arc interruption in the thermal reignition phase. The experimental results under an interruption probability of 50% showed that free burning SF6 arcs with axially asymmetric electron density profiles were interrupted with a success rate of 88%. On the other hand, the current interruption of SF6 gas-blast arcs was reproducibly achieved under locally reduced electron densities and the interruption success rate was 100%.

A bias-tunable electron-spin filter based on a two-dimensional electron gas modulated by ferromagnetic-Schottky metal stripes

Energy Technology Data Exchange (ETDEWEB)

Lu Jianduo, E-mail: l_j316@163.co [Hubei Province Key Laboratory of Systems Science in Metallurgical Process, Wuhan University of Science and Technology, Wuhan 430081 (China); Li Yunbao; Yun Meijuan [Hubei Province Key Laboratory of Systems Science in Metallurgical Process, Wuhan University of Science and Technology, Wuhan 430081 (China); Zheng Wei [Key Laboratory of Dynamic Geodesy, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077 (China)

2011-03-28

We investigate the effect of the bias in an electron-spin filter based on a two-dimensional electron gas modulated by ferromagnetic-Schottky metal stripes. The numerical results show that the electron transmission and the conductance as well as the spin polarization are strongly dependent on the bias applied to the device. - Research highlights: We propose a bias-tunable electron-spin filter. The transmission and the conductance depend on the bias and the electron energy. The spin polarization depends on the bias and the electron energy. The results are helpful for making new types of bias-tunable spin filters.

Longitudinal and spin Hall conductance of a one-dimensional Aharonov-Bohm ring

International Nuclear Information System (INIS)

Moca, Catalin Pascu; Marinescu, D C

2006-01-01

The longitudinal and spin Hall conductances of an electron gas with Rashba-Dresselhaus spin-orbit interaction, confined to a quasi-one-dimensional Aharonov-Bohm ring, are studied as functions of disorder and magnetic flux. The system is mapped onto a one-dimensional virtual lattice and is described, in a tight binding approximation, by a Hamiltonian that depends parametrically on the nearest neighbour hopping integral t, the Rashba spin-orbit coupling V R , the Dresselhaus spin-orbit coupling V D and an Anderson-like, on-site disorder energy strength W. Numerical results are obtained within a spin dependent Landauer-Buettiker formalism

Zero-dimensional limit of the two-dimensional Lugiato-Lefever equation

Science.gov (United States)

Cardoso, Wesley B.; Salasnich, Luca; Malomed, Boris A.

2017-05-01

We study effects of tight harmonic-oscillator confinement on the electromagnetic field in a laser cavity by solving the two-dimensional Lugiato-Lefever (2D LL) equation, taking into account self-focusing or defocusing nonlinearity, losses, pump, and the trapping potential. Tightly confined (quasi-zero-dimensional) optical modes (pixels), produced by this model, are analyzed by means of the variational approximation, which provides a qualitative picture of the ensuing phenomena. This is followed by systematic simulations of the time-dependent 2D LL equation, which reveal the shape, stability, and dynamical behavior of the resulting localized patterns. In this way, we produce stability diagrams for the expected pixels. Then, we consider the LL model with the vortical pump, showing that it can produce stable pixels with embedded vorticity (vortex solitons) in remarkably broad stability areas. Alongside confined vortices with the simple single-ring structure, in the latter case the LL model gives rise to stable multi-ring states, with a spiral phase field. In addition to the numerical results, a qualitatively correct description of the vortex solitons is provided by the Thomas-Fermi approximation. Contribution to the Topical Issue: "Theory and Applications of the Lugiato-Lefever Equation", edited by Yanne K. Chembo, Damia Gomila, Mustapha Tlidi, Curtis R. Menyuk.

Microscopic theory of the superconducting gap in the quasi-one-dimensional organic conductor (TMTSF) 2ClO4 : Model derivation and two-particle self-consistent analysis

Science.gov (United States)

Aizawa, Hirohito; Kuroki, Kazuhiko

2018-03-01

We present a first-principles band calculation for the quasi-one-dimensional (Q1D) organic superconductor (TMTSF) 2ClO4 . An effective tight-binding model with the TMTSF molecule to be regarded as the site is derived from a calculation based on maximally localized Wannier orbitals. We apply a two-particle self-consistent (TPSC) analysis by using a four-site Hubbard model, which is composed of the tight-binding model and an onsite (intramolecular) repulsive interaction, which serves as a variable parameter. We assume that the pairing mechanism is mediated by the spin fluctuation, and the sign of the superconducting gap changes between the inner and outer Fermi surfaces, which correspond to a d -wave gap function in a simplified Q1D model. With the parameters we adopt, the critical temperature for superconductivity estimated by the TPSC approach is approximately 1 K, which is consistent with experiment.

Development of constraint algorithm for the number of electrons in molecular orbitals consisting mainly 4f atomic orbitals of rare-earth elements and its introduction to tight-binding quantum chemical molecular dynamics method

International Nuclear Information System (INIS)

Endou, Akira; Onuma, Hiroaki; Jung, Sun-ho

2007-01-01

Our original tight-binding quantum chemical molecular dynamics code, Colors', has been successfully applied to the theoretical investigation of complex materials including rare-earth elements, e.g., metal catalysts supported on a CeO 2 surface. To expand our code so as to obtain a good convergence for the electronic structure of a calculation system including a rare-earth element, we developed a novel algorithm to provide a constraint condition for the number of electrons occupying the selected molecular orbitals that mainly consist of 4f atomic orbitals of the rare-earth element. This novel algorithm was introduced in Colors. Using Colors, we succeeded in obtaining the classified electronic configurations of the 4f atomic orbitals of Ce 4+ and reduced Ce ions in a CeO 2 bulk model with one oxygen defect, which makes it difficult to obtain a good convergence using a conventional first-principles quantum chemical calculation code. (author)

Direct Measurement of the Band Structure of a Buried Two-Dimensional Electron Gas

DEFF Research Database (Denmark)

Miwa, Jill; Hofmann, Philip; Simmons, Michelle Y.

2013-01-01

We directly measure the band structure of a buried two dimensional electron gas (2DEG) using angle resolved photoemission spectroscopy. The buried 2DEG forms 2 nm beneath the surface of p-type silicon, because of a dense delta-type layer of phosphorus n-type dopants which have been placed there...

Reentrant behavior in the superconducting phase-dependent resistance of a disordered two-dimensional electron gas

NARCIS (Netherlands)

den Hartog, S.G.; Wees, B.J.van; Klapwijk, T.M; Nazarov, Y.V.; Borghs, G.

1997-01-01

We have investigated the bias-voltage dependence of the phase-dependent differential resistance of a disordered T-shaped two-dimensional electron gas coupled to two superconducting terminals. The resistance oscillations first increase upon lowering the energy. For bias voltages below the Thouless

Regular and chaotic motion of two dimensional electrons in a strong magnetic field

International Nuclear Information System (INIS)

Bar-Lev, Oded; Levit, Shimon.

1992-05-01

For two dimensional system of electrons in a strong magnetic field a standard approximation is the projection on a single Landau level. The resulting Hamiltonian is commonly treated semiclassically. An important element in applying the semiclassical approximation is the integrability of the corresponding classical system. We discuss the relevant integrability conditions and give a simple example of a non-integrable system-two interacting electrons in the presence of two impurities-which exhibits a coexistence of regular and chaotic classical motions. Since the inverse of the magnetic field plays the role of the Planck constant in these problems, one has the opportunity to control the 'closeness' of chaotic physical systems to the classical limit. (author)

General solution of the Dirac equation for quasi-two-dimensional electrons

Energy Technology Data Exchange (ETDEWEB)

Eremko, Alexander, E-mail: eremko@bitp.kiev.ua [Bogolyubov Institute for Theoretical Physics, Metrologichna Str., 14-b, Kyiv, 03680 (Ukraine); Brizhik, Larissa, E-mail: brizhik@bitp.kiev.ua [Bogolyubov Institute for Theoretical Physics, Metrologichna Str., 14-b, Kyiv, 03680 (Ukraine); Loktev, Vadim, E-mail: vloktev@bitp.kiev.ua [Bogolyubov Institute for Theoretical Physics, Metrologichna Str., 14-b, Kyiv, 03680 (Ukraine); National Technical University of Ukraine “KPI”, Peremohy av., 37, Kyiv, 03056 (Ukraine)

2016-06-15

The general solution of the Dirac equation for quasi-two-dimensional electrons confined in an asymmetric quantum well, is found. The energy spectrum of such a system is exactly calculated using special unitary operator and is shown to depend on the electron spin polarization. This solution contains free parameters, whose variation continuously transforms one known particular solution into another. As an example, two different cases are considered in detail: electron in a deep and in a strongly asymmetric shallow quantum well. The effective mass renormalized by relativistic corrections and Bychkov–Rashba coefficients are analytically obtained for both cases. It is demonstrated that the general solution transforms to the particular solutions, found previously (Eremko et al., 2015) with the use of spin invariants. The general solution allows to establish conditions at which a specific (accompanied or non-accompanied by Rashba splitting) spin state can be realized. These results can prompt the ways to control the spin degree of freedom via the synthesis of spintronic heterostructures with the required properties.

Two-electron states of a group-V donor in silicon from atomistic full configuration interactions

Science.gov (United States)

Tankasala, Archana; Salfi, Joseph; Bocquel, Juanita; Voisin, Benoit; Usman, Muhammad; Klimeck, Gerhard; Simmons, Michelle Y.; Hollenberg, Lloyd C. L.; Rogge, Sven; Rahman, Rajib

2018-05-01

Two-electron states bound to donors in silicon are important for both two-qubit gates and spin readout. We present a full configuration interaction technique in the atomistic tight-binding basis to capture multielectron exchange and correlation effects taking into account the full band structure of silicon and the atomic-scale granularity of a nanoscale device. Excited s -like states of A1 symmetry are found to strongly influence the charging energy of a negative donor center. We apply the technique on subsurface dopants subjected to gate electric fields and show that bound triplet states appear in the spectrum as a result of decreased charging energy. The exchange energy, obtained for the two-electron states in various confinement regimes, may enable engineering electrical control of spins in donor-dot hybrid qubits.

Structure and Stability of Molecular Crystals with Many-Body Dispersion-Inclusive Density Functional Tight Binding.

Science.gov (United States)

Mortazavi, Majid; Brandenburg, Jan Gerit; Maurer, Reinhard J; Tkatchenko, Alexandre

2018-01-18

Accurate prediction of structure and stability of molecular crystals is crucial in materials science and requires reliable modeling of long-range dispersion interactions. Semiempirical electronic structure methods are computationally more efficient than their ab initio counterparts, allowing structure sampling with significant speedups. We combine the Tkatchenko-Scheffler van der Waals method (TS) and the many-body dispersion method (MBD) with third-order density functional tight-binding (DFTB3) via a charge population-based method. We find an overall good performance for the X23 benchmark database of molecular crystals, despite an underestimation of crystal volume that can be traced to the DFTB parametrization. We achieve accurate lattice energy predictions with DFT+MBD energetics on top of vdW-inclusive DFTB3 structures, resulting in a speedup of up to 3000 times compared with a full DFT treatment. This suggests that vdW-inclusive DFTB3 can serve as a viable structural prescreening tool in crystal structure prediction.

Coulomb interactions in dense two-dimensional electron systems in a magnetic field

International Nuclear Information System (INIS)

Cheng, Szucheng.

1988-01-01

The simplest model of a two-dimensional system ignores the Coulomb interactions between the electrons. In this approximation, the electrons occupy the Landau levels, broadened by impurities and irregularities in the lattice. This independent electron approximation has usually been used to discuss observations for electron densities ρ and magnetic fields B where bar ν > 1 (bar ν triple-bond the number of Landau levels occupied). The most famous example is the theory of the integral Quantum Hall effect. However, when bar ν 1, electron-electron interactions should become important through the mixing of Landau levels. This thesis describes calculations for bar ν > 1 on phenomena which should be sensitive to electron-electron interactions: Wigner crystallization, the stability of the Landau levels under electron-electron interactions, the existence of quasiparticles and quasiholes, and the densities of states. The main results obtained concern: (1) The values of ρ and B where crystallization should occur when bar ν > 1. (2) The effect of electron-electron interactions in broadening the individual Landau levels, and in distributing the amplitudes for the excitation of independent electrons over many Landau levels. (3) The existence of quasiparticles and quasiholes whose lifetime is infinite near the Fermi level

Effects of disorder on the electron pairing

International Nuclear Information System (INIS)

Oviedo-Roa, R.; Wang, C.; Navarro, O.

1996-01-01

The electron pairing in randomly disordered lattices is studied by using an attractive Hubbard model, and by mapping the many-body problem onto a tight-binding one in a higher dimensional space, where a diagonal disorder is considered within the coherent-potential approximation. The results show an enhancement of the pair-binding energy as the self-energy difference increases in a binary alloy A x B 1-x . This fact suggests that the pairing process is highly sensitive to the one-particle localization condition. A ground-state phase diagram for on-site interaction disorder shows regions where pairing is avoided for ordered diatomic systems but not for disordered case

Direct current hopping conductance in one-dimensional diagonal disordered systems

Institute of Scientific and Technical Information of China (English)

Ma Song-Shan; Xu Hui; Liu Xiao-Liang; Xiao Jian-Rong

2006-01-01

Based on a tight-binding disordered model describing a single electron band, we establish a direct current (dc) electronic hopping transport conductance model of one-dimensional diagonal disordered systems, and also derive a dc conductance formula. By calculating the dc conductivity, the relationships between electric field and conductivity and between temperature and conductivity are analysed, and the role played by the degree of disorder in electronic transport is studied. The results indicate the conductivity of systems decreasing with the increase of the degree of disorder, characteristics of negative differential dependence of resistance on temperature at low temperatures in diagonal disordered systems, and the conductivity of systems decreasing with the increase of electric field, featuring the non-Ohm's law conductivity.

Ultra-Broadband Two-Dimensional Electronic Spectroscopy and Pump-Probe Microscopy of Molecular Systems

Science.gov (United States)

Spokoyny, Boris M.

Ultrafast spectroscopy offers an unprecedented view on the dynamic nature of chemical reactions. From charge transfer in semiconductors to folding and isomerization of proteins, these all important processes can now be monitored and in some instances even controlled on real, physical timescales. One of the biggest challenges of ultrafast science is the incredible energetic complexity of most systems. It is not uncommon to encounter macromolecules or materials with absorption spectra spanning significant portions of the visible spectrum. Monitoring a multitude of electronic and vibrational transitions, all dynamically interacting with each other on femtosecond timescales poses a truly daunting experimental task. The first part of this thesis deals with the development of a novel Two-Dimensional Electronic Spectroscopy (2DES) and its associated, advanced detection methodologies. Owing to its ultra-broadband implementation, this technique enables us to monitor femtosecond chemical dynamics that span the energetic landscape of the entire visible spectrum. In order to demonstrate the utility of our method, we apply it to two laser dye molecules, IR-144 and Cresyl Violet. Variation of photophysical properties on a microscopic scale in either man-made or naturally occurring systems can have profound implications on how we understand their macroscopic properties. Recently, inorganic hybrid perovskites have been tapped as the next generation solar energy harvesting materials. Their remarkable properties include low exciton binding energy, low exciton recombination rates and long carrier diffusion lengths. Nevertheless, considerable variability in device properties made with nearly identical preparation methods has puzzled the community. In the second part of this thesis we use non-linear pump probe microscopy to study the heterogeneous nature of femtosecond carrier dynamics in thin film perovskites. We show that the local morphology of the perovskite thin films has a

Temperature dependent transport of two dimensional electrons in the integral quantum Hall regime

International Nuclear Information System (INIS)

Wi, H.P.

1986-01-01

This thesis is concerned with the temperature dependent electronic transport properties of a two dimensional electron gas subject to background potential fluctuations and a perpendicular magnetic field. The author carried out an extensive temperature dependent study of the transport coefficients, in the region of an integral quantum plateau, in an In/sub x/Ga/sub 1-x/As/InP heterostructure for 4.2K 10 cm -2 meV -1 ) even at the middle between two Landau levels, which is unexpected from model calculations based on short ranged randomness. In addition, the different T dependent behavior of rho/sub xx/ between the states in the tails and those near the center of a Landau level, indicates the existence of different electron states in a Landau level. Additionally, the author reports T-dependent transport measurements in the transition region between two quantum plateaus in several different materials

Spin injection into a two-dimensional electron gas using inter-digital-ferromagnetic contacts

DEFF Research Database (Denmark)

Hu, C.M.; Nitta, J.; Jensen, Ane

2002-01-01

We present a model that describes the spin injection across a single interface with two electrodes. The spin-injection rate across a typical hybrid junction made of ferromagnet (FM) and a two-dimensional electron gas (2DEG) is found at the percentage level. We perforin spin-injection-detection ex......-injection-detection experiment on devices with two ferromagnetic contacts on a 2DEG confined in an InAs quantum well. A spin-injection rate of 4.5% is estimated from the measured magnetoresistance....

Many-particle correlations in quasi-two-dimensional electron-hole systems

International Nuclear Information System (INIS)

Nikolaev, Valentin

2002-01-01

This thesis reports a theoretical investigation of many-particle correlation effects in semiconductor heterostructures containing quantum wells. Particular attention is paid towards quasi-particle pair correlations. Using the Green's function technique and the ladder approximation as a basis, the generalized mass action law, which describes the redistribution of particles between correlated and uncorrelated states in quasi-two-dimensional systems for different temperatures and total densities, is derived. The expression is valid beyond the low-density limit, which allows us to investigate the transition of the system from a dilute exciton gas to a dense electron-hole plasma. A generalized Levinson theorem, which takes k-space filling into account, is formulated. Screening in quasi-two-dimensional systems is analyzed rigorously. Firstly, the qualitatively new mechanism of static local screening by indirect excitons is studied using the simple Thomas-Fermi approximation. Then, a detailed many-body description suitable for a proper account of dynamic screening by a quasi-2D electron-hole plasma, and consistent with the previously derived mass action law, is provided. The generalized Lindhard approximation and excitonic plasmon-pole approximations are also derived. The theory is applied to single and double quantum wells. A self-consistent procedure is developed for numerical investigation of the ionization degree of an electron-hole plasma at different values of temperature/exciton Rydberg ratios. This procedure accounts for screening, k-space filling (exciton bleaching), and the formation of excitons. An abrupt jump in the value of the ionization degree that happens with an increase of the carrier density or temperature (Mott transition) is found in a certain density-temperature region. It has been found that the critical density of the Mott transition for indirect excitons may be much smaller than that for direct excitons. A suggestion has been made that some of the

Empirical tight-binding modeling of ordered and disordered semiconductor structures; Empirische Tight-Binding-Modellierung geordneter und ungeordneter Halbleiterstrukturen

Energy Technology Data Exchange (ETDEWEB)

Mourad, Daniel

2010-11-30

In this thesis, we investigate the electronic and optical properties of pure as well as of substitutionally alloyed II-VI and III-V bulk semiconductors and corresponding semiconductor quantum dots by means of an empirical tight-binding (TB) model. In the case of the alloyed systems of the type A{sub x}B{sub 1-x}, where A and B are the pure compound semiconductor materials, we study the influence of the disorder by means of several extensions of the TB model with different levels of sophistication. Our methods range from rather simple mean-field approaches (virtual crystal approximation, VCA) over a dynamical mean-field approach (coherent potential approximation, CPA) up to calculations where substitutional disorder is incorporated on a finite ensemble of microscopically distinct configurations. In the first part of this thesis, we cover the necessary fundamentals in order to properly introduce the TB model of our choice, the effective bond-orbital model (EBOM). In this model, one s- and three p-orbitals per spin direction are localized on the sites of the underlying Bravais lattice. The matrix elements between these orbitals are treated as free parameters in order to reproduce the properties of one conduction and three valence bands per spin direction and can then be used in supercell calculations in order to model mixed bulk materials or pure as well as mixed quantum dots. Part II of this thesis deals with unalloyed systems. Here, we use the EBOM in combination with configuration interaction calculations for the investigation of the electronic and optical properties of truncated pyramidal GaN quantum dots embedded in AlN with an underlying zincblende structure. Furthermore, we develop a parametrization of the EBOM for materials with a wurtzite structure, which allows for a fit of one conduction and three valence bands per spin direction throughout the whole Brillouin zone of the hexagonal system. In Part III, we focus on the influence of alloying on the electronic

Two dimensional molecular electronics spectroscopy for molecular fingerprinting, DNA sequencing, and cancerous DNA recognition.

Science.gov (United States)

Rajan, Arunkumar Chitteth; Rezapour, Mohammad Reza; Yun, Jeonghun; Cho, Yeonchoo; Cho, Woo Jong; Min, Seung Kyu; Lee, Geunsik; Kim, Kwang S

2014-02-25

Laser-driven molecular spectroscopy of low spatial resolution is widely used, while electronic current-driven molecular spectroscopy of atomic scale resolution has been limited because currents provide only minimal information. However, electron transmission of a graphene nanoribbon on which a molecule is adsorbed shows molecular fingerprints of Fano resonances, i.e., characteristic features of frontier orbitals and conformations of physisorbed molecules. Utilizing these resonance profiles, here we demonstrate two-dimensional molecular electronics spectroscopy (2D MES). The differential conductance with respect to bias and gate voltages not only distinguishes different types of nucleobases for DNA sequencing but also recognizes methylated nucleobases which could be related to cancerous cell growth. This 2D MES could open an exciting field to recognize single molecule signatures at atomic resolution. The advantages of the 2D MES over the one-dimensional (1D) current analysis can be comparable to those of 2D NMR over 1D NMR analysis.

Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping

NARCIS (Netherlands)

Chen, Yunzhong; Trier, F.; Wijnands, Tom; Green, R.J.; Gauquelin, N.; Egoavil, R.; Christensen, D.V.; Koster, Gertjan; Huijben, Mark; Bovet, N.; Macke, S.; He, F.; Sutarto, R.; Andersen, N.H.; Sulpizio, J.A.; Honig, M.; Prawiroatmodjo, G.E.D.K.; Jespersen, T.S.; Linderoth, S.; Ilani, S.; Verbeeck, J.; van Tendeloo, G.; Rijnders, Augustinus J.H.M.; Sawatzky, G.A.; Pryds, N.

2015-01-01

Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable

Temperature Dependence of the Spin-Hall Conductivity of a Two-Dimensional Impure Rashba Electron Gas in the Presence of Electron-Phonon and Electron-Electron Interactions

Science.gov (United States)

Yavari, H.; Mokhtari, M.; Bayervand, A.

2015-03-01

Based on Kubo's linear response formalism, temperature dependence of the spin-Hall conductivity of a two-dimensional impure (magnetic and nonmagnetic impurities) Rashba electron gas in the presence of electron-electron and electron-phonon interactions is analyzed theoretically. We will show that the temperature dependence of the spin-Hall conductivity is determined by the relaxation rates due to these interactions. At low temperature, the elastic lifetimes ( and are determined by magnetic and nonmagnetic impurity concentrations which are independent of the temperature, while the inelastic lifetimes ( and related to the electron-electron and electron-phonon interactions, decrease when the temperature increases. We will also show that since the spin-Hall conductivity is sensitive to temperature, we can distinguish the intrinsic and extrinsic contributions.

Reentrant resistance and giant Andreev back scattering in a two-dimensional electron gas coupled to superconductors

NARCIS (Netherlands)

den Hartog, Sander; Wees, B.J. van; Nazarov, Yu.V.; Klapwijk, T.M.; Borghs, G.

1998-01-01

We first present the bias-voltage dependence of the superconducting phase-dependent reduction in the differential resistance of a disordered T-shaped two-dimensional electron gas (2DEG) coupled to two superconductors. This reduction exhibits a reentrant behavior, since it first increases upon

Dimer and cluster approach for the evaluation of electronic couplings governing charge transport: Application to two pentacene polymorphs

International Nuclear Information System (INIS)

Canola, Sofia; Pecoraro, Claudia; Negri, Fabrizia

2016-01-01

Hole transport properties are modeled for two polymorphs of pentacene: the single crystal polymorph and the thin film polymorph relevant for organic thin-film transistor applications. Electronic couplings are evaluated in the standard dimer approach but also considering a cluster approach in which the central molecule is surrounded by a large number of molecules quantum-chemically described. The effective electronic couplings suitable for the parametrization of a tight-binding model are derived either from the orthogonalization scheme limited to HOMO orbitals and from the orthogonalization of the full basis of molecular orbitals. The angular dependent mobilities estimated for the two polymorphs using the predicted pattern of couplings display different anisotropy characteristics as suggested from experimental investigations.

Dimer and cluster approach for the evaluation of electronic couplings governing charge transport: Application to two pentacene polymorphs

Energy Technology Data Exchange (ETDEWEB)

Canola, Sofia; Pecoraro, Claudia; Negri, Fabrizia

2016-10-20

Hole transport properties are modeled for two polymorphs of pentacene: the single crystal polymorph and the thin film polymorph relevant for organic thin-film transistor applications. Electronic couplings are evaluated in the standard dimer approach but also considering a cluster approach in which the central molecule is surrounded by a large number of molecules quantum-chemically described. The effective electronic couplings suitable for the parametrization of a tight-binding model are derived either from the orthogonalization scheme limited to HOMO orbitals and from the orthogonalization of the full basis of molecular orbitals. The angular dependent mobilities estimated for the two polymorphs using the predicted pattern of couplings display different anisotropy characteristics as suggested from experimental investigations.

Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy.

Science.gov (United States)

Richter, Johannes M; Branchi, Federico; Valduga de Almeida Camargo, Franco; Zhao, Baodan; Friend, Richard H; Cerullo, Giulio; Deschler, Felix

2017-08-29

In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. However, the initial build-up of the thermal distribution proved difficult to resolve with pump-probe techniques due to the requirement of high resolution, both in time and pump energy. Here, we use two-dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density. We extract characteristic carrier thermalization times from below 10 to 85 fs. These values allow for mobilities of 500 cm 2  V -1  s -1 at carrier densities lower than 2 × 10 19  cm -3 and limit the time for carrier extraction in hot carrier solar cells.Carrier-carrier scattering rates determine the fundamental limits of carrier transport and electronic coherence. Using two-dimensional electronic spectroscopy with sub-10 fs resolution, Richter and Branchi et al. extract carrier thermalization times of 10 to 85 fs in hybrid perovskites.

ELECTRON ACCELERATIONS AT HIGH MACH NUMBER SHOCKS: TWO-DIMENSIONAL PARTICLE-IN-CELL SIMULATIONS IN VARIOUS PARAMETER REGIMES

Energy Technology Data Exchange (ETDEWEB)

Matsumoto, Yosuke [Department of Physics, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522 (Japan); Amano, Takanobu; Hoshino, Masahiro, E-mail: ymatumot@astro.s.chiba-u.ac.jp [Department of Earth and Planetary Science, University of Tokyo, Hongo 1-33, Bunkyo-ku, Tokyo 113-0033 (Japan)

2012-08-20

Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfven Mach numbers, ion-to-electron mass ratios, and the upstream electron {beta}{sub e} (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (M{sub A} {approx} 30) with a mass ratio of M/m = 100 and {beta}{sub e} = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.

A comparative study of density functional and density functional tight binding calculations of defects in graphene

Energy Technology Data Exchange (ETDEWEB)

Zobelli, Alberto [Laboratoire de Physique des Solides, Univ. Paris Sud, CNRS UMR, Orsay (France); Ivanovskaya, Viktoria; Wagner, Philipp; Yaya, Abu; Ewels, Chris P. [Institut des Materiaux Jean Rouxel (IMN), CNRS UMR, University of Nantes (France); Suarez-Martinez, Irene [Nanochemistry Research Institute, Curtin University of Technology, Perth, Western Australia (Australia)

2012-02-15

The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies, and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localized Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone-Wales defects in graphene, as well as various unfunctionalized and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone-Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene. DFTB-MD derived sputtering energy threshold map for a carbon atom in a graphene plane. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Explicit expressions for masses and bindings of multibaryons in two dimensional quantum chromodynamics

International Nuclear Information System (INIS)

Frishman, Y.; Zakrewski, W.J.

1989-07-01

We derive explicit expressions for the masses and the binding energies of k-baryons states in two dimensional (one space and one time) Quantum Chromodynamics (QCD(2)). The expressions are given using the parameters n 1 ,n 2 ,...,nN f -1 which characterize the representation of SU(N f ), where N f is the number of flavours, in terms of its Young tableau description. We find that the difference between the mass of the k-baryon state and the sum of masses of any combination of its constituents, is independent of the value N f (ie the number of flavors). These results hold within a certain bosonized form of QCD(2) and within the strong coupling limit of (G/m) → ∞, where G is the gauge coupling constant and m the quark mass. (authors)

Interplay between magnetism and energetics in Fe-Cr alloys from a predictive noncollinear magnetic tight-binding model

DEFF Research Database (Denmark)

Soulairol, R.; Barreteau, Cyrille; Fu, Chu-Chun

2016-01-01

Magnetism is a key driving force controlling several thermodynamic and kinetic properties of Fe-Cr systems. We present a tight-binding model for Fe-Cr, where magnetism is treated beyond the usual collinear approximation. A major advantage of this model consists in a rather simple fitting procedur...

Magnetoresistance of a two-dimensional electron gas in a random magnetic field

DEFF Research Database (Denmark)

Smith, Anders; Taboryski, Rafael Jozef; Hansen, Luise Theil

1994-01-01

We report magnetoresistance measurements on a two-dimensional electron gas made from a high-mobility GaAs/AlxGa1-xAs heterostructure, where the externally applied magnetic field was expelled from regions of the semiconductor by means of superconducting lead grains randomly distributed on the surf...... on the surface of the sample. A theoretical explanation in excellent agreement with the experiment is given within the framework of the semiclassical Boltzmann equation. © 1994 The American Physical Society...

Prediction of two-dimensional diluted magnetic semiconductors: Doped monolayer MoS2 systems

KAUST Repository

Cheng, Yingchun

2013-03-05

Using first-principles calculations, we propose a two-dimensional diluted magnetic semiconductor: monolayer MoS2 doped by transition metals. Doping of transition metal atoms from the IIIB to VIB groups results in nonmagnetic states, since the number of valence electrons is smaller or equal to that of Mo. Doping of atoms from the VIIB to IIB groups becomes energetically less and less favorable. Magnetism is observed for Mn, Fe, Co, Zn, Cd, and Hg doping, while for the other dopants from these groups it is suppressed by Jahn-Teller distortions. Analysis of the binding energies and magnetic properties indicates that (Mo,X)S2 (X=Mn, Fe, Co, and Zn) are promising systems to explore two-dimensional diluted magnetic semiconductors.

Electronic structure engineering in silicene via atom substitution and a new two-dimensional Dirac structure Si3C

Science.gov (United States)

Yin, Na; Dai, Ying; Wei, Wei; Huang, Baibiao

2018-04-01

A lot of efforts have been made towards the band gap opening in two-dimensional silicene, the silicon version of graphene. In the present work, the electronic structures of single atom doped (B, N, Al and P) and codoped (B/N and Al/P) silicene monolayers are systematically examined on the base of density functional electronic calculations. Our results demonstrate that single atom doping can realize electron or hole doping in the silicene; while codoping, due to the syergistic effects, results in finite band gap in silicene at the Dirac point without significantly degrading the electronic properties. In addition, the characteristic of band gap shows dependence on the doping concentration. Importantly, we predict a new two-dimensional Dirac structure, the graphene-like Si3C, which also shows linear band dispersion relation around the Fermi level. Our results demonstrates an important perspective to engineer the electronic and optical properties of silicene.

Exchange electron-hole interaction of two-dimensional magnetoexcitons under the influence of the Rashba spin-orbit coupling

International Nuclear Information System (INIS)

Moskalenko, S.A.; Podlesny, I.V.; Lelyakov, I.A.; Novikov, B.V.; Kiselyova, E.S.; Gherciu, L.

2011-01-01

The Rashba spin-orbit coupling (RSOC) in the case of two-dimensional (2D) electrons and holes in a strong perpendicular magnetic field was studied. The spinor-type wave functions are characterized by different numbers of Landau levels in different spin projections. For electrons they differ by 1 as was established earlier by Rashba, whereas for holes they differ by 3. Two lowest electron states and four lowest hole states of Landau quantization give rise to eight 2D magnetoexciton states. The exchange electron-hole interaction in the frame of these states is investigated.

Tight-binding analysis of Si and GaAs ultrathin bodies with subatomic wave-function resolution

Science.gov (United States)

Tan, Yaohua P.; Povolotskyi, Michael; Kubis, Tillmann; Boykin, Timothy B.; Klimeck, Gerhard

2015-08-01

Empirical tight-binding (ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultrasmall structures like the As-terminated GaAs ultrathin bodies (UTBs). In this work, it is shown that more transferable ETB parameters with a short interaction range can be obtained by a process of mapping ab initio bands and wave functions to ETB models. This process enables the calibration of not only the ETB energy bands but also the ETB wave functions with corresponding ab initio calculations. Based on the mapping process, ETB models of Si and GaAs are parameterized with respect to hybrid functional calculations. Highly localized ETB basis functions are obtained. Both the ETB energy bands and wave functions with subatomic resolution of UTBs show good agreement with the corresponding hybrid functional calculations. The ETB methods can then be used to explain realistically extended devices in nonequilibrium that cannot be tackled with ab initio methods.

Magnetoresistance calculations for a two-dimensional electron gas with unilateral short-period strong modulation

Czech Academy of Sciences Publication Activity Database

Výborný, Karel; Smrčka, Ludvík

2002-01-01

Roč. 66, č. 20 (2002), s. 205318-1 - 205318-8 ISSN 0163-1829 R&D Projects: GA ČR GA202/01/0754 Institutional research plan: CEZ:AV0Z1010914 Keywords : magnetoresistance * short-period superlattices * two-dimensional electron gas Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.327, year: 2002

Two-dimensional multiferroics in monolayer group IV monochalcogenides

Science.gov (United States)

Wang, Hua; Qian, Xiaofeng

2017-03-01

Low-dimensional multiferroic materials hold great promises in miniaturized device applications such as nanoscale transducers, actuators, sensors, photovoltaics, and nonvolatile memories. Here, using first-principles theory we predict that two-dimensional (2D) monolayer group IV monochalcogenides including GeS, GeSe, SnS, and SnSe are a class of 2D semiconducting multiferroics with giant strongly-coupled in-plane spontaneous ferroelectric polarization and spontaneous ferroelastic lattice strain that are thermodynamically stable at room temperature and beyond, and can be effectively modulated by elastic strain engineering. Their optical absorption spectra exhibit strong in-plane anisotropy with visible-spectrum excitonic gaps and sizable exciton binding energies, rendering the unique characteristics of low-dimensional semiconductors. More importantly, the predicted low domain wall energy and small migration barrier together with the coupled multiferroic order and anisotropic electronic structures suggest their great potentials for tunable multiferroic functional devices by manipulating external electrical, mechanical, and optical field to control the internal responses, and enable the development of four device concepts including 2D ferroelectric memory, 2D ferroelastic memory, and 2D ferroelastoelectric nonvolatile photonic memory as well as 2D ferroelectric excitonic photovoltaics.

Ballistic magnetotransport in a suspended two-dimensional electron gas with periodic antidot lattices

Energy Technology Data Exchange (ETDEWEB)

Zhdanov, E. Yu., E-mail: zhdanov@isp.nsc.ru; Pogosov, A. G.; Budantsev, M. V.; Pokhabov, D. A.; Bakarov, A. K. [Siberian Branch of the Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics (Russian Federation)

2017-01-15

The magnetoresistance of suspended semiconductor nanostructures with a two-dimensional electron gas structured by periodic square antidot lattices is studied. It is shown that the ballistic regime of electron transport is retained after detaching the sample from the substrate. Direct comparative analysis of commensurability oscillations of magnetoresistance and their temperature dependences in samples before and after suspension is performed. It is found that the temperature dependences are almost identical for non-suspended and suspended samples, whereas significant differences are observed in the nonlinear regime, caused by direct current passage. Commensurability oscillations in the suspended samples are more stable with respect to exposure to direct current, which can be presumably explained by electron–electron interaction enhancement after detaching nanostructures from the high-permittivity substrate.

Chirality dependence of dipole matrix element of carbon nanotubes in axial magnetic field: A third neighbor tight binding approach

Science.gov (United States)

Chegel, Raad; Behzad, Somayeh

2014-02-01

We have studied the electronic structure and dipole matrix element, D, of carbon nanotubes (CNTs) under magnetic field, using the third nearest neighbor tight binding model. It is shown that the 1NN and 3NN-TB band structures show differences such as the spacing and mixing of neighbor subbands. Applying the magnetic field leads to breaking the degeneracy behavior in the D transitions and creates new allowed transitions corresponding to the band modifications. It is found that |D| is proportional to the inverse tube radius and chiral angle. Our numerical results show that amount of filed induced splitting for the first optical peak is proportional to the magnetic field by the splitting rate ν11. It is shown that ν11 changes linearly and parabolicly with the chiral angle and radius, respectively.

Three-dimensional structures of the mammalian multidrug resistance P-glycoprotein demonstrate major conformational changes in the transmembrane domains upon nucleotide binding.

Science.gov (United States)

Rosenberg, Mark F; Kamis, Alhaji Bukar; Callaghan, Richard; Higgins, Christopher F; Ford, Robert C

2003-03-07

P-glycoprotein is an ATP-binding cassette transporter that is associated with multidrug resistance and the failure of chemotherapy in human patients. We have previously shown, based on two-dimensional projection maps, that P-glycoprotein undergoes conformational changes upon binding of nucleotide to the intracellular nucleotide binding domains. Here we present the three-dimensional structures of P-glycoprotein in the presence and absence of nucleotide, at a resolution limit of approximately 2 nm, determined by electron crystallography of negatively stained crystals. The data reveal a major reorganization of the transmembrane domains throughout the entire depth of the membrane upon binding of nucleotide. In the absence of nucleotide, the two transmembrane domains form a single barrel 5-6 nm in diameter and about 5 nm deep with a central pore that is open to the extracellular surface and spans much of the membrane depth. Upon binding nucleotide, the transmembrane domains reorganize into three compact domains that are each 2-3 nm in diameter and 5-6 nm deep. This reorganization opens the central pore along its length in a manner that could allow access of hydrophobic drugs (transport substrates) directly from the lipid bilayer to the central pore of the transporter.

Exciton Scattering approach for conjugated macromolecules: from electronic spectra to electron-phonon coupling

Science.gov (United States)

Tretiak, Sergei

2014-03-01

The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.

High-Current Gain Two-Dimensional MoS 2 -Base Hot-Electron Transistors

KAUST Repository

Torres, Carlos M.

2015-12-09

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications. © 2015 American Chemical Society.

High-Current Gain Two-Dimensional MoS 2 -Base Hot-Electron Transistors

KAUST Repository

Torres, Carlos M.; Lan, Yann Wen; Zeng, Caifu; Chen, Jyun Hong; Kou, Xufeng; Navabi, Aryan; Tang, Jianshi; Montazeri, Mohammad; Adleman, James R.; Lerner, Mitchell B.; Zhong, Yuan Liang; Li, Lain-Jong; Chen, Chii Dong; Wang, Kang L.

2015-01-01

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications. © 2015 American Chemical Society.

Electron tomography, three-dimensional Fourier analysis and colour prediction of a three-dimensional amorphous biophotonic nanostructure

Science.gov (United States)

Shawkey, Matthew D.; Saranathan, Vinodkumar; Pálsdóttir, Hildur; Crum, John; Ellisman, Mark H.; Auer, Manfred; Prum, Richard O.

2009-01-01

Organismal colour can be created by selective absorption of light by pigments or light scattering by photonic nanostructures. Photonic nanostructures may vary in refractive index over one, two or three dimensions and may be periodic over large spatial scales or amorphous with short-range order. Theoretical optical analysis of three-dimensional amorphous nanostructures has been challenging because these structures are difficult to describe accurately from conventional two-dimensional electron microscopy alone. Intermediate voltage electron microscopy (IVEM) with tomographic reconstruction adds three-dimensional data by using a high-power electron beam to penetrate and image sections of material sufficiently thick to contain a significant portion of the structure. Here, we use IVEM tomography to characterize a non-iridescent, three-dimensional biophotonic nanostructure: the spongy medullary layer from eastern bluebird Sialia sialis feather barbs. Tomography and three-dimensional Fourier analysis reveal that it is an amorphous, interconnected bicontinuous matrix that is appropriately ordered at local spatial scales in all three dimensions to coherently scatter light. The predicted reflectance spectra from the three-dimensional Fourier analysis are more precise than those predicted by previous two-dimensional Fourier analysis of transmission electron microscopy sections. These results highlight the usefulness, and obstacles, of tomography in the description and analysis of three-dimensional photonic structures. PMID:19158016

Two-Dimensional Materials for Sensing: Graphene and Beyond

Directory of Open Access Journals (Sweden)

Seba Sara Varghese

2015-09-01

Full Text Available Two-dimensional materials have attracted great scientific attention due to their unusual and fascinating properties for use in electronics, spintronics, photovoltaics, medicine, composites, etc. Graphene, transition metal dichalcogenides such as MoS2, phosphorene, etc., which belong to the family of two-dimensional materials, have shown great promise for gas sensing applications due to their high surface-to-volume ratio, low noise and sensitivity of electronic properties to the changes in the surroundings. Two-dimensional nanostructured semiconducting metal oxide based gas sensors have also been recognized as successful gas detection devices. This review aims to provide the latest advancements in the field of gas sensors based on various two-dimensional materials with the main focus on sensor performance metrics such as sensitivity, specificity, detection limit, response time, and reversibility. Both experimental and theoretical studies on the gas sensing properties of graphene and other two-dimensional materials beyond graphene are also discussed. The article concludes with the current challenges and future prospects for two-dimensional materials in gas sensor applications.

Two dimensional electron transport in disordered and ordered distributions of magnetic flux vortices

International Nuclear Information System (INIS)

Nielsen, M.; Hedegaard, P.

1994-04-01

We have considered the conductivity properties of a two dimensional electron gas (2DEG) in two different kinds of inhomogeneous magnetic fields, i.e. a disordered distribution of magnetic flux vortices, and a periodic array of magnetic flux vortices. The work falls in two parts. In the first part we show how the phase shifts for an electron scattering on an isolated vortex, can be calculated analytically, and related to the transport properties through the differential cross section. In the second part we present numerical results for the Hall conductivity of the 2DEG in a periodic array of flux vortices found by exact diagonalization. We find characteristic spikes in the Hall conductance, when it is plotted against the filling fraction. It is argued that the spikes can be interpreted in terms of ''topological charge'' piling up across local and global gaps in the energy spectrum. (au) (23 refs.)

Spontaneous breaking of time-reversal symmetry in strongly interacting two-dimensional electron layers in silicon and germanium.

Science.gov (United States)

Shamim, S; Mahapatra, S; Scappucci, G; Klesse, W M; Simmons, M Y; Ghosh, A

2014-06-13

We report experimental evidence of a remarkable spontaneous time-reversal symmetry breaking in two-dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P delta layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time-reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two-dimensional electrons are confined to narrow half-filled impurity bands.

Resolving molecular vibronic structure using high-sensitivity two-dimensional electronic spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Bizimana, Laurie A.; Brazard, Johanna; Carbery, William P.; Gellen, Tobias; Turner, Daniel B., E-mail: dturner@nyu.edu [Department of Chemistry, New York University, 100 Washington Square East, New York, New York 10003 (United States)

2015-10-28

Coherent multidimensional optical spectroscopy is an emerging technique for resolving structure and ultrafast dynamics of molecules, proteins, semiconductors, and other materials. A current challenge is the quality of kinetics that are examined as a function of waiting time. Inspired by noise-suppression methods of transient absorption, here we incorporate shot-by-shot acquisitions and balanced detection into coherent multidimensional optical spectroscopy. We demonstrate that implementing noise-suppression methods in two-dimensional electronic spectroscopy not only improves the quality of features in individual spectra but also increases the sensitivity to ultrafast time-dependent changes in the spectral features. Measurements on cresyl violet perchlorate are consistent with the vibronic pattern predicted by theoretical models of a highly displaced harmonic oscillator. The noise-suppression methods should benefit research into coherent electronic dynamics, and they can be adapted to multidimensional spectroscopies across the infrared and ultraviolet frequency ranges.

Semiclassical magnetotransport in strongly spin-orbit coupled Rashba two-dimensional electron systems

Science.gov (United States)

Xiao, Cong; Li, Dingping

2016-06-01

Semiclassical magnetoelectric and magnetothermoelectric transport in strongly spin-orbit coupled Rashba two-dimensional electron systems is investigated. In the presence of a perpendicular classically weak magnetic field and short-range impurity scattering, we solve the linearized Boltzmann equation self-consistently. Using the solution, it is found that when Fermi energy E F locates below the band crossing point (BCP), the Hall coefficient is a nonmonotonic function of electron density n e and not inversely proportional to n e. While the magnetoresistance (MR) and Nernst coefficient vanish when E F locates above the BCP, non-zero MR and enhanced Nernst coefficient emerge when E F decreases below the BCP. Both of them are nonmonotonic functions of E F below the BCP. The different semiclassical magnetotransport behaviors between the two sides of the BCP can be helpful to experimental identifications of the band valley regime and topological change of Fermi surface in considered systems.

Semiclassical magnetotransport in strongly spin–orbit coupled Rashba two-dimensional electron systems

International Nuclear Information System (INIS)

Xiao, Cong; Li, Dingping

2016-01-01

Semiclassical magnetoelectric and magnetothermoelectric transport in strongly spin–orbit coupled Rashba two-dimensional electron systems is investigated. In the presence of a perpendicular classically weak magnetic field and short-range impurity scattering, we solve the linearized Boltzmann equation self-consistently. Using the solution, it is found that when Fermi energy E F locates below the band crossing point (BCP), the Hall coefficient is a nonmonotonic function of electron density n e and not inversely proportional to n e . While the magnetoresistance (MR) and Nernst coefficient vanish when E F locates above the BCP, non-zero MR and enhanced Nernst coefficient emerge when E F decreases below the BCP. Both of them are nonmonotonic functions of E F below the BCP. The different semiclassical magnetotransport behaviors between the two sides of the BCP can be helpful to experimental identifications of the band valley regime and topological change of Fermi surface in considered systems. (paper)

Bandstructure modulation for Si-h and Si-g nanotubes in a transverse electric field: Tight binding approach

Science.gov (United States)

Chegel, Raad; Behzad, Somayeh

2013-11-01

We have investigated the electronic properties of SiNTs, under the external electric field, using Tight Binding (TB) approximation. It was found that the energy levels, energy gaps, and density of states (DOS) strongly depend on the electric field strength. The large electric strength leads to coupling the neighbor subbands and induce destruction of subband degeneracy, increase of low-energy states, and strong modulation of energy gap which these effects reflect in the DOS spectrum. It has been shown that, the band gap reduction of Si g-NTs is linearly proportional to the electric field strength. The band gap variation for Si h-NTs increases first and later decreases (Metallic) or first remains constant and then decreases (semiconductor). Also we show that the larger diameter tubes are more sensitive to the field strength than smaller ones. The semiconducting metallic transition or vice versa can be achieved through an increasing of applied fields. Number and position of peaks in DOS spectrum are dependent on electric field strength.

Generation and control of electronic hybrid entanglement via a two-dimensional Rashba anisotropic nanodot

Energy Technology Data Exchange (ETDEWEB)

Amiri, F.; Rastgoo, S.; Golshan, M.M., E-mail: golshan@susc.ac.ir

2014-06-13

In the present article we report the dynamics of electronic spin–subbands, as well as subband–subband, hybrid entanglements in a two-dimensional anisotropic quantum dot. The dot is under the influence of Rashba effect and an external magnetic field. To study the hybrid entanglements, we partition the system into two categories in which either spatial degrees of freedom, subbands, entangle with the spin or the subbands become entangled amongst themselves. For the first case we calculate the von Neumann entropy, while for the latter the negativity is calculated. Our calculations show that for both cases information is periodically distributed between the corresponding subspaces. Effects of Rashba parameter and magnetic field on the characteristics of such oscillatory behavior are also discussed. For spin–subband entanglement the oscillations include dips, surrounded by plateaus of maximal entanglement. The subband–subband entanglement shows vanishingly small plateaus. The duration of plateaus is controlled by Rashba coupling and the external field. - Highlights: • Dynamics of hybrid entanglements in a parabolic 2-dimensional electron gas is reported. • The electron gas is influenced by the Rashba spin–orbit coupling and a magnetic field. • Spin–subband entanglement exhibits oscillations with dips and maximal plateaus. • Subband–subband entanglement also oscillates, but with vanishingly small plateaus. • The vigilance of plateaus is controllable by the Rashba effect and/or the field.

Hall field-induced magnetoresistance oscillations of a two-dimensional electron system

International Nuclear Information System (INIS)

Kunold, A.; Torres, M.

2008-01-01

We develop a model of the nonlinear response to a dc electrical current of a two-dimensional electron system (2DES) placed on a magnetic field. Based on the exact solution to the Schroedinger equation in arbitrarily strong electric and magnetic fields, and separating the relative and guiding center coordinates, a Kubo-like formula for the current is worked out as a response to the impurity scattering. Self-consistent expressions determine the longitudinal and Hall components of the electric field in terms of the dc current. The differential resistivity displays strong Hall field-induced oscillations, in agreement with the main features of the phenomenon observed in recent experiments

Assessment of the Density Functional Tight Binding Method for Protic Ionic Liquids.

Science.gov (United States)

Addicoat, Matthew A; Stefanovic, Ryan; Webber, Grant B; Atkin, Rob; Page, Alister J

2014-10-14

Density functional tight binding (DFTB), which is ∼100-1000 times faster than full density functional theory (DFT), has been used to simulate the structure and properties of protic ionic liquid (IL) ions, clusters of ions and the bulk liquid. Proton affinities for a wide range of IL cations and anions determined using DFTB generally reproduce G3B3 values to within 5-10 kcal/mol. The structures and thermodynamic stabilities of n -alkyl ammonium nitrate clusters (up to 450 quantum chemical atoms) predicted with DFTB are in excellent agreement with those determined using DFT. The IL bulk structure simulated using DFTB with periodic boundary conditions is in excellent agreement with published neutron diffraction data.

Two-dimensional flexible nanoelectronics

Science.gov (United States)

Akinwande, Deji; Petrone, Nicholas; Hone, James

2014-12-01

2014/2015 represents the tenth anniversary of modern graphene research. Over this decade, graphene has proven to be attractive for thin-film transistors owing to its remarkable electronic, optical, mechanical and thermal properties. Even its major drawback--zero bandgap--has resulted in something positive: a resurgence of interest in two-dimensional semiconductors, such as dichalcogenides and buckled nanomaterials with sizeable bandgaps. With the discovery of hexagonal boron nitride as an ideal dielectric, the materials are now in place to advance integrated flexible nanoelectronics, which uniquely take advantage of the unmatched portfolio of properties of two-dimensional crystals, beyond the capability of conventional thin films for ubiquitous flexible systems.

Resonances in a two-dimensional electron waveguide with a single δ-function scatterer

International Nuclear Information System (INIS)

Boese, Daniel; Lischka, Markus; Reichl, L. E.

2000-01-01

We study the conductance properties of a straight two-dimensional electron waveguide with an s-like scatterer modeled by a single δ-function potential with a finite number of modes. Even such a simple system exhibits interesting resonance phenomena. These resonances are explained in terms of quasibound states both by using a direct solution of the Schroedinger equation and by studying the Green's function of the system. Using the Green's function we calculate the survival probability as well as the power absorption, and show the influence of the quasibound states on these two quantities. (c) 2000 The American Physical Society

Electronic band structure of Two-Dimensional WS2/Graphene van der Waals Heterostructures

Science.gov (United States)

Henck, Hugo; Ben Aziza, Zeineb; Pierucci, Debora; Laourine, Feriel; Reale, Francesco; Palczynski, Pawel; Chaste, Julien; Silly, Mathieu G.; Bertran, François; Le Fèvre, Patrick; Lhuillier, Emmanuel; Wakamura, Taro; Mattevi, Cecilia; Rault, Julien E.; Calandra, Matteo; Ouerghi, Abdelkarim

2018-04-01

Combining single-layer two-dimensional semiconducting transition-metal dichalcogenides (TMDs) with a graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single-layer W S2 on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements, and density functional theory (DFT) calculations. The results show good electronic properties as well as a well-defined band arising from the strong splitting of the single-layer W S2 valence band at the K points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single-layer W S2 . Our results provide an important reference for future studies of electronic properties of W S2 and its applications in valleytronic devices.

A theoretical study on the performances of thermoelectric heat engine and refrigerator with two-dimensional electron reservoirs

International Nuclear Information System (INIS)

Luo, Xiaoguang; Long, Kailin; Wang, Jun; Qiu, Teng; He, Jizhou; Liu, Nian

2014-01-01

Theoretical thermoelectric nanophysics models of low-dimensional electronic heat engine and refrigerator devices, comprising two-dimensional hot and cold reservoirs and an interconnecting filtered electron transport mechanism have been established. The models were used to numerically simulate and evaluate the thermoelectric performance and energy conversion efficiencies of these low-dimensional devices, based on three different types of electron transport momentum-dependent filters, referred to herein as k x , k y , and k r filters. Assuming the Fermi-Dirac distribution of electrons, expressions for key thermoelectric performance parameters were derived for the resonant transport processes, in which the transmission of electrons has been approximated as a Lorentzian resonance function. Optimizations were carried out and the corresponding optimized design parameters have been determined, including but not limited to the universal theoretical upper bound of the efficiency at maximum power for heat engines, and the maximum coefficient of performance for refrigerators. From the results, it was determined that k r filter delivers the best thermoelectric performance, followed by the k x filter, and then the k y filter. For refrigerators with any one of three filters, an optimum range for the full width at half maximum of the transport resonance was found to be B T.

Experiments on melting in classical and quantum two dimensional electron systems

International Nuclear Information System (INIS)

Williams, F.I.B.

1991-01-01

''Two dimensional electron system'' (2DES) here refers to electrons whose dynamics is free in 2 dimensions but blocked in the third. Experiments have been performed in two limiting situations: the classical, low density, limit realised by electrons deposited on a liquid helium surface and the quantum, high density, limit realised by electrons at an interface between two epitaxially matched semiconductors. In the classical system, where T Q c so that the thermodynamic state is determined by the competition between the temperature and the Coulomb interaction, melting is induced either by raising the temperature at constant density or by lowering the density at finite temperature. In the quantum system, it is not possible to lower the density below about 100n W without the Coulomb interaction losing out to the random field representing the extrinsic disorder imposed by the semiconductor host. Instead one has to induce crystallisation with the help of the Lorentz force, by applying a perpendicular magnetic field B [2] . As the quantum magnetic length l c = (Planck constant c/eB) 1/2 is reduced with respect to the interelectronic spacing a, expressed by the filling factor ν 2l c 2 /a 2 , the system exhibits the quantum Hall effect (QHE), first for integer then for fractional values of ν. The fractional quantum Hall effect (FQHE) is a result of Coulomb induced correlation in the quantum liquid, but as ν is decreased still further the correlations are expected to take on long-range crystal-like periodicity accompanied by elastic shear rigidity. Such a state can nonetheless be destroyed by the disordering effect of temperature, giving rise to a phase boundary in a (T, B) plane. The aim of experiment is first to determine the phase diagram and then to help elucidate the mechanism of the melting. (author)

Electronic configuration of the c(2 x 2)MnCu two-dimensional alloy in layered structures supported on Cu(100)

International Nuclear Information System (INIS)

Gallego, S; Munoz, M C; Huttel, Y; Avila, J; Asensio, M C

2003-01-01

The c(2 x 2)MnCu surface alloy on Cu(100) can be considered as a purely two-dimensional magnetic system where the Mn atoms exhibit a large corrugation closely related to their high spin moment. In this paper we investigate the influence of the atomic environment on the electronic and magnetic properties of the two-dimensional alloyed layer, extending our study to the less known multilayered system made of MnCu two-dimensional alloy layers embedded in a Cu crystal. The analysis is based on angle-resolved photoelectron spectroscopy measurements and calculations using the Green function matching method, which allows us to treat exactly the projection of the three-dimensional lattice on the c(2 x 2) plane. A complete study of the valence band is performed along the two-dimensional Brillouin zone in a wide energy range. We show that the presence of Mn results in an important redistribution of the spin-polarized electronic states of the neighbouring Cu atoms. This redistribution is not accompanied by a net charge transfer between different atoms, and also the spin moment of Cu remains small. Most of the new features induced by Mn in the surface alloy are also present in the multilayered system, evidencing that they are specific to the two-dimensional alloyed layer and not surface effects

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. II. Application—Effect of quantum confinement and homogeneous strain on Cu conductance

Science.gov (United States)

Hegde, Ganesh; Povolotskyi, Michael; Kubis, Tillmann; Charles, James; Klimeck, Gerhard

2014-03-01

The Semi-Empirical tight binding model developed in Part I Hegde et al. [J. Appl. Phys. 115, 123703 (2014)] is applied to metal transport problems of current relevance in Part II. A systematic study of the effect of quantum confinement, transport orientation, and homogeneous strain on electronic transport properties of Cu is carried out. It is found that quantum confinement from bulk to nanowire boundary conditions leads to significant anisotropy in conductance of Cu along different transport orientations. Compressive homogeneous strain is found to reduce resistivity by increasing the density of conducting modes in Cu. The [110] transport orientation in Cu nanowires is found to be the most favorable for mitigating conductivity degradation since it shows least reduction in conductance with confinement and responds most favorably to compressive strain.

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. II. Application—Effect of quantum confinement and homogeneous strain on Cu conductance

International Nuclear Information System (INIS)

Hegde, Ganesh; Povolotskyi, Michael; Kubis, Tillmann; Charles, James; Klimeck, Gerhard

2014-01-01

The Semi-Empirical tight binding model developed in Part I Hegde et al. [J. Appl. Phys. 115, 123703 (2014)] is applied to metal transport problems of current relevance in Part II. A systematic study of the effect of quantum confinement, transport orientation, and homogeneous strain on electronic transport properties of Cu is carried out. It is found that quantum confinement from bulk to nanowire boundary conditions leads to significant anisotropy in conductance of Cu along different transport orientations. Compressive homogeneous strain is found to reduce resistivity by increasing the density of conducting modes in Cu. The [110] transport orientation in Cu nanowires is found to be the most favorable for mitigating conductivity degradation since it shows least reduction in conductance with confinement and responds most favorably to compressive strain

An environment-dependent semi-empirical tight binding model suitable for electron transport in bulk metals, metal alloys, metallic interfaces, and metallic nanostructures. II. Application—Effect of quantum confinement and homogeneous strain on Cu conductance

Energy Technology Data Exchange (ETDEWEB)

Hegde, Ganesh, E-mail: ghegde@purdue.edu; Povolotskyi, Michael; Kubis, Tillmann; Charles, James; Klimeck, Gerhard, E-mail: gekco@purdue.edu [Network for Computational Nanotechnology (NCN), Department of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907 (United States)

2014-03-28

The Semi-Empirical tight binding model developed in Part I Hegde et al. [J. Appl. Phys. 115, 123703 (2014)] is applied to metal transport problems of current relevance in Part II. A systematic study of the effect of quantum confinement, transport orientation, and homogeneous strain on electronic transport properties of Cu is carried out. It is found that quantum confinement from bulk to nanowire boundary conditions leads to significant anisotropy in conductance of Cu along different transport orientations. Compressive homogeneous strain is found to reduce resistivity by increasing the density of conducting modes in Cu. The [110] transport orientation in Cu nanowires is found to be the most favorable for mitigating conductivity degradation since it shows least reduction in conductance with confinement and responds most favorably to compressive strain.

Exploring two-dimensional electron gases with two-dimensional Fourier transform spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Paul, J.; Dey, P.; Karaiskaj, D., E-mail: karaiskaj@usf.edu [Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620 (United States); Tokumoto, T.; Hilton, D. J. [Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294 (United States); Reno, J. L. [CINT, Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)

2014-10-07

The dephasing of the Fermi edge singularity excitations in two modulation doped single quantum wells of 12 nm and 18 nm thickness and in-well carrier concentration of ∼4 × 10{sup 11} cm{sup −2} was carefully measured using spectrally resolved four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. Although the absorption at the Fermi edge is broad at this doping level, the spectrally resolved FWM shows narrow resonances. Two peaks are observed separated by the heavy hole/light hole energy splitting. Temperature dependent “rephasing” (S{sub 1}) 2DFT spectra show a rapid linear increase of the homogeneous linewidth with temperature. The dephasing rate increases faster with temperature in the narrower 12 nm quantum well, likely due to an increased carrier-phonon scattering rate. The S{sub 1} 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations. Distinct 2DFT lineshapes were observed for co-linear and cross-linear polarizations, suggesting the existence of polarization dependent contributions. The “two-quantum coherence” (S{sub 3}) 2DFT spectra for the 12 nm quantum well show a single peak for both co-linear and co-circular polarizations.

Absence of zero-temperature transmission rate of a double-chain tight-binding model for DNA with random sequence of nucleotides in thermodynamic limit

International Nuclear Information System (INIS)

Xiong Gang; Wang, X.R.

2005-01-01

The zero-temperature transmission rate spectrum of a double-chain tight-binding model for real DNA is calculated. It is shown that a band of extended-like states exists only for finite chain length with strong inter-chain coupling. While the whole spectrum tends to zero in thermodynamic limit, regardless of the strength of inter-chain coupling. It is also shown that a more faithful model for real DNA with periodic sugar-phosphate chains in backbone structures can be mapped into the above simple double-chain tight-binding model. Combined with above results, the transmission rate of real DNA with long random sequence of nucleotides is expected to be poor

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

International Nuclear Information System (INIS)

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

1998-01-01

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 (ωapproximately0 - 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

Band gap engineering for poly(p-phenylene) and poly(p-phenylene vinylene) copolymers using the tight-binding approach

Science.gov (United States)

Giro, R.; Caldas, M. J.; Galvão, D. S.

The interest in poly(p-phenylene) (PPP) and poly(p-phenylene vinylene) (PPV) copolymers stems from the fact that these homopolymers present interesting optical and electronic properties that allow a great variety of technological applications. Combining different numbers of PPP and PPV units it is possible, in principle, to obtain new structures presenting intermediate gap values (2.8 eV and 2.4 eV for PPP and PPV, respectively). For this study we used a Hückel Hamiltonian tight-binding coupled to the negative factor counting (NFC) technique. We carried out a systematic search to determine optimum relative concentrations for disordered binary polymeric alloys with predefined gap values. Once these structures were obtained, we used the semiempirical methods AM1/PM3 and ZINDO/S-CI for geometrical and optical studies, respectively. Our theoretical results show that it is possible to obtain copolymers of PPP and PPV with intermediate gap values of their parent structures.

Terahertz Radiation Heterodyne Detector Using Two-Dimensional Electron Gas in a GaN Heterostructure

Science.gov (United States)

Karasik, Boris S.; Gill, John J.; Mehdi, Imran; Crawford, Timothy J.; Sergeev, Andrei V.; Mitin, Vladimir V.

2012-01-01

High-resolution submillimeter/terahertz spectroscopy is important for studying atmospheric and interstellar molecular gaseous species. It typically uses heterodyne receivers where an unknown (weak) signal is mixed with a strong signal from the local oscillator (LO) operating at a slightly different frequency. The non-linear mixer devices for this frequency range are unique and are not off-the-shelf commercial products. Three types of THz mixers are commonly used: Schottky diode, superconducting hot-electron bolometer (HEB), and superconductor-insulation-superconductor (SIS) junction. A HEB mixer based on the two-dimensional electron gas (2DEG) formed at the interface of two slightly dissimilar semiconductors was developed. This mixer can operate at temperatures between 100 and 300 K, and thus can be used with just passive radiative cooling available even on small spacecraft.

Theoretical study of the structure, energetics, and dynamics of silicon and carbon systems using tight-binding approaches

International Nuclear Information System (INIS)

Xu, Chunhui.

1991-01-01

Semiempirical interatomic potentials are developed for silicon and carbon by modeling the total energy of the system using tight-binding approaches. The parameters of the models were obtained by fitting to results from accurate first-principles Local Density Functional calculations. Applications to the computation of phonons as a function of volume for diamond-structured silicon and carbon and the thermal expansions for silicon and diamond yields results which agree well with experiment. The physical origin of the negative thermal expansion observed in silicon is explained. A tight-binding total energy model is generated capable of describing carbon systems with a variety of atomic coordinations and topologies. The model reproduces the total energy versus volume curves of various carbon polytypes as well as phonons and elastic constants of diamond and graphite. The model has also been used in the molecular-dynamics simulation of the properties of carbon clusters. The calculated ground-state geometries of small clusters (C 2 --C 10 ) correlates well with results from accurate quantum chemical calculations, and the structural trend of clusters from C 2 to C 60 are investigated. 67 refs., 19 figs

The electronic conductance of polypyrrole (PPy molecular wires and emergence of Fano resonance phenomena

Directory of Open Access Journals (Sweden)

M Mardaani

2012-06-01

Full Text Available In this paper, we studied the electronic conductance of a polypyrrole polymer, which is embedded between two semi-infinite simple chains by using Green’s function technique in tight-binding approach. We first reduced the center polymer to a one dimensional chain with renormalized onsite and hopping energies by renormalization method. Then, we calculated the system conductivity as a function of incoming electron energy, polymer length and contact hopping terms. The results showed that by increasing polymer length and decreasing contact hopping energies, the conductance decreases in the gap regions. This means that for larger gaps, the electron tunneling happens with more difficulty. Moreover, at the resonance area, due to the existence of nitrogen atom in the polymer cyclic structure, the Fano resonance will emerge. Furthermore, the polymer can behave like a metallic chain by variation of the value of nitrogen on-site term.

DFTB3: Extension of the self-consistent-charge density-functional tight-binding method (SCC-DFTB).

Science.gov (United States)

Gaus, Michael; Cui, Qiang; Elstner, Marcus

2012-04-10

The self-consistent-charge density-functional tight-binding method (SCC-DFTB) is an approximate quantum chemical method derived from density functional theory (DFT) based on a second-order expansion of the DFT total energy around a reference density. In the present study we combine earlier extensions and improve them consistently with, first, an improved Coulomb interaction between atomic partial charges, and second, the complete third-order expansion of the DFT total energy. These modifications lead us to the next generation of the DFTB methodology called DFTB3, which substantially improves the description of charged systems containing elements C, H, N, O, and P, especially regarding hydrogen binding energies and proton affinities. As a result, DFTB3 is particularly applicable to biomolecular systems. Remaining challenges and possible solutions are also briefly discussed.

Dimensional perturbation theory for the two-electron atom

International Nuclear Information System (INIS)

Goodson, D.Z.

1987-01-01

Perturbation theory in δ = 1/D, where D is the dimensionality of space, is applied to the two-electron atom. In Chapter 1 an efficient procedure for calculating the coefficients of the perturbation series for the ground-state energy is developed using recursion relations between the moments of the coordinate operators. Results through tenth order are presented. The series is divergent, but Pade summation gives results comparable in accuracy to the best configuration-interaction calculations. The singularity structure of the Pade approximants confirms the hypothesis that the energy as a function of δ has an infinite sequence of poles on the negative real axis that approaches an essential singularity at δ = O. The essential singularity causes the divergence of the perturbation series. There are also two poles at δ = 1 that slow the asymptotic convergence of the low-order terms. In Chapter 2, various techniques are demonstrated for removing the effect of these poles, and accurate results are thereby obtained, even at very low order. In Chapter 3, the large D limit of the correlation energy (CE) is investigated. In the limit D → infinity it is only 35% smaller than at D = 3. It can be made to vanish in the limit by modifying the Hartree-Fock (HF) wavefunction. In Chapter 4, perturbation theory is applied to the Hooke's-law model of the atom. Prospects for treating more-complicated systems are briefly discussed

Physicochemical and structural characterization of a two-dimensional polymer performed by using the Langmuir-Blodgett technique

International Nuclear Information System (INIS)

Lefevre, Didier

1995-01-01

This research thesis addresses the physicochemical and structural characterization of two-dimensional polymer made of polymerizable macro-cycles pre-organised in-plane by using the Langmuir-Blodgett technique. Macro-cycles are porphyrins with four acetylenic functions which bind in both plane directions by formation of diacetylenic covalent bonds. These porphyrins are adsorbed under a single layer of dihexadecyl-phosphoric acid to build up a monomer amphiphilic film. The author reports the characterization of the Langmuir film by the study of compression isotherms and by Brewster angle microscopy. Other techniques are used (UV, visible and infrared spectroscopy, Raman spectroscopy) to highlight the polymerization in LB film. X photo-electronic spectroscopy and secondary ion mass spectroscopy are also used. The author reports the study of the orientation of macro-cycles before and after polymerization by using linear dichroism, electronic paramagnetic resonance and X ray diffraction. The in-plane LB film structure is studied by transmission X ray diffraction, atomic force microscopy in correlation with molecular simulation. The two-dimensional feature of the polymer formed at the water surface is highlighted. The membrane is visualized by electronic and optic microscopy, and characterized by EDXS and electronic diffraction [fr

Emergent gravity in the cubic tight-binding model of Weyl semimetal in the presence of elastic deformations

Energy Technology Data Exchange (ETDEWEB)

Cortijo, Alberto [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid (Spain); Zubkov, M.A., E-mail: zubkov@itep.ru [ITEP, B. Cheremushkinskaya 25, Moscow, 117259 (Russian Federation); Moscow Institute of Physics and Technology, 9, Institutskii per., Dolgoprudny, Moscow Region, 141700 (Russian Federation); Far Eastern Federal University, School of Biomedicine, 690950 Vladivostok (Russian Federation); National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe highway 31, 115409 Moscow (Russian Federation)

2016-03-15

We consider the tight-binding model with cubic symmetry that may be relevant for the description of a certain class of Weyl semimetals. We take into account elastic deformations of the semimetal through the modification of hopping parameters. This modification results in the appearance of emergent gauge field and the coordinate dependent anisotropic Fermi velocity. The latter may be interpreted as emergent gravitational field.

Consistent calculation of the stopping power for slow ions in two-dimensional electron gases

International Nuclear Information System (INIS)

Wang, You-Nian; Ma, Teng-Gai

1997-01-01

Within the framework of quantum scattering theory, we present a consistent calculation of the stopping power for slow protons and antiprotons moving in two-dimensional electron gases. The Friedel sum rule is used to determine the screening constant in the scattering potential. For the stopping power our results are compared with that of the random-phase approximation dielectric theory and that predicted by the linear Thomas-Fermi potential. copyright 1997 The American Physical Society

Effects of non-local electron transport in one-dimensional and two-dimensional simulations of shock-ignited inertial confinement fusion targets

Energy Technology Data Exchange (ETDEWEB)

Marocchino, A.; Atzeni, S.; Schiavi, A. [Dipartimento SBAI, Università di Roma “La Sapienza” and CNISM, Roma 00161 (Italy)

2014-01-15

In some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolaï-Busquet [G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000)] has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies.

Effects of non-local electron transport in one-dimensional and two-dimensional simulations of shock-ignited inertial confinement fusion targets

International Nuclear Information System (INIS)

Marocchino, A.; Atzeni, S.; Schiavi, A.

2014-01-01

In some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolaï-Busquet [G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000)] has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies

Effects of non-local electron transport in one-dimensional and two-dimensional simulations of shock-ignited inertial confinement fusion targets

Science.gov (United States)

Marocchino, A.; Atzeni, S.; Schiavi, A.

2014-01-01

In some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolaï-Busquet [G. P. Schurtz et al., Phys. Plasmas 7, 4238 (2000)] has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies.

Electronic properties of moire superlattice bands in layered two dimensional materials

Science.gov (United States)

Jung, Jeil

2014-03-01

When atomically thin two-dimensional materials are layered they often form incommensurate non-crystalline structures that exhibit long period moiré patterns when examined by scanning probes. In this talk, I will present a theoretical method which can be used to derive an effective Hamiltonian for these twisted van der Waals heterostructures using input from ab initio calculations performed on short-period crystalline structures. I will argue that the effective Hamiltonian can quantitatively describe the electronic properties of these layered systems for arbitrary twist angle and lattice constants. Applying this method to the important cases of graphene on graphene and graphene on hexagonal-boron nitride, I will present a series of experimentally observable quantities that can be extracted from their electronic structure, including their density of states and local density of states as a function of twist angle, and compare with available experiments. Work done in collaboration with Allan MacDonald, Shaffique Adam, Arnaud Raoux, Zhenhua Qiao, and Ashley DaSilva; and supported by the Singapore National Research Foundation Fellowship NRF-NRFF2012-01.

Traditional Semiconductors in the Two-Dimensional Limit.

Science.gov (United States)

Lucking, Michael C; Xie, Weiyu; Choe, Duk-Hyun; West, Damien; Lu, Toh-Ming; Zhang, S B

2018-02-23

Interest in two-dimensional materials has exploded in recent years. Not only are they studied due to their novel electronic properties, such as the emergent Dirac fermion in graphene, but also as a new paradigm in which stacking layers of distinct two-dimensional materials may enable different functionality or devices. Here, through first-principles theory, we reveal a large new class of two-dimensional materials which are derived from traditional III-V, II-VI, and I-VII semiconductors. It is found that in the ultrathin limit the great majority of traditional binary semiconductors studied (a series of 28 semiconductors) are not only kinetically stable in a two-dimensional double layer honeycomb structure, but more energetically stable than the truncated wurtzite or zinc-blende structures associated with three dimensional bulk. These findings both greatly increase the landscape of two-dimensional materials and also demonstrate that in the double layer honeycomb form, even ordinary semiconductors, such as GaAs, can exhibit exotic topological properties.

Optimal block-tridiagonalization of matrices for coherent charge transport

International Nuclear Information System (INIS)

Wimmer, Michael; Richter, Klaus

2009-01-01

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms require the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrarily complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green's function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.

Electronic bandstructure of an incommensurate crystal

International Nuclear Information System (INIS)

Rasing, T.

1984-06-01

The consequences of an incommensurate lattice modulation on the electronic energy levels have been studied by optical transmission experiments on Rb 2 ZnBr 4 . The results are analyzed with a simple tight-binding model in which the superspace symmetry of the crystal is taken into account. The lattice translational symmetry of crystalline matter leads to the well known concepts of the Brillouin zones, Bloch electrons, phonons and the like. In a crystal where the lattice is periodically distorted with a period that is incommensurate with the underlying lattice, this translational symmetry is broken. Nonetheless, incommensurate crystals are perfectly ordered and can be described by higher dimensional so-called superspace groups. In this paper we will show how this superspace approach provides a natural framework to understand their electronic bandstructure as well. 5 references, 3 figures

New edge magnetoplasmon for a two-dimensional electron gas in a ring geometry

International Nuclear Information System (INIS)

Proetto, C.R.

1992-09-01

The dynamical response of a classical two-dimensional electron gas confined in a ring geometry under a perpendicular magnetic field is analysed. Within the hydrodynamical approach and in the strong magnetic field limit, a new set of antidot edge magnetoplasmons is obtained, corresponding to density oscillations circulating along the inner boundary of the ring and whose frequency increases with magnetic field. The associated self-induced distribution of densities and currents are presented, together with an analysis of the size dependence of these perimeter waves. (author). 15 refs, 4 figs

Fullerene Derived Molecular Electronic Devices

Science.gov (United States)

Menon, Madhu; Srivastava, Deepak; Saini, Subbash

1998-01-01

The carbon Nanotube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale electronic devices. While the simple joint of two dissimilar tubes can be generated by the introduction of a pair of heptagon-pentagon defects in an otherwise perfect hexagonal grapheme sheet, more complex joints require other mechanisms. In this work we explore structural and electronic properties of complex 3-point junctions of carbon nanotubes using a generalized tight-binding molecular-dynamics scheme.

FPGA Implementation of one-dimensional and two-dimensional cellular automata

International Nuclear Information System (INIS)

D'Antone, I.

1999-01-01

This report describes the hardware implementation of one-dimensional and two-dimensional cellular automata (CAs). After a general introduction to the cellular automata, we consider a one-dimensional CA used to implement pseudo-random techniques in built-in self test for VLSI. Due to the increase in digital ASIC complexity, testing is becoming one of the major costs in the VLSI production. The high electronics complexity, used in particle physics experiments, demands higher reliability than in the past time. General criterions are given to evaluate the feasibility of the circuit used for testing and some quantitative parameters are underlined to optimize the architecture of the cellular automaton. Furthermore, we propose a two-dimensional CA that performs a peak finding algorithm in a matrix of cells mapping a sub-region of a calorimeter. As in a two-dimensional filtering process, the peaks of the energy clusters are found in one evolution step. This CA belongs to Wolfram class II cellular automata. Some quantitative parameters are given to optimize the architecture of the cellular automaton implemented in a commercial field programmable gate array (FPGA)

Graphene/MoS2 hybrid technology for large-scale two-dimensional electronics.

Science.gov (United States)

Yu, Lili; Lee, Yi-Hsien; Ling, Xi; Santos, Elton J G; Shin, Yong Cheol; Lin, Yuxuan; Dubey, Madan; Kaxiras, Efthimios; Kong, Jing; Wang, Han; Palacios, Tomás

2014-06-11

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

Control of two-dimensional electronic states at anatase Ti O2(001 ) surface by K adsorption

Science.gov (United States)

Yukawa, R.; Minohara, M.; Shiga, D.; Kitamura, M.; Mitsuhashi, T.; Kobayashi, M.; Horiba, K.; Kumigashira, H.

2018-04-01

The nature of the intriguing metallic electronic structures appearing at the surface of anatase titanium dioxide (a-Ti O2 ) remains to be elucidated, mainly owing to the difficulty of controlling the depth distribution of the oxygen vacancies generated by photoirradiation. In this study, K atoms were adsorbed onto the (001) surface of a-Ti O2 to dope electrons into the a-Ti O2 and to confine the electrons in the surface region. The success of the electron doping and its controllability were confirmed by performing in situ angle-resolved photoemission spectroscopy as well as core-level measurements. Clear subband structures were observed in the surface metallic states, indicating the creation of quasi-two-dimensional electron liquid (q2DEL) states in a controllable fashion. With increasing electron doping (K adsorption), the q2DEL states exhibited crossover from polaronic liquid states with multiple phonon-loss structures originating from the long-range Fröhlich interaction to "weakly correlated metallic" states. In the q2DEL states in the weakly correlated metallic region, a kink due to short-range electron-phonon coupling was clearly observed at about 80 ±10 meV . The characteristic energy is smaller than that previously observed for the metallic states of a-Ti O2 with three-dimensional nature (˜110 meV ) . These results suggest that the dominant electron-phonon coupling is modulated by anisotropic carrier screening in the q2DEL states.

Chaotic dynamics in two-dimensional noninvertible maps

CERN Document Server

Mira, Christian; Cathala, Jean-Claude; Gardini, Laura

1996-01-01

This book is essentially devoted to complex properties (Phase plane structure and bifurcations) of two-dimensional noninvertible maps, i.e. maps having either a non-unique inverse, or no real inverse, according to the plane point. They constitute models of sets of discrete dynamical systems encountered in Engineering (Control, Signal Processing, Electronics), Physics, Economics, Life Sciences. Compared to the studies made in the one-dimensional case, the two-dimensional situation remained a long time in an underdeveloped state. It is only since these last years that the interest for this resea

Effects of SiNx on two-dimensional electron gas and current collapse of AlGaN/GaN high electron mobility transistors

International Nuclear Information System (INIS)

Fan, Ren; Zhi-Biao, Hao; Lei, Wang; Lai, Wang; Hong-Tao, Li; Yi, Luo

2010-01-01

SiN x is commonly used as a passivation material for AlGaN/GaN high electron mobility transistors (HEMTs). In this paper, the effects of SiN x passivation film on both two-dimensional electron gas characteristics and current collapse of AlGaN/GaN HEMTs are investigated. The SiN x films are deposited by high- and low-frequency plasma-enhanced chemical vapour deposition, and they display different strains on the AlGaN/GaN heterostructure, which can explain the experiment results. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

Evolution of ferromagnetism in two-dimensional electron gas of LaTiO3/SrTiO3

Science.gov (United States)

Wen, Fangdi; Cao, Yanwei; Liu, Xiaoran; Pal, B.; Middey, S.; Kareev, M.; Chakhalian, J.

2018-03-01

Understanding, creating, and manipulating spin polarization of two-dimensional electron gases at complex oxide interfaces present an experimental challenge. For example, despite almost a decade long research effort, the microscopic origin of ferromagnetism in LaAlO3/SrTiO3 heterojunctions is still an open question. Here, by using a prototypical two-dimensional electron gas (2DEG) which emerges at the interface between band insulator SrTiO3 and antiferromagnetic Mott insulator LaTiO3, the experiment reveals the evidence for magnetic phase separation in a hole-doped Ti d1 t2g system, resulting in spin-polarized 2DEG. The details of electronic and magnetic properties of the 2DEG were investigated by temperature-dependent d.c. transport, angle-dependent X-ray photoemission spectroscopy, and temperature-dependent magnetoresistance. The observation of clear hysteresis in magnetotransport at low magnetic fields implies spin-polarization from magnetic islands in the hole rich LaTiO3 near the interface. These findings emphasize the role of magnetic instabilities in doped Mott insulators, thus providing another path for designing all-oxide structures relevant to spintronic applications.

Influence of magnetic disorders on quantum anomalous Hall effect in magnetic topological insulator films beyond the two-dimensional limit

Science.gov (United States)

Xing, Yanxia; Xu, Fuming; Cheung, King Tai; Sun, Qing-feng; Wang, Jian; Yao, Yugui

2018-04-01

Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetic topological insulator (MTI) thin films fabricated on magnetically doped {({{Bi}},{{Sb}})}2{{{Te}}}3. In an MTI thin film with the magnetic easy axis along the normal direction (z-direction), orientations of magnetic dopants are randomly distributed around the magnetic easy axis, acting as magnetic disorders. With the aid of the non-equilibrium Green's function and Landauer–Büttiker formalism, we numerically study the influence of magnetic disorders on QAHE in an MTI thin film modeled by a three-dimensional tight-binding Hamiltonian. It is found that, due to the existence of gapless side surface states, QAHE is protected even in the presence of magnetic disorders as long as the z-component of magnetic moment of all magnetic dopants are positive. More importantly, such magnetic disorders also suppress the dissipation of the chiral edge states and enhance the quality of QAHE in MTI films. In addition, the effect of magnetic disorders depends very much on the film thickness, and the optimal influence is achieved at certain thickness. These findings are new features for QAHE in three-dimensional systems, not present in two-dimensional systems.

Asymptotics for Two-dimensional Atoms

DEFF Research Database (Denmark)

Nam, Phan Thanh; Portmann, Fabian; Solovej, Jan Philip

2012-01-01

We prove that the ground state energy of an atom confined to two dimensions with an infinitely heavy nucleus of charge $Z>0$ and $N$ quantum electrons of charge -1 is $E(N,Z)=-{1/2}Z^2\\ln Z+(E^{\\TF}(\\lambda)+{1/2}c^{\\rm H})Z^2+o(Z^2)$ when $Z\\to \\infty$ and $N/Z\\to \\lambda$, where $E^{\\TF}(\\lambd......We prove that the ground state energy of an atom confined to two dimensions with an infinitely heavy nucleus of charge $Z>0$ and $N$ quantum electrons of charge -1 is $E(N,Z)=-{1/2}Z^2\\ln Z+(E^{\\TF}(\\lambda)+{1/2}c^{\\rm H})Z^2+o(Z^2)$ when $Z\\to \\infty$ and $N/Z\\to \\lambda$, where $E......^{\\TF}(\\lambda)$ is given by a Thomas-Fermi type variational problem and $c^{\\rm H}\\approx -2.2339$ is an explicit constant. We also show that the radius of a two-dimensional neutral atom is unbounded when $Z\\to \\infty$, which is contrary to the expected behavior of three-dimensional atoms....

Coxibs interfere with the action of aspirin by binding tightly to one monomer of cyclooxygenase-1

Energy Technology Data Exchange (ETDEWEB)

Rimon, Gilad; Sidhu, Ranjinder S.; Lauver, D. Adam; Lee, Jullia Y.; Sharma, Narayan P.; Yuan, Chong; Frieler, Ryan A.; Trievel, Raymond C.; Lucchesi, Benedict R.; Smith, William L. (Michigan)

2010-02-11

Pain associated with inflammation involves prostaglandins synthesized from arachidonic acid (AA) through cyclooxygenase-2 (COX-2) pathways while thromboxane A{sub 2} formed by platelets from AA via cyclooxygenase-1 (COX-1) mediates thrombosis. COX-1 and COX-2 are both targets of nonselective nonsteroidal antiinflammatory drugs (nsNSAIDs) including aspirin whereas COX-2 activity is preferentially blocked by COX-2 inhibitors called coxibs. COXs are homodimers composed of identical subunits, but we have shown that only one subunit is active at a time during catalysis; moreover, many nsNSAIDS bind to a single subunit of a COX dimer to inhibit the COX activity of the entire dimer. Here, we report the surprising observation that celecoxib and other coxibs bind tightly to a subunit of COX-1. Although celecoxib binding to one monomer of COX-1 does not affect the normal catalytic processing of AA by the second, partner subunit, celecoxib does interfere with the inhibition of COX-1 by aspirin in vitro. X-ray crystallographic results obtained with a celecoxib/COX-1 complex show how celecoxib can bind to one of the two available COX sites of the COX-1 dimer. Finally, we find that administration of celecoxib to dogs interferes with the ability of a low dose of aspirin to inhibit AA-induced ex vivo platelet aggregation. COX-2 inhibitors such as celecoxib are widely used for pain relief. Because coxibs exhibit cardiovascular side effects, they are often prescribed in combination with low-dose aspirin to prevent thrombosis. Our studies predict that the cardioprotective effect of low-dose aspirin on COX-1 may be blunted when taken with coxibs.

Few helium atoms in quasi two-dimensional space

International Nuclear Information System (INIS)

Kilic, Srecko; Vranjes, Leandra

2003-01-01

Two, three and four 3 He and 4 He atoms in quasi two-dimensional space above graphite and cesium surfaces and in 'harmonic' potential perpendicular to the surface have been studied. Using some previously examined variational wave functions and the Diffusion Monte Carlo procedure, it has been shown that all molecules: dimers, trimers and tetramers, are bound more strongly than in pure two- and three-dimensional space. The enhancement of binding with respect to unrestricted space is more pronounced on cesium than on graphite. Furthermore, for 3 He 3 ( 3 He 4 ) on all studied surfaces, there is an indication that the configuration of a dimer and a 'free' particle (two dimers) may be equivalently established

Multifractal character of the electronic states in disordered two-dimensional systems

International Nuclear Information System (INIS)

Tit, N.; Schreiber, M.

1994-08-01

The nature of electronic states in disordered two-dimensional (2D) systems is investigated. To this aim, we present our calculations of both density of states and dc-conductivity for square lattices modelling the Anderson Hamiltonian with on-site energies randomly chosen from a box distribution of width W. For weak disorder (W), the eigenfunctions calculated by means of the Lanczos diagonalization algorithm display spatial fluctuations reflecting their (multi)fractal behaviour. For increasing disorder or energy the observed increase of the curdling of the wavefunction reflects its stronger localization. Our dc-conductivity results suggest a critical fractal dimension d * c =1.48±0.05 to discriminate between the exponentially and the power-law localized states. Consequences of the localization on transport properties are also discussed. (author). 30 refs, 10 figs, 1 tab

Piecewise parabolic negative magnetoresistance of two-dimensional electron gas with triangular antidot lattice

International Nuclear Information System (INIS)

Budantsev, M. V.; Lavrov, R. A.; Pogosov, A. G.; Zhdanov, E. Yu.; Pokhabov, D. A.

2011-01-01

Extraordinary piecewise parabolic behavior of the magnetoresistance has been experimentally detected in the two-dimensional electron gas with a dense triangular lattice of antidots, where commensurability magnetoresistance oscillations are suppressed. The magnetic field range of 0–0.6 T can be divided into three wide regions, in each of which the magnetoresistance is described by parabolic dependences with high accuracy (comparable to the experimental accuracy) and the transition regions between adjacent regions are much narrower than the regions themselves. In the region corresponding to the weakest magnetic fields, the parabolic behavior becomes almost linear. The observed behavior is reproducible as the electron gas density changes, which results in a change in the resistance by more than an order of magnitude. Possible physical mechanisms responsible for the observed behavior, including so-called “memory effects,” are discussed.

Coupling effect of topological states and Chern insulators in two-dimensional triangular lattices

Science.gov (United States)

Zhang, Jiayong; Zhao, Bao; Xue, Yang; Zhou, Tong; Yang, Zhongqin

2018-03-01

We investigate topological states of two-dimensional (2D) triangular lattices with multiorbitals. Tight-binding model calculations of a 2D triangular lattice based on px and py orbitals exhibit very interesting doubly degenerate energy points at different positions (Γ and K /K' ) in momentum space, with quadratic non-Dirac and linear Dirac band dispersions, respectively. Counterintuitively, the system shows a global topologically trivial rather than nontrivial state with consideration of spin-orbit coupling due to the "destructive interference effect" between the topological states at the Γ and K /K' points. The topologically nontrivial state can emerge by introducing another set of triangular lattices to the system (bitriangular lattices) due to the breakdown of the interference effect. With first-principles calculations, we predict an intrinsic Chern insulating behavior (quantum anomalous Hall effect) in a family of the 2D triangular lattice metal-organic framework of Co(C21N3H15) (TPyB-Co) from this scheme. Our results provide a different path and theoretical guidance for the search for and design of new 2D topological quantum materials.

Tight-binding study of the hole subband structure properties of p-type delta-doped quantum wells in Si by using a Thomas-Fermi-Dirac potential

International Nuclear Information System (INIS)

Rodriguez-Vargas, I; Madrigal-Melchor, J; Vlaev, S J

2009-01-01

We present the hole subband structure of p-type delta-doped single, double, multiple and superlattice quantum wells in Si. We use the first neighbors sp 3 s' tight-binding approximation including spin for the hole level structure analysis. The parameters of the tight-binding hamiltonian were taken from Klimeck et al. [Klimeck G, Bowen R C, Boykin T B, Salazar-Lazaro C, Cwik T A and Stoica A 2000 Superlattice. Microst. 27 77], first neighbors parameters that give realiable results for the valence band of Si. The calculations are based on a scheme previously proposed and applied to delta-doped quantum well systems [Vlaev S J and Gaggero-Sager L M 1998 Phys. Rev. B 58 1142]. The scheme relies on the incorporation of the delta-doped quantum well potential in the diagonal terms of the tight-binding hamiltonian. We give a detail description of the delta-doped quantum well structures, this is, we study the hole subband structure behavior as a function of the impurity density, the interwell distance of the doped planes and the superlattice period. We also compare our results with the available theoretical and experimental data, obtaining a reasonable agreement.

Tight-binding study of the hole subband structure properties of p-type delta-doped quantum wells in Si by using a Thomas-Fermi-Dirac potential

Energy Technology Data Exchange (ETDEWEB)

Rodriguez-Vargas, I; Madrigal-Melchor, J; Vlaev, S J, E-mail: isaac@planck.reduaz.m [Unidad Academica de Fisica, Universidad Autonoma de Zacatecas, Calzada Solidaridad Esquina Con Paseo La Bufa S/N, 98060 Zacatecas, ZAC. (Mexico)

2009-05-01

We present the hole subband structure of p-type delta-doped single, double, multiple and superlattice quantum wells in Si. We use the first neighbors sp{sup 3}s' tight-binding approximation including spin for the hole level structure analysis. The parameters of the tight-binding hamiltonian were taken from Klimeck et al. [Klimeck G, Bowen R C, Boykin T B, Salazar-Lazaro C, Cwik T A and Stoica A 2000 Superlattice. Microst. 27 77], first neighbors parameters that give realiable results for the valence band of Si. The calculations are based on a scheme previously proposed and applied to delta-doped quantum well systems [Vlaev S J and Gaggero-Sager L M 1998 Phys. Rev. B 58 1142]. The scheme relies on the incorporation of the delta-doped quantum well potential in the diagonal terms of the tight-binding hamiltonian. We give a detail description of the delta-doped quantum well structures, this is, we study the hole subband structure behavior as a function of the impurity density, the interwell distance of the doped planes and the superlattice period. We also compare our results with the available theoretical and experimental data, obtaining a reasonable agreement.

A Study of the Surface Structure of Polymorphic Graphene and Other Two-Dimensional Materials for Use in Novel Electronics and Organic Photovoltaics

Science.gov (United States)

Grady, Maxwell

For some time there has been interest in the fundamental physical properties of low- dimensional material systems. The discovery of graphene as a stable two-dimensional form of solid carbon lead to an exponential increase in research in two-dimensional and other re- duced dimensional systems. It is now known that there is a wide range of materials which are stable in two-dimensional form. These materials span a large configuration space of struc- tural, mechanical, and electronic properties, which results in the potential to create novel electronic devices from nano-scale heterostructures with exactly tailored device properties. Understanding the material properties at the nanoscale level requires specialized tools to probe materials with atomic precision. Here I present the growth and analysis of a novel graphene-ruthenium system which exhibits unique polymorphism in its surface structure, hereby referred to as polymorphic graphene. Scanning Tunneling Microscopy (STM) investigations of the polymorphic graphene surface reveal a periodically rippled structure with a vast array of domains, each exhibiting xvia unique moire period. The majority of moire domains found in this polymorphic graphene system are previously unreported in past studies of the structure of graphene on ruthenium. To better understand many of the structural properties of this system, characterization methods beyond those available at the UNH surface science lab are employed. Further investigation using Low Energy Electron Microscopy (LEEM) has been carried out at Sandia National Laboratory's Center for Integrated Nanotechnology and the Brookhaven National Laboratory Center for Functional Nanomaterials. To aid in analysis of the LEEM data, I have developed an open source software package to automate extraction of electron reflectivity curves from real space and reciprocal space data sets. This software has been used in the study of numerous other two-dimensional materials beyond graphene. When

Anomalous behavior of a confined two-dimensional electron within an external magnetic field

Energy Technology Data Exchange (ETDEWEB)

Rosas, R; Riera R; Marin, J. L. [Universidad de Sonora, Hermosillo, Sonora (Mexico); Leon, H. [Instituto Superior Jose Antonio Echeverria, La Habana (Cuba)

2001-10-01

An anomalous diamagnetic behavior of a confined two-dimensional electron within an external magnetic field (perpendicular to the confining plane) is discussed in this letter. Although this finding is consistent with the pioneering work of Robnik, it has not been previously reported. When this effect occurs, the ratio between the typical length of spatial and magnetic confinement is an integer number. This property leads also to a quantization of the magnetic flux across the confining circle. The possible consequences of the peculiar behavior of the electron within such a structure are discussed. [Spanish] Se estudia una posible anomalia en las propiedades diamagneticas de un electron bidimensional confinado en presencia de un campo magnetico externo perpendicular al plano de confinamiento. Aunque los resultados obtenidos son consistentes con el trabajo pionero de Robnik, no han sido reportados anteriormente, a pesar de sus posibles aplicaciones, ya que cuando ocurre, el cociente entre la longitud magnetica y el tamano de la region de confinamiento es un numero entero, propiedad que establece una cuantizacion del flujo magnetico que atraviesa el circulo confinante. Se discuten las posibles consecuencias del comportamiento peculiar del electron en este tipo de estructura.

Optical Conductivity in a Two-Dimensional Extended Hubbard Model for an Organic Dirac Electron System α-(BEDT-TTF2I3

Directory of Open Access Journals (Sweden)

Daigo Ohki

2018-03-01

Full Text Available The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system α -(BEDT-TTF 2 I 3 using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in a two-dimensional Dirac electron system, a peak structure is found above the charge order gap. It is shown that the peak structure originates from the Van Hove singularities of the conduction and valence bands, where those singularities are located at a saddle point between two Dirac cones in momentum space. The frequency of the peak structure exhibits drastic change in the vicinity of the charge order transition.

Two-Dimensional Electron Density Measurement of Positive Streamer Discharge in Atmospheric-Pressure Air

Science.gov (United States)

Inada, Yuki; Ono, Ryo; Kumada, Akiko; Hidaka, Kunihiko; Maeyama, Mitsuaki

2016-09-01

The electron density of streamer discharges propagating in atmospheric-pressure air is crucially important for systematic understanding of the production mechanisms of reactive species utilized in wide ranging applications such as medical treatment, plasma-assisted ignition and combustion, ozone production and environmental pollutant processing. However, electron density measurement during the propagation of the atmospheric-pressure streamers is extremely difficult by using the conventional localized type measurement systems due to the streamer initiation jitters and the irreproducibility in the discharge paths. In order to overcome the difficulties, single-shot two-dimensional electron density measurement was conducted by using a Shack-Hartmann type laser wavefront sensor. The Shack-Hartmann sensor with a temporal resolution of 2 ns was applied to pulsed positive streamer discharges generated in an air gap between pin-to-plate electrodes. The electron density a few ns after the streamer initiation was 7*1021m-3 and uniformly distributed along the streamer channel. The electron density and its distribution profile were compared with a previous study simulating similar streamers, demonstrating good agreement. This work was supported in part by JKA and its promotion funds from KEIRIN RACE. The authors like to thank Mr. Kazuaki Ogura and Mr. Kaiho Aono of The University of Tokyo for their support during this work.

Coordination-resolved local bond contraction and electron binding-energy entrapment of Si atomic clusters and solid skins

Energy Technology Data Exchange (ETDEWEB)

Bo, Maolin; Huang, Yongli; Zhang, Ting [Key Laboratory of Low-Dimensional Materials and Application Technologies, Xiangtan University, Hunan 411105 (China); Wang, Yan, E-mail: ywang8@hnust.edu.cn, E-mail: ecqsun@ntu.edu.sg [Key Laboratory of Low-Dimensional Materials and Application Technologies, Xiangtan University, Hunan 411105 (China); School of Information and Electronic Engineering, Hunan University of Science and Technology, Hunan 411201 (China); Zhang, Xi [School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (Singapore); Li, Can [Center for Coordination Bond Engineering, School of Materials Science and Engineering, China Jiliang University, Hangzhou 330018 (China); Sun, Chang Q., E-mail: ywang8@hnust.edu.cn, E-mail: ecqsun@ntu.edu.sg [Key Laboratory of Low-Dimensional Materials and Application Technologies, Xiangtan University, Hunan 411105 (China); School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (Singapore); Center for Coordination Bond Engineering, School of Materials Science and Engineering, China Jiliang University, Hangzhou 330018 (China)

2014-04-14

Consistency between x-ray photoelectron spectroscopy measurements and density-function theory calculations confirms our bond order-length-strength notation-incorporated tight-binding theory predictions on the quantum entrapment of Si solid skin and atomic clusters. It has been revealed that bond-order deficiency shortens and strengthens the Si-Si bond, which results in the local densification and quantum entrapment of the core and valence electrons. Unifying Si clusters and Si(001) and (111) skins, this mechanism has led to quantification of the 2p binding energy of 96.089 eV for an isolated Si atom, and their bulk shifts of 2.461 eV. Findings evidence the significance of atomic undercoordination that is of great importance to device performance.

Single-shot ultrabroadband two-dimensional electronic spectroscopy of the light-harvesting complex LH2.

Science.gov (United States)

Harel, Elad; Long, Phillip D; Engel, Gregory S

2011-05-01

Here we present two-dimensional (2D) electronic spectra of the light-harvesting complex LH2 from purple bacteria using coherent pulses with bandwidth of over 100 nm FWHM. This broadband excitation and detection has allowed the simultaneous capture of both the B800 and B850 bands using a single light source. We demonstrate that one laser pulse is sufficient to capture the entire 2D electronic spectrum with a high signal-to-noise ratio. At a waiting time of 800 fs, we observe population transfer from the B800 to B850 band as manifested by a prominent cross peak. These results will enable observation of the dynamics of biological systems across both ultrafast (1 ms) timescales simultaneously.

Electrons, pseudoparticles, and quasiparticles in the one-dimensional many-electron problem

International Nuclear Information System (INIS)

Carmelo, J.M.; Castro Neto, A.H.

1996-01-01

We generalize the concept of quasiparticle for one-dimensional (1D) interacting electronic systems. The ↑ and ↓ quasiparticles recombine the pseudoparticle colors c and s (charge and spin at zero-magnetic field) and are constituted by one many-pseudoparticle topological-momentum shift and one or two pseudoparticles. These excitations cannot be separated. We consider the case of the Hubbard chain. We show that the low-energy electron-quasiparticle transformation has a singular character which justifies the perturbative and nonperturbative nature of the quantum problem in the pseudoparticle and electronic basis, respectively. This follows from the absence of zero-energy electron-quasiparticle overlap in 1D. The existence of Fermi-surface quasiparticles both in 1D and three dimensional (3D) many-electron systems suggests their existence in quantum liquids in dimensions 1 1 or whether it becomes finite as soon as we leave 1D remains an unsolved question. copyright 1996 The American Physical Society

Nonequilibrium chemical potential in a two-dimensional electron gas in the quantum-Hall-effect regime

Energy Technology Data Exchange (ETDEWEB)

Pokhabov, D. A., E-mail: pokhabov@isp.nsc.ru; Pogosov, A. G.; Budantsev, M. V.; Zhdanov, E. Yu.; Bakarov, A. K. [Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics, Siberian Branch (Russian Federation)

2016-08-15

The nonequilibrium state of a two-dimensional electron gas in the quantum-Hall-effect regime is studied in Hall bars equipped with additional inner contacts situated within the bar. The magnetic-field dependence of the voltage drop between different contact pairs are studied at various temperatures. It was found that the voltage between the inner and outer contacts exhibits peaks of significant amplitude in narrow magnetic-field intervals near integer filling factors. Furthermore, the magnetic-field dependence of the voltage in these intervals exhibits a hysteresis, whereas the voltage between the outer contacts remains zero in the entire magnetic-field range. The appearance of the observed voltage peaks and their hysteretic behavior can be explained by an imbalance between the chemical potentials of edge and bulk states, resulting from nonequilibrium charge redistribution between the edge and bulk states when the magnetic field sweeps under conditions of the quantum Hall effect. The results of the study significantly complement the conventional picture of the quantum Hall effect, explicitly indicating the existence of a significant imbalance at the edge of the two-dimensional electron gas: the experimentally observed difference between the electrochemical potentials of the edge and bulk exceeds the distance between Landau levels by tens of times.

Sub-Nanometer Channels Embedded in Two-Dimensional Materials

KAUST Repository

Han, Yimo; Li, Ming-yang; Jung, Gang-Seob; Marsalis, Mark A.; Qin, Zhao; Buehler, Markus J.; Li, Lain-Jong; Muller, David A.

2017-01-01

Two-dimensional (2D) materials are among the most promising candidates for next-generation electronics due to their atomic thinness, allowing for flexible transparent electronics and ultimate length scaling1. Thus far, atomically-thin p-n junctions2

Few quantum particles on one dimensional lattices

Energy Technology Data Exchange (ETDEWEB)

Valiente Cifuentes, Manuel

2010-06-18

There is currently a great interest in the physics of degenerate quantum gases and low-energy few-body scattering due to the recent experimental advances in manipulation of ultracold atoms by light. In particular, almost perfect periodic potentials, called optical lattices, can be generated. The lattice spacing is fixed by the wavelength of the laser field employed and the angle betwen the pair of laser beams; the lattice depth, defining the magnitude of the different band gaps, is tunable within a large interval of values. This flexibility permits the exploration of different regimes, ranging from the ''free-electron'' picture, modified by the effective mass for shallow optical lattices, to the tight-binding regime of a very deep periodic potential. In the latter case, effective single-band theories, widely used in condensed matter physics, can be implemented with unprecedent accuracy. The tunability of the lattice depth is nowadays complemented by the use of magnetic Feshbach resonances which, at very low temperatures, can vary the relevant atom-atom scattering properties at will. Moreover, optical lattices loaded with gases of effectively reduced dimensionality are experimentally accessible. This is especially important for one spatial dimension, since most of the exactly solvable models in many-body quantum mechanics deal with particles on a line; therefore, experiments with one-dimensional gases serve as a testing ground for many old and new theories which were regarded as purely academic not so long ago. The physics of few quantum particles on a one-dimensional lattice is the topic of this thesis. Most of the results are obtained in the tight-binding approximation, which is amenable to exact numerical or analytical treatment. For the two-body problem, theoretical methods for calculating the stationary scattering and bound states are developed. These are used to obtain, in closed form, the two-particle solutions of both the Hubbard and

Few quantum particles on one dimensional lattices

International Nuclear Information System (INIS)

Valiente Cifuentes, Manuel

2010-01-01

There is currently a great interest in the physics of degenerate quantum gases and low-energy few-body scattering due to the recent experimental advances in manipulation of ultracold atoms by light. In particular, almost perfect periodic potentials, called optical lattices, can be generated. The lattice spacing is fixed by the wavelength of the laser field employed and the angle betwen the pair of laser beams; the lattice depth, defining the magnitude of the different band gaps, is tunable within a large interval of values. This flexibility permits the exploration of different regimes, ranging from the ''free-electron'' picture, modified by the effective mass for shallow optical lattices, to the tight-binding regime of a very deep periodic potential. In the latter case, effective single-band theories, widely used in condensed matter physics, can be implemented with unprecedent accuracy. The tunability of the lattice depth is nowadays complemented by the use of magnetic Feshbach resonances which, at very low temperatures, can vary the relevant atom-atom scattering properties at will. Moreover, optical lattices loaded with gases of effectively reduced dimensionality are experimentally accessible. This is especially important for one spatial dimension, since most of the exactly solvable models in many-body quantum mechanics deal with particles on a line; therefore, experiments with one-dimensional gases serve as a testing ground for many old and new theories which were regarded as purely academic not so long ago. The physics of few quantum particles on a one-dimensional lattice is the topic of this thesis. Most of the results are obtained in the tight-binding approximation, which is amenable to exact numerical or analytical treatment. For the two-body problem, theoretical methods for calculating the stationary scattering and bound states are developed. These are used to obtain, in closed form, the two-particle solutions of both the Hubbard and extended Hubbard models

Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional Mo S 2

KAUST Repository

Hong, Jinhua

2016-02-29

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly investigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the electronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low-energy valence electrons in the freestanding MoS2 layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4 ± 0.2 eV in monolayer MoS2. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS2 systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement effect.

Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional Mo S 2

KAUST Repository

Hong, Jinhua; Li, Kun; Jin, Chuanhong; Zhang, Xixiang; Zhang, Ze; Yuan, Jun

2016-01-01

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly investigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the electronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low-energy valence electrons in the freestanding MoS2 layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4 ± 0.2 eV in monolayer MoS2. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS2 systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement effect.

Assessment of Density-Functional Tight-Binding Ionization Potentials and Electron Affinities of Molecules of Interest for Organic Solar Cells Against First-Principles GW Calculations

Directory of Open Access Journals (Sweden)

Ala Aldin M. H. M. Darghouth

2015-12-01

Full Text Available Ionization potentials (IPs and electron affinities (EAs are important quantities input into most models for calculating the open-circuit voltage (Voc of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB method with the third-order self-consistent charge (SCC correction and the 3ob parameter set (the third-order DFTB (DFTB3 organic and biochemistry parameter set against experiments (for smaller molecules and against first-principles GW (Green’s function, G, times the screened potential, W calculations (for larger molecules of interest in organic electronics for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT, but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ΔSCF (Δ self-consistent field SCC-DFTB calculations for first IPs (good to ± 0.649 eV. When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans’ theorem (KT IPs as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ΔSCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ΔSCF calculations and of ± 0.326/0.458 eV from (KT orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15–20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of

Observation of Electronic Excitation Transfer Through Light Harvesting Complex II Using Two-Dimensional Electronic-Vibrational Spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Lewis, NHC; Gruenke, NL; Oliver, TAA; Ballottari, M; Bassi, R; Fleming, GR

2016-10-05

Light-harvesting complex II (LHCII) serves a central role in light harvesting for oxygenic photosynthesis and is arguably the most important photosynthetic antenna complex. In this article, we present two-dimensional electronic–vibrational (2DEV) spectra of LHCII isolated from spinach, demonstrating the possibility of using this technique to track the transfer of electronic excitation energy between specific pigments within the complex. We assign the spectral bands via comparison with the 2DEV spectra of the isolated chromophores, chlorophyll a and b, and present evidence that excitation energy between the pigments of the complex are observed in these spectra. Lastly, we analyze the essential components of the 2DEV spectra using singular value decomposition, which makes it possible to reveal the relaxation pathways within this complex.

FDTD method for computing the off-plane band structure in a two-dimensional photonic crystal consisting of nearly free-electron metals

Energy Technology Data Exchange (ETDEWEB)

Xiao Sanshui; He Sailing

2002-12-01

An FDTD numerical method for computing the off-plane band structure of a two-dimensional photonic crystal consisting of nearly free-electron metals is presented. The method requires only a two-dimensional discretization mesh for a given off-plane wave number k{sub z} although the off-plane propagation is a three-dimensional problem. The off-plane band structures of a square lattice of metallic rods with the high-frequency metallic model in the air are studied, and a complete band gap for some nonzero off-plane wave number k{sub z} is founded.

FDTD method for computing the off-plane band structure in a two-dimensional photonic crystal consisting of nearly free-electron metals

International Nuclear Information System (INIS)

Xiao Sanshui; He Sailing

2002-01-01

An FDTD numerical method for computing the off-plane band structure of a two-dimensional photonic crystal consisting of nearly free-electron metals is presented. The method requires only a two-dimensional discretization mesh for a given off-plane wave number k z although the off-plane propagation is a three-dimensional problem. The off-plane band structures of a square lattice of metallic rods with the high-frequency metallic model in the air are studied, and a complete band gap for some nonzero off-plane wave number k z is founded

A method for the direct measurement of electronic site populations in a molecular aggregate using two-dimensional electronic-vibrational spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Lewis, Nicholas H. C.; Dong, Hui; Oliver, Thomas A. A.; Fleming, Graham R., E-mail: grfleming@lbl.gov [Department of Chemistry, University of California, Berkeley, California 94720 (United States); Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States); Kavli Energy Nanosciences Institute at Berkeley, Berkeley, California 94720 (United States)

2015-09-28

Two dimensional electronic spectroscopy has proved to be a valuable experimental technique to reveal electronic excitation dynamics in photosynthetic pigment-protein complexes, nanoscale semiconductors, organic photovoltaic materials, and many other types of systems. It does not, however, provide direct information concerning the spatial structure and dynamics of excitons. 2D infrared spectroscopy has become a widely used tool for studying structural dynamics but is incapable of directly providing information concerning electronic excited states. 2D electronic-vibrational (2DEV) spectroscopy provides a link between these domains, directly connecting the electronic excitation with the vibrational structure of the system under study. In this work, we derive response functions for the 2DEV spectrum of a molecular dimer and propose a method by which 2DEV spectra could be used to directly measure the electronic site populations as a function of time following the initial electronic excitation. We present results from the response function simulations which show that our proposed approach is substantially valid. This method provides, to our knowledge, the first direct experimental method for measuring the electronic excited state dynamics in the spatial domain, on the molecular scale.

Extended Polymorphism of Two-Dimensional Material

NARCIS (Netherlands)

Yoshida, Masaro; Ye, Jianting; Zhang, Yijin; Imai, Yasuhiko; Kimura, Shigeru; Fujiwara, Akihiko; Nishizaki, Terukazu; Kobayashi, Norio; Nakano, Masaki; Iwasa, Yoshihiro

When controlling electronic properties of bulk materials, we usually assume that the basic crystal structure is fixed. However, in two-dimensional (2D) materials, atomic structure or to functionalize their properties. Various polymorphs can exist in transition metal dichalcogenides (TMDCs) from

Simulated two-dimensional electronic spectroscopy of the eight-bacteriochlorophyll FMO complex

Energy Technology Data Exchange (ETDEWEB)

Yeh, Shu-Hao [Department of Chemistry and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States); Kais, Sabre, E-mail: kais@purdue.edu [Department of Chemistry and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States); Qatar Environment and Energy Research Institute, Qatar Foundation, Doha (Qatar)

2014-12-21

The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire conducting energy between the outer antenna system and the reaction center; it is an important photosynthetic system to study the transfer of excitonic energy. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a) in some of the FMO monomers. To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways: (1) it is directly involved in the first apo form pathway (6 → 3 → 1) by passing the excitonic energy to exciton 6; and (2) it facilitates an increase in the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 → 4,5 → 2 → 1) and thus increases the possible downward sampling routes across the BChls.

Photocatalytic oxidation dynamics of acetone on TiO2: tight-binding quantum chemical molecular dynamics study

International Nuclear Information System (INIS)

Lv Chen; Wang Xiaojing; Agalya, Govindasamy; Koyama, Michihisa; Kubo, Momoji; Miyamoto, Akira

2005-01-01

The clarification of the excited states dynamics on TiO 2 surface is important subject for the design of the highly active photocatalysts. In the present study, we applied our novel tight-binding quantum chemical molecular dynamics method to the investigation on the photocatalytic oxidation dynamics of acetone by photogenerated OH radicals on the hydrated anatase TiO 2 surface. The elucidated photocatalytic reaction mechanism strongly supports the previous experimental proposal and finally the effectiveness of our new approach for the clarification of the photocatalytic reaction dynamics employing the large simulation model was confirmed

Physics Colloquium - Tight-binding in a new light: Photons in optical lattices

CERN Multimedia

Ecole de Physique - Université de Genève

2011-01-01

Geneva University Physics Department 24, Quai Ernest Ansermet CH-1211 Geneva 4   Lundi 21 mars 2011, 17h00 Ecole de Physique, Auditoire Stueckelberg Tight-binding in a new light: Photons in optical lattices Dr. Niels Madsen Department of Physics, Swansea University, Singleton Park, Swansea, United Kingdom   Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom, subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen co...

Two-dimensional Cu2Si sheet: a promising electrode material for nanoscale electronics

Science.gov (United States)

Meng Yam, Kah; Guo, Na; Zhang, Chun

2018-06-01

Building electronic devices on top of two-dimensional (2D) materials has recently become one of most interesting topics in nanoelectronics. Finding high-performance 2D electrode materials is one central issue in 2D nanoelectronics. In the current study, based on first-principles calculations, we compare the electronic and transport properties of two nanoscale devices. One device consists of two single-atom-thick planar Cu2Si electrodes, and a nickel phthalocyanine (NiPc) molecule in the middle. The other device is made of often-used graphene electrodes and a NiPc molecule. Planer Cu2Si is a new type of 2D material that was recently predicted to exist and be stable under room temperature [11]. We found that at low bias voltages, the electric current through the Cu2Si–NiPc–Cu2Si junction is about three orders higher than that through graphene–NiPc–graphene. Detailed analysis shows that the surprisingly high conductivity of Cu2Si–NiPc–Cu2Si originates from the mixing of the Cu2Si state near Fermi energy and the highest occupied molecular orbital of NiPc. These results suggest that 2D Cu2Si may be an excellent candidate for electrode materials for future nanoscale devices.

New Computational Approach to Electron Transport in Irregular Graphene Nanostructures

Science.gov (United States)

Mason, Douglas; Heller, Eric; Prendergast, David; Neaton, Jeffrey

2009-03-01

For novel graphene devices of nanoscale-to-macroscopic scale, many aspects of their transport properties are not easily understood due to difficulties in fabricating devices with regular edges. Here we develop a framework to efficiently calculate and potentially screen electronic transport properties of arbitrary nanoscale graphene device structures. A generalization of the established recursive Green's function method is presented, providing access to arbitrary device and lead geometries with substantial computer-time savings. Using single-orbital nearest-neighbor tight-binding models and the Green's function-Landauer scattering formalism, we will explore the transmission function of irregular two-dimensional graphene-based nanostructures with arbitrary lead orientation. Prepared by LBNL under contract DE-AC02-05CH11231 and supported by the U.S. Dept. of Energy Computer Science Graduate Fellowship under grant DE-FG02-97ER25308.

Quantum pump effect induced by a linearly polarized microwave in a two-dimensional electron gas.

Science.gov (United States)

Song, Juntao; Liu, Haiwen; Jiang, Hua

2012-05-30

A quantum pump effect is predicted in an ideal homogeneous two-dimensional electron gas (2DEG) that is normally irradiated by linearly polarized microwaves (MW). Without considering effects from spin-orbital coupling or the magnetic field, it is found that a polarized MW can continuously pump electrons from the longitudinal to the transverse direction, or from the transverse to the longitudinal direction, in the central irradiated region. The large pump current is obtained for both the low frequency limit and the high frequency case. Its magnitude depends on sample properties such as the size of the radiated region, the power and frequency of the MW, etc. Through the calculated results, the pump current should be attributed to the dominant photon-assisted tunneling processes as well as the asymmetry of the electron density of states with respect to the Fermi energy.

Electronic properties of a biased graphene bilayer

International Nuclear Information System (INIS)

Castro, Eduardo V; Lopes dos Santos, J M B; Novoselov, K S; Morozov, S V; Geim, A K; Peres, N M R; Nilsson, Johan; Castro Neto, A H; Guinea, F

2010-01-01

We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system-a biased bilayer. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its four-band and two-band continuum approximations, and the four-band model is shown to always be a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, made out of either SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point for understanding the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, such as the second-nearest-neighbour hopping energies t' (in-plane) and γ 4 (inter-layer), and the on-site energy Δ.

Collective excitations in semiconductor superlattices and plasma modes of a two-dimensional electron gas with spatially modulated charge density

International Nuclear Information System (INIS)

Eliasson, G.L.

1987-01-01

The theory of collective excitations in semiconductor superlattices is formulated by using linear response theory. Different kinds of collective excitations in type I (GaAs-GaAlAs) and type II (GaSb-InAs) superlattices are surveyed. Special attention is paid to the presence of surface and finite-size effects. In calculating the dielectric matrix, the effect of different approximations of the system is discussed. The theory for inelastic length scattering (Raman scattering), and for Electron Energy Loss (EEL) due to collective excitations, is formulated. Calculations for several model systems are presented and the main features of the spectra are discussed. In part II the theory of collective excitations of a two-dimensional electron gas with a spatially periodic equilibrium density is formulated. As a first example a periodic array of two-dimensional electron gas strips with constant equilibrium density is studied. The integral equation that describes the charge fluctuations on the strips is derived and solved numerically. The spatial dependence of the density fluctuation across a single strip can be in the form of either propagating or evanescent waves

Structural properties of metal-organic frameworks within the density-functional based tight-binding method

Energy Technology Data Exchange (ETDEWEB)

Lukose, Binit; Supronowicz, Barbara; Kuc, Agnieszka B.; Heine, Thomas [School of Engineering and Science, Jacobs University Bremen (Germany); Petkov, Petko S.; Vayssilov, Georgi N. [Faculty of Chemistry, University of Sofia (Bulgaria); Frenzel, Johannes [Lehrstuhl fuer Theoretische Chemie, Ruhr-Universitaet Bochum (Germany); Seifert, Gotthard [Physikalische Chemie, Technische Universitaet Dresden (Germany)

2012-02-15

Density-functional based tight-binding (DFTB) is a powerful method to describe large molecules and materials. Metal-organic frameworks (MOFs), materials with interesting catalytic properties and with very large surface areas, have been developed and have become commercially available. Unit cells of MOFs typically include hundreds of atoms, which make the application of standard density-functional methods computationally very expensive, sometimes even unfeasible. The aim of this paper is to prepare and to validate the self-consistent charge-DFTB (SCC-DFTB) method for MOFs containing Cu, Zn, and Al metal centers. The method has been validated against full hybrid density-functional calculations for model clusters, against gradient corrected density-functional calculations for supercells, and against experiment. Moreover, the modular concept of MOF chemistry has been discussed on the basis of their electronic properties. We concentrate on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn{sub 4}O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)), and aluminum oxide AlO{sub 4}(OH){sub 2} octahedron (MIL-53). We show that SCC-DFTB predicts structural parameters with a very good accuracy (with less than 5% deviation, even for adsorbed CO and H{sub 2}O on HKUST-1), while adsorption energies differ by 12 kJ mol{sup -1} or less for CO and water compared to DFT benchmark calculations. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Two-Finger Tightness: What Is It? Measuring Torque and Reproducibility in a Simulated Model.

Science.gov (United States)

Acker, William B; Tai, Bruce L; Belmont, Barry; Shih, Albert J; Irwin, Todd A; Holmes, James R

2016-05-01

Residents in training are often directed to insert screws using "two-finger tightness" to impart adequate torque but minimize the chance of a screw stripping in bone. This study seeks to quantify and describe two-finger tightness and to assess the variability of its application by residents in training. Cortical bone was simulated using a polyurethane foam block (30-pcf density) that was prepared with predrilled holes for tightening 3.5 × 14-mm long cortical screws and mounted to a custom-built apparatus on a load cell to capture torque data. Thirty-three residents in training, ranging from the first through fifth years of residency, along with 8 staff members, were directed to tighten 6 screws to two-finger tightness in the test block, and peak torque values were recorded. The participants were blinded to their torque values. Stripping torque (2.73 ± 0.56 N·m) was determined from 36 trials and served as a threshold for failed screw placement. The average torques varied substantially with regard to absolute torque values, thus poorly defining two-finger tightness. Junior residents less consistently reproduced torque compared with other groups (0.29 and 0.32, respectively). These data quantify absolute values of two-finger tightness but demonstrate considerable variability in absolute torque values, percentage of stripping torque, and ability to consistently reproduce given torque levels. Increased years in training are weakly correlated with reproducibility, but experience does not seem to affect absolute torque levels. These results question the usefulness of two-finger tightness as a teaching tool and highlight the need for improvement in resident motor skill training and development within a teaching curriculum. Torque measuring devices may be a useful simulation tools for this purpose.

IrPd nanoalloys: simulations, from surface segregation to local electronic properties

Energy Technology Data Exchange (ETDEWEB)

Andriamiharintsoa, T. H. [Institut de Physique et Chimie des Matériaux de Strasbourg CNRS-UDS UMR 7504 (France); Rakotomahevitra, A. [Institut pour la Maîtrise de l’Énergie, Faculté des sciences d’Antananarivo (Madagascar); Piccolo, L. [Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256 CNRS and Université Lyon 1 (France); Goyhenex, C., E-mail: christine.goyhenex@ipcms.unistra.fr [Institut de Physique et Chimie des Matériaux de Strasbourg CNRS-UDS UMR 7504 (France)

2015-05-15

Using semi-empirical modeling, namely tight-binding at different levels of accuracy, the chemical, crystallographic, and electronic structures of bimetallic IrPd nanoparticles are characterized. For the purpose, model cuboctahedral particles containing 561 atoms are considered. Atomistic simulations show that core–shell nanoparticles are highly stable, with a strong surface segregation of Pd, at least for one atomic shell thickness. Within self-consistent tight-binding calculations founded on the density functional theory, an accurate insight is given into the electronic structure of these materials which have a high potential as catalysts.

Two- to three-dimensional crossover in a dense electron liquid in silicon

Science.gov (United States)

Matmon, Guy; Ginossar, Eran; Villis, Byron J.; Kölker, Alex; Lim, Tingbin; Solanki, Hari; Schofield, Steven R.; Curson, Neil J.; Li, Juerong; Murdin, Ben N.; Fisher, Andrew J.; Aeppli, Gabriel

2018-04-01

Doping of silicon via phosphine exposures alternating with molecular beam epitaxy overgrowth is a path to Si:P substrates for conventional microelectronics and quantum information technologies. The technique also provides a well-controlled material for systematic studies of two-dimensional lattices with a half-filled band. We show here that for a dense (ns=2.8 ×1014 cm-2) disordered two-dimensional array of P atoms, the full field magnitude and angle-dependent magnetotransport is remarkably well described by classic weak localization theory with no corrections due to interaction. The two- to three-dimensional crossover seen upon warming can also be interpreted using scaling concepts developed for anistropic three-dimensional materials, which work remarkably except when the applied fields are nearly parallel to the conducting planes.

Zonula occludens toxin structure-function analysis. Identification of the fragment biologically active on tight junctions and of the zonulin receptor binding domain.

Science.gov (United States)

Di Pierro, M; Lu, R; Uzzau, S; Wang, W; Margaretten, K; Pazzani, C; Maimone, F; Fasano, A

2001-06-01

Zonula occludens toxin (Zot) is an enterotoxin elaborated by Vibrio cholerae that increases intestinal permeability by interacting with a mammalian cell receptor with subsequent activation of intracellular signaling leading to the disassembly of the intercellular tight junctions. Zot localizes in the bacterial outer membrane of V. cholerae with subsequent cleavage and secretion of a carboxyl-terminal fragment in the host intestinal milieu. To identify the Zot domain(s) directly involved in the protein permeating effect, several zot gene deletion mutants were constructed and tested for their biological activity in the Ussing chamber assay and their ability to bind to the target receptor on intestinal epithelial cell cultures. The Zot biologically active domain was localized toward the carboxyl terminus of the protein and coincided with the predicted cleavage product generated by V. cholerae. This domain shared a putative receptor-binding motif with zonulin, the Zot mammalian analogue involved in tight junction modulation. Amino acid comparison between the Zot active fragment and zonulin, combined with site-directed mutagenesis experiments, confirmed the presence of an octapeptide receptor-binding domain toward the amino terminus of the processed Zot.

Pairing in a two-dimensional two-band very anisotropic model in the mean field approximation

International Nuclear Information System (INIS)

Fazakas, A.B.; Pitis, R.

1993-09-01

A two-dimensional model is proposed: there are two kinds of sites, with one electronic state per site; tunneling takes place only in one direction; the interaction involves only electrons on different sites. The existence of a phase transition involving interband pairing of electrons is discussed in the mean field approximation. (author)

Two dimensional crystals of LH2 light-harvesting complexes from Ectothiorhodospira sp. and Rhodobacter capsulatus investigated by electron microscopy

NARCIS (Netherlands)

Oling, Frank; Boekema, EJ; deZarate, IO; Visschers, R; vanGrondelle, R; Keegstra, W; Brisson, A; Picorel, R

1996-01-01

Two-dimensional crystals of LH2 (B800-850) light-harvesting complexes from Ectothiorhodospira sp, and Rhodobacter capsulatus were obtained by reconstitution of purified protein into phospholipid vesicles and characterized by electron microscopy. The size of the crystals was up to several

Two-dimensional crystals of LH2 light-harvesting complexes from Ectothiorhodospira sp. and Rhodobacter capsulatus investigated by electron microscopy.

NARCIS (Netherlands)

Oling, F.; Boekema, E.J.; Ortiz de Zarate, I.; Visschers, R.W.; van Grondelle, R.; Keegstra, W.; Brisson, A.; Picorel, R.

1996-01-01

Two-dimensional crystals of LH2 (B800-850) light-harvesting complexes from Ectothiorhodospira sp. and Rhodobacter capsulatus were obtained by reconstitution of purified protein into phospholipid vesicles and characterized by electron microscopy. The size of the crystals was up to several

Electron cryomicroscopy of two-dimensional crystals of the H+-ATPase from chloroplasts

NARCIS (Netherlands)

Böttcher, Bettina; Gräber, Peter; Boekema, Egbert J.; Lücken, Uwe

1995-01-01

The H+-ATPase from spinach chloroplasts was isolated and purified. Two-dimensional crystals were obtained from the protein/lipid/detergent micelles by treatment with phospholipase and simultaneous removal of detergent and fatty acids by Biobeads. The resulting two-dimensionally ordered arrays were

A high-order corrected description of ultra-short and tightly focused laser pulses, and their electron acceleration in vacuum

International Nuclear Information System (INIS)

Zhang, J.T.; Wang, P.X.; Kong, Q.; Chen, Z.; Ho, Y.K.

2007-01-01

Field expressions are derived for ultra-short, tightly focused laser pulses up to the second-order temporal correction and seventh-order spatial correction. To evaluate the importance of these corrections, we simulate these fields and investigate the final energy of the accelerated electrons. We vary the order of the corrected expressions, the pulse duration, and the beam waist. We find that electron capture is still an important and generic phenomenon in ultra-short, tightly focused laser pulses. While small differences in the electron acceleration are obtained for various orders of the corrected field equations relative to the paraxial field equations, there is no qualitative difference in the behavior of the electron. Furthermore, the temporal and spatial corrections are found to be correlated

Consistent two-dimensional visualization of protein-ligand complex series

Directory of Open Access Journals (Sweden)

Stierand Katrin

2011-06-01

Full Text Available Abstract Background The comparative two-dimensional graphical representation of protein-ligand complex series featuring different ligands bound to the same active site offers a quick insight in their binding mode differences. In comparison to arbitrary orientations of the residue molecules in the individual complex depictions a consistent placement improves the legibility and comparability within the series. The automatic generation of such consistent layouts offers the possibility to apply it to large data sets originating from computer-aided drug design methods. Results We developed a new approach, which automatically generates a consistent layout of interacting residues for a given series of complexes. Based on the structural three-dimensional input information, a global two-dimensional layout for all residues of the complex ensemble is computed. The algorithm incorporates the three-dimensional adjacencies of the active site residues in order to find an universally valid circular arrangement of the residues around the ligand. Subsequent to a two-dimensional ligand superimposition step, a global placement for each residue is derived from the set of already placed ligands. The method generates high-quality layouts, showing mostly overlap-free solutions with molecules which are displayed as structure diagrams providing interaction information in atomic detail. Application examples document an improved legibility compared to series of diagrams whose layouts are calculated independently from each other. Conclusions The presented method extends the field of complex series visualizations. A series of molecules binding to the same protein active site is drawn in a graphically consistent way. Compared to existing approaches these drawings substantially simplify the visual analysis of large compound series.

Human zonulin, a potential modulator of intestinal tight junctions.

Science.gov (United States)

Wang, W; Uzzau, S; Goldblum, S E; Fasano, A

2000-12-01

Intercellular tight junctions are dynamic structures involved in vectorial transport of water and electrolytes across the intestinal epithelium. Zonula occludens toxin derived from Vibrio cholerae interacts with a specific intestinal epithelial surface receptor, with subsequent activation of a complex intracellular cascade of events that regulate tight junction permeability. We postulated that this toxin may mimic the effect of a functionally and immunologically related endogenous modulator of intestinal tight junctions. Affinity-purified anti-zonula occludens toxin antibodies and the Ussing chamber assay were used to screen for one or more mammalian zonula occludens toxin analogues in both fetal and adult human intestine. A novel protein, zonulin, was identified that induces tight junction disassembly in non-human primate intestinal epithelia mounted in Ussing chambers. Comparison of amino acids in the active zonula occludens toxin fragment and zonulin permitted the identification of the putative receptor binding domain within the N-terminal region of the two proteins. Zonulin likely plays a pivotal role in tight junction regulation during developmental, physiological, and pathological processes, including tissue morphogenesis, movement of fluid, macromolecules and leukocytes between the intestinal lumen and the interstitium, and inflammatory/autoimmune disorders.

Exchange Enhancement of the Electron-Phonon Interaction: The Case of Weakly Doped Two-Dimensional Multivalley Semiconductors

Science.gov (United States)

Pamuk, Betül; Zoccante, Paolo; Baima, Jacopo; Mauri, Francesco; Calandra, Matteo

2018-04-01

The effect of the exchange interaction on the vibrational properties and on the electron-phonon coupling were investigated in several recent works. In most of the cases, exchange tends to enhance the electron-phonon interaction, although the motivations for such behaviour are not completely understood. Here we consider the class of weakly doped two-dimensional multivalley semiconductors and we demonstrate that a more global picture emerges. In particular we show that in these systems, at low enough doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. We demonstrate the applicability of the theory by performing random phase approximation and first principles calculations in transition metal chloronitrides. We find that exchange is responsible for the enhancement of the superconducting critical temperature in LixZrNCl and that much larger Tcs could be obtained in intercalated HfNCl if the synthesis of cleaner samples could remove the Anderson insulating state competing with superconductivity.

Band nesting, massive Dirac fermions, and valley Landé and Zeeman effects in transition metal dichalcogenides: A tight-binding model

Science.gov (United States)

Bieniek, Maciej; Korkusiński, Marek; Szulakowska, Ludmiła; Potasz, Paweł; Ozfidan, Isil; Hawrylak, Paweł

2018-02-01

We present here the minimal tight-binding model for a single layer of transition metal dichalcogenides (TMDCs) MX 2(M , metal; X , chalcogen) which illuminates the physics and captures band nesting, massive Dirac fermions, and valley Landé and Zeeman magnetic field effects. TMDCs share the hexagonal lattice with graphene but their electronic bands require much more complex atomic orbitals. Using symmetry arguments, a minimal basis consisting of three metal d orbitals and three chalcogen dimer p orbitals is constructed. The tunneling matrix elements between nearest-neighbor metal and chalcogen orbitals are explicitly derived at K ,-K , and Γ points of the Brillouin zone. The nearest-neighbor tunneling matrix elements connect specific metal and sulfur orbitals yielding an effective 6 ×6 Hamiltonian giving correct composition of metal and chalcogen orbitals but not the direct gap at K points. The direct gap at K , correct masses, and conduction band minima at Q points responsible for band nesting are obtained by inclusion of next-neighbor Mo-Mo tunneling. The parameters of the next-nearest-neighbor model are successfully fitted to MX 2(M =Mo ; X =S ) density functional ab initio calculations of the highest valence and lowest conduction band dispersion along K -Γ line in the Brillouin zone. The effective two-band massive Dirac Hamiltonian for MoS2, Landé g factors, and valley Zeeman splitting are obtained.

Empirical tight-binding modeling of ordered and disordered semiconductor structures

International Nuclear Information System (INIS)

Mourad, Daniel

2010-01-01

In this thesis, we investigate the electronic and optical properties of pure as well as of substitutionally alloyed II-VI and III-V bulk semiconductors and corresponding semiconductor quantum dots by means of an empirical tight-binding (TB) model. In the case of the alloyed systems of the type A x B 1-x , where A and B are the pure compound semiconductor materials, we study the influence of the disorder by means of several extensions of the TB model with different levels of sophistication. Our methods range from rather simple mean-field approaches (virtual crystal approximation, VCA) over a dynamical mean-field approach (coherent potential approximation, CPA) up to calculations where substitutional disorder is incorporated on a finite ensemble of microscopically distinct configurations. In the first part of this thesis, we cover the necessary fundamentals in order to properly introduce the TB model of our choice, the effective bond-orbital model (EBOM). In this model, one s- and three p-orbitals per spin direction are localized on the sites of the underlying Bravais lattice. The matrix elements between these orbitals are treated as free parameters in order to reproduce the properties of one conduction and three valence bands per spin direction and can then be used in supercell calculations in order to model mixed bulk materials or pure as well as mixed quantum dots. Part II of this thesis deals with unalloyed systems. Here, we use the EBOM in combination with configuration interaction calculations for the investigation of the electronic and optical properties of truncated pyramidal GaN quantum dots embedded in AlN with an underlying zincblende structure. Furthermore, we develop a parametrization of the EBOM for materials with a wurtzite structure, which allows for a fit of one conduction and three valence bands per spin direction throughout the whole Brillouin zone of the hexagonal system. In Part III, we focus on the influence of alloying on the electronic and

Strain-engineered growth of two-dimensional materials.

Science.gov (United States)

Ahn, Geun Ho; Amani, Matin; Rasool, Haider; Lien, Der-Hsien; Mastandrea, James P; Ager Iii, Joel W; Dubey, Madan; Chrzan, Daryl C; Minor, Andrew M; Javey, Ali

2017-09-20

The application of strain to semiconductors allows for controlled modification of their band structure. This principle is employed for the manufacturing of devices ranging from high-performance transistors to solid-state lasers. Traditionally, strain is typically achieved via growth on lattice-mismatched substrates. For two-dimensional (2D) semiconductors, this is not feasible as they typically do not interact epitaxially with the substrate. Here, we demonstrate controlled strain engineering of 2D semiconductors during synthesis by utilizing the thermal coefficient of expansion mismatch between the substrate and semiconductor. Using WSe 2 as a model system, we demonstrate stable built-in strains ranging from 1% tensile to 0.2% compressive on substrates with different thermal coefficient of expansion. Consequently, we observe a dramatic modulation of the band structure, manifested by a strain-driven indirect-to-direct bandgap transition and brightening of the dark exciton in bilayer and monolayer WSe 2 , respectively. The growth method developed here should enable flexibility in design of more sophisticated devices based on 2D materials.Strain engineering is an essential tool for modifying local electronic properties in silicon-based electronics. Here, Ahn et al. demonstrate control of biaxial strain in two-dimensional materials based on the growth substrate, enabling more complex low-dimensional electronics.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy.

Science.gov (United States)

Akhtar, Parveen; Zhang, Cheng; Liu, Zhengtang; Tan, Howe-Siang; Lambrev, Petar H

2018-03-01

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3-4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67-75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4 ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P 700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

Two-dimensional ion effects in relativistic diodes

International Nuclear Information System (INIS)

Poukey, J.W.

1975-01-01

In relativistic diodes, ions are emitted from the anode plasma. The effects and properties of these ions are studied via a two-dimensional particle simulation code. The space charge of these ions enhances the electron emission, and this additional current (including that of the ions, themselves) aids in obtaining superpinched electron beams for use in pellet fusion studies. (U.S.)

PatchSurfers: Two methods for local molecular property-based binding ligand prediction.

Science.gov (United States)

Shin, Woong-Hee; Bures, Mark Gregory; Kihara, Daisuke

2016-01-15

Protein function prediction is an active area of research in computational biology. Function prediction can help biologists make hypotheses for characterization of genes and help interpret biological assays, and thus is a productive area for collaboration between experimental and computational biologists. Among various function prediction methods, predicting binding ligand molecules for a target protein is an important class because ligand binding events for a protein are usually closely intertwined with the proteins' biological function, and also because predicted binding ligands can often be directly tested by biochemical assays. Binding ligand prediction methods can be classified into two types: those which are based on protein-protein (or pocket-pocket) comparison, and those that compare a target pocket directly to ligands. Recently, our group proposed two computational binding ligand prediction methods, Patch-Surfer, which is a pocket-pocket comparison method, and PL-PatchSurfer, which compares a pocket to ligand molecules. The two programs apply surface patch-based descriptions to calculate similarity or complementarity between molecules. A surface patch is characterized by physicochemical properties such as shape, hydrophobicity, and electrostatic potentials. These properties on the surface are represented using three-dimensional Zernike descriptors (3DZD), which are based on a series expansion of a 3 dimensional function. Utilizing 3DZD for describing the physicochemical properties has two main advantages: (1) rotational invariance and (2) fast comparison. Here, we introduce Patch-Surfer and PL-PatchSurfer with an emphasis on PL-PatchSurfer, which is more recently developed. Illustrative examples of PL-PatchSurfer performance on binding ligand prediction as well as virtual drug screening are also provided. Copyright © 2015 Elsevier Inc. All rights reserved.

Ab initio electronic structure of quasi-two-dimensional materials: A “native” Gaussian–plane wave approach

Energy Technology Data Exchange (ETDEWEB)

Trevisanutto, Paolo E. [Graphene Research Centre and CA2DM, National University of Singapore, Singapore 117542, Singapore and Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603 (Singapore); Vignale, Giovanni, E-mail: vignaleg@missouri.edu [Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211 (United States)

2016-05-28

Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is “native” to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory — density functional theory, GW approximation and Bethe-Salpeter equation — are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields.

Center Line Slope Analysis in Two-Dimensional Electronic Spectroscopy

OpenAIRE

?anda, Franti?ek; Perl?k, V?clav; Lincoln, Craig N.; Hauer, J?rgen

2015-01-01

Center line slope (CLS) analysis in 2D infrared spectroscopy has been extensively used to extract frequency?frequency correlation functions of vibrational transitions. We apply this concept to 2D electronic spectroscopy, where CLS is a measure of electronic gap fluctuations. The two domains, infrared and electronic, possess differences: In the infrared, the frequency fluctuations are classical, often slow and Gaussian. In contrast, electronic spectra are subject to fast spectral diffusion and...

Magnetic two-dimensional electron gas at the manganite-buffered LaAlO3/SrTiO3 interface

DEFF Research Database (Denmark)

R. Zhang, H.; Zhang, Y.; Zhang, H.

2017-01-01

Fabrication of highly mobile spin-polarized two-dimensional electron gas (2DEG) is crucially important for both fundamental and applied research. Usually, spin polarization appears below 10 K for the 2DEG of LaAlO3/SrTiO3 interface, stemming from the magnetic ordering of Ti3+ ions with the mediat......Fabrication of highly mobile spin-polarized two-dimensional electron gas (2DEG) is crucially important for both fundamental and applied research. Usually, spin polarization appears below 10 K for the 2DEG of LaAlO3/SrTiO3 interface, stemming from the magnetic ordering of Ti3+ ions...... with the mediation of itinerant electrons. Herein, we report a magnetic 2DEG at a La7/8Sr1/8MnO3-buffered LaAlO3/SrTiO3 interface, which simultaneously shows electrically tunable anomalous Hall effect and high conductivity. The spin-polarized temperature for the 2DEG is promoted to 30 K while the mobility remains...... high. The magnetism likely results from a gradient manganese interdiffusion into SrTiO3. The present work demonstrates the great potential of manganite-buffered LaAlO3/SrTiO3 interfaces for spintronic applications....

Spin currents in a normal two-dimensional electron gas in contact with a spin-orbit interaction region

International Nuclear Information System (INIS)

Sukhanov, Aleksei A; Sablikov, Vladimir A; Tkach, Yurii Ya

2009-01-01

Spin effects in a normal two-dimensional (2D) electron gas in lateral contact with a 2D region with spin-orbit interaction are studied. The peculiarity of this system is the presence of spin-dependent scattering of electrons from the interface. This results in an equilibrium edge spin current and nontrivial spin responses to a particle current. We investigate the spatial distribution of the spin currents and spin density under non-equilibrium conditions caused by a ballistic electron current flowing normal or parallel to the interface. The parallel electron current is found to generate a spin density near the interface and to change the edge spin current. The perpendicular electron current changes the edge spin current proportionally to the electron current and produces a bulk spin current penetrating deep into the normal region. This spin current has two components, one of which is directed normal to the interface and polarized parallel to it, and the second is parallel to the interface and is polarized in the plane perpendicular to the contact line. Both spin currents have a high degree of polarization (∼40-60%).

Communication: Coherences observed in vivo in photosynthetic bacteria using two-dimensional electronic spectroscopy

International Nuclear Information System (INIS)

Dahlberg, Peter D.; Norris, Graham J.; Wang, Cheng; Viswanathan, Subha; Singh, Ved P.; Engel, Gregory S.

2015-01-01

Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850 ∗ states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs

Communication: Coherences observed in vivo in photosynthetic bacteria using two-dimensional electronic spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Dahlberg, Peter D. [Graduate Program in the Biophysical Sciences, Institute for Biophysical Dynamics, and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (United States); Norris, Graham J.; Wang, Cheng; Viswanathan, Subha; Singh, Ved P.; Engel, Gregory S., E-mail: gsengel@uchicago.edu [Department of Chemistry, Institute for Biophysical Dynamics, and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (United States)

2015-09-14

Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850{sup ∗} states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs.

Electronic transport properties of carbon nanotube metal-semiconductor-metal

Directory of Open Access Journals (Sweden)

F Khoeini

2008-07-01

Full Text Available  In this work, we study electronic transport properties of a quasi-one dimensional pure semi-conducting Zigzag Carbon Nanotube (CNT attached to semi-infinite clean metallic Zigzag CNT leads, taking into account the influence of topological defect in junctions. This structure may behave like a field effect transistor. The calculations are based on the tight-binding model and Green’s function method, in which the local density of states(LDOS in the metallic section to semi-conducting section, and muli-channel conductance of the system are calculated in the coherent and linear response regime, numerically. Also we have introduced a circuit model for the system and investigated its current. The theoretical results obtained, can be a base, for developments in designing nano-electronic devices.

Prime tight frames

DEFF Research Database (Denmark)

Lemvig, Jakob; Miller, Christopher; Okoudjou, Kasso A.

2014-01-01

to suggest effective analysis and synthesis computation strategies for such frames. Finally, we describe all prime frames constructed from the spectral tetris method, and, as a byproduct, we obtain a characterization of when the spectral tetris construction works for redundancies below two.......We introduce a class of finite tight frames called prime tight frames and prove some of their elementary properties. In particular, we show that any finite tight frame can be written as a union of prime tight frames. We then characterize all prime harmonic tight frames and use thischaracterization...

Spin dynamics in high-mobility two-dimensional electron systems embedded in GaAs/AlGaAs quantum wells

Energy Technology Data Exchange (ETDEWEB)

Griesbeck, Michael

2012-11-22

Since many years there has been great effort to explore the spin dynamics in low-dimensional electron systems embedded in GaAs/AlGaAs based heterostructures for the purpose of quantum computation and spintronics applications. Advances in technology allow for the design of high quality and well-defined two-dimensional electron systems (2DES), which are perfectly suited for the study of the underlying physics that govern the dynamics of the electron spin system. In this work, spin dynamics in high-mobility 2DES is studied by means of the all-optical time-resolved Kerr/Faraday rotation technique. In (001)-grown 2DES, a strong in-plane spin dephasing anisotropy is studied, resulting from the interference of comparable Rashba and Dresselhaus contributions to the spin-orbit field (SOF). The dependence of this anisotropy on parameters like the confinement length of the 2DES, the sample temperature, as well as the electron density is demonstrated. Furthermore, coherent spin dynamics of an ensemble of ballistically moving electrons is studied without and within an applied weak magnetic field perpendicular to the sample plane, which forces the electrons to move on cyclotron orbits. Finally, strongly anisotropic spin dynamics is investigated in symmetric (110)-grown 2DES, using the resonant spin amplification method. Here, extremely long out-of-plane spin dephasing times can be achieved, in consequence of the special symmetry of the Dresselhaus SOF.

Real-space mapping of a disordered two-dimensional electron system in the quantum Hall regime

International Nuclear Information System (INIS)

Hashimoto, K; Hirayama, Y; Wiebe, J; Wiesendanger, R; Inaoka, T; Morgenstern, M

2011-01-01

By using scanning tunnelling spectroscopy, we study the influence of potential disorder on an adsorbate-induced two-dimensional electron system in the integer quantum Hall regime. The real-space imaged local density of states exhibits transition from localized drift states encircling the potential minima to another type of localized drift states encircling the potential maxima. While the former states show regular round shapes, the latter have irregular-shaped patterns. This difference is induced by different sources for the potential minima and maxima, i.e., substrate donors and an inhomogeneous distribution of the adsorbates, respectively.

Quantum Monte Carlo calculation of the Fermi-liquid parameters in the two-dimensional electron gas

International Nuclear Information System (INIS)

Kwon, Y.; Ceperley, D.M.; Martin, R.M.

1994-01-01

Excitations of the two-dimensional electron gas, including many-body effects, are calculated with a variational Monte Carlo method. Correlated sampling is introduced to calculate small energy differences between different excitations. The usual pair-product (Slater-Jastrow) trial wave function is found to lack certain correlations entirely so that backflow correlation is crucial. From the excitation energies calculated here, we determine Fermi-liquid parameters and related physical quantities such as the effective mass and the Lande g factor of the system. Our results for the effective mass are compared with previous analytic calculations

Nonlinear effects in microwave photoconductivity of two-dimensional electron systems

International Nuclear Information System (INIS)

Ryzhii, V; Suris, R

2003-01-01

We present a model for microwave photoconductivity of two-dimensional electron systems in a magnetic field which describes the effects of strong microwave and steady-state electric fields. Using this model, we derive an analytical formula for the photoconductivity associated with photon- and multi-photon-assisted impurity scattering as a function of the frequency and power of microwave radiation. According to the developed model, the microwave conductivity is an oscillatory function of the frequency of microwave radiation and the cyclotron frequency which becomes zero at the cyclotron resonance and its harmonics. It exhibits maxima and minima (with absolute negative conductivity) at microwave frequencies somewhat different from the resonant frequencies. The calculated power dependence of the amplitude of the microwave photoconductivity oscillations exhibits pronounced sublinear behaviour similar to a logarithmic function. The height of the microwave photoconductivity maxima and the depth of its minima are nonmonotonic functions of the electric field. The possibility of a strong widening of the maxima and minima due to a strong sensitivity of their parameters on the electric field and the presence of strong long-range electric-field fluctuations is pointed to. The obtained dependences are consistent with the results of the experimental observations

Band Alignment Determination of Two-Dimensional Heterojunctions and Their Electronic Applications

KAUST Repository

Chiu, Ming-Hui

2018-05-09

Two-dimensional (2D) layered materials such as MoS2 have been recognized as high on-off ratio semiconductors which are promising candidates for electronic and optoelectronic devices. In addition to the use of individual 2D materials, the accelerated field of 2D heterostructures enables even greater functionalities. Device designs differ, and they are strongly controlled by the electronic band alignment. For example, photovoltaic cells require type II heterostructures for light harvesting, and light-emitting diodes benefit from multiple quantum wells with the type I band alignment for high emission efficiency. The vertical tunneling field-effect transistor for next-generation electronics depends on nearly broken-gap band alignment for boosting its performance. To tailor these 2D layered materials toward possible future applications, the understanding of 2D heterostructure band alignment becomes critically important. In the first part of this thesis, we discuss the band alignment of 2D heterostructures. To do so, we firstly study the interlayer coupling between two dissimilar 2D materials. We conclude that a post-anneal process could enhance the interlayer coupling of as-transferred 2D heterostructures, and heterostructural stacking imposes similar symmetry changes as homostructural stacking. Later, we precisely determine the quasi particle bandgap and band alignment of the MoS2/WSe2 heterostructure by using scan tunneling microscopy/spectroscopy (STM/S) and micron-beam X-ray photoelectron spectroscopy (μ-XPS) techniques. Lastly, we prove that the band alignment of 2D heterojunctions can be accurately predicted by Anderson’s model, which has previously failed to predict conventional bulk heterostructures. In the second part of this thesis, we develop a new Chemical Vapor Deposition (CVD) method capable of precisely controlling the growth area of p- and n-type transition metal dichalcogenides (TMDCs) and further form lateral or vertical 2D heterostructures. This

Two-Dimensional Free Energy Surfaces for Electron Transfer Reactions in Solution

Directory of Open Access Journals (Sweden)

Shigeo Murata

2008-01-01

Full Text Available Change in intermolecular distance between electron donor (D and acceptor (A can induce intermolecular electron transfer (ET even in nonpolar solvent, where solvent orientational polarization is absent. This was shown by making simple calculations of the energies of the initial and final states of ET. In the case of polar solvent, the free energies are functions of both D-A distance and solvent orientational polarization. On the basis of 2-dimensional free energy surfaces, the relation of Marcus ET and exciplex formation is discussed. The transient effect in fluorescence quenching was measured for several D-A pairs in a nonpolar solvent. The results were analyzed by assuming a distance dependence of the ET rate that is consistent with the above model.

Electronic, Optical, and Thermal Properties of Reduced-Dimensional Semiconductors

Science.gov (United States)

Huang, Shouting

Reduced-dimensional materials have attracted tremendous attention because of their new physics and exotic properties, which are of great interests for fundamental science. More importantly, the manipulation and engineering of matter on an atomic scale yield promising applications for many fields including nanoelectronics, nanobiotechnology, environments, and renewable energy. Because of the unusual quantum confinement and enhanced surface effect of reduced-dimensional materials, traditional empirical models suffer from necessary but unreliable parameters extracted from previously-studied bulk materials. In this sense, quantitative, parameter-free approaches are highly useful for understanding properties of reduced-dimensional materials and, furthermore, predicting their novel applications. The first-principles density functional theory (DFT) is proven to be a reliable and convenient tool. In particular, recent progress in many-body perturbation theory (MBPT) makes it possible to calculate excited-state properties, e.g., quasiparticle (QP) band gap and optical excitations, by the first-principles approach based on DFT. Therefore, during my PhD study, I employed first-principles calculations based on DFT and MBPT to systematically study fundamental properties of typical reduced-dimensional semiconductors, i.e., the electronic structure, phonons, and optical excitations of core-shell nanowires (NWs) and graphene-like two-dimensional (2D) structures of current interests. First, I present first-principles studies on how to engineer band alignments of nano-sized radial heterojunctions, Si/Ge core-shell NWs. Our calculation reveals that band offsets in these one-dimensional (1D) nanostructures can be tailored by applying axial strain or varying core-shell sizes. In particular, the valence band offset can be efficiently tuned across a wide range and even be diminished via applied strain. Two mechanisms contribute to this tuning of band offsets. Furthermore, varying the

Relativistic band-structure calculations for CeTIn sub 5 (T=Ir and Co) and analysis of the energy bands by using tight-binding method

CERN Document Server

Maehira, T; Ueda, K; Hasegawa, A

2003-01-01

In order to investigate electronic properties of recently discovered heavy fermion superconductors CeTIn sub 5 (T=Ir and Co), we employ the relativistic linear augmented-plane-wave (RLAPW) method to clarify the energy band structures and Fermi surfaces of those materials. The obtained energy bands mainly due to the large hybridization between Ce 4 f and In 5 p states well reproduce the Fermi surfaces consistent with the de Haas-van Alphen experimental results. However, when we attempt to understand magnetism and superconductively in CeTIn sub 5 from the microscopic viewpoint, the energy bands obtained in the RLAPW method are too complicated to analyze the system by further including electron correlations. Thus, it is necessary to prepare a more simplified model, keeping correctly the essential characters of the energy bands obtained in the band-structure calculation. For the purpose, we construct a tight-binding model for CeTIn sub 5 by including f-f and p-p hoppings as well as f-p hybridization, which are ex...

Spin–orbit coupling induced magnetoresistance oscillation in a dc biased two-dimensional electron system

International Nuclear Information System (INIS)

Wang, C M; Lei, X L

2014-01-01

We study dc-current effects on the magnetoresistance oscillation in a two-dimensional electron gas with Rashba spin-orbit coupling, using the balance-equation approach to nonlinear magnetotransport. In the weak current limit the magnetoresistance exhibits periodical Shubnikov-de Haas oscillation with changing Rashba coupling strength for a fixed magnetic field. At finite dc bias, the period of the oscillation halves when the interbranch contribution to resistivity dominates. With further increasing current density, the oscillatory resistivity exhibits phase inversion, i.e., magnetoresistivity minima (maxima) invert to maxima (minima) at certain values of the dc bias, which is due to the current-induced magnetoresistance oscillation. (paper)

The Two-Beam Free Electron Laser Oscillator

CERN Document Server

Thompson, Neil R

2004-01-01

A one-dimensional model of a free-electron laser operating simultaneously with two electron beams of different energies [1] is extended to an oscillator configuration. The electron beam energies are chosen so that an harmonic of the lower energy beam is at the fundamental radiation wavelength of the higher energy beam. Potential benefits over a single-beam free-electron laser oscillator are discussed.

Two dimensional CCD [charged coupled device] arrays as parallel detectors in electron energy loss and x-ray wavelength dispersive spectroscopy

International Nuclear Information System (INIS)

Zaluzec, N.J.

1988-08-01

Parallel detection systems for spectroscopy have generally been based upon linear detector arrays. Replacing the linear arrays with two dimensional systems yields more complicated devices; however, there are corresponding benefits which can be realized for both x-ray and electron energy loss spectroscopy. The operational design of these systems, as well as preliminary results from the construction of such a device used for electron spectroscopy, are presented. 10 refs., 8 figs

Evolution of function in the "two dinucleotide binding domains" flavoproteins.

Directory of Open Access Journals (Sweden)

Sunil Ojha

2007-07-01

Full Text Available Structural and biochemical constraints force some segments of proteins to evolve more slowly than others, often allowing identification of conserved structural or sequence motifs that can be associated with substrate binding properties, chemical mechanisms, and molecular functions. We have assessed the functional and structural constraints imposed by cofactors on the evolution of new functions in a superfamily of flavoproteins characterized by two-dinucleotide binding domains, the "two dinucleotide binding domains" flavoproteins (tDBDF superfamily. Although these enzymes catalyze many different types of oxidation/reduction reactions, each is initiated by a stereospecific hydride transfer reaction between two cofactors, a pyridine nucleotide and flavin adenine dinucleotide (FAD. Sequence and structural analysis of more than 1,600 members of the superfamily reveals new members and identifies details of the evolutionary connections among them. Our analysis shows that in all of the highly divergent families within the superfamily, these cofactors adopt a conserved configuration optimal for stereospecific hydride transfer that is stabilized by specific interactions with amino acids from several motifs distributed among both dinucleotide binding domains. The conservation of cofactor configuration in the active site restricts the pyridine nucleotide to interact with FAD from the re-side, limiting the flow of electrons from the re-side to the si-side. This directionality of electron flow constrains interactions with the different partner proteins of different families to occur on the same face of the cofactor binding domains. As a result, superimposing the structures of tDBDFs aligns not only these interacting proteins, but also their constituent electron acceptors, including heme and iron-sulfur clusters. Thus, not only are specific aspects of the cofactor-directed chemical mechanism conserved across the superfamily, the constraints they impose are

Structural organization of the human glucocorticoid receptor determined by one- and two-dimensional gel electrophoresis of proteolytic receptor fragments

International Nuclear Information System (INIS)

Smith, A.C.; Harmon, J.M.

1987-01-01

The structural organization of the steroid-binding protein of the IM-9 cell glucocorticoid receptor was investigated by using one- and two-dimensional gel electrophoresis of proteolytic receptor fragments. One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of receptor fragments isolated after trypsin digestion of immunopurified [ 3 H]dexamethasone 21-mesylate ([ 3 H]DM-) labeled receptor revealed the presence of a stable 26.5-kilodalton (kDa) steroid-containing non-DNA-binding fragment, derived from a larger, less stable, 29-kDa fragment. The 26.5-kDa tryptic fragment appeared to be completely contained within a 41-kDa, steroid-containing, DNA-binding species isolated after chymotrypsin digestion of the intact protein. Two-dimensional electrophoretic analysis of the [ 3 H]DM-labeled tryptic fragments resolved two 26.5-kDa and two 29-kDa components. This was the same number of isoforms seen in the intact protein, indicating that the charge heterogeneity of the steroid-binding protein is the result of modification within the steroid-containing, non-DNA-binding, 26.5-kDa tryptic fragment. Two-dimensional analysis of the 41-kDa [ 3 H]DM-labeled chymotryptic species revealed a pattern of isoforms more complex than that seen either in the intact protein or in the steroid-containing tryptic fragments. These results suggest that the 41-kDa [ 3 H]DM-labeled species resolved by one-dimensional SDS-PAGE after chymotrypsin digestion may be composed of several distinct proteolytic fragments

Two dimensional layered materials: First-principle investigation

Science.gov (United States)

Tang, Youjian

Two-dimensional layered materials have emerged as a fascinating research area due to their unique physical and chemical properties, which differ from those of their bulk counterparts. Some of these unique properties are due to carriers and transport being confined to 2 dimensions, some are due to lattice symmetry, and some arise from their large surface area, gateability, stackability, high mobility, spin transport, or optical accessibility. How to modify the electronic and magnetic properties of two-dimensional layered materials for desirable long-term applications or fundamental physics is the main focus of this thesis. We explored the methods of adsorption, intercalation, and doping as ways to modify two-dimensional layered materials, using density functional theory as the main computational methodology. Chapter 1 gives a brief review of density functional theory. Due to the difficulty of solving the many-particle Schrodinger equation, density functional theory was developed to find the ground-state properties of many-electron systems through an examination of their charge density, rather than their wavefunction. This method has great application throughout the chemical and material sciences, such as modeling nano-scale systems, analyzing electronic, mechanical, thermal, optical and magnetic properties, and predicting reaction mechanisms. Graphene and transition metal dichalcogenides are arguably the two most important two-dimensional layered materials in terms of the scope and interest of their physical properties. Thus they are the main focus of this thesis. In chapter 2, the structure and electronic properties of graphene and transition metal dichalcogenides are described. Alkali adsorption onto the surface of bulk graphite and metal intecalation into transition metal dichalcogenides -- two methods of modifying properties through the introduction of metallic atoms into layered systems -- are described in chapter 2. Chapter 3 presents a new method of tuning

Local imaging of high mobility two-dimensional electron systems with virtual scanning tunneling microscopy

Energy Technology Data Exchange (ETDEWEB)

Pelliccione, M. [Department of Applied Physics, Stanford University, 348 Via Pueblo Mall, Stanford, California 94305 (United States); Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025 (United States); Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106 (United States); Bartel, J.; Goldhaber-Gordon, D. [Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025 (United States); Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305 (United States); Sciambi, A. [Department of Applied Physics, Stanford University, 348 Via Pueblo Mall, Stanford, California 94305 (United States); Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025 (United States); Pfeiffer, L. N.; West, K. W. [Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 (United States)

2014-11-03

Correlated electron states in high mobility two-dimensional electron systems (2DESs), including charge density waves and microemulsion phases intermediate between a Fermi liquid and Wigner crystal, are predicted to exhibit complex local charge order. Existing experimental studies, however, have mainly probed these systems at micron to millimeter scales rather than directly mapping spatial organization. Scanning probes should be well-suited to study the spatial structure of these states, but high mobility 2DESs are found at buried semiconductor interfaces, beyond the reach of conventional scanning tunneling microscopy. Scanning techniques based on electrostatic coupling to the 2DES deliver important insights, but generally with resolution limited by the depth of the 2DES. In this letter, we present our progress in developing a technique called “virtual scanning tunneling microscopy” that allows local tunneling into a high mobility 2DES. Using a specially designed bilayer GaAs/AlGaAs heterostructure where the tunnel coupling between two separate 2DESs is tunable via electrostatic gating, combined with a scanning gate, we show that the local tunneling can be controlled with sub-250 nm resolution.

Dimensional and correlation effects of charged excitons in low-dimensional semiconductors

Energy Technology Data Exchange (ETDEWEB)

Roennow, Troels F; Pedersen, Thomas G [Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, 9220 Aalborg Oest (Denmark); Cornean, Horia D, E-mail: tfr@nanophysics.d [Department of Mathematical Sciences, Aalborg University, Frederik Bajers Vej 7G, 9220 Aalborg (Denmark)

2010-11-26

In this paper, we investigate the existence of bound trion states in fractional dimensional nanostructures, in terms of variational calculus. We start with trial states, then we refine the result with the help of the Hartree-Fock approximation and finally we use a partial basis expansion. We show that Hartree-Fock significantly underestimates the trion binding energy and that the correlation energy is comparable with the trion binding energy. Furthermore we calculate the binding energies of positive and negative trions restricted to a large subspace of functions, which we expect to span the low-lying eigenstates of the full Hamiltonian. We find that the difference between the positive and negative trion binding energies varies very little for the electron-hole mass fractions m{sub e}/m{sub h} = {sigma} in [0.8; 1.0] and that the difference between the positive and negative trion energies grows as the dimension decreases. Finally, we compare a cylindrical effective-mass model of a typical carbon nanotube, with a fractional dimensional model with D = 1.71. We find very good agreement between the trion binding energies predicted by the two models.

Dimensional and correlation effects of charged excitons in low-dimensional semiconductors

International Nuclear Information System (INIS)

Roennow, Troels F; Pedersen, Thomas G; Cornean, Horia D

2010-01-01

In this paper, we investigate the existence of bound trion states in fractional dimensional nanostructures, in terms of variational calculus. We start with trial states, then we refine the result with the help of the Hartree-Fock approximation and finally we use a partial basis expansion. We show that Hartree-Fock significantly underestimates the trion binding energy and that the correlation energy is comparable with the trion binding energy. Furthermore we calculate the binding energies of positive and negative trions restricted to a large subspace of functions, which we expect to span the low-lying eigenstates of the full Hamiltonian. We find that the difference between the positive and negative trion binding energies varies very little for the electron-hole mass fractions m e /m h = σ in [0.8; 1.0] and that the difference between the positive and negative trion energies grows as the dimension decreases. Finally, we compare a cylindrical effective-mass model of a typical carbon nanotube, with a fractional dimensional model with D = 1.71. We find very good agreement between the trion binding energies predicted by the two models.

Effects of magnetic barriers on transport and magnetoresistance in a two-dimensional electronic device

Energy Technology Data Exchange (ETDEWEB)

He, H. L.; Zhang, X. W., E-mail: hedge80@sina.com.cn; Dai, B.; Ren, Y. [State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010 (China); Wang, Z. P. [Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900 (China)

2016-05-15

We study theoretically the giant magnetoresistance (GMR) effect of 2-dimensional electron system (2DES) by the transfer matrix method. To produce the inhomogeneous magnetic field, two magnetic strips are pre-deposited on the surface of 2DES. In our work, we fix the magnetization M in one magnetic strip and adjust the tilting angle θ of magnetization in the other. The result shows that the electronic transmission and conductance vary significantly for different θ. The minimum conductance can be obtained at θ = π which corresponds to the magnetization anti-parallel alignment. The magnetoresistance ratio (MRR) calculation also indicates we would get the maximum in that case. Furthermore, we consider the magnetization M dependence of MRR in this work. When M increases, MRR peaks get higher and broader and more numbers of peaks can be observed. These results offer an alternative to get a tunable GMR device which can be controlled by adjusting the magnetization M and the magnetized angle θ.

Effects of magnetic barriers on transport and magnetoresistance in a two-dimensional electronic device

Directory of Open Access Journals (Sweden)

H. L. He

2016-05-01

Full Text Available We study theoretically the giant magnetoresistance (GMR effect of 2-dimensional electron system (2DES by the transfer matrix method. To produce the inhomogeneous magnetic field, two magnetic strips are pre-deposited on the surface of 2DES. In our work, we fix the magnetization M in one magnetic strip and adjust the tilting angle θ of magnetization in the other. The result shows that the electronic transmission and conductance vary significantly for different θ. The minimum conductance can be obtained at θ = π which corresponds to the magnetization anti-parallel alignment. The magnetoresistance ratio (MRR calculation also indicates we would get the maximum in that case. Furthermore, we consider the magnetization M dependence of MRR in this work. When M increases, MRR peaks get higher and broader and more numbers of peaks can be observed. These results offer an alternative to get a tunable GMR device which can be controlled by adjusting the magnetization M and the magnetized angle θ.

Wedge-Shaped GaN Nanowalls: A Potential Candidate for Two-Dimensional Electronics and Spintronics

Science.gov (United States)

Deb, Swarup; Dhar, Subhabrata

Schrödingerand Poisson equations are solved self-consistently in order to obtain the potential and charge density distribution in n-type GaN nanowalls tapered along c-axis by different angles. The study shows two-dimensional (2D) quantum confinement of electrons in the central vertical plane of the wall for the entire range of tapering. Calculation of room temperature electron mobility in the 2D channel shows a steady decrease with the increase of the inclination angle of the side facets with respect to the base. However, it is interesting to note that the mobility remains to be much larger than that of bulk GaN even for the inclination angle of 65∘. The properties of high mobility and the vertical orientation of the 2DEG plane in this system can be exploited in fabricating highly conducting transparent interconnects and field effect transistors, which can lead to large scale integration of 2D devices in future.

Stokes shift and fine structure splitting in composition-tunable Zn{sub x}Cd{sub 1−x}Se nanocrystals: Atomistic tight-binding theory

Energy Technology Data Exchange (ETDEWEB)

Sukkabot, Worasak, E-mail: w.sukkabot@gmail.com

2017-02-01

I report on the atomistic correlation of the structural properties and excitonic splitting of ternary alloy Zn{sub x}Cd{sub 1−x}Se wurtzite nanocrystals using the sp{sup 3}s* empirical tight-binding method with the description of the first nearest neighbouring interaction and bowing effect. Based on a successful model, the computations are presented under various Zn compositions (x) and diameters of alloy Zn{sub x}Cd{sub 1−x}Se nanocrystals with the experimentally synthesized compositions and sizes. With increasing Zn contents (x), the optical band gaps and electron-hole coulomb energies are improved, while ground electron-hole wave function overlaps, electron-hole exchange energies, stokes shift and fine structure splitting are reduced. A composition-tunable emission from blue to yellow wavelength is obviously demonstrated. The optical band gaps, ground electron-hole wave function overlaps, electron-hole interactions, stokes shift and fine structure splitting are progressively decreased with the increasing diameters. Alloy Zn{sub x}Cd{sub 1−x}Se nanocrystal with Zn rich and large diameter is the best candidate to optimistically be used as a source of entangled photon pairs. The agreement with the experimental data is remarkable. Finally, the present systematic study on the structural properties and excitonic splitting predominantly opens a new perspective to understand the size- and composition-dependent properties of Zn{sub x}Cd{sub 1−x}Se nanocrystals with a comprehensive strategy to design the optoelectronic devices.

Charge-density-wave instabilities expected in monophosphate tungsten bronzes

International Nuclear Information System (INIS)

Canadell, E.; Whangbo, M.

1991-01-01

On the basis of tight-binding band calculations, we examined the electronic structures of the tungsten oxide layers found in the monophosphate tungsten bronze (MPTB) phases. The Fermi surfaces of these MPTB phases consist of five well-nested one- and two-dimensional pieces. We calculated the nesting vectors of these Fermi surfaces and discussed the expected charge-density-wave instabilities

Tuning the two-dimensional electron liquid at oxide interfaces by buffer-layer-engineered redox reactions

DEFF Research Database (Denmark)

Chen, Yunzhong; Green, Robert J.; Sutarto, Ronny

2017-01-01

Polar discontinuities and redox reactions provide alternative paths to create two-dimensional electron liquids (2DELs) at oxide interfaces. Herein, we report high mobility 2DELs at interfaces involving SrTiO3 (STO) achieved using polar La7/8Sr1/8MnO3 (LSMO) buffer layers to manipulate both...... polarities and redox reactions from disordered overlayers grown at room temperature. Using resonant x-ray reflectometry experiments, we quantify redox reactions from oxide overlayers on STO as well as polarity induced electronic reconstruction at epitaxial LSMO/STO interfaces. The analysis reveals how...... these effects can be combined in a STO/LSMO/disordered film trilayer system to yield high mobility modulation doped 2DELs, where the buffer layer undergoes a partial transformation from perovskite to brownmillerite structure. This uncovered interplay between polar discontinuities and redox reactions via buffer...

Electron-hole liquid in semiconductors and low-dimensional structures

Science.gov (United States)

Sibeldin, N. N.

2017-11-01

The condensation of excitons into an electron-hole liquid (EHL) and the main EHL properties in bulk semiconductors and low-dimensional structures are considered. The EHL properties in bulk materials are discussed primarily in qualitative terms based on the experimental results obtained for germanium and silicon. Some of the experiments in which the main EHL thermodynamic parameters (density and binding energy) have been obtained are described and the basic factors that determine these parameters are considered. Topics covered include the effect of external perturbations (uniaxial strain and magnetic field) on EHL stability; phase diagrams for a nonequilibrium exciton-gas-EHL system; information on the size and concentration of electron-hole drops (EHDs) under various experimental conditions; the kinetics of exciton condensation and of recombination in the exciton-gas-EHD system; dynamic EHD properties and the motion of EHDs under the action of external forces; the properties of giant EHDs that form in potential wells produced by applying an inhomogeneous strain to the crystal; and effects associated with the drag of EHDs by nonequilibrium phonons (phonon wind), including the dynamics and formation of an anisotropic spatial structure of the EHD cloud. In discussing EHLs in low-dimensional structures, a number of studies are reviewed on the observation and experimental investigation of phenomena such as spatially indirect (dipolar) electron-hole and exciton (dielectric) liquids in GaAs/AlGaAs structures with double quantum wells (QWs), EHDs containing only a few electron-hole pairs (dropletons), EHLs in type-I silicon QWs, and spatially direct and dipolar EHLs in type-II silicon-germanium heterostructures.

Transferable tight binding model for strained group IV and III-V heterostructures

Science.gov (United States)

Tan, Yaohua; Povolotskyi, Micheal; Kubis, Tillmann; Boykin, Timothy; Klimeck, Gerhard

Modern semiconductor devices have reached critical device dimensions in the range of several nanometers. For reliable prediction of device performance, it is critical to have a numerical efficient model that are transferable to material interfaces. In this work, we present an empirical tight binding (ETB) model with transferable parameters for strained IV and III-V group semiconductors. The ETB model is numerically highly efficient as it make use of an orthogonal sp3d5s* basis set with nearest neighbor inter-atomic interactions. The ETB parameters are generated from HSE06 hybrid functional calculations. Band structures of strained group IV and III-V materials by ETB model are in good agreement with corresponding HSE06 calculations. Furthermore, the ETB model is applied to strained superlattices which consist of group IV and III-V elements. The ETB model turns out to be transferable to nano-scale hetero-structure. The ETB band structures agree with the corresponding HSE06 results in the whole Brillouin zone. The ETB band gaps of superlattices with common cations or common anions have discrepancies within 0.05eV.

Development of tight-binding, chemical-reaction-dynamics simulator for combinatorial computational chemistry

International Nuclear Information System (INIS)

Kubo, Momoji; Ando, Minako; Sakahara, Satoshi; Jung, Changho; Seki, Kotaro; Kusagaya, Tomonori; Endou, Akira; Takami, Seiichi; Imamura, Akira; Miyamoto, Akira

2004-01-01

Recently, we have proposed a new concept called 'combinatorial computational chemistry' to realize a theoretical, high-throughput screening of catalysts and materials. We have already applied our combinatorial, computational-chemistry approach, mainly based on static first-principles calculations, to various catalysts and materials systems and its applicability to the catalysts and materials design was strongly confirmed. In order to realize more effective and efficient combinatorial, computational-chemistry screening, a high-speed, chemical-reaction-dynamics simulator based on quantum-chemical, molecular-dynamics method is essential. However, to the best of our knowledge, there is no chemical-reaction-dynamics simulator, which has an enough high-speed ability to perform a high-throughput screening. In the present study, we have succeeded in the development of a chemical-reaction-dynamics simulator based on our original, tight-binding, quantum-chemical, molecular-dynamics method, which is more than 5000 times faster than the regular first-principles, molecular-dynamics method. Moreover, its applicability and effectiveness to the atomistic clarification of the methanol-synthesis dynamics at reaction temperature were demonstrated

Effect of disorder on the density of states of a two-dimensional electron gas under magnetic field

International Nuclear Information System (INIS)

Bonifacie, S.; Meziani, Y.M.; Chaubet, C.; Jouault, B.; Raymond, A.

2004-01-01

We have calculated the density of states (DOS) of a two-dimensional electron gas in a perpendicular magnetic field, using a multiple scattering method, in the ultraquantum limit. We have considered doped and disordered 2D systems. The results of the scattering method are compared with direct simulations of disordered samples. Using the DOS, we have studied the metal-insulator transition and the magnetic freeze-out including a comparison with experimental results

The inversion layer of electric fields and electron phase-space-hole structure during two-dimensional collisionless magnetic reconnection

International Nuclear Information System (INIS)

Chen Lijen; Lefebvre, Bertrand; Torbert, Roy B.; Daughton, William S.

2011-01-01

Based on two-dimensional fully kinetic simulations that resolve the electron diffusion layer in undriven collisionless magnetic reconnection with zero guide field, this paper reports the existence and evolution of an inversion layer of bipolar electric fields, its corresponding phase-space structure (an electron-hole layer), and the implication to collisionless dissipation. The inversion electric field layer is embedded in the layer of bipolar Hall electric field and extends throughout the entire length of the electron diffusion layer. The electron phase-space hole structure spontaneously arises during the explosive growth phase when there exist significant inflows into the reconnection layer, and electrons perform meandering orbits across the layer while being cyclotron-turned toward the outflow directions. The cyclotron turning of meandering electrons by the magnetic field normal to the reconnection layer is shown to be a primary factor limiting the current density in the region where the reconnection electric field is balanced by the gradient (along the current sheet normal) of the off-diagonal electron pressure-tensor.

Oscillation mode transformation of edge magnetoplasmons in two-dimensional electron system on liquid-helium surface

International Nuclear Information System (INIS)

Yamanaka, Shuji; Yayama, Hideki; Arai, Toshikazau; Anju Sawada, Anju; Fukuda, Akira

2013-01-01

We measured the resonance spectra of edge magnetoplasmon (EMP) oscillations in a two-dimensional (2D) electron system located on a liquid-helium surface below 1.1 K. Systematic measurements of the resonance frequency and the damping rate as a function of the lateral confinement electric field strength shows clear evidence of the oscillation mode transformation. A pronounced change corresponding to the mode transformation was observed in the damping rate. When 2D electrons are confined in a strong lateral electric field, the damping is weak. As the lateral confinement electric field is reduced below a certain threshold value, an abrupt enhancement of the damping rate is observed. We hypothesize that the weak damping mode in the strong lateral confinement electric field is the compressive density oscillation of the electrons near the edge (conventional EMP) and the strong damping mode in the weak confinement field is the coupled mode of conventional EMP and the boundary displacement wave (BDW). The observation of the strong damping in the BDW-EMP coupled mode is a manifestation of the nearly incompressible feature of strongly interacting classical electrons, which agrees with earlier theoretical predictions.

Mechanical Control of ATP Synthase Function: Activation Energy Difference between Tight and Loose Binding Sites

KAUST Repository

Beke-Somfai, Tamás

2010-01-26

Despite exhaustive chemical and crystal structure studies, the mechanistic details of how FoF1-ATP synthase can convert mechanical energy to chemical, producing ATP, are still not fully understood. On the basis of quantum mechanical calculations using a recent highresolution X-ray structure, we conclude that formation of the P-O bond may be achieved through a transition state (TS) with a planar PO3 - ion. Surprisingly, there is a more than 40 kJ/mol difference between barrier heights of the loose and tight binding sites of the enzyme. This indicates that even a relatively small change in active site conformation, induced by the γ-subunit rotation, may effectively block the back reaction in βTP and, thus, promote ATP. © 2009 American Chemical Society.

Creation of High Mobility Two-Dimensional Electron Gases via Strain Induced Polarization at an Otherwise Nonpolar Complex Oxide Interface

DEFF Research Database (Denmark)

Chen, Yunzhong; Trier, Felix; Kasama, Takeshi

2015-01-01

The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Rem...

Nonlinear transport behavior of low dimensional electron systems

Science.gov (United States)

Zhang, Jingqiao

The nonlinear behavior of low-dimensional electron systems attracts a great deal of attention for its fundamental interest as well as for potentially important applications in nanoelectronics. In response to microwave radiation and dc bias, strongly nonlinear electron transport that gives rise to unusual electron states has been reported in two-dimensional systems of electrons in high magnetic fields. There has also been great interest in the nonlinear response of quantum ballistic constrictions, where the effects of quantum interference, spatial dispersion and electron-electron interactions play crucial roles. In this thesis, experimental results of the research of low dimensional electron gas systems are presented. The first nonlinear phenomena were observed in samples of highly mobile two dimensional electrons in GaAs heavily doped quantum wells at different magnitudes of DC and AC (10 KHz to 20 GHz) excitations. We found that in the DC excitation regime the differential resistance oscillates with the DC current and external magnetic field, similar behavior was observed earlier in AlGaAs/GaAs heterostructures [C.L. Yang et al. ]. At external AC excitations the resistance is found to be also oscillating as a function of the magnetic field. However the form of the oscillations is considerably different from the DC case. We show that at frequencies below 100 KHz the difference is a result of a specific average of the DC differential resistance during the period of the external AC excitations. Secondly, in similar samples, strong suppression of the resistance by the electric field is observed in magnetic fields at which the Landau quantization of electron motion occurs. The phenomenon survives at high temperatures at which the Shubnikov de Haas oscillations are absent. The scale of the electric fields essential for the effect, is found to be proportional to temperature in the low temperature limit. We suggest that the strong reduction of the longitudinal resistance

Intrinsic two-dimensional states on the pristine surface of tellurium

Science.gov (United States)

Li, Pengke; Appelbaum, Ian

2018-05-01

Atomic chains configured in a helical geometry have fascinating properties, including phases hosting localized bound states in their electronic structure. We show how the zero-dimensional state—bound to the edge of a single one-dimensional helical chain of tellurium atoms—evolves into two-dimensional bands on the c -axis surface of the three-dimensional trigonal bulk. We give an effective Hamiltonian description of its dispersion in k space by exploiting confinement to a virtual bilayer, and elaborate on the diminished role of spin-orbit coupling. These intrinsic gap-penetrating surface bands were neglected in the interpretation of seminal experiments, where two-dimensional transport was otherwise attributed to extrinsic accumulation layers.

Magnetoresistance oscillations of two-dimensional electron systems in lateral superlattices with structured unit cells

Science.gov (United States)

Gerhardts, Rolf R.

2015-11-01

Model calculations for commensurability oscillations of the low-field magnetoresistance of two-dimensional electron systems (2DES) in lateral superlattices, consisting of unit cells with an internal structure, are compared with recent experiments. The relevant harmonics of the effective modulation potential depend not only on the geometrical structure of the modulated unit cell, but also strongly on the nature of the modulation. While higher harmonics of an electrostatically generated surface modulation are exponentially damped at the position of the 2DES about 90 nm below the surface, no such damping appears for strain-induced modulation generated, e.g., by the deposition of stripes of calixarene resist on the surface before cooling down the sample.

Direct Observation of Energy Detrapping in LH1-RC Complex by Two-Dimensional Electronic Spectroscopy.

Science.gov (United States)

Ma, Fei; Yu, Long-Jiang; Hendrikx, Ruud; Wang-Otomo, Zheng-Yu; van Grondelle, Rienk

2017-01-18

The purple bacterial core light harvesting antenna-reaction center (LH1-RC) complex is the simplest system able to achieve the entire primary function of photosynthesis. During the past decade, a variety of photosynthetic proteins were studied by a powerful technique, two-dimensional electronic spectroscopy (2DES). However, little attention has been paid to LH1-RC, although its reversible uphill energy transfer, trapping, and backward detrapping processes, represent a crucial step in the early photosynthetic reaction dynamics. Thus, in this work, we employed 2DES to study two LH1-RC complexes of Thermochromatium (Tch.) tepidum. By direct observation of detrapping, the complex reversible process was clearly identified and an overall scheme of the excitation evolution in LH1-RC was obtained.

Electronic Properties of III-V Semiconductors under [111] Uniaxial Strain; a Tight-Binding Approach: I. Arsenides and Gallium Phosphide

Directory of Open Access Journals (Sweden)

Miguel E. Mora-Ramos

2009-01-01

Full Text Available Empleando un esquema de cálculo tight-binding que usa una base de orbitales sp3s*d5, se estudian propiedades de la estructura electrónica de un grupo de materiales semiconductores IIIV los cuales son de notable interés para la tecnología de dispositivos electrónicos y optoelectrónicos. En específico, se analiza la influencia sobre estas propiedades de una tensión aplicada según la dirección cristalográfica [111], haciendo uso de una formulación basada en la teoría de la elasticidad para establecer las posiciones relativas de los iones vecinos más próximos. Especial atención se presta a la inclusión del efecto de deformación interna de la red cristalina. Para cada material de los estudiados presentamos las dependencias de las brechas energéticas asociadas a los puntos L, X y L de la zona de Brillouin como funciones de la tensión uniaxial en AlAs, GaAs, InAs y GaP. Asimismo, reportamos expresiones de ajuste para los valores de las masas efectivas de conducción en esos cuatro materiales. La comparación de la variación de la brecha de energía en X para el GaP, calculada con nuestro modelo, y recientes resultados experimentales para la transición indirecta entre la banda de huecos pesados y la banda X de conducción arroja una muy buena concordancia.

Reflectance distribution in optimal transmittance cavities: The remains of a higher dimensional space

International Nuclear Information System (INIS)

Naumis, Gerardo G.; Bazan, A.; Torres, M.; Aragon, J.L.; Quintero-Torres, R.

2008-01-01

One of the few examples in which the physical properties of an incommensurable system reflect an underlying higher dimensionality is presented. Specifically, we show that the reflectivity distribution of an incommensurable one-dimensional cavity is given by the density of states of a tight-binding Hamiltonian in a two-dimensional triangular lattice. Such effect is due to an independent phase decoupling of the scattered waves, produced by the incommensurable nature of the system, which mimics a random noise generator. This principle can be applied to design a cavity that avoids resonant reflections for almost any incident wave. An optical analogy, by using three mirrors with incommensurable distances between them, is also presented. Such array produces a countable infinite fractal set of reflections, a phenomena which is opposite to the effect of optical invisibility

Carbon Nanotubes: Molecular Electronic Components

Science.gov (United States)

Srivastava, Deepak; Saini, Subhash; Menon, Madhu

1997-01-01

The carbon Nanotube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale molecular electronic networks. While the simple joint of two dissimilar tubes can be generated by the introduction of a pair of heptagon-pentagon defects in an otherwise perfect hexagonal graphene sheet, more complex joints require other mechanisms. In this work we explore structural characteristics of complex 3-point junctions of carbon nanotubes using a generalized tight-binding molecular-dynamics scheme. The study of pi-electron local densities of states (LDOS) of these junctions reveal many interesting features, most prominent among them being the defect-induced states in the gap.

Chern-Simons field theory of two-dimensional electrons in the lowest Landau level

International Nuclear Information System (INIS)

Zhang, L.

1996-01-01

We propose a fermion Chern-Simons field theory describing two-dimensional electrons in the lowest Landau level. This theory is constructed with a complete set of states, and the lowest-Landau-level constraint is enforced through a δ functional described by an auxiliary field λ. Unlike the field theory constructed directly with the states in the lowest Landau level, this theory allows one, utilizing the physical picture of open-quote open-quote composite fermion,close-quote close-quote to study the fractional quantum Hall states by mapping them onto certain integer quantum Hall states; but, unlike its application in the unconstrained theory, such a mapping is sensible only when interactions between electrons are present. An open-quote open-quote effective mass,close-quote close-quote which characterizes the scale of low energy excitations in the fractional quantum Hall systems, emerges naturally from our theory. We study a Gaussian effective theory and interpret physically the dressed stationary point equation for λ as an equation for the open-quote open-quote mass renormalization close-quote close-quote of composite fermions. copyright 1996 The American Physical Society

Quantum oscillations in quasi-two-dimensional conductors

CERN Document Server

Galbova, O

2002-01-01

The electronic absorption of sound waves in quasi-two-dimensional conductors in strong magnetic fields, is investigated theoretically. A longitudinal acoustic wave, propagating along the normal n-> to the layer of quasi-two-dimensional conductor (k-> = left brace 0,0,k right brace; u-> = left brace 0,0,u right brace) in magnetic field (B-> = left brace 0, 0, B right brace), is considered. The quasiclassical approach for this geometry is of no interest, due to the absence of interaction between electromagnetic and acoustic waves. The problem is of interest in strong magnetic field when quantization of the charge carriers energy levels takes place. The quantum oscillations in the sound absorption coefficient, as a function of the magnetic field, are theoretically observed. The experimental study of the quantum oscillations in quasi-two-dimensional conductors makes it possible to solve the inverse problem of determining from experimental data the extrema closed sections of the Fermi surface by a plane p sub z = ...

Two-dimensional MoS2-graphene hybrid nanosheets for high gravimetric and volumetric lithium storage

Science.gov (United States)

Deng, Yakai; Ding, Lixin; Liu, Qixing; Zhan, Liang; Wang, Yanli; Yang, Shubin

2018-04-01

Two-dimensional (2D) MoS2-graphene (MoS2-G) hybrid is fabricated simultaneously and scalablely with an efficient electrochemical exfoliation approach from the combined bulk MoS2-graphite wafer. The as-prepared 2D MoS2-G hybrid is tightly covered with each other with lateral sizes of 600 nm to few micrometers and can be directly assembled to flexible films for lithium storage. When used as anode material for lithium ion battery, the resultant MoS2-G hybrid film exhibits both high gravimetric (750 mA h g-1 at 50 mA g-1) and volumetric capacities (1200 mA h cm-3 at 0.1 mA cm-2). Such excellent electrochemical performance should attributed to the unique 2D structure and good conductive graphene network, which not only facilitates the diffusion of lithium ions, but also improves the fast transfer of electrons, satisfying the kinetics requirements for rapid lithium storage.

The adsorption effect of C6H5 on density of states for double wall carbon nanotubes by tight binding model

International Nuclear Information System (INIS)

Fathalian, A.

2012-01-01

A theoretical approach based on a tight-binding model is developed to study the effects of the adsorption of finite concentrations of C 6 H 5 gas molecules on double-walled carbon nanotube (DWCNT) electronic properties. To obtain proper hopping integrals and random on-site energies for the case of one molecule adsorption, the local density of states for various hopping integrals and random on-site energies are calculated. Since C 6 H 5 molecule is a donor with respect to the carbon nanotubes and their states should appear near the conduction band of the system, effects of various hopping integral deviations and on-site energies for one molecule adsorption are considered to find proper hopping and on-site energies consistent with expected n-type semiconductor. We found that adsorption of C 6 H 5 gas molecules could lead to a (8.0)-(20.0) DWCNT n-type semiconductor. The width of impurity adsorbed gas states in the density of states could be controlled by adsorbed gas concentration.

Improving density functional tight binding predictions of free energy surfaces for peptide condensation reactions in solution

Science.gov (United States)

Kroonblawd, Matthew; Goldman, Nir

First principles molecular dynamics using highly accurate density functional theory (DFT) is a common tool for predicting chemistry, but the accessible time and space scales are often orders of magnitude beyond the resolution of experiments. Semi-empirical methods such as density functional tight binding (DFTB) offer up to a thousand-fold reduction in required CPU hours and can approach experimental scales. However, standard DFTB parameter sets lack good transferability and calibration for a particular system is usually necessary. Force matching the pairwise repulsive energy term in DFTB to short DFT trajectories can improve the former's accuracy for chemistry that is fast relative to DFT simulation times (Contract DE-AC52-07NA27344.

Improving Density Functional Tight Binding Predictions of Free Energy Surfaces for Slow Chemical Reactions in Solution

Science.gov (United States)

Kroonblawd, Matthew; Goldman, Nir

2017-06-01

First principles molecular dynamics using highly accurate density functional theory (DFT) is a common tool for predicting chemistry, but the accessible time and space scales are often orders of magnitude beyond the resolution of experiments. Semi-empirical methods such as density functional tight binding (DFTB) offer up to a thousand-fold reduction in required CPU hours and can approach experimental scales. However, standard DFTB parameter sets lack good transferability and calibration for a particular system is usually necessary. Force matching the pairwise repulsive energy term in DFTB to short DFT trajectories can improve the former's accuracy for reactions that are fast relative to DFT simulation times (Contract DE-AC52-07NA27344.

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...

Catalystlike effect of orbital angular momentum on the conversion of transverse to three-dimensional spin states within tightly focused radially polarized beams

Science.gov (United States)

Han, Lei; Liu, Sheng; Li, Peng; Zhang, Yi; Cheng, Huachao; Zhao, Jianlin

2018-05-01

We report on the catalystlike effect of orbital angular momentum (OAM) on local spin-state conversion within the tightly focused radially polarized beams associated with optical spin-orbit interaction. It is theoretically demonstrated that the incident OAM can lead to a conversion of purely transverse spin state to a three-dimensional spin state on the focal plane. This conversion can be conveniently manipulated by altering the sign and value of the OAM. By comparing the total OAM and spin angular momentum (SAM) on the incident plane to those on the focal plane, it is indicated that the incident OAM have no participation in the angular momentum intertransfer, and just play a role as a catalyst of local SAM conversion. Such an effect of OAM sheds new light on the optical spin-orbit interaction in tight-focusing processes. The resultant three-dimensional spin states may provide more degrees of freedom in optical manipulation and spin-dependent directive coupling.

Electronic transport in methylated fragments of DNA

Energy Technology Data Exchange (ETDEWEB)

Almeida, M. L. de; Oliveira, J. I. N.; Lima Neto, J. X.; Gomes, C. E. M.; Fulco, U. L., E-mail: umbertofulco@gmail.com; Albuquerque, E. L. [Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970 Natal-RN (Brazil); Freire, V. N. [Departamento de Física, Universidade Federal do Ceará, 60455-760 Fortaleza, CE (Brazil); Caetano, E. W. S. [Instituto Federal de Educação, Ciência e Tecnologia do Ceará, 60040-531 Fortaleza, CE (Brazil); Moura, F. A. B. F. de; Lyra, M. L. [Instituto de Física, Universidade Federal de Alagoas, 57072-900 Maceió-AL (Brazil)

2015-11-16

We investigate the electronic transport properties of methylated deoxyribonucleic-acid (DNA) strands, a biological system in which methyl groups are added to DNA (a major epigenetic modification in gene expression), sandwiched between two metallic platinum electrodes. Our theoretical simulations apply an effective Hamiltonian based on a tight-binding model to obtain current-voltage curves related to the non-methylated/methylated DNA strands. The results suggest potential applications in the development of novel biosensors for molecular diagnostics.

Electronic transport in methylated fragments of DNA

International Nuclear Information System (INIS)

Almeida, M. L. de; Oliveira, J. I. N.; Lima Neto, J. X.; Gomes, C. E. M.; Fulco, U. L.; Albuquerque, E. L.; Freire, V. N.; Caetano, E. W. S.; Moura, F. A. B. F. de; Lyra, M. L.

2015-01-01

We investigate the electronic transport properties of methylated deoxyribonucleic-acid (DNA) strands, a biological system in which methyl groups are added to DNA (a major epigenetic modification in gene expression), sandwiched between two metallic platinum electrodes. Our theoretical simulations apply an effective Hamiltonian based on a tight-binding model to obtain current-voltage curves related to the non-methylated/methylated DNA strands. The results suggest potential applications in the development of novel biosensors for molecular diagnostics

Two-dimensional mapping of underdosed areas using radiochromic film for patients undergoing total skin electron beam radiotherapy

International Nuclear Information System (INIS)

Gamble, Lisa M.; Farrell, Thomas J.; Jones, Glenn W.; Hayward, Joseph E.

2005-01-01

Purpose: To demonstrate the viability of radiochromic film as an in vivo, two-dimensional dosimeter for the measurement of underdosed areas in patients undergoing total skin electron beam (TSEB) radiotherapy. The results were compared with thermoluminescent dosimeter measurements. Methods and Materials: Dosimetry results are reported for an inframammary fold of 2 patients treated using a modified version of the Stanford six-position (i.e., six-field and dual-beam) TSEB technique. The results are presented as contour plots of film optical density and percentage of dose. A linear dose profile measured from film was compared with the thermoluminescent dosimeter measurements. Results: The results showed that the percentage doses as measured by film are in good agreement with those measured by the thermoluminescent dosimeters. The isodose contour plots provided by film can be used as a two-dimensional dose map for a patient when determining the size of the supplemental patch fields. Conclusion: Radiochromic film is a viable dosimetry tool that the radiation oncologist can use to understand the surface dose heterogeneity better across complex concave regions of skin to help establish more appropriate margins to patch underdosed areas. Film could be used for patients undergoing TSEB for disorders such as mycosis fungoides or undergoing TSEB or regional skin electron beam for widespread skin metastases from breast cancer and other malignancies

Effect of Sr-doping of LaMnO3 spacer on modulation-doped two-dimensional electron gases at oxide interfaces

DEFF Research Database (Denmark)

Chen, Yunzhong; Gan, Yulin; Christensen, Dennis Valbjørn

2017-01-01

Modulation-doped oxide two-dimensional electron gas formed at the LaMnO3 (LMO) buffered disorderd-LaAlO3/SrTiO3 (d-LAO/LMO/STO) heterointerface provides new opportunities for electronics as well as quantum physics. Herein, we studied the dependence of Sr-doping of La1-xSrxMnO3 (LSMO, x = 0, 1/8, ...... of LSMO during the deposition of disordered LAO or that the energy levels of Mn 3d electrons at the interface of LSMO/STO are hardly varied even when changing the LSMO composition from LMO to SrMnO3....

Two-dimensional errors

International Nuclear Information System (INIS)

Anon.

1991-01-01

This chapter addresses the extension of previous work in one-dimensional (linear) error theory to two-dimensional error analysis. The topics of the chapter include the definition of two-dimensional error, the probability ellipse, the probability circle, elliptical (circular) error evaluation, the application to position accuracy, and the use of control systems (points) in measurements

Maximally localized Wannier functions in LaMnO3 within PBE + U, hybrid functionals and partially self-consistent GW: an efficient route to construct ab initio tight-binding parameters for eg perovskites.

Science.gov (United States)

Franchini, C; Kováčik, R; Marsman, M; Murthy, S Sathyanarayana; He, J; Ederer, C; Kresse, G

2012-06-13

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions (MLWFs) for the e(g) states of the prototypical Jahn-Teller magnetic perovskite LaMnO(3) at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without the additional on-site Hubbard U term, hybrid DFT and partially self-consistent GW. By suitably mapping the MLWFs onto an effective e(g) tight-binding (TB) Hamiltonian we have computed a complete set of TB parameters which should serve as guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. The method-dependent changes of the calculated TB parameters and their interplay with the electron-electron (el-el) interaction term are discussed and interpreted. We discuss two alternative model parameterizations: one in which the effects of the el-el interaction are implicitly incorporated in the otherwise 'noninteracting' TB parameters and a second where we include an explicit mean-field el-el interaction term in the TB Hamiltonian. Both models yield a set of tabulated TB parameters which provide the band dispersion in excellent agreement with the underlying ab initio and MLWF bands.

A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al₂O₃/SrTiO₃

DEFF Research Database (Denmark)

Chen, Yunzhong; Bovet, N.; Trier, Felix

2013-01-01

The discovery of two-dimensional electron gases at the heterointerface between two insulating perovskite-type oxides, such as LaAlO3 and SrTiO3, provides opportunities for a new generation of all-oxide electronic devices. Key challenges remain for achieving interfacial electron mobilities much...

Mechanical exfoliation of two-dimensional materials

Science.gov (United States)

Gao, Enlai; Lin, Shao-Zhen; Qin, Zhao; Buehler, Markus J.; Feng, Xi-Qiao; Xu, Zhiping

2018-06-01

Two-dimensional materials such as graphene and transition metal dichalcogenides have been identified and drawn much attention over the last few years for their unique structural and electronic properties. However, their rise begins only after these materials are successfully isolated from their layered assemblies or adhesive substrates into individual monolayers. Mechanical exfoliation and transfer are the most successful techniques to obtain high-quality single- or few-layer nanocrystals from their native multi-layer structures or their substrate for growth, which involves interfacial peeling and intralayer tearing processes that are controlled by material properties, geometry and the kinetics of exfoliation. This procedure is rationalized in this work through theoretical analysis and atomistic simulations. We propose a criterion to assess the feasibility for the exfoliation of two-dimensional sheets from an adhesive substrate without fracturing itself, and explore the effects of material and interface properties, as well as the geometrical, kinetic factors on the peeling behaviors and the torn morphology. This multi-scale approach elucidates the microscopic mechanism of the mechanical processes, offering predictive models and tools for the design of experimental procedures to obtain single- or few-layer two-dimensional materials and structures.

Engineering topological phases with a three-dimensional nodal-loop semimetal

Science.gov (United States)

Li, Linhu; Yap, Han Hoe; Araújo, Miguel A. N.; Gong, Jiangbin

2017-12-01

A three-dimensional (3D) nodal-loop semimetal phase is exploited to engineer a number of intriguing phases featuring different peculiar topological surface states. In particular, by introducing various two-dimensional gap terms to a 3D tight-binding model of a nodal-loop semimetal, we obtain a rich variety of topological phases of great interest to ongoing theoretical and experimental studies, including a chiral insulator, degenerate-surface-loop insulator, and second-order topological insulator, as well as a Weyl semimetal with tunable Fermi arc profiles. The unique concept underlying our approach is to engineer topological surface states that inherit their dispersion relations from a gap term. The results provide one rather unified principle for the creation of novel topological phases and can guide the search for new topological materials. Two-terminal transport studies are also carried out to distinguish the engineered topological phases.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Science.gov (United States)

Mounet, Nicolas; Gibertini, Marco; Schwaller, Philippe; Campi, Davide; Merkys, Andrius; Marrazzo, Antimo; Sohier, Thibault; Castelli, Ivano Eligio; Cepellotti, Andrea; Pizzi, Giovanni; Marzari, Nicola

2018-02-01

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

One- and zero-dimensional electron systems over liquid helium (Review article)

CERN Document Server

Kovdrya, Y Z

2003-01-01

Experimental and theoretical investigations of one-dimensional and zero-dimensional electron systems near the liquid helium surface are surveyed. The properties of electron states over the plane surface of liquid helium including thin layers of helium are considered. The methods of realization of one- and zero-dimensional electron systems are discussed, and the results of experimental and theoretical investigations of their properties are given. The experiments with localization processes in a quasi-one-dimensional electron systems on liquid helium are described. The collective effects in one-dimensional and quasi-one-dimensional electron systems are considered, and the point of possible application of low-dimensional electron systems on liquid helium in electron devices and quantum computers is discussed.

Mesoscopic Field-Effect-Induced Devices in Depleted Two-Dimensional Electron Systems

Science.gov (United States)

Bachsoliani, N.; Platonov, S.; Wieck, A. D.; Ludwig, S.

2017-12-01

Nanoelectronic devices embedded in the two-dimensional electron system (2DES) of a GaAs /(Al ,Ga )As heterostructure enable a large variety of applications ranging from fundamental research to high-speed transistors. Electrical circuits are thereby commonly defined by creating barriers for carriers by the selective depletion of a preexisting 2DES. We explore an alternative approach: we deplete the 2DES globally by applying a negative voltage to a global top gate and screen the electric field of the top gate only locally using nanoscale gates placed on the wafer surface between the plane of the 2DES and the top gate. Free carriers are located beneath the screen gates, and their properties can be controlled by means of geometry and applied voltages. This method promises considerable advantages for the definition of complex circuits by the electric-field effect, as it allows us to reduce the number of gates and simplify gate geometries. Examples are carrier systems with ring topology or large arrays of quantum dots. We present a first exploration of this method pursuing field effect, Hall effect, and Aharonov-Bohm measurements to study electrostatic, dynamic, and coherent properties.

Electronic transport in large systems through a QUAMBO-NEGF approach: Application to atomic carbon chains

International Nuclear Information System (INIS)

Fang, X.W.; Zhang, G.P.; Yao, Y.X.; Wang, C.Z.; Ding, Z.J.; Ho, K.M.

2011-01-01

The conductance of single-atom carbon chain (SACC) between two zigzag graphene nanoribbons (GNR) is studied by an efficient scheme utilizing tight-binding (TB) parameters generated via quasi-atomic minimal basis set orbitals (QUAMBOs) and non-equilibrium Green's function (NEGF). Large systems (SACC contains more than 50 atoms) are investigated and the electronic transport properties are found to correlate with SACC's parity. The SACCs provide a stable off or on state in broad energy region (0.1-1 eV) around Fermi energy. The off state is not sensitive to the length of SACC while the corresponding energy region decreases with the increase of the width of GNR. -- Highlights: → Graphene has many superior electronic properties. → First-principles calculation are accurate but limited to system size. → QUAMBOs construct tight-binding parameters with spatial localization, and then use divide-and-conquer method. → SACC (single carbon atom chain): structure and transport show even-odd parity, and long chains are studied.

A general intermolecular force field based on tight-binding quantum chemical calculations

Science.gov (United States)

Grimme, Stefan; Bannwarth, Christoph; Caldeweyher, Eike; Pisarek, Jana; Hansen, Andreas

2017-10-01

A black-box type procedure is presented for the generation of a molecule-specific, intermolecular potential energy function. The method uses quantum chemical (QC) information from our recently published extended tight-binding semi-empirical scheme (GFN-xTB) and can treat non-covalently bound complexes and aggregates with almost arbitrary chemical structure. The necessary QC information consists of the equilibrium structure, Mulliken atomic charges, charge centers of localized molecular orbitals, and also of frontier orbitals and orbital energies. The molecular pair potential includes model density dependent Pauli repulsion, penetration, as well as point charge electrostatics, the newly developed D4 dispersion energy model, Drude oscillators for polarization, and a charge-transfer term. Only one element-specific and about 20 global empirical parameters are needed to cover systems with nuclear charges up to radon (Z = 86). The method is tested for standard small molecule interaction energy benchmark sets where it provides accurate intermolecular energies and equilibrium distances. Examples for structures with a few hundred atoms including charged systems demonstrate the versatility of the approach. The method is implemented in a stand-alone computer code which enables rigid-body, global minimum energy searches for molecular aggregation or alignment.

Magnetization of correlated electron systems. MnSi thin films, CrB2 single crystals and two-dimensional electron systems in MgZnO/ZnO

International Nuclear Information System (INIS)

Brasse, Matthias

2014-01-01

Torque magnetometry at low temperature is performed to investigate the magnetic properties of MnSi thin films, of a CrB 2 single crystal and of a two-dimensional electron system (2DESs) formed at the interface of MgZnO/ZnO. The magnetic anisotropy and phase diagram of MnSi as well as information on the electronic structure of CrB 2 are obtained. The MgZnO/ZnO 2DESs exhibits the de Haas-van Alphen effect and non-equilibrium currents which are analyzed in order to determine ground state properties and excited states, respectively.

Electronic structure in Bi2Sr2CaCu2O8 studied by two dimensional angular correlation of positron annihilation radiations

International Nuclear Information System (INIS)

Kondo, H.; Kubota, T.; Nakashima, N.; Tanigawa, S.; Minami, F.; Takekawa, S.

1992-01-01

Electronic structure in one of high-Tc-sperconducting materials, Bi 2 Sr 2 CaCu 2 O 8 , was studied by two dimensional angular correlation of positron annihilation radiations (2D-ACAR). The measurements were performed for Bi 2 Sr 2 CaCu 2 O 8 at room temperature and 24K; in the normal and superconducting states. The three dimensional electron momentum density ρ(p) has been reconstructed by using the image reconstruction technique based on a direct Fourier transportation. The reconstructed electron momentum density ρ(p) has been reduced into the reduced electron momentum density n(k) by using the LCW folding procedure. They are compared with that for Cu and Si. The difference in the density distributions between both states was observed. This may be attributed to the smearing by the reduced thermal momenta of positrons. But there is a possibility that the difference is due to the phase transition

New hybrid lead iodides: From one-dimensional chain to two-dimensional layered perovskite structure

International Nuclear Information System (INIS)

Xiong, Kecai; Liu, Wei; Teat, Simon J.; An, Litao; Wang, Hao; Emge, Thomas J.; Li, Jing

2015-01-01

Two new hybrid lead halides (H 2 BDA)[PbI 4 ] (1) (H 2 BDA=1,4-butanediammonium dication) and (HNPEIM)[PbI 3 ] (2) (HNPEIM=N-​phenyl-ethanimidamidine cation) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that compound 1 features a two-dimensional corner-sharing perovskite layer whereas compound 2 contains one-dimensional edge-sharing double chains. The N-​phenyl-ethanimidamidine cation within compound 2 was generated in-situ under solvothermal conditions. The optical absorption spectra collected at room temperature suggest that both compounds are semiconductors having direct band gaps, with estimated values of 2.64 and 2.73 eV for 1 and 2, respectively. Results from the density functional theory (DFT) calculations are consistent with the experimental data. Density of states (DOS) analysis reveals that in both compounds 1 and 2, the energy states in the valence band maximum region are iodine 5p atomic orbitals with a small contribution from lead 6s, while in the region of conduction band minimum, the major contributions are from the inorganic (Pb 6p atomic orbitals) and organic components (C and N 2p atomic orbitals) in compound 1 and 2, respectively. - Graphical abstract: Two new hybrid lead halides built on one-dimensional edge-sharing double chains and two-dimensional corner-sharing perovskite layers are synthesized and their structural and electronic properties are analyzed. - Highlights: • Two new hybrid lead iodides are designed, synthesized, and characterized. • They are closely related to, but different from, perovskite structures. • The electronic properties of both compounds are analyzed by DFT calculations

New hybrid lead iodides: From one-dimensional chain to two-dimensional layered perovskite structure

Energy Technology Data Exchange (ETDEWEB)

Xiong, Kecai; Liu, Wei [Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 (United States); Teat, Simon J. [Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States); An, Litao; Wang, Hao; Emge, Thomas J. [Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 (United States); Li, Jing, E-mail: jingli@rutgers.edu [Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 (United States)

2015-10-15

Two new hybrid lead halides (H{sub 2}BDA)[PbI{sub 4}] (1) (H{sub 2}BDA=1,4-butanediammonium dication) and (HNPEIM)[PbI{sub 3}] (2) (HNPEIM=N-​phenyl-ethanimidamidine cation) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that compound 1 features a two-dimensional corner-sharing perovskite layer whereas compound 2 contains one-dimensional edge-sharing double chains. The N-​phenyl-ethanimidamidine cation within compound 2 was generated in-situ under solvothermal conditions. The optical absorption spectra collected at room temperature suggest that both compounds are semiconductors having direct band gaps, with estimated values of 2.64 and 2.73 eV for 1 and 2, respectively. Results from the density functional theory (DFT) calculations are consistent with the experimental data. Density of states (DOS) analysis reveals that in both compounds 1 and 2, the energy states in the valence band maximum region are iodine 5p atomic orbitals with a small contribution from lead 6s, while in the region of conduction band minimum, the major contributions are from the inorganic (Pb 6p atomic orbitals) and organic components (C and N 2p atomic orbitals) in compound 1 and 2, respectively. - Graphical abstract: Two new hybrid lead halides built on one-dimensional edge-sharing double chains and two-dimensional corner-sharing perovskite layers are synthesized and their structural and electronic properties are analyzed. - Highlights: • Two new hybrid lead iodides are designed, synthesized, and characterized. • They are closely related to, but different from, perovskite structures. • The electronic properties of both compounds are analyzed by DFT calculations.

Identification of Tight-Binding Plasmepsin II and Falcipain 2 Inhibitors in Aqueous Extracts of Marine Invertebrates by the Combination of Enzymatic and Interaction-Based Assays

Science.gov (United States)

Salas-Sarduy, Emir; Guerra, Yasel; Covaleda Cortés, Giovanni; Avilés, Francesc Xavier; Chávez Planes, María A.

2017-01-01

Natural products from marine origin constitute a very promising and underexplored source of interesting compounds for modern biotechnological and pharmaceutical industries. However, their evaluation is quite challenging and requires specifically designed assays to reliably identify the compounds of interest in a highly heterogeneous and interfering context. In the present study, we describe a general strategy for the confident identification of tight-binding protease inhibitors in the aqueous extracts of 62 Cuban marine invertebrates, using Plasmodium falciparum hemoglobinases Plasmepsin II and Falcipain 2 as model enzymes. To this end, we first developed a screening strategy that combined enzymatic with interaction-based assays and then validated screening conditions using five reference extracts. Interferences were evaluated and minimized. The results from the massive screening of such extracts, the validation of several hits by a variety of interaction-based assays and the purification and functional characterization of PhPI, a multifunctional and reversible tight-binding inhibitor for Plasmepsin II and Falcipain 2 from the gorgonian Plexaura homomalla, are presented. PMID:28430158

Two-dimensional and relativistic effects in lower-hybrid current drive

International Nuclear Information System (INIS)

Hewett, D.; Hizanidis, K.; Krapchev, V.; Bers, A.

1983-06-01

We present new numerical and analytic solutions of the two-dimensional Fokker-Planck equation supplemented by a parallel quasilinear diffusion term. The results show a large enhancement of the perpendicular temperature of both the electrons resonant with the applied RF fields and the more energetic electrons in the tail. Both the RF-generated current and power dissipated are substantially increased by the perpendicular energy broadening in the resonant region. In the presence of a small DC electric field the RF current generated is very much enhanced, much more than in a simple additive fashion. In addition, we present a relativistic formulation of the two-dimensional Fokker-Planck quasilinear equation. From conservation equations, based upon this formulation, we derive the characteristics of RF current drive with energetic electrons. These show how the RF-driven current and its figure of merit (I/P/sub d/) increase with the energy of the current-carrying electrons, and that their perpendicular, random momentum must also increase

ONE-DIMENSIONAL AND TWO-DIMENSIONAL LEADERSHIP STYLES

Directory of Open Access Journals (Sweden)

Nikola Stefanović

2007-06-01

Full Text Available In order to motivate their group members to perform certain tasks, leaders use different leadership styles. These styles are based on leaders' backgrounds, knowledge, values, experiences, and expectations. The one-dimensional styles, used by many world leaders, are autocratic and democratic styles. These styles lie on the two opposite sides of the leadership spectrum. In order to precisely define the leadership styles on the spectrum between the autocratic leadership style and the democratic leadership style, leadership theory researchers use two dimensional matrices. The two-dimensional matrices define leadership styles on the basis of different parameters. By using these parameters, one can identify two-dimensional styles.

The one-particle scenario for the metal-insulator transition in two-dimensional systems at T = 0

CERN Document Server

Tarasov, Y V

2003-01-01

The conductance of bounded disordered electron systems is calculated by reducing the original dynamic problem of arbitrary dimensionality to a set of strictly one-dimensional problems for one-particle mode propagators. The metallic ground state of a two-dimensional conductor, which is considered as a limiting case of three-dimensional quantum waveguide, is shown to result from its multi-modeness. As the waveguide thickness is reduced, e.g., by applying a 'pressing' potential, the electron system undergoes a set of continuous phase transitions related to discrete variations of the number of extended modes. The closing of the last current carrying mode is regarded as a phase transition of the electron system from metallic to dielectric state. The obtained results agree qualitatively with the observed 'anomalies' of resistivity of different two-dimensional electron and hole systems.

Directional detection of dark matter with two-dimensional targets

Science.gov (United States)

Hochberg, Yonit; Kahn, Yonatan; Lisanti, Mariangela; Tully, Christopher G.; Zurek, Kathryn M.

2017-09-01

We propose two-dimensional materials as targets for direct detection of dark matter. Using graphene as an example, we focus on the case where dark matter scattering deposits sufficient energy on a valence-band electron to eject it from the target. We show that the sensitivity of graphene to dark matter of MeV to GeV mass can be comparable, for similar exposure and background levels, to that of semiconductor targets such as silicon and germanium. Moreover, a two-dimensional target is an excellent directional detector, as the ejected electron retains information about the angular dependence of the incident dark matter particle. This proposal can be implemented by the PTOLEMY experiment, presenting for the first time an opportunity for directional detection of sub-GeV dark matter.

Probing temperature-driven flow lines in a gated two-dimensional electron gas with tunable spin-splitting

International Nuclear Information System (INIS)

Wang, Yi-Ting; Huang, C F; Chen, Wei-Jen; Chang, Y H; Liang, C-T; Kim, Gil-Ho; Lo, Shun-Tsung; Nicholls, J T; Lin, Li-Hung; Ritchie, D A; Dolan, B P

2012-01-01

We study the temperature flow of conductivities in a gated GaAs two-dimensional electron gas (2DEG) containing self-assembled InAs dots and compare the results with recent theoretical predictions. By changing the gate voltage, we are able to tune the 2DEG density and thus vary disorder and spin-splitting. Data for both the spin-resolved and spin-degenerate phase transitions are presented, the former collapsing to the latter with decreasing gate voltage and/or decreasing spin-splitting. The experimental results support a recent theory, based on modular symmetry, which predicts how the critical Hall conductivity varies with spin-splitting.

Doppler Velocimetry of Current Driven Spin Helices in a Two-Dimensional Electron Gas

Energy Technology Data Exchange (ETDEWEB)

Yang, Luyi [Univ. of California, Berkeley, CA (United States)

2013-05-17

Spins in semiconductors provide a pathway towards the development of spin-based electronics. The appeal of spin logic devices lies in the fact that the spin current is even under time reversal symmetry, yielding non-dissipative coupling to the electric field. To exploit the energy-saving potential of spin current it is essential to be able to control it. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices. This dissertation presents the demonstration and application of a powerful new optical technique, Doppler spin velocimetry, for probing the motion of spin polarization at the level of 1 nm on a picosecond time scale. We discuss experiments in which this technique is used to measure the motion of spin helices in high mobility n-GaAs quantum wells as a function of temperature, in-plane electric field, and photoinduced spin polarization amplitude. We find that the spin helix velocity changes sign as a function of wave vector and is zero at the wave vector that yields the largest spin lifetime. This observation is quite striking, but can be explained by the random walk model that we have developed. We discover that coherent spin precession within a propagating spin density wave is lost at temperatures near 150 K. This finding is critical to understanding why room temperature operation of devices based on electrical gate control of spin current has so far remained elusive. We report that, at all temperatures, electron spin polarization co-propagates with the high-mobility electron sea, even when this requires an unusual form of separation of spin density from photoinjected electron density. Furthermore, although the spin packet co-propagates with the two-dimensional electron gas, spin diffusion is strongly

Tuning spin transport across two-dimensional organometallic junctions

Science.gov (United States)

Liu, Shuanglong; Wang, Yun-Peng; Li, Xiangguo; Fry, James N.; Cheng, Hai-Ping

2018-01-01

We study via first-principles modeling and simulation two-dimensional spintronic junctions made of metal-organic frameworks consisting of two Mn-phthalocyanine ferromagnetic metal leads and semiconducting Ni-phthalocyanine channels of various lengths. These systems exhibit a large tunneling magnetoresistance ratio; the transmission functions of such junctions can be tuned using gate voltage by three orders of magnitude. We find that the origin of this drastic change lies in the orbital alignment and hybridization between the leads and the center electronic states. With physical insight into the observed on-off phenomenon, we predict a gate-controlled spin current switch based on two-dimensional crystallines and offer general guidelines for designing spin junctions using 2D materials.

Controlled Interactions between Two Dimensional Layered Inorganic Nanosheets and Polymers

Science.gov (United States)

2016-06-15

AFRL-AFOSR-JP-TR-2016-0071 Controlled Interactions between Two Dimensional Layered Inorganic Nanosheets and Polymers Cheolmin Park YONSEI UNIVERSITY...Interactions between Two Dimensional Layered Inorganic Nanosheets and Polymers 5a.  CONTRACT NUMBER 5b.  GRANT NUMBER FA2386-14-1-4054 5c.  PROGRAM ELEMENT...prospects for a variety of emerging applications in a broad range of fields, such as electronics, energy conversion and storage, catalysis and polymer

Imaginary time density-density correlations for two-dimensional electron gases at high density

Energy Technology Data Exchange (ETDEWEB)

Motta, M.; Galli, D. E. [Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano (Italy); Moroni, S. [IOM-CNR DEMOCRITOS National Simulation Center and SISSA, Via Bonomea 265, 34136 Trieste (Italy); Vitali, E. [Department of Physics, College of William and Mary, Williamsburg, Virginia 23187-8795 (United States)

2015-10-28

We evaluate imaginary time density-density correlation functions for two-dimensional homogeneous electron gases of up to 42 particles in the continuum using the phaseless auxiliary field quantum Monte Carlo method. We use periodic boundary conditions and up to 300 plane waves as basis set elements. We show that such methodology, once equipped with suitable numerical stabilization techniques necessary to deal with exponentials, products, and inversions of large matrices, gives access to the calculation of imaginary time correlation functions for medium-sized systems. We discuss the numerical stabilization techniques and the computational complexity of the methodology and we present the limitations related to the size of the systems on a quantitative basis. We perform the inverse Laplace transform of the obtained density-density correlation functions, assessing the ability of the phaseless auxiliary field quantum Monte Carlo method to evaluate dynamical properties of medium-sized homogeneous fermion systems.

Ferromagnetism in the two-dimensional periodic Anderson model

International Nuclear Information System (INIS)

Batista, C. D.; Bonca, J.; Gubernatis, J. E.

2001-01-01

Using the constrained-path Monte Carlo method, we studied the magnetic properties of the two-dimensional periodic Anderson model for electron fillings between 1/4 and 1/2. We also derived two effective low-energy theories to assist in interpreting the numerical results. For 1/4 filling, we found that the system can be a Mott or a charge-transfer insulator, depending on the relative values of the Coulomb interaction and the charge-transfer gap between the two noninteracting bands. The insulator may be a paramagnet or antiferromagnet. We concentrated on the effect of electron doping on these insulating phases. Upon doping we obtained a partially saturated ferromagnetic phase for low concentrations of conduction electrons. If the system were a charge-transfer insulator, we would find that the ferromagnetism is induced by the well-known Ruderman-Kittel-Kasuya-Yosida interaction. However, we found a novel correlated hopping mechanism inducing the ferromagnetism in the region where the nondoped system is a Mott insulator. Our regions of ferromagnetism spanned a much smaller doping range than suggested by recent slave boson and dynamical mean-field theory calculations, but they were consistent with that obtained by density-matrix renormalization group calculations of the one-dimensional periodic Anderson model

The Interplay of Rashba Spin-Orbit Interaction and Landau Level Broadening on a Two-Dimensional Electron Gas Under a Tilted Magnetic Field

International Nuclear Information System (INIS)

Gammag, Rayda; Villagonzalo, Cristine

2012-01-01

A two-dimensional electron gas in a tilted magnetic field with Rashba spin-orbit interaction (RSOI) was studied. The RSOI is accredited to the asymmetry of the heterostructure where the two-dimensional electron gas is found. The effects of the disorder-attributed Landau level broadening and the RSOI on the spin splitting were identified by simulating the density of states which was assumed to take a Gaussian shape. Increased Landau level broadening obscures the spin splitting and increases the overlap between spin states resulting to stout Gaussian peaks. On the other hand, stronger RSOI amplifies the splitting and lessens the overlap between spin states of the Landau levels. The splitting, however, results to stouter peaks. The similarity in the RSOI and Landau level broadening effects can be explained by recognizing that the asymmetry of the heterostructure is in itself a form of structural disorder.

Two- and three dimensional electrons and photons and their supersymmetric partners

International Nuclear Information System (INIS)

Steringa, J.J.

1989-01-01

This thesis contains a study of supersymmetric gauge theories in two and tree spacetime dimensions. Supersymmetric gauge theories in less than four spacetime dimensions are useful for trying out field theoretical methods which ultimately will be applied to realistic models. In ch. 1 all the aspects of field theory that are necessary for later chapters are treated. In ch. 2 sypersymmetry in two- and three-dimensional space time is treated, and superfields and superspace techniques are introduced. With these a simple Abelian supersymmetric gauge theory in two spacetime dimensions is constructed, the Schwinger model. Ch. 3 deals with general properties and a perturbative analysis of the model. Ch. 4 contains a non-perturbative analysis by means of Dyson-Schwinger equations. A supersummetric extension of theSalam-Delbourgo Gauge Technique is presented and is applied with some seccess to the supersymmetric Schwinger model. In ch. 5 prperties of three-dimensional supersymmetric gauge theories are investigated. (author). 55 refs.; 7 figs.; schemes

High mobility two-dimensional electron gases in nitride heterostructures with high Al composition AlGaN alloy barriers

International Nuclear Information System (INIS)

Li Guowang; Cao Yu; Xing Huili Grace; Jena, Debdeep

2010-01-01

We report high-electron mobility nitride heterostructures with >70% Al composition AlGaN alloy barriers grown by molecular beam epitaxy. Direct growth of such AlGaN layers on GaN resulted in hexagonal trenches and a low mobility polarization-induced charge. By applying growth interruption at the heterojunction, the surface morphology improved dramatically and the room temperature two-dimensional electron gas (2DEG) mobility increased by an order of magnitude, exceeding 1300 cm 2 /V s. The 2DEG density was tunable at 0.4-3.7x10 13 /cm 2 by varying the total barrier thickness (t). Surface barrier heights of the heterostructures were extracted and exhibited dependence on t.

Electrostatic modulation of periodic potentials in a two-dimensional electron gas: From antidot lattice to quantum dot lattice

Energy Technology Data Exchange (ETDEWEB)

Goswami, Srijit; Aamir, Mohammed Ali; Shamim, Saquib; Ghosh, Arindam [Department of Physics, Indian Institute of Science, Bangalore 560 012 (India); Siegert, Christoph; Farrer, Ian; Ritchie, David A. [Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Pepper, Michael [Department of Electrical and Electronic Engineering, University College, London WC1E 7JE (United Kingdom)

2013-12-04

We use a dual gated device structure to introduce a gate-tuneable periodic potential in a GaAs/AlGaAs two dimensional electron gas (2DEG). Using only a suitable choice of gate voltages we can controllably alter the potential landscape of the bare 2DEG, inducing either a periodic array of antidots or quantum dots. Antidots are artificial scattering centers, and therefore allow for a study of electron dynamics. In particular, we show that the thermovoltage of an antidot lattice is particularly sensitive to the relative positions of the Fermi level and the antidot potential. A quantum dot lattice, on the other hand, provides the opportunity to study correlated electron physics. We find that its current-voltage characteristics display a voltage threshold, as well as a power law scaling, indicative of collective Coulomb blockade in a disordered background.

Electrostatic modulation of periodic potentials in a two-dimensional electron gas: From antidot lattice to quantum dot lattice

International Nuclear Information System (INIS)

Goswami, Srijit; Aamir, Mohammed Ali; Shamim, Saquib; Ghosh, Arindam; Siegert, Christoph; Farrer, Ian; Ritchie, David A.; Pepper, Michael

2013-01-01

We use a dual gated device structure to introduce a gate-tuneable periodic potential in a GaAs/AlGaAs two dimensional electron gas (2DEG). Using only a suitable choice of gate voltages we can controllably alter the potential landscape of the bare 2DEG, inducing either a periodic array of antidots or quantum dots. Antidots are artificial scattering centers, and therefore allow for a study of electron dynamics. In particular, we show that the thermovoltage of an antidot lattice is particularly sensitive to the relative positions of the Fermi level and the antidot potential. A quantum dot lattice, on the other hand, provides the opportunity to study correlated electron physics. We find that its current-voltage characteristics display a voltage threshold, as well as a power law scaling, indicative of collective Coulomb blockade in a disordered background

Interaction of a two-dimensional electromagnetic breather with an electron inhomogeneity in an array of carbon nanotubes

International Nuclear Information System (INIS)

Zhukov, Alexander V.; Bouffanais, Roland; Fedorov, E. G.; Belonenko, Mikhail B.

2014-01-01

Propagation of ultrashort laser pulses through various nano-objects has recently became an attractive topic for both theoretical and experimental studies due to its promising perspectives in a variety of problems of modern nanoelectronics. Here, we study the propagation of extremely short two-dimensional bipolar electromagnetic pulses in a heterogeneous array of semiconductor carbon nanotubes. Heterogeneity is defined as a region of enhanced electron density. The electromagnetic field in an array of nanotubes is described by Maxwell's equations, reduced to a multidimensional wave equation. Our numerical analysis shows the possibility of stable propagation of an electromagnetic pulse in a heterogeneous array of nanotubes. Furthermore, we establish that, depending on its speed of propagation, the pulse can pass through the area of increased electron concentration or be reflected therefrom.

Structural consequences of cutting a binding loop: two circularly permuted variants of streptavidin

International Nuclear Information System (INIS)

Le Trong, Isolde; Chu, Vano; Xing, Yi; Lybrand, Terry P.; Stayton, Patrick S.; Stenkamp, Ronald E.

2013-01-01

The crystal structures of two circularly permuted streptavidins probe the role of a flexible loop in the tight binding of biotin. Molecular-dynamics calculations for one of the mutants suggests that increased fluctuations in a hydrogen bond between the protein and biotin are associated with cleavage of the binding loop. Circular permutation of streptavidin was carried out in order to investigate the role of a main-chain amide in stabilizing the high-affinity complex of the protein and biotin. Mutant proteins CP49/48 and CP50/49 were constructed to place new N-termini at residues 49 and 50 in a flexible loop involved in stabilizing the biotin complex. Crystal structures of the two mutants show that half of each loop closes over the binding site, as observed in wild-type streptavidin, while the other half adopts the open conformation found in the unliganded state. The structures are consistent with kinetic and thermodynamic data and indicate that the loop plays a role in enthalpic stabilization of the bound state via the Asn49 amide–biotin hydrogen bond. In wild-type streptavidin, the entropic penalties of immobilizing a flexible portion of the protein to enhance binding are kept to a manageable level by using a contiguous loop of medium length (six residues) which is already constrained by its anchorage to strands of the β-barrel protein. A molecular-dynamics simulation for CP50/49 shows that cleavage of the binding loop results in increased structural fluctuations for Ser45 and that these fluctuations destabilize the streptavidin–biotin complex

Two-dimensional NMR spectrometry

International Nuclear Information System (INIS)

Farrar, T.C.

1987-01-01

This article is the second in a two-part series. In part one (ANALYTICAL CHEMISTRY, May 15) the authors discussed one-dimensional nuclear magnetic resonance (NMR) spectra and some relatively advanced nuclear spin gymnastics experiments that provide a capability for selective sensitivity enhancements. In this article and overview and some applications of two-dimensional NMR experiments are presented. These powerful experiments are important complements to the one-dimensional experiments. As in the more sophisticated one-dimensional experiments, the two-dimensional experiments involve three distinct time periods: a preparation period, t 0 ; an evolution period, t 1 ; and a detection period, t 2

Electron transport in a double quantum ring: Evidence of an AND gate

International Nuclear Information System (INIS)

Maiti, Santanu K.

2009-01-01

We explore AND gate response in a double quantum ring where each ring is threaded by a magnetic flux φ. The double quantum ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, V a and V b , are applied, respectively, in the lower arms of the two rings which are treated as two inputs of the AND gate. The system is described in the tight-binding framework and the calculations are done using the Green's function formalism. Here we numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our study suggests that, for a typical value of the magnetic flux φ=φ 0 /2 (φ 0 =ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears only if both the two inputs to the gate are high (1), while if neither or only one input to the gate is high (1), a low output current (0) results. It clearly demonstrates the AND gate behavior and this aspect may be utilized in designing an electronic logic gate.

Modeling of full-Heusler alloys within tight-binding approximation: Case study of Fe2MnAl

Science.gov (United States)

Azhar, A.; Majidi, M. A.; Nanto, D.

2017-07-01

Heusler alloys have been known for about a century, and predictions of magnetic moment values using Slater-Pauling rule have been successful for many such materials. However, such a simple counting rule has been found not to always work for all Heusler alloys. For instance, Fe2CuAl has been found to have magnetic moment of 3.30 µB per formula unit although the Slater-Pauling rule suggests the value of 2 µB. On the other hand, a recent experiment shows that a non-stoichiometric Heusler compound Fe2Mn0.5Cu0.5Al possesses magnetic moment of 4 µB, closer to the Slater-Pauling prediction for the stoichiometric compound. Such discrepancies signify that the theory to predict the magnetic moment of Heusler alloys in general is still far from being complete. Motivated by this issue, we propose to do a theoretical study on a full-Heusler alloy Fe2MnAl to understand the formation of magnetic moment microscopically. We model the system by constructing a density-functional-theory-based tight-binding Hamiltonian and incorporating Hubbard repulsive as well as spin-spin interactions for the electrons occupying the d-orbitals. Then, we solve the model using Green's function approach, and treat the interaction terms within the mean-field approximation. At this stage, we aim to formulate the computational algorithm for the overall calculation process. Our final goal is to compute the total magnetic moment per unit cell of this system and compare it with the experimental data.

Novel effects of strains in graphene and other two dimensional materials

Energy Technology Data Exchange (ETDEWEB)

Amorim, B., E-mail: amorim.bac@icmm.csic.es [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); Department of Physics and Center of Physics, University of Minho, P-4710-057, Braga (Portugal); Cortijo, A. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); Juan, F. de [Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, CA 94720 (United States); Department of Physics, University of California, Berkeley, CA 94720 (United States); Grushin, A.G. [Max-Planck-Institut fur Physik komplexer Systeme, 01187 Dresden (Germany); Guinea, F. [School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL (United Kingdom); IMDEA Nanociencia Calle de Faraday, 9, Cantoblanco, 28049, Madrid (Spain); Donostia International Physics Center (DIPC), 20018 San Sebastián (Spain); Gutiérrez-Rubio, A. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); Ochoa, H. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); Donostia International Physics Center (DIPC), 20018 San Sebastián (Spain); Parente, V. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); IMDEA Nanociencia Calle de Faraday, 9, Cantoblanco, 28049, Madrid (Spain); Roldán, R.; San-Jose, P.; Schiefele, J. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain); Sturla, M. [IFLP-CONICET. Departamento de Física, Universidad Nacional de La Plata, (1900) La Plata (Argentina); Vozmediano, M.A.H. [Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid (Spain)

2016-03-03

The analysis of the electronic properties of strained or lattice deformed graphene combines ideas from classical condensed matter physics, soft matter, and geometrical aspects of quantum field theory (QFT) in curved spaces. Recent theoretical and experimental work shows the influence of strains in many properties of graphene not considered before, such as electronic transport, spin–orbit coupling, the formation of Moiré patterns and optics. There is also significant evidence of anharmonic effects, which can modify the structural properties of graphene. These phenomena are not restricted to graphene, and they are being intensively studied in other two dimensional materials, such as the transition metal dichalcogenides. We review here recent developments related to the role of strains in the structural and electronic properties of graphene and other two dimensional compounds.

Empirical pseudo-potential studies on electronic structure

Indian Academy of Sciences (India)

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the ...

Tunneling conductance of a two-dimensional electron gas with Dresselhaus spin-orbit coupling

International Nuclear Information System (INIS)

Srisongmuang, B.; Ka-oey, A.

2012-01-01

We theoretically studied the spin-dependent charge transport in a two-dimensional electron gas with Dresselhaus spin-orbit coupling (DSOC) and metal junctions. It is shown that the DSOC energy can be directly measured from the tunneling conductance spectrum. We found that spin polarization of the conductance in the propagation direction can be obtained by injecting from the DSOC system. We also considered the effect of the interfacial scattering barrier (both spin-flip and non-spin-flip scattering) on the overall conductance and the spin polarization of the conductance. It is found that the increase of spin-flip scattering can enhance the conductance under certain conditions. Moreover, both types of scattering can increase the spin polarization below the branches crossing of the energy band. - Highlights: → DSOC energy can be directly measured from tunneling conductance spectrum. → Spin polarization of conductance in the propagation direction can be obtained by injecting from DSOC system. → Both types of scattering can increase spin polarization.

Towards molecular doping effect on the electronic properties of two-dimensional layered materials

International Nuclear Information System (INIS)

Arramel; Wang, Q.; Zheng, Y.; Zhang, W.; Wee, A. T. S.

2016-01-01

In recent advancements of an atomically-thick, flat, and flexible two-dimensional (2D) material has attracted tremendous interest. Graphene and 2D layered semiconductors such as transition-metal dichalcogenides (TMDs) pave the way on the exploration of their unique layer-number dependent electronic and optical properties. The latter have a promising future on the microelectronics due to their sizeable bandgaps, i.e., the crossover from indirect-direct bandgap transition occurs as the thickness of TMDs is decreased to a monolayer. In this work, we systematically investigated the optimum growth parameter of chemical vapor deposition of MoS2 and WSe2, respectively. It turns out that the temperature and the duration growth plays role to produce a large area of TMDs monolayers. Our studies suggest that a well-controlled high quality of TMDs could serves as template and interlayer in the TMD-organic heterointerfaces. Thus it is potentially an attractive approach towards a wide-ranging application in optoelectronics, nanoelectronics and energy-harvesting applications. (paper)

Induced magnetization spiral in a nonmagnetic metal sandwiched between two ferromagnets

CERN Document Server

Mathon, J; Villeret, M; Muniz, R B; Edwards, D M

2000-01-01

Calculation of the magnetic moment induced in a non-magnetic metal, sandwiched between two ferromagnets with magnetizations at an arbitrary angle, is reported. It is found that the induced magnetization rotates along a complex three-dimensional spiral and can undergo many complete 360 deg. rotations. A simple free-electron model is used to derive an analytic formula for the twist angle phi inside the spacer. This demonstrates that, contrary to the behavior of magnetization inside a domain wall in a ferromagnet, phi varies non-uniformly inside the spacer and exhibits plateaus of almost constant rotation separated by regions of sharp rotations by large angles. The calculation is extended to the case of a realistic Co/Cu/Co(0 0 1) trilayer described by s, p, d tight-binding bands fitted to an ab initio band structure. An analytic formula for the components of the induced moment (and hence, for phi) is derived using the stationary phase approximation. Its validity is tested against a fully numerical calculation u...

Solitary wave solutions of two-dimensional nonlinear Kadomtsev ...

Indian Academy of Sciences (India)

Aly R Seadawy

2017-09-13

Sep 13, 2017 ... We considered the two-dimensional DASWs in colli- sionless, unmagnetized cold plasma consisting of dust fluid, ions and electrons. The dynamics of DASWs is governed by the normalized fluid equations of nonlin- ear continuity (1), nonlinear motion of system (2) and. (3) and linear Poisson equation (4) as.

Solution-Based Processing and Applications of Two-Dimensional Heterostructures

Science.gov (United States)

Hersam, Mark

Two-dimensional materials have emerged as promising candidates for next-generation electronics and optoelectronics, but advances in scalable nanomanufacturing are required to exploit this potential in real-world technology. This talk will explore methods for improving the uniformity of solution-processed two-dimensional materials with an eye toward realizing dispersions and inks that can be deposited into large-area thin-films. In particular, density gradient ultracentrifugation allows the solution-based isolation of graphene, boron nitride, montmorillonite, and transition metal dichalcogenides (e.g., MoS2, WS2, ReS2, MoSe2, WSe2) with homogeneous thickness down to the atomically thin limit. Similarly, two-dimensional black phosphorus is isolated in organic solvents or deoxygenated aqueous surfactant solutions with the resulting phosphorene nanosheets showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes. By adding cellulosic polymer stabilizers to these dispersions, the rheological properties can be tuned by orders of magnitude, thereby enabling two-dimensional material inks that are compatible with a range of additive manufacturing methods including inkjet, gravure, screen, and 3D printing. The resulting solution-processed two-dimensional heterostructures show promise in several device applications including photodiodes, anti-ambipolar transistors, gate-tunable memristors, and heterojunction photovoltaics.

Electronic transport of molecular nanowires by considering of electron hopping energy between the second neighbors

Directory of Open Access Journals (Sweden)

H Rabani

2015-07-01

Full Text Available In this paper, we study the electronic conductance of molecular nanowires by considering the electron hopping between the first and second neighbors with the help Green’s function method at the tight-binding approach. We investigate three types of structures including linear uniform and periodic chains as well as poly(p-phenylene molecule which are embedded between two semi-infinite metallic leads. The results show that in the second neighbor approximation, the resonance, anti-resonance and Fano phenomena occur in the conductance spectra of these structures. Moreover, a new gap is observed at edge of the lead energy band wich its width depends on the value of the electron hopping energy between the second neighbors. In the systems including intrinsic gap, this hopping energy shifts the gap in the energy spectra.

Acoustic phonon emission by two dimensional plasmons

International Nuclear Information System (INIS)

Mishonov, T.M.

1990-06-01

Acoustic wave emission of the two dimensional plasmons in a semiconductor or superconductor microstructure is investigated by using the phenomenological deformation potential within the jellium model. The plasmons are excited by the external electromagnetic (e.m.) field. The power conversion coefficient of e.m. energy into acoustic wave energy is also estimated. It is shown, the coherent transformation has a sharp resonance at the plasmon frequency of the two dimensional electron gas (2DEG). The incoherent transformation of the e.m. energy is generated by ohmic dissipation of 2DEG. The method proposed for coherent phonon beam generation can be very effective for high mobility 2DEG and for thin superconducting layers if the plasmon frequency ω is smaller than the superconducting gap 2Δ. (author). 21 refs, 1 fig

Benchmarking density functional tight binding models for barrier heights and reaction energetics of organic molecules.

Science.gov (United States)

Gruden, Maja; Andjeklović, Ljubica; Jissy, Akkarapattiakal Kuriappan; Stepanović, Stepan; Zlatar, Matija; Cui, Qiang; Elstner, Marcus

2017-09-30

Density Functional Tight Binding (DFTB) models are two to three orders of magnitude faster than ab initio and Density Functional Theory (DFT) methods and therefore are particularly attractive in applications to large molecules and condensed phase systems. To establish the applicability of DFTB models to general chemical reactions, we conduct benchmark calculations for barrier heights and reaction energetics of organic molecules using existing databases and several new ones compiled in this study. Structures for the transition states and stable species have been fully optimized at the DFTB level, making it possible to characterize the reliability of DFTB models in a more thorough fashion compared to conducting single point energy calculations as done in previous benchmark studies. The encouraging results for the diverse sets of reactions studied here suggest that DFTB models, especially the most recent third-order version (DFTB3/3OB augmented with dispersion correction), in most cases provide satisfactory description of organic chemical reactions with accuracy almost comparable to popular DFT methods with large basis sets, although larger errors are also seen for certain cases. Therefore, DFTB models can be effective for mechanistic analysis (e.g., transition state search) of large (bio)molecules, especially when coupled with single point energy calculations at higher levels of theory. © 2017 Wiley Periodicals, Inc. © 2017 Wiley Periodicals, Inc.

Lanthanide 4f-electron binding energies and the nephelauxetic effect in wide band gap compounds

International Nuclear Information System (INIS)

Dorenbos, Pieter

2013-01-01

Employing data from luminescence spectroscopy, the inter 4f-electron Coulomb repulsion energy U(6, A) in Eu 2+/3+ impurities together with the 5d-centroid energy shift ϵ c (1,3+,A) in Ce 3+ impurities in 40 different fluoride, chloride, bromide, iodide, oxide, sulfide, and nitride compounds has been determined. This work demonstrates that the chemical environment A affects the two energies in a similar fashion; a fashion that follows the anion nephelauxetic sequence F, O, Cl, Br, N, I, S, Se. One may then calculate U(6, A) from well established and accurate ϵ c (1,3+,A) values which are then used as input to the chemical shift model proposed in Dorenbos (2012) [19]. As output it provides the chemical shift of 4f-electron binding energy and therewith the 4f-electron binding energy relative to the vacuum energy. In addition this method provides a tool to routinely establish the binding energy of electrons at the top of the valence band (work function) and the bottom of the conduction band (electron affinity) throughout the entire family of inorganic compounds. How the electronic structure of the compound and lanthanide impurities therein change with type of compound and type of lanthanide is demonstrated. -- Highlights: ► A relationship between 5d centroid shift and 4f-electron Coulomb repulsion energy is established. ► Information on the absolute 4f-electron binding energy of lanthanides in 40 compounds is provided. ► A new tool to determine absolute binding energies of electrons in valence and conduction bands is demonstrated

Triazatriangulene as binding group for molecular electronics

DEFF Research Database (Denmark)

Wei, Zhongming; Wang, Xintai; Borges, Anders

2014-01-01

The triazatriangulene (TATA) ring system was investigated as a binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated, and their electrical conductance was recorded ...... with its high stability and directionality make this binding group very attractive for molecular electronic measurements and devices. (Figure Presented)....

A tight-binding model of the transmission probability through a molecular junction; a single molecule vs. a molecular layer

International Nuclear Information System (INIS)

Landau, A.; Nitzan, A.

2006-01-01

Full Text: Molecular electronics, one of the major fields of the current effort in nano-science, may be de ed as the study of electronic behaviors, devices and applications that depend on the properties of matter at the molecular scale. If the miniaturization trend of microelectronic devices is to continue, elements such as transistors and contacts will soon shrink to single molecules. The promise of these new technological breakthroughs has been major driving force in this ld. Moreover, the consideration of molecular systems as electronic devices has raised new fundamental questions. In particular, while traditional quantum chemistry deals with electronically closed systems, we now face problems involving molecular systems that are open to their electronic environment, moreover, function in far from equilibrium situations. A generic molecular junction is made of two electrodes connected by a molecular spacer that takes the form of a molecular chain of varying length or a molecular layer of varying thickness. We use a simple nearest-neighbors tight-biding model with the non-equilibrium Green's function (NEGF) method to investigate and compare between a self-assembled monolayer (SAM), finite molecular layer (FML), and single molecule (SM) chemisorption to a surface of a metal substrate. In addition, we examine the difference in the transmission probability through a SAM, FML and SM sandwiched between two metallic electrodes. Dramatic differences are observed between the SM, FML and SAM density of electronic states and transmission functions. In addition, we analyze the effects of changing different physical parameters such as molecule-substrate interaction, molecule-molecule interactions, etc; interesting effects that pertain to the conduction properties of single molecules and molecular layers are observed. Intriguing results are attained when we investigate the commensurability of the SAM with the metallic surface

All-optical evaluation of spin-orbit interaction based on diffusive spin motion in a two-dimensional electron gas