Spin Flipping and Polarization Lifetimes of a 270 MeV Deuteron Beam

International Nuclear Information System (INIS)

Morozov, V.S.; Crawford, M.Q.; Etienne, Z.B.; Kandes, M.C.; Krisch, A.D.; Leonova, M.A.; Sivers, D.W.; Wong, V.K.; Yonehara, K.; Anferov, V.A.; Meyer, H.O.; Schwandt, P.; Stephenson, E.J.; Przewoski, B. von

2003-01-01

We recently studied the spin flipping of a 270 MeV vertically polarized deuteron beam stored in the IUCF Cooler Ring. We swept an rf solenoid's frequency through an rf-induced spin resonance and observed the effect on the beam's vector and tensor polarizations. After optimizing the resonance crossing rate and setting the solenoid's voltage to its maximum value, we obtained a spin-flip efficiency of about 94 ± 1% for the vector polarization; we also observed a partial spin-flip of the tensor polarization. We then used the rf-induced resonance to measure the vector and tensor polarizations' lifetimes at different distances from the resonance; the polarization lifetime ratio τvector/τtensor was about 1.9 ± 0.4

Quasilinear theory of a spin-flip laser

International Nuclear Information System (INIS)

Arunasalam, V.

1973-09-01

A discussion of the nonlinear electrodynamic behavior of a gas of spin 1/2 particles in a uniform external magnetic field is presented. In particular, the quasilinear time evolution of a spin-flip laser system is examined in detail both from the point of view of the thermodynamics of negative temperature systems and the quantum kinetic methods of nonequilibrium statistical mechanics. It is shown that the quasilinear steady state of a spin-flip laser system is that state at which the populations of the spin-up and the spin-down states are equal to each other, and this quasilinear steady state is the state of minimum entropy production. The maximum output power of the spin-flip laser predicted by the theory presented in this paper is shown to be in reasonably good agreement with experimental results. The method used here is based on the general principles of nonrelativistic quantum theory and takes account of the Doppler broadening, collisional broadening, and Compton recoil effects. 30 refs., 1 fig

Single-shot quantum nondemolition measurement of a quantum-dot electron spin using cavity exciton-polaritons

Science.gov (United States)

Puri, Shruti; McMahon, Peter L.; Yamamoto, Yoshihisa

2014-10-01

We propose a scheme to perform single-shot quantum nondemolition (QND) readout of the spin of an electron trapped in a semiconductor quantum dot (QD). Our proposal relies on the interaction of the QD electron spin with optically excited, quantum well (QW) microcavity exciton-polaritons. The spin-dependent Coulomb exchange interaction between the QD electron and cavity polaritons causes the phase and intensity response of left circularly polarized light to be different than that of right circularly polarized light, in such a way that the QD electron's spin can be inferred from the response to a linearly polarized probe reflected or transmitted from the cavity. We show that with careful device design it is possible to essentially eliminate spin-flip Raman transitions. Thus a QND measurement of the QD electron spin can be performed within a few tens of nanoseconds with fidelity ˜99.95%. This improves upon current optical QD spin readout techniques across multiple metrics, including speed and scalability.

Spin-flipping a stored polarized proton beam with an rf dipole

International Nuclear Information System (INIS)

Blinov, B.B.; Derbenev, Ya.S.; Kageya, T.; Kantsyrev, D.Yu.; Krisch, A.D.; Morozov, V.S.; Sivers, D.W.; Wong, V.K.; Anferov, V.A.; Schwandt, P.; Przewoski, B. von

2000-01-01

Frequent polarization reversals, or spin-flips, of a stored polarized high-energy beam may greatly reduce systematic errors of spin asymmetry measurements in a scattering asymmetry experiment. We studied the spin-flipping of a 120 MeV horizontally-polarized proton beam stored in the IUCF Cooler Ring by ramping an rf-dipole magnet's frequency through an rf-induced depolarizing resonance in the presence of a nearly-full Siberian snake. After optimizing the frequency ramp parameters, we used multiple spin-flips to measure a spin-flip efficiency of 86.5±0.5%. The spin-flip efficiency was apparently limited by the rf-dipole's field strength. This result indicates that an efficient spin-flipping a stored polarized beam should be possible in high energy rings such as RHIC and HERA where Siberian snakes are certainly needed and only dipole rf-flipper-magnets are practical

Polarizing a stored proton beam by spin flip?

International Nuclear Information System (INIS)

Oellers, D.; Barion, L.; Barsov, S.; Bechstedt, U.; Benati, P.; Bertelli, S.; Chiladze, D.; Ciullo, G.; Contalbrigo, M.; Dalpiaz, P.F.; Dietrich, J.; Dolfus, N.; Dymov, S.; Engels, R.; Erven, W.; Garishvili, A.; Gebel, R.; Goslawski, P.

2009-01-01

We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement of the spin-flip cross section in low-energy electron-proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.

Variable-flip-angle spin-echo imaging (VFSE)

International Nuclear Information System (INIS)

Kasai, Toshifumi; Sugimura, Kazuro; Kawamitsu, Hideaki; Yasui, Kiyoshi; Ishida, Tetsuya; Tsukamoto, Tetsuji.

1990-01-01

T 2 weighted imaging provides images with high object contrast for pathologic conditions in which the water content of tissues is increased. The authors predicted theoretical analysis of the effects of changing flip angle, and analyzed the effects in MR imaging of both phantoms and humans. Variable flip angle spin echo MR imaging (VFSE) with a 1,000/80 (repetition time msec/echo time msec) can obtain T 2 weighted image when flip angle is smaller than 80 degrees. VFSE with 40 to 60 degrees flip angle have higher contrast than other flip angle images. Signal to noise ratio (S/N) of VFSE are 55% at a 30 degree, 76% at a 45 degree, 92% at a 60 degree respectively as compared with conventional spin echo image (2000/80, flip angle 90 degree). VFSE is applicable to obtain T 2 weighted image reduced imaging time. (author)

Multi spin-flip dynamics: a solution of the one-dimensional Ising model

International Nuclear Information System (INIS)

Novak, I.

1990-01-01

The Glauber dynamics of interacting Ising spins (the single spin-flip dynamics) is generalized to p spin-flip dynamics with a simultaneous flip of up to p spins in a single configuration move. The p spin-flip dynamics is studied of the one-dimensional Ising model with uniform nearest-neighbour interaction. For this case, an exact relation is given for the time dependence of magnetization. It was found that the critical slowing down in this model could be avoided when p spin-flip dynamics with p>2 was considered. (author). 17 refs

Spin-flip inelastic scattering in electron energy loss spectroscopy of a ferromagnetic metal

International Nuclear Information System (INIS)

Yin, S.; Tosatti, E.

1981-08-01

We calculate the spin polarization occuring during electron inelastic scattering from electron-hole pairs in a model ferromagnetic metal. The polarization is found to have contributions from unequal spin flip as well as non-flip energy loss rates. Our results indicate an asymmetry of the order of a few percent with parameters roughly modeling Fsub(e). The possibilities of comparison with experiments in the presence of simultaneous spin-polarizing elastic scattering are discussed. (author)

Spin Flipping in the Presence of a Full Siberian Snake

International Nuclear Information System (INIS)

Blinov, B.B.; Anferov, V.A.; Derbenev, Y.S.; Kageya, T.; Krisch, A.D.; Lorenzon, W.; Ratner, L.G.; Sivers, D.W.; Sourkont, K.V.; Wong, V.K.; Chu, C.M.; Lee, S.Y.; Rinckel, T.; Schwandt, P.; Sperisen, F.; Przewoski, B. von; Sato, H.

1998-01-01

We have demonstrated for the first time spin flipping of a polarized proton beam stored in a ring containing a nearly 100% Siberian snake; we did this using a 'snake' depolarizing resonance induced by an rf solenoid magnet. By varying the rf solenoid close-quote s ramp time, frequency range, and voltage, we reached a spin-flip efficiency of about 91% . This spin-flip efficiency was probably reduced because the horizontal stable spin direction was not perpendicular to the longitudinal field of the rf solenoid, and was possibly reduced by nearby synchrotron sideband resonances. The planned use of a vertical rf dipole may improve the spin-flip efficiency. copyright 1998 The American Physical Society

Spin flip inelastic scattering in electron energy loss spectroscopy of a ferromagnetic metal

International Nuclear Information System (INIS)

Bocchetta, C.J.; Tosatti, E.; Yin, S.

1986-11-01

A model ferromagnetic metal is used to calculate the spin-polarization which occurs during inelastic electron-metal scattering with the production of an electron-hole pair. The polarization is found to have contributions from unequal spin-flip as well as non-flip energy loss rates. Our results indicate an asymmetry of the order of a few percent with parameters roughly modelling iron. (author)

Spin-dependent exciton-exciton interaction potential in two- and three-dimensional structure semiconductors under excitation

International Nuclear Information System (INIS)

Nguyen Ba An; Hoang Ngoc Cam; Nguyen Trung Dan

1990-08-01

Analytical expressions of the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D structure materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass ratio and two-exciton state spin symmetry. Numerical calculations show that the character of the exciton-exciton interaction is determined by all of the above-mentioned dependences. (author). 32 refs, 7 figs

Spin-flip processes in low-energy Fe17+ + He collisions

International Nuclear Information System (INIS)

Bruch, R.; Altick, P.L.; Rauscher, E.; Wang, H.; Schneider, D.

1993-01-01

Spin-nonconserving electron transfer processes violating the ''Wigner rule'' have been studied for slow multiply charged ion-atom collisions. Experimentally a strong population of highly metastable sodium-like quartet states in low energy Fe 17+ + He single collision events has been observed. The possibility of double-electron capture plus spin-flip mechanisms has been discussed experimentally and theoretically, Our theoretical model using time dependent perturbation theory predicts that spin-flip processes are as likely as no spin flip under the conditions of our experiment

Interaction-flip identities in spin glasses

NARCIS (Netherlands)

Contucci, P.; Giardinà, C.; Giberti, C.

2009-01-01

We study the properties of fluctuation for the free energies and internal energies of two spin glass systems that differ for having some set of interactions flipped. We show that their difference has a variance that grows like the volume of the flipped region. Using a new interpolation method, which

Polarizing a stored proton beam by spin-flip?

International Nuclear Information System (INIS)

Oellers, Dieter Gerd Christian

2010-01-01

The present thesis discusses the extraction of the electron-proton spin-flip cross-section. The experimental setup, the data analysis and the results are pictured in detail. The proton is described by a QCD-based parton model. In leading twist three functions are needed. The quark distribution, the helicity distribution and the transversity distribution. While the first two are well-known, the transversity distribution is largely unknown. A self-sufficient measurement of the transversity is possible in double polarized proton-antiproton scattering. This rises the need of a polarized antiproton beam. So far spin filtering is the only tested method to produce a polarized proton beam, which may be capable to hold also for antiprotons. In-situ polarization build-up of a stored beam either by selective removal or by spin-flip of a spin-(1)/(2) beam is mathematically described. A high spin-flip cross-section would create an effective method to produce a polarized antiproton beam by polarized positrons. Prompted by conflicting calculations, a measurement of the spin-flip cross-section in low-energy electron-proton scattering was carried out. This experiment uses the electron beam of the electron cooler at COSY as an electron target. The depolarization of the stored proton beam is detected. An overview of the experiment is followed by detailed descriptions of the cycle setup, of the electron target and the ANKE silicon tracking telescopes acting as a beam polarimeter. Elastic protondeuteron scattering is the analyzing reaction. The event selection is depicted and the beam polarization is calculated. Upper limits of the two electron-proton spin-flip cross-sections σ parallel and σ perpendicular to are deduced using the likelihood method. (orig.)

RECOILING SUPERMASSIVE BLACK HOLES IN SPIN-FLIP RADIO GALAXIES

International Nuclear Information System (INIS)

Liu, F. K.; Wang Dong; Chen Xian

2012-01-01

Numerical relativity simulations predict that coalescence of supermassive black hole (SMBH) binaries leads not only to a spin flip but also to a recoiling of the merger remnant SMBHs. In the literature, X-shaped radio sources are popularly suggested to be candidates for SMBH mergers with spin flip of jet-ejecting SMBHs. Here we investigate the spectral and spatial observational signatures of the recoiling SMBHs in radio sources undergoing black hole spin flip. Our results show that SMBHs in most spin-flip radio sources have mass ratio q ∼> 0.3 with a minimum possible value q min ≅ 0.05. For major mergers, the remnant SMBHs can get a kick velocity as high as 2100 km s –1 in the direction within an angle ∼< 40° relative to the spin axes of remnant SMBHs, implying that recoiling quasars are biased to be with high Doppler-shifted broad emission lines while recoiling radio galaxies are biased to large apparent spatial off-center displacements. We also calculate the distribution functions of line-of-sight velocity and apparent spatial off-center displacements for spin-flip radio sources with different apparent jet reorientation angles. Our results show that the larger the apparent jet reorientation angle is, the larger the Doppler-shifting recoiling velocity and apparent spatial off-center displacement will be. We investigate the effects of recoiling velocity on the dust torus in spin-flip radio sources and suggest that recoiling of SMBHs would lead to 'dust-poor' active galactic nuclei. Finally, we collect a sample of 19 X-shaped radio objects and for each object give the probability of detecting the predicted signatures of recoiling SMBH.

Quark loops and spin-flip effects in pomeron exchange

International Nuclear Information System (INIS)

Goloskokov, S.V.

1991-01-01

On the basis of QCD at large distances with taking account of some nonperturbative properties of the theory, the possibility of spin-flip effects in high energy hadron processes at fixed momenta transfer is investigated. It is shown that the diagrams with the quark loops in QCD at large distances may lead to the spin-flip amplitude growing as s for s→∞, t-fixed. The confirmation of this result is obtained by calculations of the nonleading contributions from quark loops in t-channel exchange in QED up to the end. Physical mechanisms leading to that behaviour of the spin-flip amplitude is discussed. So we conclude that the pomeron has a complicated spin structure. (orig.)

Spin flipping a stored polarized proton beam at the IUCF cooler ring

International Nuclear Information System (INIS)

Phelps, R.A.

1995-01-01

We recently studied the spin flip of a vertically polarized 139 MeV proton beam stored in the IUCF Cooler Ring. We used an rf solenoid to induce a depolarizing resonance in the ring; we flipped the spin by varying the solenoid field's frequency through this resonance. We found a polarization loss after multiple spin flips less than 0.1% per flip; we also found that this loss increased for very slow frequency changes. This spin flip could reduce systematic errors in stored polarization beam experiments by allowing frequent beam polarization reversals during the experiment. copyright 1995 American Institute of Physics

Polarizing a stored proton beam by spin-flip?

Energy Technology Data Exchange (ETDEWEB)

Oellers, Dieter Gerd Christian

2010-04-15

The present thesis discusses the extraction of the electron-proton spin-flip cross-section. The experimental setup, the data analysis and the results are pictured in detail. The proton is described by a QCD-based parton model. In leading twist three functions are needed. The quark distribution, the helicity distribution and the transversity distribution. While the first two are well-known, the transversity distribution is largely unknown. A self-sufficient measurement of the transversity is possible in double polarized proton-antiproton scattering. This rises the need of a polarized antiproton beam. So far spin filtering is the only tested method to produce a polarized proton beam, which may be capable to hold also for antiprotons. In-situ polarization build-up of a stored beam either by selective removal or by spin-flip of a spin-(1)/(2) beam is mathematically described. A high spin-flip cross-section would create an effective method to produce a polarized antiproton beam by polarized positrons. Prompted by conflicting calculations, a measurement of the spin-flip cross-section in low-energy electron-proton scattering was carried out. This experiment uses the electron beam of the electron cooler at COSY as an electron target. The depolarization of the stored proton beam is detected. An overview of the experiment is followed by detailed descriptions of the cycle setup, of the electron target and the ANKE silicon tracking telescopes acting as a beam polarimeter. Elastic protondeuteron scattering is the analyzing reaction. The event selection is depicted and the beam polarization is calculated. Upper limits of the two electron-proton spin-flip cross-sections {sigma} {sub parallel} and {sigma} {sub perpendicular} {sub to} are deduced using the likelihood method. (orig.)

One and two-phonon processes of the spin-flip relaxation in quantum dots: Spin-phonon coupling mechanism

Science.gov (United States)

Wang, Zi-Wu; Li, Shu-Shen

2012-07-01

We investigate the spin-flip relaxation in quantum dots using a non-radiation transition approach based on the descriptions for the electron-phonon deformation potential and Fröhlich interaction in the Pavlov-Firsov spin-phonon Hamiltonian. We give the comparisons of the electron relaxations with and without spin-flip assisted by one and two-phonon processes. Calculations are performed for the dependence of the relaxation time on the external magnetic field, the temperature and the energy separation between the Zeeman sublevels of the ground and first-excited state. We find that the electron relaxation time of the spin-flip process is more longer by three orders of magnitudes than that of no spin-flip process.

Accessing the dark exciton spin in deterministic quantum-dot microlenses

Science.gov (United States)

Heindel, Tobias; Thoma, Alexander; Schwartz, Ido; Schmidgall, Emma R.; Gantz, Liron; Cogan, Dan; Strauß, Max; Schnauber, Peter; Gschrey, Manuel; Schulze, Jan-Hindrik; Strittmatter, Andre; Rodt, Sven; Gershoni, David; Reitzenstein, Stephan

2017-12-01

The dark exciton state in semiconductor quantum dots (QDs) constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state-based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses which promise enhanced photon extraction, to optically prepare and read out the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding states, which are identified by deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of (0.82 ± 0.01) ns corresponding to a fine-structure splitting of (5.0 ± 0.7) μeV between its eigenstates |↑ ⇑ ±↓ ⇓ ⟩. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory.

Supermassive black hole spin-flip during the inspiral

International Nuclear Information System (INIS)

Gergely, Laszlo A; Biermann, Peter L; Caramete, Laurentiu I

2010-01-01

During post-Newtonian evolution of a compact binary, a mass ratio ν different from 1 provides a second small parameter, which can lead to unexpected results. We present a statistics of supermassive black hole candidates, which enables us first to derive their mass distribution, and then to establish a logarithmically even probability in ν of the mass ratios at their encounter. In the mass ratio range ν in (1/30, 1/3) of supermassive black hole mergers representing 40% of all possible cases, the combined effect of spin-orbit precession and gravitational radiation leads to a spin-flip of the dominant spin during the inspiral phase of the merger. This provides a mechanism for explaining a large set of observations on X-shaped radio galaxies. In another 40% with mass ratios ν in (1/30, 1/1000) a spin-flip never occurs, while in the remaining 20% of mergers with mass ratios ν in (1/3, 1) it may occur during the plunge. We analyze the magnitude of the spin-flip angle occurring during the inspiral as a function of the mass ratio and original relative orientation of the spin and orbital angular momentum. We also derive a formula for the final spin at the end of the inspiral in this mass ratio range.

Spin transport dynamics of excitons in CdTe/Cd1-xMnxTe quantum wells

International Nuclear Information System (INIS)

Kayanuma, Kentaro; Shirado, Eiji; Debnath, Mukul C.; Souma, Izuru; Chen, Zhanghai; Oka, Yasuo

2001-01-01

Transport properties of spin-polarized excitons were studied in the double quantum well system composed of Cd 0.95 Mn 0.05 Te and CdTe wells. Circular polarization degrees of the time resolved exciton photoluminescence in magnetic field showed that the spin-polarized excitons diffused from the magnetic quantum well and injected to the non-magnetic quantum well by conserving their spins. The spin-polarized excitons injected into the nonmagnetic well reaches 18% of the nonmagnetic well excitons. From the circular polarization degree and the lifetime of the magnetic quantum well excitons, the spin relaxation time of the excitons in the Cd 0.95 Mn 0.05 Te well was determined as 275 - 10 ps depending on the magnetic field strength. [copyright] 2001 American Institute of Physics

Formation of very short pulse by neutron spin flip chopper for J-PARC

International Nuclear Information System (INIS)

Ebisawa, T.; Soyama, K.; Yamazaki, D.; Tasaki, S.; Sakai, K.; Oku, T.; Maruyama, R.; Hino, M.

2004-01-01

We have developed neutron spin flip choppers with high S/N ratio and high intensity for pulsed sources using multi-stage spin flip choppers. It is not easy for us to obtain a very short neutron pulse less than 10 μs using a spin flip chopper, due to the time constant L/R in the normal LR circuit. We will discuss a method obtaining a very short neutron pulse applying the modified push-pull circuit proposed by Ito and Takahashi [4] to the double spin flip chopper with polarizing guides

99.9% Spin-Flip Efficiency in the Presence of a Strong Siberian Snake

International Nuclear Information System (INIS)

Morozov, V.S.; Blinov, B.B.; Etienne, Z.B.; Krisch, A.D.; Leonova, M.A.; Lin, A.M.T.; Lorenzon, W.; Peters, C.C.; Sivers, D.W.; Wong, V.K.; Yonehara, K.; Anferov, V. A.; Schwandt, P.; Stephenson, E.J.; Przewoski, B. von; Sato, H.

2003-01-01

We recently studied the spin-flipping efficiency of an rf-dipole magnet using a 120-MeV horizontally polarized proton beam stored in the Indiana University Cyclotron Facility Cooler Ring, which contained a full Siberian snake. We flipped the spin by ramping the rf dipole's frequency through an rf-induced depolarizing resonance. By adiabatically turning on the rf dipole, we minimized the beam loss, while preserving almost all of the beam's polarization. After optimizing the frequency ramp parameters, we used up to 400 multiple spin flips to measure a spin-flip efficiency of 99.93 ± 0.02%. This result indicates that spin flipping should be possible in very-high-energy polarized storage rings, where Siberian snakes are certainly needed and only dipole rf-flipper magnets are practical

Spin-flip scattering effect on the current-induced spin torque in ferromagnet-insulator-ferromagnet tunnel junctions

International Nuclear Information System (INIS)

Zhu Zhengang; Su Gang; Jin Biao; Zheng Qingrong

2003-01-01

We have investigated the current-induced spin transfer torque of a ferromagnet-insulator-ferromagnet tunnel junction by taking the spin-flip scatterings into account. It is found that the spin-flip scattering can induce an additional spin torque, enhancing the maximum of the spin torque and giving rise to an angular shift compared to the case when the spin-flip scatterings are neglected. The effects of the molecular fields of the left and right ferromagnets on the spin torque are also studied. It is found that τ Rx /I e (τ Rx is the spin-transfer torque acting on the right ferromagnet and I e is the tunneling electrical current) does vary with the molecular fields. At two certain angles, τ Rx /I e is independent of the molecular field of the right ferromagnet, resulting in two crossing points in the curve of τ Rx /I e versus the relevant orientation for different molecular fields

Spin flip in inelastic scattering of protons on 28Si nuclei

International Nuclear Information System (INIS)

Wang Syn Chan; Komsan, M.N.Kh.; Osetinskij, G.M.; Golubev, S.L.; Kurepin, A.B.; Likhosherstov, V.N.

1975-01-01

We measured the energy and angular dependences of the spin-flip probability and of the differential cross section for inelastic scattering of protons in the resonance region of the reaction 28 Si(p,p') 23 Si* (2 + , 1.78 MeV) at E sub(p) = 3.095 and 3.34 MeV. The energy dependence of the spin-flip probability was found to have a resonance character. The angular distribution of the inelastic scattering and of the spin-flip probability is asymmetrical with respect to 90 deg in the c.m.s

Computational quantum chemistry for single Heisenberg spin couplings made simple: Just one spin flip required

International Nuclear Information System (INIS)

Mayhall, Nicholas J.; Head-Gordon, Martin

2014-01-01

We highlight a simple strategy for computing the magnetic coupling constants, J, for a complex containing two multiradical centers. On the assumption that the system follows Heisenberg Hamiltonian physics, J is obtained from a spin-flip electronic structure calculation where only a single electron is excited (and spin-flipped), from the single reference with maximum S ^ z , M, to the M − 1 manifold, regardless of the number of unpaired electrons, 2M, on the radical centers. In an active space picture involving 2M orbitals, only one β electron is required, together with only one α hole. While this observation is extremely simple, the reduction in the number of essential configurations from exponential in M to only linear provides dramatic computational benefits. This (M, M − 1) strategy for evaluating J is an unambiguous, spin-pure, wave function theory counterpart of the various projected broken symmetry density functional theory schemes, and likewise gives explicit energies for each possible spin-state that enable evaluation of properties. The approach is illustrated on five complexes with varying numbers of unpaired electrons, for which one spin-flip calculations are used to compute J. Some implications for further development of spin-flip methods are discussed

Markov chain analysis of single spin flip Ising simulations

International Nuclear Information System (INIS)

Hennecke, M.

1997-01-01

The Markov processes defined by random and loop-based schemes for single spin flip attempts in Monte Carlo simulations of the 2D Ising model are investigated, by explicitly constructing their transition matrices. Their analysis reveals that loops over all lattice sites using a Metropolis-type single spin flip probability often do not define ergodic Markov chains, and have distorted dynamical properties even if they are ergodic. The transition matrices also enable a comparison of the dynamics of random versus loop spin selection and Glauber versus Metropolis probabilities

Spin-diffusion lengths in metals and alloys, and spin-flipping at metal/metal interfaces: an experimentalist's critical review

International Nuclear Information System (INIS)

Bass, Jack; Pratt, William P Jr

2007-01-01

In magnetoresistance (MR) studies of magnetic multilayers composed of combinations of ferromagnetic (F) and non-magnetic (N) metals, the magnetic moment (or related 'spin') of each conduction electron plays a crucial role, supplementary to that of its charge. While initial analyses of MR in such multilayers assumed that the direction of the spin of each electron stayed fixed as the electron transited the multilayer, we now know that this is true only in a certain limit. Generally, the spins 'flip' in a distance characteristic of the metal, its purity, and the temperature. They can also flip at F/N or N1/N2 interfaces. In this review we describe how to measure the lengths over which electron moments flip in pure metals and alloys, and the probability of spin-flipping at metallic interfaces. Spin-flipping within metals is described by a spin-diffusion length, l sf M , where the metal M F or N. Spin-diffusion lengths are the characteristic lengths in the current-perpendicular-to-plane (CPP) and lateral non-local (LNL) geometries that we focus upon in this review. In certain simple cases, l sf N sets the distance over which the CPP-MR and LNL-MR decrease as the N-layer thickness (CPP-MR) or N-film length (LNL) increases, and l sf F does the same for increase of the CPP-MR with increasing F-layer thickness. Spin-flipping at M1/M2 interfaces can be described by a parameter, δ M1/M2 , which determines the spin-flipping probability, P = 1-exp(-δ). Increasing δ M1/M2 usually decreases the MR. We list measured values of these parameters and discuss the limitations on their determinations. (topical review)

Thermally excited proton spin-flip laser emission in tokamaks

International Nuclear Information System (INIS)

Arunasalam, V.; Greene, G.J.

1993-07-01

Based on statistical thermodynamic fluctuation arguments, it is shown here for the first time that thermally excited spin-flip laser emission from the fusion product protons can occur in large tokamak devices that are entering the reactor regime of operation. Existing experimental data from TFTR supports this conjecture, in the sense that these measurements are in complete agreement with the predictions of the quasilinear theory of the spin-flip laser

Spin flip statistics and spin wave interference patterns in Ising ferromagnetic films: A Monte Carlo study.

Science.gov (United States)

Acharyya, Muktish

2017-07-01

The spin wave interference is studied in two dimensional Ising ferromagnet driven by two coherent spherical magnetic field waves by Monte Carlo simulation. The spin waves are found to propagate and interfere according to the classic rule of interference pattern generated by two point sources. The interference pattern of spin wave is observed in one boundary of the lattice. The interference pattern is detected and studied by spin flip statistics at high and low temperatures. The destructive interference is manifested as the large number of spin flips and vice versa.

Polarizing a stored proton beam by spin flip? - A high statistic reanalysis

International Nuclear Information System (INIS)

Oellers, Dieter

2011-01-01

Prompted by recent, conflicting calculations, we have carried out a measurement of the spin flip cross section in low-energy electron-proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. A reanalysis of the data leeds to a reduced statistical errors resulting in a factor of 4 reduced upper limit for the spin flip cross section. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam.

NMR investigation of spin flip in TmCrO3

International Nuclear Information System (INIS)

Karnachev, A.S.; Klechin, Yu.I.; Kovtun, N.M.; Moskvin, A.S.; Solov'ev, E.E.; Tkachenko, A.A.

1987-01-01

Spin flip in the rare earth orthochromate TmCrO 3 is studied by the double-pulse NMR technique. It is shown that below 5.6 K spin flip in the absence of an external magnetic field takes place as a first order phase transition from the high temperature phase Γ 2 to the low temperature phase Γ 4 with a region of coexistence of the two phases of more than 3.8 K. The spin flip phase transitions Γ 2 ↔ Γ 4 induced by an external magnetic field and the attendant phenomenon of magnetic and electric nonequivalence of 53 Cr nuclei from different magnetic sublattices are investigated. The anisotropy parameters of the hyperfine interactions and nuclear quadrupole interactions are calculated on the basis of the experimental data

Double-spin-flip resonance of rhodium nuclei at positive and negative spin temperatures

DEFF Research Database (Denmark)

Tuoriniemi, J.T.; Knuuttila, T.A.; Lefmann, K.

2000-01-01

Sensitive SQUID-NMR measurements were used to study the mutual interactions in the highly polarized nuclear-spin system of rhodium metal. The dipolar coupling gives rise to a weak double-spin-flip resonance. The observed frequency shifts allow deducing separately the dipolarlike contribution...

Effective coupling constants for spin-flip and non spin-flip E1 transitions in A--90 nuclei

International Nuclear Information System (INIS)

Nakayama, Shintaro; Shibata, Tokushi; Kishimoto, Tadafumi; Sasao, Mamiko; Ejiri, Hiroyasu

1983-01-01

Radiative proton capture reactions through two isobaric analogue resonances (IAR) in 89 Y were studied, one was the 12.07 MeV 2dsub(5/2) state lying just above the neutron threshold energy Bsub(n) and another was the 14.48 MeV 2dsub(3/2) state lying well above Bsub(n). E1 transitions from these IAR's were studied for favoured cases with no spin-flip and no change of radial nodes, and for unfavoured cases spin-flip and/or change of radial nodes. At the 2dsub(3/2) IAR lying well above Bsub(n), the favoured transitions show the resonance feature, but the unfavoured ones not. At the 2dsub(5/2) IAR near Bsub(n), however, both the favoured and unfavoured transitions show the resonance feature. Anormalous resonant feature of the unfavoured transitions is interpreted mainly due to the compound process. Favoured transitions are all found to be reduced by factors -- 0.3 over the shell model values. (author)

Resolution of Single Spin-Flips of a Single Proton

CERN Document Server

Mooser, A.; Blaum, K.; Bräuninger, S.A.; Franke, K.; Leiteritz, C.; Quint, W.; Rodegheri, C.C.; Ulmer, S.; Walz, J.

2013-04-04

The spin magnetic moment of a single proton in a cryogenic Penning trap was coupled to the particle's axial motion with a superimposed magnetic bottle. Jumps in the oscillation frequency indicate spin-flips and were identified using a Bayesian analysis.

Spin-flip configuration interaction singles with exact spin-projection: Theory and applications to strongly correlated systems.

Science.gov (United States)

Tsuchimochi, Takashi

2015-10-14

Spin-flip approaches capture static correlation with the same computational scaling as the ordinary single reference methods. Here, we extend spin-flip configuration interaction singles (SFCIS) by projecting out intrinsic spin-contamination to make it spin-complete, rather than by explicitly complementing it with spin-coupled configurations. We give a general formalism of spin-projection for SFCIS, applicable to any spin states. The proposed method is viewed as a natural unification of SFCIS and spin-projected CIS to achieve a better qualitative accuracy at a low computational cost. While our wave function ansatz is more compact than previously proposed spin-complete SF approaches, it successfully offers more general static correlation beyond biradicals without sacrificing good quantum numbers. It is also shown that our method is invariant with respect to open-shell orbital rotations, due to the uniqueness of spin-projection. We will report benchmark calculations to demonstrate its qualitative performance on strongly correlated systems, including conical intersections that appear both in ground-excited and excited-excited degeneracies.

Excitonic magnet in external field: Complex order parameter and spin currents

Science.gov (United States)

Geffroy, D.; Hariki, A.; Kuneš, J.

2018-04-01

We investigate spin-triplet exciton condensation in the two-orbital Hubbard model close to half-filling by means of dynamical mean-field theory. Employing an impurity solver that handles complex off-diagonal hybridization functions, we study the behavior of excitonic condensate in stoichiometric and doped systems subject to external magnetic field. We find a general tendency of the triplet order parameter to lie perpendicular with the applied field and identify exceptions from this rule. For solutions exhibiting k -odd spin textures, we discuss the Bloch theorem, which, in the absence of spin-orbit coupling, forbids the appearance of spontaneous net spin current. We demonstrate that the Bloch theorem is not obeyed by the dynamical mean-field theory.

Spin-flip effects on the supercurrent through mesoscopic superconducting junctions

International Nuclear Information System (INIS)

Pan Hui; Lin Tsunghan

2005-01-01

We investigate the spin-flip effects on the Andreev bound states and the supercurrent in a superconductor/quantum-dot/superconductor system, theoretically. The spin-flip scattering in the quantum dot can reverse the supercurrent flowing through the system, which results in a π-junction transition. By controlling the energy level of the quantum dot, the π-junction transition can be caused to occur again. The two mechanisms of the π-junction transitions are interpreted within the picture of Andreev bound states

Achieving 99.9% proton spin-flip efficiency at higher energy with a small rf dipole

CERN Document Server

Leonova, M A; Gebel, R; Hinterberger, F; Krisch, A D; Lehrach, A; Lorentz, B; Maier, R; Morozov, V S; Prasuhn, D; Raymond, R S; Schnase, A; Stockhorst, H; Ulbrich, K; Wong, V K; 10.1103/PhysRevLett.93.224801

2004-01-01

We recently used a new ferrite rf dipole to study spin flipping of a 2.1 GeV/c vertically polarized proton beam stored in the COSY Cooler Synchrotron in Julich, Germany. We swept the rf dipole's frequency through an rf-induced spin resonance to flip the beam's polarization direction. After determining the resonance's frequency, we varied the frequency range, frequency ramp time, and number of flips. At the rf dipole's maximum strength and optimum frequency range and ramp time, we measured a spin-flip efficiency of 99.92+or-0.04%. This result, along with a similar 0.49 GeV/c IUCF result, indicates that, due to the Lorentz invariance of an rf dipole's transverse integral Bdl and the weak energy dependence of its spin-resonance strength, an only 35% stronger rf dipole should allow efficient spin flipping in the 100 GeV BNL RHIC Collider or even the 7 TeV CERN Large Hadron Collider.

High-energy hadron spin-flip amplitude at small momentum transfer and new AN data from RHIC

International Nuclear Information System (INIS)

Cudell, J.-R.; Selyugin, O.V.; Predazzi, E.

2004-01-01

In the case of elastic high-energy hadron-hadron scattering, the impact of the large-distance contributions on the behaviour of the slopes of the spin-non-flip and of the spin-flip amplitudes is analysed. It is shown that the long tail of the hadronic potential in impact parameter space leads to a value of the slope of the reduced spin-flip amplitude larger than that of the spin-non-flip amplitude. This effect is taken into account in the calculation of the analysing power in proton-nucleus reactions at high energies. It is shown that the preliminary measurement of A N for p 12 C obtained by the E950 Collaboration indeed favours a spin-flip amplitude with a large slope. Predictions for A N at p L =250/ GeV/c are given. (orig.)

Spin-flip tunneling in quantum dots

Energy Technology Data Exchange (ETDEWEB)

Schreiber, Lars; Braakman, Floris; Meunier, Tristan; Calado, Victor; Vandersypen, Lieven [Kavli Institute of NanoScience, Delft (Netherlands); Wegscheider, Werner [Institute for Experimental and Applied Physics, University of Regensburg (Germany)

2010-07-01

Electron spins in a gate-defined double quantum dot formed in a GaAs/(Al,Ga)As 2DEG are promising candidates for quantum information processing as coherent single spin rotation and spin swap has been demonstrated recently. In this system we investigate the two-electron spin dynamics in the presence of microwaves (5.20 GHz) applied to one side gate. During microwave excitation we observe characteristic photon assisted tunneling (PAT) peaks at the (1,1) to (0,2) charge transition. Some of the PAT peaks are attributed to photon tunneling events between the singlet S(0,2) and the singlet S(1,1) states, a spin-conserving transition. Surprisingly, other PAT peaks stand out by their different external magnetic field dependence. They correspond to tunneling involving a spin-flip, from the (0,2) singlet to a (1,1) triplet. The full spectrum of the observed PAT lines is captured by simulations. This process offers novel possibilities for 2-electron spin manipulation and read-out.

Spin Flips versus Spin Transport in Nonthermal Electrons Excited by Ultrashort Optical Pulses in Transition Metals

Science.gov (United States)

Shokeen, V.; Sanchez Piaia, M.; Bigot, J.-Y.; Müller, T.; Elliott, P.; Dewhurst, J. K.; Sharma, S.; Gross, E. K. U.

2017-09-01

A joint theoretical and experimental investigation is performed to understand the underlying physics of laser-induced demagnetization in Ni and Co films with varying thicknesses excited by 10 fs optical pulses. Experimentally, the dynamics of spins is studied by determining the time-dependent amplitude of the Voigt vector, retrieved from a full set of magnetic and nonmagnetic quantities performed on both sides of films, with absolute time reference. Theoretically, ab initio calculations are performed using time-dependent density functional theory. Overall, we demonstrate that spin-orbit induced spin flips are the most significant contributors with superdiffusive spin transport, which assumes only that the transport of majority spins without spin flips induced by scattering does not apply in Ni. In Co it plays a significant role during the first ˜20 fs only. Our study highlights the material dependent nature of the demagnetization during the process of thermalization of nonequilibrium spins.

Spin-flip transitions in self-assembled quantum dots

Science.gov (United States)

Stavrou, V. N.

2017-12-01

Detailed realistic calculations of the spin-flip time (T 1) for an electron in a self-assembled quantum dot (SAQD) due to emission of an acoustic phonon, using only bulk properties with no fitting parameters, are presented. Ellipsoidal lens shaped Inx Ga1-x As quantum dots, with electronic states calculated using 8-band strain dependent {k \\cdot p} theory, are considered. The phonons are treated as bulk acoustic phonons coupled to the electron by both deformation potential and piezoelectric interactions. The dependence of T 1 on the geometry of SAQD, on the applied external magnetic field and on the lattice temperature is highlighted. The theoretical results are close to the experimental measurements on the spin-flip times for a single electron in QD.

Increasing spin-flips and decreasing cost: Perturbative corrections for external singles to the complete active space spin flip model for low-lying excited states and strong correlation

International Nuclear Information System (INIS)

Mayhall, Nicholas J.; Head-Gordon, Martin

2014-01-01

An approximation to the spin-flip extended configuration interaction singles method is developed using a second-order perturbation theory approach. In addition to providing significant efficiency advantages, the new framework is general for an arbitrary number of spin-flips, with the current implementation being applicable for up to around 4 spin-flips. Two new methods are introduced: one which is developed using non-degenerate perturbation theory, spin-flip complete active-space (SF-CAS(S)), and a second quasidegenerate perturbation theory method, SF-CAS(S) 1 . These two approaches take the SF-CAS wavefunction as the reference, and then perturbatively includes the effect of single excitations. For the quasidegenerate perturbation theory method, SF-CAS(S) 1 , the subscripted “1” in the acronym indicates that a truncated denominator expansion is used to obtain an energy-independent down-folded Hamiltonian. We also show how this can alternatively be formulated in terms of an extended Lagrangian, by introducing an orthonormality constraint on the first-order wavefunction. Several numerical examples are provided, which demonstrate the ability of SF-CAS(S) and SF-CAS(S) 1 to describe bond dissociations, singlet-triplet gaps of organic molecules, and exchange coupling parameters for binuclear transition metal complexes

Vortex Flipping in Superconductor-Ferromagnet Spin Valve Structures

Science.gov (United States)

Patino, Edgar J.; Aprili, Marco; Blamire, Mark; Maeno, Yoshiteru

2014-03-01

We report in plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni, below the superconducting transition Tc, our results show strong evidence of vortex flipping driven by the ferromagnets magnetization. This is a direct consequence of proximity effect that leads to vortex supercurrents leakage into the ferromagnets. Here the polarized electron spins are subject to vortices magnetic field occasioning vortex flipping. Such novel mechanism has been made possible for the first time by fabrication of the F/S/F/AF multilayered spin valves with a thin-enough S layer to barely confine vortices inside as well as thin-enough F layers to align and control the magnetization within the plane. When Co is used there is no observation of vortex flipping effect. This is attributed to Co shorter coherence length. Interestingly instead a reduction in pinning field of about 400 Oe is observed when the Nb layer is in superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts any explanation must be directly related to proximity effect and thus a remarkable phenomenon that deserves further investigation. Programa Nacional de Ciencias Basicas COLCIENCIAS (No. 120452128168).

Slope of differential cross sections and size of hadron spin-flip amplitude

International Nuclear Information System (INIS)

Selyugin, O.V.

1998-01-01

A possibility to obtain restrictions of the size of the elastic spin-flip hadron scattering amplitude from the exactly measured experimental data on the differential cross sections of elastic hadron-hadron scattering is shown and possible sizes are calculated. Appropriate estimations confirm the previous analysis of experimental data at √s = 540 GeV and a probable contribution of the hadron spin-flip amplitude

The nuclear deformation versus spin-flip like excitations and the suppression of the 2νββ amplitude

International Nuclear Information System (INIS)

Raduta, A.A.; Delion, D.S.; Faessler, Amand

1997-01-01

We were the first who investigated the influence of spin-flip and non-spin-flip configuration on the separation of the transition amplitude of the Gamow-Teller double beta decay. A realistic Hamiltonian and a projected spherical single particle basis is considered, while the effects are generated by three antagonistic sources: spin-flip, non-spin-flip like excitation and nuclear deformation. Moreover, by a smooth variation of a deformation parameter one could bridge the spherical and deformed pictures. Although our application is not aimed at describing the experimental situation we chose as input data those corresponding to the transition 82 Se → 82 Kr. For near spherical case there are two resonances in M GT , one having a spin-flip structure and identified as GT resonance and one of non-spin-flip structure, placed at low energy. For large deformation and vanishing g pp coupling constant there are two resonances of spin-flip and non-spin-flip natures (ΔI = 1 and 0, respectively) and located at the same energy, what indicates that the deformation acts against the separation of this resonances. In conclusion, our calculation showed that the mechanism of M GT suppression is different for spherical and deformed nuclei. In both cases approaching the critical value of g pp where the RPA breaks down, a lot of strength is accumulated in lowest RPA state and, while in the spherical case this has a non spin-flip nature, in the deformed case the state is a mixture of both types of configurations

Collective Behavior of a Spin-Aligned Gas of Interwell Excitons in Double Quantum Wells

DEFF Research Database (Denmark)

Larionov, A. V.; Bayer, M.; Hvam, Jørn Märcher

2005-01-01

The kinetics of a spin-aligned gas of interwell excitons in GaAs/AlGaAs double quantum wells (n–i–n heterostructure) is studied. The temperature dependence of the spin relaxation time for excitons, in which a photoexcited electron and hole are spatially separated between two adjacent quantum wells...

Spin-flip transition of L10-type MnPt alloy single crystal studied by neutron scattering

International Nuclear Information System (INIS)

Hama, Hiroaki; Motomura, Ryo; Shinozaki, Tatsuya; Tsunoda, Yorihiko

2007-01-01

Magnetic structure, tetragonality, and the spin-flip transition for an L1 0 -type MnPt ordered alloy were studied by neutron scattering using a single-crystal specimen. Tetragonality of the lattice showed strong correlation with the spin-flip transition. Although the spin-flip transition looks like a gradual change of the easy axis in the temperature range between 580 and 770 K, two modes of magnon-gap peaks with different energies were observed in this transition temperature range. Thus, the crystal consists of two regions with different anisotropy energies and the volume fractions of these regions with different spin directions change gradually with temperature. The tetragonality and spin-flip transition are discussed using the hard-sphere model for atomic radii of Pt and Mn. The Invar effect of Mn atoms is proposed using high- and low-spin transitions of Mn moments in analogy with the two-γ model of Fe moments in FeNi Invar alloy

Low flip angle spin-echo MR imaging to obtain better Gd-DTPA enhanced imaging with ECG gating

International Nuclear Information System (INIS)

Sugimura, Kazuro; Kawamitsu, Hideaki; Yoshikawa, Kazuaki; Kasai, Toshifumi; Yuasa, Koji; Ishida, Tetsuya

1992-01-01

ECG-gated spin-echo imaging (ECG-SE) can reduce physiological motion artifact. However, ECG-SE does not provide strong T1-weighted images because repetition time (TR) depends on heart rate (HR). We investigated the usefulness of low flip angle spin-echo imaging (LFSE) in obtaining more T1-dependent contrast with ECG gating. In computer simulation, the predicted image contrast and single-to-noise ratio (SNR) obtained for each flip angle (0-180deg) and each TR (300 msec-1200 msec) were compared with those obtained by conventional T1-weighted spin-echo imaging (CSE: TR=500 msec, TE=20 msec). In clinical evaluation, tissue contrast [contrast index (CI): (SI of lesion-SI of muslce) 2* 100/SI of muscle] obtained by CSE and LFSE were compared in 17 patients. At a TR of 1,000 msec, T1-dependent contrast increased with decreasing flip angle and that at 38deg was identical to that with T1-weighted spin-echo. SNR increased with the flip angle until 100deg, and that at 53deg was identical to that with T1-weighted spin-echo. CI on LFSE (74.0±52.0) was significantly higher than CI on CSE (40.9±35.9). ECG-gated LFSE imaging provides better T1-dependent contrast than conventional ECG-SE. This method was especially useful for Gd-DTPA enhanced MR imaging. (author)

Charged particle spin flip in a storage ring with HF-electromagnetic field

International Nuclear Information System (INIS)

Polunin, A.A.; Shatupov, Yu.M.

1982-01-01

An experiment for revealing a possibility of adiabatic electron spin flip in the VEPP-2M storage ring is described. High frequency longitudinal magnetic field up to 100 Gs at the length of 40 cm and frequency of 7.95 MHz was produced by a spiral of 10 coils supplied from HF-generator with 5 kW power. The control system permitted to vary generator frequency within +-3x10 - 3 f range during 10 - 3 -10 s. Determination of beam polarization degree was exercised by detection of electron elastic scattering inside the bunch. A possibility of changing the polarization sign at preservation of other beam parameters (dimensions, currents, energy, etc.) is of interest in experiments with polarized particles in storage rings. Spin flip can be exercised by effect on the beam of high frequency electromagnetic field, resonance with spin precession frequency around the leading field of the storage ring. The polarized 5 mA beam was produced due to radiation polarization at which electron spins are alinged along the direction of the magnetic field. Processing of the experimental results revealed good correspondence to analytical dependence. The depolarization value at the spin flip did not exceed 10%

Behavior of the hadron potential at large distances and properties of the hadron spin-flip amplitude

International Nuclear Information System (INIS)

Predazzi, E.; Selyugin, O.V.

2002-01-01

The impact of the form of the hadron potential at large distances on the behavior of the hadron spin-flip amplitude at small angles is examined. The t-dependence of the spin-flip amplitude of high-energy hadron elastic scattering is analyzed under different assumptions on the hadron interaction. It is shown that the long tail of the nonGaussian form of the hadron potential of the hadron interaction in the impact parameter representation leads to a large value of the slope of the spin-flip amplitude (without the kinematical factor √(vertical stroke t vertical stroke)) as compared with the slope of the spin-nonflip amplitude. This effect can explain the form of the differential cross-section and the analyzing power at small transfer momenta. The methods for the definition of the spin-dependent part of the hadron scattering amplitude are presented. A possibility to investigate the structure of the hadron spin-flip amplitude from the accurate measure of the differential cross-section and the spin correlation parameters is shown. (orig.)

A Spin-Flip Cavity for Microwave Spectroscopy of Antihydrogen

CERN Document Server

Federmann, Silke; Widmann, Eberhard

The present thesis is a contribution to the Asacusa (Atomic Spectroscopy And Collisions Using Slow Antiprotons) experiment. The aim of this experiment is to measure the ground-state hyperfine structure of antihydrogen. This is done using a Rabi-like spectrometer line consisting of an antihydrogen source, a microwave cavity, a sextupole magnet and a detector. The cavity induces spin-flip transitions in the ground-state hyperfine levels of antihydrogen whereas the sextupole magnet selects the antihydrogen atoms according to their spin state. Such a configuration allows the measurements of the hyperfine transition in antihydrogen with very high precision. A comparison with the corresponding transitions in hydrogen would thus provide a very sensitive test of the charge-parity-time (Cpt) symmetry. In the context of this thesis, the central piece of this spectrometer line, the spin flip cavity, was designed and implemented. The delicacy of this task was achieving the required field homogeneity: It needs to be bette...

Triplet exciton dynamics

International Nuclear Information System (INIS)

Strien, A.J. van.

1981-01-01

Results are presented of electron spin echo experiments combined with laser flash excitation on triplet states of aromatic molecules. Some of the theoretical and experimental aspects of the photoexcited triplet state are discussed in detail and the electron spin echo spectrometers and laser systems are described. All the experiments described in this thesis were performed at liquid helium temperatures. An account is given of the ESE experiments performed on the photoexcited, non-radiative, triplet state of pentacene in napthalene. This is an example of the ESE technique being used to ascertain the zero-field splitting parameters, the populating and depopulating rates, and the orientation of the pentacene molecules in the naphthalene host. A combination of high resolution laser flash excitation and electron-spin echoes in zero-magnetic field allowed the author to observe directly k(vector)→k(vector)' exciton scattering processes in the one-dimensional triplet excitons in tetrachlorobenzene for the first time. Additional experimental data about exciton scattering is provided and a study of the orientational dependence of the spin-lattice relaxation of the triplet excitons in an external magnetic field is described. (Auth.)

Localized excitons in quantum wells show spin relaxation without coherence loss

DEFF Research Database (Denmark)

Zimmermann, R.; Langbein, W.; Runge, E.

2001-01-01

The coherence in the secondary emission from quantum well excitons is studied using the speckle method. Analysing the different polarization channels allows to conclude that (i) no coherence loss occurs in the cross-polarized emission, favouring spin beating instead of spin dephasing, and that (i...

First evidence for spin-flip M1 strength in 40Ar

International Nuclear Information System (INIS)

Li, T.C.; Rainovski, G.; Pietralla, N.; Ahn, T.; Costin, A.; Tonchev, A.P.; Ahmed, M.W.; Blackston, M.A.; Parpottas, Y.; Perdue, B.A.; Tornow, W.; Weller, H.R.; Angell, C.; Keeter, K.J.; Li, J.; Mikhailov, S.; Wu, Y.K.; Lisetskiy, A.

2006-01-01

The 40 Ar(γ→,γ ' ) photon scattering reaction was used to search for spin-flip M1 strength in 40 Ar. The nearly monochromatic, linearly polarized photon beam of HIγS, in an energy region from 7.7 to 11 MeV, was employed in this study. 28 dipole excitations were observed. The azimuthal intensity asymmetry indicated that all of these states were E1 except for the state at E x =9.757 MeV. Shell-model calculations were used to interpret this state as one fragment of the spin-flip M1 strength in 40 Ar

Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics

Science.gov (United States)

Wang, Huihui; Bokarev, Sergey I.; Aziz, Saadullah G.; Kühn, Oliver

2017-08-01

Recent developments in attosecond spectroscopy yield access to the correlated motion of electrons on their intrinsic timescales. Spin-flip dynamics is usually considered in the context of valence electronic states, where spin-orbit coupling is weak and processes related to the electron spin are usually driven by nuclear motion. However, for core-excited states, where the core-hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin-flips on a much shorter timescale. Using density matrix-based time-dependent restricted active space configuration interaction including spin-orbit coupling, we address an unprecedentedly short spin-crossover for the example of L-edge (2p→3d) excited states of a prototypical Fe(II) complex. This process occurs on a timescale, which is faster than that of Auger decay (∼4 fs) treated here explicitly. Modest variations of carrier frequency and pulse duration can lead to substantial changes in the spin-state yield, suggesting its control by soft X-ray light.

Spatial resolution properties in 3D fast spin-echo using variable refocusing flip angles

International Nuclear Information System (INIS)

Ozaki, Masanori; Mizukami, Shinya; Hata, Hirofumi; Sato, Mayumi; Komi, Syotaro; Miyati, Tosiaki; Nozaki, Atsushi

2011-01-01

A new 3-dimensional fast spin-echo (3D FSE) method that uses a variable refocusing flip angle technique has recently been applied to imaging. The imaging pulse sequence can inhibit T 2 decay by varying the refocusing flip angle. Use of a long echo train length allows acquisition of 3D T 2 -weighted images with less blurring in a short scan time. The smaller refocusing flip angle in the new 3D FSE method than in the conventional method can reduce the specific absorption rate. However, T 2 decay differs between the new and conventional 3D FSE methods, so the resolution properties of the 2 methods may differ. We investigated the resolution properties of the new 3D FSE method using a variable refocusing flip angle technique. Varying the refocusing flip angle resulted in different resolution properties for the new 3D FSE method compared to the conventional method, a difference particularly noticeable when the imaging parameters were set for obtaining proton density weighted images. (author)

Intradot spin-flip Andreev reflection tunneling through a ferromagnet-quantum dot-superconductor system with ac field

International Nuclear Information System (INIS)

Song Hongyan; Zhou Shiping

2008-01-01

We investigate Andreev reflection (AR) tunneling through a ferromagnet-quantum dot-superconductor (F-QD-S) system in the presence of an external ac field. The intradot spin-flip scattering in the QD is involved. Using the nonequilibrium Green function and BCS quasiparticle spectrum for superconductor, time-averaged AR conductance is formulated. The competition between the intradot spin-flip scattering and photon-assisted tunneling dominates the resonant behaviors of the time-averaged AR conductance. For weak intradot spin-flip scattering strengths, the AR conductance shows a series of equal interval resonant levels. However, the single-peak at main resonant level develops into a well-resolved double-peak resonance at a strong intradot spin-flip scattering strength. Remarkable, multiple-photon-assisted tunneling that generates photonic sideband peaks with a variable interval has been found. In addition, the AR conductance-bias voltage characteristic shows a transition between the single-peak to double-peak resonance as the ratio of the two tunneling strengths varies

Design of a 1.42 GHZ spin-flip cavity for antihydrogen atoms

CERN Document Server

Caspers, F; Juhasz, B; Mahner, E; Widmann, E

2010-01-01

The ground state hyperfine transition frequency of hydrogen is known to a very high precision and therefore the measurement of this transition frequency in antihydrogen is offering one of the most accurate tests of CPT symmetry. The ASACUSA collaboration at CERN will run an experiment designed to produce ground state antihydrogen atoms in a cusp trap. These antihydrogen atoms will pass with a low rate in the order of 1 per second through a spin-flip cavity where they get excited depending on their polarization by a 1.42 GHz magnetic field. Due to the small amount of antihydrogen atoms that will be available the requirement of good field homogeneity is imposed in order to obtain an interaction with as many antihydrogen atoms as possible. This leads to a requirement of an RF field deviation of less than ±10% transverse to the beam direction over a beam aperture with 10 cm diameter. All design aspects of this new spin-flip cavity, including the required field homogeneity and vacuum aspects, are discussed.

Modeling the neutron spin-flip process in a time-of-flight spin-resonance energy filter

CERN Document Server

Parizzi, A A; Klose, F

2002-01-01

A computer program for modeling the neutron spin-flip process in a novel time-of-flight (TOF) spin-resonance energy filter has been developed. The software allows studying the applicability of the device in various areas of spallation neutron scattering instrumentation, for example as a dynamic TOF monochromator. The program uses a quantum-mechanical approach to calculate the local spin-dependent spectra and is essential for optimizing the magnetic field profiles along the resonator axis. (orig.)

Skyrmion formation and optical spin-Hall effect in an expanding coherent cloud of indirect excitons.

Science.gov (United States)

Vishnevsky, D V; Flayac, H; Nalitov, A V; Solnyshkov, D D; Gippius, N A; Malpuech, G

2013-06-14

We provide a theoretical description of the polarization pattern and phase singularities experimentally evidenced recently in a condensate of indirect excitons [H. High et al., Nature 483, 584 (2012)]. We show that the averaging of the electron and hole orbital motion leads to a comparable spin-orbit interaction for both types of carriers. We demonstrate that the interplay between a radial coherent flux of bright indirect excitons and the Dresselhaus spin-orbit interaction results in the formation of spin domains and of topological defects similar to Skyrmions. We reproduce qualitatively all the features of the experimental data and obtain a polarization pattern as in the optical spin-Hall effect despite the different symmetry of the spin-orbit interactions.

Laser resolution of unpolarized-electron scattering cross sections into spin-conserved and spin-flip components

International Nuclear Information System (INIS)

Ritchie, B.

1981-01-01

The theory is presented for one-photon free-free absorption by electrons scattering from high-Z atoms. The absorption cross section provides sufficient information to resolve the unpolarized-electron total cross section, Vertical Barf(theta)Vertical Bar 2 +Vertical Barg(theta)Vertical Bar 2 , into its individual components for spin-nonflip, Vertical Barf(theta)Vertical Bar 2 , and spin-flip, Vertical Barg(theta)Vertical Bar 2 , scattering. The observation of a spin-polarization effect for a spin-independent process (free-free absorption) is analogous to the Fano effect for bound-free absorption

Energy-level repulsion by spin-orbit coupling in two-dimensional Rydberg excitons

Science.gov (United States)

Stephanovich, V. A.; Sherman, E. Ya.; Zinner, N. T.; Marchukov, O. V.

2018-05-01

We study the effects of Rashba spin-orbit coupling on two-dimensional Rydberg exciton systems. Using analytical and numerical arguments we demonstrate that this coupling considerably modifies the wave functions and leads to a level repulsion that results in a deviation from the Poissonian statistics of the adjacent level distance distribution. This signifies the crossover to nonintegrability of the system and hints at the possibility of quantum chaos emerging. Such behavior strongly differs from the classical realization, where spin-orbit coupling produces highly entangled, chaotic electron trajectories in an exciton. We also calculate the oscillator strengths and show that randomization appears in the transitions between states with different total momenta.

Topological Hall effect in diffusive ferromagnetic thin films with spin-flip scattering

Science.gov (United States)

Zhang, Steven S.-L.; Heinonen, Olle

2018-04-01

We study the topological Hall (TH) effect in a diffusive ferromagnetic metal thin film by solving a Boltzmann transport equation in the presence of spin-flip scattering. A generalized spin-diffusion equation is derived which contains an additional source term associated with the gradient of the emergent magnetic field that arises from skyrmions. Because of the source term, spin accumulation may build up in the vicinity of the skyrmions. This gives rise to a spin-polarized diffusion current that in general suppresses the bulk TH current. Only when the spin-diffusion length is much smaller than the skyrmion size does the TH resistivity approach the value derived by Bruno et al. [Phys. Rev. Lett. 93, 096806 (2004), 10.1103/PhysRevLett.93.096806]. We derive a general expression of the TH resistivity that applies to thin-film geometries with spin-flip scattering, and show that the corrections to the TH resistivity become large when the size of room temperature skyrmions is further reduced to tens of nanometers.

Study on proton spin flip in scattering by Ti and Fe nuclei

International Nuclear Information System (INIS)

Korbetskij, E.V.; Prokopenko, V.S.; Sklyarenko, V.D.; Chernievskij, V.K.; Shustov, A.V.

1981-01-01

Spin-orbital effects and mechanisms of inelastic scattering of protons with energy of 6.9 MeV by sup(14, 48)Ti and sup(54, 56, 58)Fe are studied by the analysis of experimental results within the framework of the method of coupled channels. Simultaneously angular dependences of cross sections of elastic and inelastic (two first 2 + levels) scatterings and a probability of spin flip of proton at inelastic scattering are analysed. Experimental data were used for analysis, obtained in the given work, as well as the data published earlier. Targets are used in experiment which are in the form of self-sustaining fine (1-2 mg cm -2 ) films, enriched with corresponding isotope. Cross section determination error is 8% in the average. Obtained angular dependences of spin flip probability for sup(54, 56)Fe at Esub(p)=6.9 MeV are very similar in form and close in value to analogous at Esub(p)= 6 MeV, and differ greatly for 56 Fe at Esub(p)=5.88 MeV. Angular distributions of spin flip probabilities of protons from sup(54, 56)Fe (ppsub(1)) reaction at energies of 10, 11 and 12 MeV show the sufficient energy dependence of their shape and value. Experimental data are described satisfactorily witohin the framework of the cupled channel method namely - differential cross sections of elastic and inelastic scattering and angular dependences of the probability of spin flip at the interaction of protons with 6.9 MeV energy with sup(46, 48)Ti and sup(54, 56, 58)Fe nuclei. Difficulties, appearing in the description of cross sections of elastic scattering in case of sup(46, 48)Ti and of inelastic one for 56 Fe show that indirect processes are of importance in the present energy range and they should be taken into consideration [ru

Dominant spin-flip effects for the hadronic-produced J/ψ polarization at the Tevatron

International Nuclear Information System (INIS)

Wu Xinggang; Fang Zhenyun

2009-01-01

Dominant spin-flip effects for the direct and prompt J/ψ polarizations at Tevatron run II with collision energy 1.96 TeV and rapidity cut |y J/ψ | 8 [ 3 S 1 ] into J/ψ is especially discussed with care. It is found that the spin-flip effect shall always dilute the J/ψ polarization, and with a suitable choice of the parameters a 0,1 and c 0,1,2 , the J/ψ polarization puzzle can be solved to a certain degree. At large transverse momentum p t , α for the prompt J/ψ is reduced by ∼50% for f 0 =v 2 and by ∼80% for f 0 =1. We also study the indirect J/ψ polarization from the b decays, which however is slightly affected by the same spin-flip effect and then shall provide a better platform to determine the color-octet matrix elements.

The nuclear deformation versus the spin-flip like excitations and the suppression of the 2 νββ decay amplitude

International Nuclear Information System (INIS)

Raduta, A. A.; Delion, D. S.; Faessler, A.

1998-01-01

The suppression mechanism of the Gamow-Teller double beta decay amplitude M GT is studied using a many body Hamiltonian which describes a composite system of protons and neutrons moving in a projected spherical single particle basis. Alike nucleons interact through pairing, while protons and neutrons by a separable dipole-dipole force both in the particle-hole (ph) and particle-particle (pp) channels. The spin-flip and non-spin-flip components of the QRPA phonons have different contributions to the M GT value. The relative magnitudes and phases depend on both the strength of the particle-particle interaction (g pp ) and nuclear deformation. The deformation yields a fragmentation of the M GT value on one hand and washes out the separation of states of pure spin-flip and non spin-flip structures. Due to this effect, M GT has only one fragmented resonance structure in the low part of the spectrum. The mechanism of M GT suppression is different for spherical and deformed nuclei. While for spherical situation the resonances of pure spin-flip and non spin-flip character are separated in energy, for deformed case the two resonances coincide. In both cases, approaching the critical value of g pp , where the Random Phase Approximation (RPA) breaks down, a lot of strength is accumulated in the lowest RPA state. The difference is that, while in the spherical case this has a non spin-flip nature, in the deformed case the state is a mixture of both types of configurations. (authors)

Changing the cubic ferrimagnetic domain structure in temperature region of spin flip transition

International Nuclear Information System (INIS)

Djuraev, D.R.; Niyazov, L.N.; Saidov, K.S.; Sokolov, B.Yu.

2011-01-01

The transformation of cubic ferrimagnetic Tb 0.2 Y 2.8 Fe 5 O 12 domain structure has been studied by magneto optic method in the temperature region of spontaneous spin flip phase transition (SPT). It has been found that SPT occurs in a finite temperature interval where the coexistence of low- and high- temperature magnetic phase domains has observed. A character of domain structure evolution in temperature region of spin flip essentially depends on the presence of mechanical stresses in crystal. Interpretation of experimental results has been carried out within the framework of SPT theory for a cubic crystal. (authors)

Dynamics of chiral oscillations: a comparative analysis with spin flipping

International Nuclear Information System (INIS)

Bernardini, A E

2006-01-01

Chiral oscillation as well as spin flipping effects correspond to quantum phenomena of fundamental importance in the context of particle physics and, in particular, of neutrino physics. From the point of view of first quantized theories, we are specifically interested in pointing out the differences between chirality and helicity by obtaining their dynamic equations for a fermionic Dirac-type particle (neutrino). We also identify both effects when the non-minimal coupling with an external (electro)magnetic field in the neutrino interacting Lagrangian is taken into account. We demonstrate that, however, there is no constraint between chiral oscillations, when it takes place in vacuum, and the process of spin flipping related to the helicity quantum number, which does not take place in vacuum. To conclude, we show that the origin of chiral oscillations (in vacuum) can be interpreted as projections of very rapid oscillations of position onto the longitudinal direction of momentum

Coherent spin dynamics of an interwell excitonic gas in GaAs/AlGaAs coupled quantum wells

DEFF Research Database (Denmark)

Larionov, A. V.; Bisti, V. E.; Bayer, M.

2006-01-01

The spin dynamics of an interwell exciton gas has been investigated in n-i-n GaAs/AlGaAs coupled quantum wells. The time evolution kinetics of the interwell exciton photoluminescence has been measured under resonant excitation of the 1s heavy-hole intrawell exciton, using a pulsed tunable laser...

Spin polarized semimagnetic exciton-polariton condensate in magnetic field.

Science.gov (United States)

Król, Mateusz; Mirek, Rafał; Lekenta, Katarzyna; Rousset, Jean-Guy; Stephan, Daniel; Nawrocki, Michał; Matuszewski, Michał; Szczytko, Jacek; Pacuski, Wojciech; Piętka, Barbara

2018-04-27

Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to the quantum wells, and consequently forming semimagnetic polaritons. In this system, increasing magnetic field can induce polariton condensation at constant excitation power. Here we evidence the spin polarization of a semimagnetic polaritons condensate exhibiting a circularly polarized emission over 95% even in a moderate magnetic field of about 3 T. Furthermore, we show that unlike nonmagnetic polaritons, an increase on excitation power results in an increase of the semimagnetic polaritons condensate spin polarization. These properties open new possibilities for testing theoretically predicted phenomena of spin polarized condensate.

Spin-flip induced magnetoresistance in positionally disordered organic solids.

Science.gov (United States)

Harmon, N J; Flatté, M E

2012-05-04

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.

Spin flip in single quantum ring with Rashba spin–orbit interation

Science.gov (United States)

Liu, Duan-Yang; Xia, Jian-Bai

2018-03-01

We theoretically investigate spin transport in the elliptical ring and the circular ring with Rashba spin–orbit interaction. It is shown that when Rashba spin–orbit interaction is relatively weak, a single circular ring can not realize spin flip, however an elliptical ring may work as a spin-inverter at this time, and the influence of the defect of the geometry is not obvious. Howerver if a giant Rashba spin–orbit interaction strength has been obtained, a circular ring can work as a spin-inverter with a high stability. Project supported by the National Natural Science Foundation of China (Grant No. 11504016).

Optically detected electron spin-flip resonance in CdMnTe

International Nuclear Information System (INIS)

Zeng, S.; Smith, L.C.; Davies, J.J.; Wolverson, D.; Bingham, S.J.; Aliev, G.N.

2006-01-01

We show that the spin-flip of electrons at neutral donors in a dilute magnetic semiconductor can be observed directly by means of optically-detected magnetic resonance (ODMR). Spectra obtained at 105 GHz for a bulk crystal of Cd 1-x Mn x Te with x = 0.005 showed strong signals with g -values ranging between 12 (at 4.2 K) and 35 (at 1.7 K), with magnetic resonance linewidths ranging from 0.3 Tesla to 0.1 Tesla at the lowest temperature. In energy terms, these linewidths are independent of temperature and agree with those in spin-flip Raman spectra from the same specimen. The line broadening is caused by fluctuations in the number of manganese ions that interact with a particular donor and an analysis of this leads to a value for the donor Bohr radius of 4.5 nm. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Spin-polarized current and shot noise in the presence of spin flip in a quantum dot via nonequilibrium Green's functions

DEFF Research Database (Denmark)

De Souza, Fabricio; Jauho, Antti-Pekka; Egues, J.C.

2008-01-01

Using nonequilibrium Green's functions we calculate the spin-polarized current and shot noise in a ferromagnet-quantum-dot-ferromagnet system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and coherent spin flip (similar to a transverse magnetic...... field) are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization p. In particular, p can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage....... The coherent spin flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression...

Spin-exciton interaction and related micro-photoluminescence spectra of ZnSe:Mn DMS nanoribbon.

Science.gov (United States)

Hou, Lipeng; Zhou, Weichang; Zou, Bingsuo; Zhang, Yu; Han, Junbo; Yang, Xinxin; Gong, Zhihong; Li, Jingbo; Xie, Sishen; Shi, Li-Jie

2017-03-10

For their spintronic applications the magnetic and optical properties of diluted magnetic semiconductors (DMS) have been studied widely. However, the exact relationships between the magnetic interactions and optical emission behaviors in DMS are not well understood yet due to their complicated microstructural and compositional characters from different growth and preparation techniques. Manganese (Mn) doped ZnSe nanoribbons with high quality were obtained by using the chemical vapor deposition (CVD) method. Successful Mn ion doping in a single ZnSe nanoribbon was identified by elemental energy-dispersive x-ray spectroscopy mapping and micro-photoluminescence (PL) mapping of intrinsic d-d optical transition at 580 nm, i.e. the transition of 4 T 1 ( 4 G) →  6 A 1 ( 6 s),. Besides the d-d transition PL peak at 580 nm, two other PL peaks related to Mn ion aggregates in the ZnSe lattice were detected at 664 nm and 530 nm, which were assigned to the d-d transitions from the Mn 2+ -Mn 2+ pairs with ferromagnetic (FM) coupling and antiferromagnetic (AFM) coupling, respectively. Moreover, AFM pair formation goes along with strong coupling with acoustic phonon or structural defects. These arguments were supported by temperature-dependent PL spectra, power-dependent PL lifetimes, and first-principle calculations. Due to the ferromagnetic pair existence, an exciton magnetic polaron (EMP) is formed and emits at 460 nm. Defect existence favors the AFM pair, which also can account for its giant enhancement of spin-orbital coupling and the spin Hall effect observed in PRL 97, 126603(2006) and PRL 96, 196404(2006). These emission results of DMS reflect their relation to local sp-d hybridization, spin-spin magnetic coupling, exciton-spin or phonon interactions covering structural relaxations. This kind of material can be used to study the exciton-spin interaction and may find applications in spin-related photonic devices besides spintronics.

Stochastic local operations and classical communication (SLOCC) and local unitary operations (LU) classifications of n qubits via ranks and singular values of the spin-flipping matrices

Science.gov (United States)

Li, Dafa

2018-06-01

We construct ℓ -spin-flipping matrices from the coefficient matrices of pure states of n qubits and show that the ℓ -spin-flipping matrices are congruent and unitary congruent whenever two pure states of n qubits are SLOCC and LU equivalent, respectively. The congruence implies the invariance of ranks of the ℓ -spin-flipping matrices under SLOCC and then permits a reduction of SLOCC classification of n qubits to calculation of ranks of the ℓ -spin-flipping matrices. The unitary congruence implies the invariance of singular values of the ℓ -spin-flipping matrices under LU and then permits a reduction of LU classification of n qubits to calculation of singular values of the ℓ -spin-flipping matrices. Furthermore, we show that the invariance of singular values of the ℓ -spin-flipping matrices Ω 1^{(n)} implies the invariance of the concurrence for even n qubits and the invariance of the n-tangle for odd n qubits. Thus, the concurrence and the n-tangle can be used for LU classification and computing the concurrence and the n-tangle only performs additions and multiplications of coefficients of states.

The nuclear deformation versus the spin-flip like excitations and the suppression of the 2νββ decay amplitude

International Nuclear Information System (INIS)

Raduta, A.A.; Delion, D.S.; Faessler, A.

1997-01-01

The suppression mechanism of the Gamow-Teller double beta decay amplitude M GT is studied using a many body Hamiltonian which describes a composite system of protons and neutrons moving in a projected spherical single particle basis. Alike nucleons interact through pairing while protons and neutrons by a separable dipole-dipole force both in the particle-hole (ph) and particle-particle (pp) channels. The spin-flip and non-spin-flip components of the QRPA phonons have a differents contribution to the M GT values. The relative magnitudes and phases depend both on the strength of the particle-particle interaction (g pp ) and on the nuclear deformation. The deformation yields a fragmentation of the M GT value on one hand and washes out the separation of states of pure spin-flip and non-spin-flip structures. Due to this effect M GT has only one fragmented resonance structure in the low part of the spectrum. (orig.)

Microwave-induced direct spin-flip transitions in mesoscopic Pd/Co heterojunctions

Science.gov (United States)

Pietsch, Torsten; Egle, Stefan; Keller, Martin; Fridtjof-Pernau, Hans; Strigl, Florian; Scheer, Elke

2016-09-01

We experimentally investigate the effect of resonant microwave absorption on the magneto-conductance of tunable Co/Pd point contacts. At the interface a non-equilibrium spin accumulation is created via microwave absorption and can be probed via point contact spectroscopy. We interpret the results as a signature of direct spin-flip excitations in Zeeman-split spin-subbands within the Pd normal metal part of the junction. The inverse effect, which is associated with the emission of a microwave photon in a ferromagnet/normal metal point contact, can also be detected via its unique signature in transport spectroscopy.

Microwave-induced direct spin-flip transitions in mesoscopic Pd/Co heterojunctions

International Nuclear Information System (INIS)

Pietsch, Torsten; Egle, Stefan; Keller, Martin; Fridtjof-Pernau, Hans; Strigl, Florian; Scheer, Elke

2016-01-01

We experimentally investigate the effect of resonant microwave absorption on the magneto-conductance of tunable Co/Pd point contacts. At the interface a non-equilibrium spin accumulation is created via microwave absorption and can be probed via point contact spectroscopy. We interpret the results as a signature of direct spin-flip excitations in Zeeman-split spin-subbands within the Pd normal metal part of the junction. The inverse effect, which is associated with the emission of a microwave photon in a ferromagnet/normal metal point contact, can also be detected via its unique signature in transport spectroscopy. (paper)

First observation of spin flips with a single proton stored in a cryogenic Penning trap

International Nuclear Information System (INIS)

Ulmer, Stefan

2011-01-01

In this thesis the very first observation of spin transitions of a single proton stored in a cryogenic double-Penning trap is presented. The experimental observation of spin transitions is based on the continuous Stern-Gerlach effect, which couples the spin of the single trapped proton to its axial eigenfrequency, by means of an inhomogeneous magnetic field. A spin transition causes a change of the axial frequency, which can be measured non-destructively. Due to the tiny magnetic moment of the proton, the direct detection of proton spin-flips is an exceeding challenge. To achieve spin-flip resolution, the proton was stored in the largest magnetic field inhomogeneity, which has ever been superimposed to a Penning trap, and its axial frequency was detected non-destructively. Therefore, superconducting detection systems with ultrahigh-sensitivity were developed, allowing the direct observation of the single trapped proton, as well as the high-precision determination of its eigenfrequencies. Based on novel experimental methods, which were developed in the framework of this thesis, the axial frequency of the particle was stabilized to a level, where the observation of single-proton spin-flips is possible, which was demonstrated. This experimental success is one of the most important steps towards the high-precision determination of the magnetic moment of the free proton. With the very first observation of spin transitions with a single trapped proton, a highly exciting perspective opens. All experimental techniques which were developed in this thesis can be directly applied to the antiproton. Thus, the first high-precision measurement of the magnetic moment of the antiproton becomes possible. This will provide a new high-precision test of the matterantimatter symmetry. (orig.)

Study of organic light emitting devices (OLEDs) with optimal emission efficiency

Energy Technology Data Exchange (ETDEWEB)

Singh, Jai [School of Engineering and IT, B-purple 12, Faculty of EHS, Charles Darwin University, Darwin, NT 0909 (Australia)

2010-04-15

The external emission efficiency of organic light emitting devices (OLEDs) is analysed by studying the rate of spontaneous emission of both singlet and triplet excitons and their corresponding radiative lifetimes. Rates of spontaneous emissions are calculated from the first order perturbation theory using the newly discovered time-dependent spin-orbit-exciton-photon interaction operator as the perturbation operator. It is clearly shown how the new interaction operator is responsible for attracting triplet excitons to a phosphor (heavy metal atom) and then it flips the spins to a singlet configuration. Thus, the spin forbidden transition becomes spin allowed. Calculated rates agree with the experimental results qualitatively. Results are of general interests for OLED studies. (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Anisotropy of exciton spectrum and spin-orbit interactions in quantum wells in tilted magnetic field

International Nuclear Information System (INIS)

Olendski, Oleg; Shahbazyan, Tigran V

2006-01-01

We study theoretically excitonic energy spectrum and optical absorption in narrowgap semiconductor quantum wells in strong magnetic field. We show that, in the presence of an in-plane field component, the absorption coefficient exhibit a double-peak structure due to hybridization of bright and dark excitons. If both Rashba and Dresselhaus spin-orbit terms are present, the spectrum is anisotropic in in-plane field orientation with respect to [100] axis. In particular, the magnitude of the splitting can be tuned in a wide interval by varying the azimuthal angle of the in-plane field. The absorption spectrrum anisotropy would allow simultaneous measurement Dresselhaus and Rashba spin-orbit coefficients

Probability of spin flipping of proton with energy 6.9 MeV at inelastic scattering with sup(54,56)Fe nuclei

International Nuclear Information System (INIS)

Prokopenko, V.S.; Sklyarenko, V.; Chernievskij, V.K.; Shustov, A.V.

1980-01-01

Spin-orbital effects of inelastic scattering of protons by nuclei with mean atomic weight are investigated along with the mechanisms of the reaction course by measuring proton spin flip. The experiment consists in measuring proton-gamma coincidences in mutually perpendicular planes by the technique of quick-slow coincidences. The excitation function of the 56 Fe(P,P 1 ) reaction is measured in the 3.5-6.2 MeV energy range. Angular dependences of probability of proton spin flip (a level of 2 + , 0.847 MeV) are measured at energies of incident protons of 4.96; 5.58 and 5.88 MeV. Measurements of probabilities of proton spin flipping at inelastic scattering by sup(54,56)Fe nuclei are performed in the process of studying spin-orbital effects and mechanisms of the reaction course. A conclusion is made that the inelastic scattering process in the energy range under investigation is mainly realized by two equivalent mechanisms: direct interaction and formation of a compound nucleus. Angular dependences for 54 Fe and 56 Fe noticeably differ in the values of probability of spin flip in the angular range of 50-150 deg

Spin-flip-Raman studies of semimagnetic II-VI heterostructures; Spin-flip-Raman-Untersuchungen an semimagnetischen II-VI-Halbleiter-Quantentroegen und Volumenproben

Energy Technology Data Exchange (ETDEWEB)

Lentze, Michael

2009-03-18

In the present doctoral thesis, spin flip Raman studies of semimagnetic (Zn,Mn)Se samples were in the focus of interest. Quantum wells as well as bulk-like materials were investigated. The main goal was a better understanding of the exchange interaction behaviour of heavily n-doped semimagnetic samples. The influence of doping on the exchange interaction is of special relevance with regard to spintronics applications. Several series of high quality MBE-grown (Zn,Mn)Se-samples samples were available. (orig.)

Observation of ESR spin flip satellite lines of trapped hydrogen atoms in solid H2 at 4.2 K

International Nuclear Information System (INIS)

Miyazaki, Tetsuo; Iwata, Nobuchika; Fueki, Kenji; Hase, Hirotomo

1990-01-01

ESR spectra of H atoms, produced in γ-irradiated solid H 2 , were studied at 4.2 K. Two main lines of the ESR spectra of H atoms that are separated by about 500 G accompanied two weak satellite lines. Both satellite lines and main lines decrease with the same decay rate. In the D 2 -H 2 mixtures, the satellite-line intensity depends upon the number of matrix protons. The spacing of the satellites from the main lines is equal to that of the NMR proton resonance frequency. It was concluded that the satellite lines were not ascribable to paired atoms but to spin flip lines due to an interaction of H atoms with matrix protons. The analysis of the spin flip lines and the main lines suggests that H atoms in solid H 2 are trapped in the substitutional site

Asymmetry of spin-flip of polarized protons in the inelastic scattering to the first 2+ states of 48Ti and 50Ti

International Nuclear Information System (INIS)

Tomizawa, M.; Aoki, T.; Aoki, Y.; Sakai, T.; Tagishi, Y.; Yagi, K.; Murayama, T.

1990-01-01

Angular distributions of differential cross section, analyzing power, spin-flip probability and spin-flip asymmetry in the excitation of the first 2 + states in 48 Ti and 50 Ti were measured at incident energies of 11 and 18 MeV using (p,p'γ) coincidence technique with polarized proton beam. The angular distributions show strong incident energy and target dependence. The results were analyzed in terms of a macroscopic coupled channels method based on the vibrational model and of the microscopic distorted wave Born approximation (DWBA) based on shell-model wave functions and effective nucleon-nucleon interactions. The spin-flip asymmetry is quite sensitive to the spin-dependent part in the interaction which causes the inelastic scattering. (author)

Magneto-optical quantum interferences in a system of spinor excitons

Science.gov (United States)

Kuan, Wen-Hsuan; Gudmundsson, Vidar

2018-04-01

In this work we investigate magneto-optical properties of two-dimensional semiconductor quantum-ring excitons with Rashba and Dresselhaus spin-orbit interactions threaded by a magnetic flux perpendicular to the plane of the ring. By calculating the excitonic Aharonov-Bohm spectrum, we study the Coulomb and spin-orbit effects on the Aharonov-Bohm features. From the light-matter interactions of the excitons, we find that for scalar excitons, there are open channels for spontaneous recombination resulting in a bright photoluminescence spectrum, whereas the forbidden recombination of dipolar excitons results in a dark photoluminescence spectrum. We investigate the generation of persistent charge and spin currents. The exploration of spin orientations manifests that by adjusting the strength of the spin-orbit interactions, the exciton can be constructed as a squeezed complex with specific spin polarization. Moreover, a coherently moving dipolar exciton acquires a nontrivial dual Aharonov-Casher phase, creating the possibility to generate persistent dipole currents and spin dipole currents. Our study reveals that in the presence of certain spin-orbit generated fields, the manipulation of the magnetic field provides a potential application for quantum-ring spinor excitons to be utilized in nano-scaled magneto-optical switches.

Spin-triplet excitons and anisotropy effects in the S=12 gapped antiferromagnet BaCuSi2O6

International Nuclear Information System (INIS)

Zvyagin, S.A.; Wosnitza, J.; Krzystek, J.; Stern, R.; Jaime, M.; Sasago, Y.; Uchinokura, K.

2007-01-01

BaCuSi 2 O 6 can be regarded as an almost ideal realization of an S=12 system of weakly interacting spin dimers with spin-singlet ground state and gapped excitation spectrum. We argue that the fine structure observed in low-temperature EPR spectra of BaCuSi 2 O 6 is a fingerprint of triplet excitations (excitons). Analyzing the angular dependence of the exciton modes allows us to precisely calculate the zero-field splitting within the triplet states and, correspondingly, the anisotropy parameter, D=0.07cm -1 . The proposed procedure can be applied for studying anisotropy effects in a large number of S=12 gapped quantum antiferromagnets with dimerized or alternating spin structure

Surface tension and Wulff shape for a lattice model without spin flip symmetry.

CERN Document Server

Bodineau, T

2003-01-01

We propose a new definition of surface tension and check it in a spin model of the Pirogov-Sinai class where the spin flip symmetry is broken. We study the model at low temperatures on the phase transitions line and prove: (i) existence of the surface tension in the thermodynamic limit, for any orientation of the surface and in all dimensions $d\\ge 2$; (ii) the Wulff shape constructed with such a surface tension coincides with the equilibrium shape of the cluster which appears when fixing the total spin magnetization (Wulff problem).

Flip-flopping binary black holes.

Science.gov (United States)

Lousto, Carlos O; Healy, James

2015-04-10

We study binary spinning black holes to display the long term individual spin dynamics. We perform a full numerical simulation starting at an initial proper separation of d≈25M between equal mass holes and evolve them down to merger for nearly 48 orbits, 3 precession cycles, and half of a flip-flop cycle. The simulation lasts for t=20 000M and displays a total change in the orientation of the spin of one of the black holes from an initial alignment with the orbital angular momentum to a complete antialignment after half of a flip-flop cycle. We compare this evolution with an integration of the 3.5 post-Newtonian equations of motion and spin evolution to show that this process continuously flip flops the spin during the lifetime of the binary until merger. We also provide lower order analytic expressions for the maximum flip-flop angle and frequency. We discuss the effects this dynamics may have on spin growth in accreting binaries and on the observational consequences for galactic and supermassive binary black holes.

Spin flipping in ring-coupled-cluster-doubles theory

DEFF Research Database (Denmark)

Klopper, Wim; M. Teale, Andrew; Coriani, Sonia

2011-01-01

We report a critical analysis and comparison of a variety of random-phase-approximation (RPA) based approaches to determine the electronic ground-state energy. Interrelations between RPA variants are examined by numerical examples with particular attention paid to the role of spin......-flipped excitations and the behaviour of the adiabatic-connection integrands where appropriate. In general, it is found that RPA variants that include Hartree–Fock exchange contributions are unsuitable as generally applicable methods for the determination of correlation energies. Of the remaining methods only...... the direct RPA and RPA with second-order screened exchange are recommended for general use....

Development of a 3He nuclear spin flip system on an in-situ SEOP 3He spin filter and demonstration for a neutron reflectometer and magnetic imaging technique

International Nuclear Information System (INIS)

Hayashida, H; Kira, H; Miyata, N; Akutsu, K; Mizusawa, M; Parker, J D; Matsumoto, Y; Oku, T; Sakai, K; Hiroi, K; Shinohara, T; Takeda, M; Yamazaki, D; Oikawa, K; Harada, M; Ino, T; Imagawa, T; Ohkawara, M; Ohoyama, K; Kakurai, K

2016-01-01

We have been developing a 3 He neutron spin filter (NSF) using the spin exchange optical pumping (SEOP) technique. The 3 He NSF provides a high-energy polarized neutron beam with large beam size. Moreover the 3 He NSF can work as a π-flipper for a polarized neutron beam by flipping the 3 He nuclear spin using a nuclear magnetic resonance (NMR) technique. For NMR with the in-situ SEOP technique, the polarization of the laser must be reversed simultaneously because a non-reversed laser reduces the polarization of the spin-flipped 3 He. To change the polarity of the laser, a half-wavelength plate was installed. The rotation angle of the half-wavelength plate was optimized, and a polarization of 97% was obtained for the circularly polarized laser. The 3 He polarization reached 70% and was stable over one week. A demonstration of the 3 He nuclear spin flip system was performed at the polarized neutron reflectometer SHARAKU (BL17) and NOBORU (BL10) at J-PARC. Off-specular measurement from a magnetic Fe/Cr thin film and magnetic imaging of a magnetic steel sheet were performed at BL17 and BL10, respectively. (paper)

Pseudo-spin flip in doubly decoupled structures and identical bands

International Nuclear Information System (INIS)

Kreiner, A.J.; Cardona, M.A.; Somacal, H.; Debray, M.E.; Hojman, D.; Davidson, J.; Davidson, M.; De Acuna, D.; Napoli, D.R.; Rico, J.; Bazzacco, D.; Burch, R.; Lenzi, S.M.; Rossi Alvarez, C.; Blasi, N.; Lo Bianco, G.

1995-01-01

Unfavored components of doubly decoupled bands are reported for the first time. They can be interpreted as having the pseudo-spin flipped relative to the orientation in the favored components, i.e. antialigned with respect to the rotation axis. In addition, the differences in consecutive transition energies along the favored and unfavored sequences are strikingly similar among them up to I π =15 + and 14 + respectively. This feature arises from a cancellation of differences in alignments and moments of inertia. ((orig.))

Generation of a third harmonic due to spin-flip transitions in non-symmetric heterostructures

CERN Document Server

Korovin, A V

2003-01-01

The third-order non-linear response due to spin-flip transitions of electrons in asymmetric narrow-gap quantum wells with a spin-split energy spectrum is calculated. The resonant spectral dependences and the gate-voltage dependences of the third-order susceptibility are obtained. The efficiency of up-conversion of the microwave pumping into submillimetre radiation in the multi-well structure is estimated and the dependences on the incidence angle and on the polarization of pumping are presented.

Non-flipping 13C spins near an NV center in diamond: hyperfine and spatial characteristics by density functional theory simulation of the C510[NV]H252 cluster

Science.gov (United States)

Nizovtsev, A. P.; Kilin, S. Ya; Pushkarchuk, A. L.; Pushkarchuk, V. A.; Kuten, S. A.; Zhikol, O. A.; Schmitt, S.; Unden, T.; Jelezko, F.

2018-02-01

Single NV centers in diamond coupled by hyperfine interaction (hfi) to neighboring 13C nuclear spins are now widely used in emerging quantum technologies as elements of quantum memory adjusted to a nitrogen-vacancy (NV) center electron spin qubit. For nuclear spins with low flip-flop rate, single shot readout was demonstrated under ambient conditions. Here we report on a systematic search for such stable NV-13C systems using density functional theory to simulate the hfi and spatial characteristics of all possible NV-13C complexes in the H-terminated cluster C510[NV]-H252 hosting the NV center. Along with the expected stable ‘NV-axial-13C’ systems wherein the 13C nuclear spin is located on the NV axis, we found for the first time new families of positions for the 13C nuclear spin exhibiting negligible hfi-induced flipping rates due to near-symmetric local spin density distribution. Spatially, these positions are located in the diamond bilayer passing through the vacancy of the NV center and being perpendicular to the NV axis. Analysis of available publications showed that, apparently, some of the predicted non-axial near-stable NV-13C systems have already been observed experimentally. A special experiment performed on one of these systems confirmed the prediction made.

Analysis of possibilities for a spin flip in high energy electron ring HERA

International Nuclear Information System (INIS)

Stres, S.; Pestotnik, R.

2007-01-01

In a high energy electron ring the spins of electrons become spontaneously polarized via the emission of spin-flip synchrotron radiation. By employing a radio frequency (RF) radial dipole field kicker, particle spin directions can be rotated slowly over many turns. A model which couples three dimensional spin motion and longitudinal particle motion was constructed to describe non-equilibrium spin dynamics in high energy electron storage rings. The effects of a stochastic synchrotron radiation on the orbital motion in the accelerator synchrotron plane and its influence on the spin motion are studied. The main contributions to the spin motion, the synchrotron oscillations and the stochastic synchrotron radiation, have different influence on the spin polarization reversal in different regions of the parameter space. The results indicate that polarization reversal might be obtained in high energy electron storage rings with a significant noise even with relatively small strengths of a perturbing magnetic field. The only experimental datum avaliable agrees with the model prediction, however further experimental data would be necessary to validate the model

Interlayer excitons in a bulk van der Waals semiconductor

DEFF Research Database (Denmark)

Arora, Ashish; Drueppel, Matthias; Schmidt, Robert

2017-01-01

Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity......, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments...

Magnetic dipole strength in 128Xe and 134Xe in the spin-flip resonance region

Science.gov (United States)

Massarczyk, R.; Rusev, G.; Schwengner, R.; Dönau, F.; Bhatia, C.; Gooden, M. Â. E.; Kelley, J. Â. H.; Tonchev, A. Â. P.; Tornow, W.

2014-11-01

The magnetic dipole strength in the energy region of the spin-flip resonance is investigated in 128Xe and 134Xe using quasimonoenergetic and linearly polarized γ -ray beams at the High-Intensity γ -Ray Source facility in Durham, North Carolina, USA. Absorption cross sections were deduced for the magnetic and electric and dipole strength distributions separately for various intervals of excitation energy, including the strength of states in the unresolved quasicontinuum. The magnetic dipole strength distributions show structures resembling a resonance in the spin-flip region around an excitation energy of 8 MeV. The electric dipole strength distributions obtained from the present experiments are in agreement with the ones deduced from an earlier experiment using broad-band bremsstrahlung instead of a quasimonoenergetic beam. The experimental magnetic and electric dipole strength distributions are compared with phenomenological approximations and with predictions of a quasiparticle random phase approximation in a deformed basis.

Electron spin exchange of shallow donor muonium states

International Nuclear Information System (INIS)

Senba, Masayoshi

2005-01-01

Shallow donor muonium states with small hyperfine frequencies, recently observed in II-VI semiconductor compounds, have a number of unique features that present both opportunities and challenges in understanding muon spin dynamics in the presence of Heisenberg spin exchange. First, the shallow muonium state in CdSe with hyperfine frequency ω 0 /2π ∼ 0.1 MHz is already in the high field regime even in the earth's magnetic field, where only two precession frequencies are observable by the muon spin rotation (μSR) technique. Second, unlike in the case of more conventional muonium species with a larger hyperfine frequency, the μSR signal of shallow muonium states can be observed even in the transition region, between the slow spin-flip regime and the fast spin-flip regime, where the spin-flip rate and the hyperfine frequency are comparable. The muon spin dynamics in the transition region has not been theoretically explored previously, mainly because normal muonium in vacuum gives no observable signal in this region. Third, in the case of shallow muonium states, the incoherent process defined to be those spin-flip collisions that cause changes in muon spin precession frequencies, becomes crucially important in the transition region, where the incoherent process is entirely negligible in more conventional muonium species. By taking incoherent multiple collisions into account, an analytical expression for the time evolution of the muon spin polarization in Mu is derived, where Mu undergoes repeated spin-flip collisions. Comparisons with Monte Carlo calculations show that the analytical expression obtained in this work can reliably be used to analyse experimental data for shallow donor states not only in the slow spin-flip regime, but also in the transition region up to the onset of the fast regime. The present work confirms a recent experimental finding that, in the transition region, the initial phases of the two precession components of shallow donor states

Magnetic dipole strength in {sup 128}Xe and {sup 134}Xe in the spin-flip resonance region

Energy Technology Data Exchange (ETDEWEB)

Massarczyk, R. [Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden (Germany); Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Rusev, G. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Schwengner, R.; Doenau, F. [Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden (Germany); Bathia, C. [McMaster University, Hamilton, Ontario L8S4L8 (Canada); Gooden, M.E.; Kelley, J.H. [Triangle Universities Nuclear Laboratory, Durham, NC 27708 (United States); North Carolina State University, Raleigh, NC 27695 (United States); Tonchev, A.P. [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States); Tornow, W. [Triangle Universities Nuclear Laboratory, Durham, NC 27708 (United States); Duke University, Durham, NC 27708 (United States)

2015-07-01

The magnetic dipole strength in the energy region of the spin-flip resonance is investigated in {sup 128}Xe and {sup 134}Xe using quasimonoenergetic and linearly polarized γ-ray beams at the High-Intensity γ-Ray Source facility in Durham, North Carolina, USA. Absorption cross sections were deduced for the magnetic and electric and dipole strength distributions separately for various intervals of excitation energy, including the strength of states in the unresolved quasicontinuum. The magnetic dipole strength distributions show structures resembling a resonance in the spin-flip region around an excitation energy of 8 MeV. The electric dipole strength distributions obtained from the present experiments are in agreement with the ones deduced from an earlier experiment using broad-band bremsstrahlung instead of a quasimonoenergetic beam. The experimental magnetic and electric dipole strength distributions are compared with model predictions.

Spin-polarization and spin-flip in a triple-quantum-dot ring by using tunable lateral bias voltage and Rashba spin-orbit interaction

Energy Technology Data Exchange (ETDEWEB)

Molavi, Mohamad, E-mail: Mo_molavi@yahoo.com [Faculty of Physics, Kharazmi University, Tehran (Iran, Islamic Republic of); Faizabadi, Edris, E-mail: Edris@iust.ac.ir [School of Physics, Iran University of Science and Technology, 16846 Tehran (Iran, Islamic Republic of)

2017-04-15

By using the Green's function formalism, we investigate the effects of single particle energy levels of a quantum dot on the spin-dependent transmission properties through a triple-quantum-dot ring structure. In this structure, one of the quantum dots has been regarded to be non-magnetic and the Rashba spin-orbit interaction is imposed locally on this dot while the two others can be magnetic. The on-site energy of dots, manipulates the interference of the electron spinors that are transmitted to output leads. Our results show that the effects of magnetic dots on spin-dependent transmission properties are the same as the difference of on-site energies of the various dots, which is applicable by a controllable lateral bias voltage externally. Besides, by tuning the parameters such as Rashba spin-orbit interaction, and on-site energy of dots and magnetic flux inside the ring, the structure can be indicated the spin-flip effect and behave as a full spin polarizer or splitter. - Highlights: • The effects of magnetic dots on spin-dependent transmission properties are the same as the difference of on-site energies of the various dots. • In the situation that the QDs have non-zero on-site energies, the system can demonstrate the full spin-polarization. • By tuning the Rashba spin-orbit strength and magnetic flux encountered by the ring the system operates as a Stern-Gerlach apparatus.

Interlayer excitons in a bulk van der Waals semiconductor.

Science.gov (United States)

Arora, Ashish; Drüppel, Matthias; Schmidt, Robert; Deilmann, Thorsten; Schneider, Robert; Molas, Maciej R; Marauhn, Philipp; Michaelis de Vasconcellos, Steffen; Potemski, Marek; Rohlfing, Michael; Bratschitsch, Rudolf

2017-09-21

Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments and ab initio calculations for 2H-MoTe 2 , we explain their salient features: the positive sign of the g-factor and the large diamagnetic shift. Our investigations solve the long-standing puzzle of positive g-factors in transition metal dichalcogenides, and pave the way for studying collective phenomena in these materials at elevated temperatures.Excitons, quasi-particles of bound electron-hole pairs, are at the core of the optoelectronic properties of layered transition metal dichalcogenides. Here, the authors unveil the presence of interlayer excitons in bulk van der Waals semiconductors, arising from strong localization and spin-valley coupling of charge carriers.

Anomalous Kondo-Switching Effect of a Spin-Flip Quantum Dot Embedded in an Aharonov-Bohm Ring

International Nuclear Information System (INIS)

Chen Xiongwen; Shi Zhengang; Song Kehui

2009-01-01

We theoretically investigate the Kondo effect of a quantum dot embedded in a mesoscopic Aharonov-Bohm (AB) ring in the presence of the spin flip processes by means of the one-impurity Anderson Hamiltonian. Based on the slave-boson mean-field theory, we find that in this system the persistent current (PC) sensitively depends on the parity and size of the AB ring and can be tuned by the spin-flip scattering (R). In the small AB ring, the PC is suppressed due to the enhancing R weakening the Kondo resonance. On the contrary, in the large AB ring, with R increasing, the peak of PC firstly moves up to max-peak and then down. Especially, the PC phase shift of π appears suddenly with the proper value of R, implying the existence of the anomalous Kondo effect in this system. Thus this system may be a candidate for quantum switch. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

Improved imaging of cochlear nerve hypoplasia using a 3-Tesla variable flip-angle turbo spin-echo sequence and a 7-cm surface coil.

Science.gov (United States)

Giesemann, Anja M; Raab, Peter; Lyutenski, Stefan; Dettmer, Sabine; Bültmann, Eva; Frömke, Cornelia; Lenarz, Thomas; Lanfermann, Heinrich; Goetz, Friedrich

2014-03-01

Magnetic resonance imaging of the temporal bone has an important role in decision making with regard to cochlea implantation, especially in children with cochlear nerve deficiency. The purpose of this study was to evaluate the usefulness of the combination of an advanced high-resolution T2-weighted sequence with a surface coil in a 3-Tesla magnetic resonance imaging scanner in cases of suspected cochlear nerve aplasia. Prospective study. Seven patients with cochlear nerve hypoplasia or aplasia were prospectively examined using a high-resolution three-dimensional variable flip-angle turbo spin-echo sequence using a surface coil, and the images were compared with the same sequence in standard resolution using a standard head coil. Three neuroradiologists evaluated the magnetic resonance images independently, rating the visibility of the nerves in diagnosing hypoplasia or aplasia. Eight ears in seven patients with hypoplasia or aplasia of the cochlear nerve were examined. The average age was 2.7 years (range, 9 months-5 years). Seven ears had accompanying malformations. The inter-rater reliability in diagnosing hypoplasia or aplasia was greater using the high-resolution three-dimensional variable flip-angle turbo spin-echo sequence (fixed-marginal kappa: 0.64) than with the same sequence in lower resolution (fixed-marginal kappa: 0.06). Examining cases of suspected cochlear nerve aplasia using the high-resolution three-dimensional variable flip-angle turbo spin-echo sequence in combination with a surface coil shows significant improvement over standard methods. © 2013 The American Laryngological, Rhinological and Otological Society, Inc.

New experimental upper limit of the electron–proton spin-flip cross-section

International Nuclear Information System (INIS)

Oellers, D.; Weidemann, C.; Lenisa, P.; Meyer, H.O.; Rathmann, F.; Trusov, S.; Augustyniak, W.; Bagdasarian, Z.; Barion, L.; Barsov, S.

2014-01-01

In a previous publication, measurements of the depolarization of a stored proton beam by interaction with a co-propagating unpolarized electron beam at low relative energy have been presented and an upper limit of about 3 ×10 7 b for the electron–proton spin-flip cross-section was determined. A refined analysis presented in this paper reduces the previous upper limit by a factor of three by the introduction of a new procedure that also makes use of non-identified particles

Ion cyclotron and spin-flip emissions from fusion products in tokamaks

International Nuclear Information System (INIS)

Arunasalam, V.; Greene, G.J.; Young, K.M.

1993-02-01

Power emission by fusion products of tokamak plasmas in their ion cyclotron range of frequencies (ICRF) and at their spin-flip resonance frequency is calculated for some specific model fusion product velocity-space distribution functions. The background plasma of say deuterium (D) is assumed to be in equilibrium with a Maxwellian distribution both for the electrons and ions. The fusion product velocity distributions analyzed here are: (1) A monoenergetic velocity space ring distribution. (2) A monoenergetic velocity space spherical shell distribution. (3) An anisotropic Maxwellian distribution with T perpendicular ≠ T parallel and with appreciable drift velocity along the confining magnetic field. Single ''dressed'' test particle spontaneous emission calculations are presented first and the radiation temperature for ion cyclotron emission (ICE) is analyzed both for black-body emission and nonequilibrium conditions. Thresholds for instability and overstability conditions are then examined and quasilinear and nonlinear theories of the electromagnetic ion cyclotron modes are discussed. Distinctions between ''kinetic or causal instabilities'' and ''hydrodynamic instabilities'' are drawn and some numerical estimates are presented for typical tokamak parameters. Semiquantitative remarks are offered on wave accessibility, mode conversion, and parametric decay instabilities as possible for spatially localized ICE. Calculations are carried out both for k parallel = 0 for k parallel ≠ 0. The effects of the temperature anisotropy and large drift velocities in the parallel direction are also examined. Finally, proton spin-flip resonance emission and absorption calculations are also presented both for thermal equilibrium conditions and for an ''inverted'' population of states

Ion cyclotron and spin-flip emissions from fusion products in tokamaks

Energy Technology Data Exchange (ETDEWEB)

Arunasalam, V.; Greene, G.J.; Young, K.M.

1993-02-01

Power emission by fusion products of tokamak plasmas in their ion cyclotron range of frequencies (ICRF) and at their spin-flip resonance frequency is calculated for some specific model fusion product velocity-space distribution functions. The background plasma of say deuterium (D) is assumed to be in equilibrium with a Maxwellian distribution both for the electrons and ions. The fusion product velocity distributions analyzed here are: (1) A monoenergetic velocity space ring distribution. (2) A monoenergetic velocity space spherical shell distribution. (3) An anisotropic Maxwellian distribution with T [perpendicular] [ne] T[parallel]and with appreciable drift velocity along the confining magnetic field. Single dressed'' test particle spontaneous emission calculations are presented first and the radiation temperature for ion cyclotron emission (ICE) is analyzed both for black-body emission and nonequilibrium conditions. Thresholds for instability and overstability conditions are then examined and quasilinear and nonlinear theories of the electromagnetic ion cyclotron modes are discussed. Distinctions between kinetic or causal instabilities'' and hydrodynamic instabilities'' are drawn and some numerical estimates are presented for typical tokamak parameters. Semiquantitative remarks are offered on wave accessibility, mode conversion, and parametric decay instabilities as possible for spatially localized ICE. Calculations are carried out both for k[parallel] = 0 for k[parallel] [ne] 0. The effects of the temperature anisotropy and large drift velocities in the parallel direction are also examined. Finally, proton spin-flip resonance emission and absorption calculations are also presented both for thermal equilibrium conditions and for an inverted'' population of states.

Analytic derivative couplings for spin-flip configuration interaction singles and spin-flip time-dependent density functional theory

International Nuclear Information System (INIS)

Zhang, Xing; Herbert, John M.

2014-01-01

We revisit the calculation of analytic derivative couplings for configuration interaction singles (CIS), and derive and implement these couplings for its spin-flip variant for the first time. Our algorithm is closely related to the CIS analytic energy gradient algorithm and should be straightforward to implement in any quantum chemistry code that has CIS analytic energy gradients. The additional cost of evaluating the derivative couplings is small in comparison to the cost of evaluating the gradients for the two electronic states in question. Incorporation of an exchange-correlation term provides an ad hoc extension of this formalism to time-dependent density functional theory within the Tamm-Dancoff approximation, without the need to invoke quadratic response theory or evaluate third derivatives of the exchange-correlation functional. Application to several different conical intersections in ethylene demonstrates that minimum-energy crossing points along conical seams can be located at substantially reduced cost when analytic derivative couplings are employed, as compared to use of a branching-plane updating algorithm that does not require these couplings. Application to H 3 near its D 3h geometry demonstrates that correct topology is obtained in the vicinity of a conical intersection involving a degenerate ground state

SETI at the spin-flip line frequency of positronium.

Science.gov (United States)

Mauersberger, R.; Wilson, T. L.; Rood, R. T.; Bania, T. M.; Hein, H.; Linhart, A.

1996-02-01

A directed search for extraterrestrial intelligence (SETI) has been carried out using the IRAM 30m telescope. Following a suggestion by Kardashev (1979), the search was conducted at the spin-flip line of the lightest atom, namely positronium, at 203GHz. Most of the 17 targets are mature stars with excess infrared radiation, which might be the waste heat of a power-rich technological civilisation. The rest frame of the cosmic background radiation was chosen as the velocity frame. The spectral resolution used was 9.7kHz. From the noise level, which was determined by the limited telescope time and weather conditions, the upper limit for the power of artificial omnidirectional transmitters at the positronium line frequency is of order 10^15^W. The relevance of this non-detection is discussed.

Robust tunable excitonic features in monolayer transition metal dichalcogenide quantum dots

Science.gov (United States)

Fouladi-Oskouei, J.; Shojaei, S.; Liu, Z.

2018-04-01

The effects of quantum confinement on excitons in parabolic quantum dots of monolayer transition metal dichalcogenides (TMDC QDs) are investigated within a massive Dirac fermion model. A giant spin-valley coupling of the TMDC QDs is obtained, larger than that of monolayer TMDC sheets and consistent with recent experimental measurements. The exciton transition energy and the binding energy are calculated, and it is found that the strong quantum confinement results in extremely high exciton binding energies. The enormously large exciton binding energy in TMDC QDs (({{E}{{B2D}}}∼ 500 meV)different kinds of TMDC QDs) ensures that the many body interactions play a significant role in the investigation of the optical properties of these novel nanostructures. The estimated oscillator strength and radiative lifetime of excitons are strongly size-dependent and indicate a giant oscillator strength enhancement and ultrafast radiative annihilation of excitons, varying from a few tens of femtoseconds to a few picoseconds. We found that the spin-dependent band gap, spin-valley coupling, binding energy and excitonic effects can be tuned by quantum confinements, leading to tunable quantum dots in monolayer TMDCs. This finding offers new functionality in engineering the interaction of a 2D material with light and creates promise for the quantum manipulation of spin and valley degrees of freedom in TMDC nanostructures, enabling versatile novel 2D quantum photonic and optoelectronic nanodevices.

Spin flip at unelastic scattering of protons with energy near 6 NeV o 50Cr and 52Cr nuclei

International Nuclear Information System (INIS)

Andronov, Yu.F.; Chubinskij, O.V.; Vinogradov, L.I.; Ehl'-Ashri, F.I.; Gustova, L.V.

1978-01-01

Angular S(Q) and energy S(E) dependences spin flip probability S were studied in inelastic scattering of protons with excitation of the 2 1 + states of 50 Cr (Q=-0.782 MeV) and 52 Cr (Q=-1.434 MeV) energy range from 5.6 to 5.95 MeV. In particular, it is elucidated how strongly the behaviour of the spin flip probability depends upon the energy and angle of scattering at Esub(p) approximately 6 MeV for 50 Cr and 52 Cr. Thereby some additional information on specific features of the mechanism of inelastic sccattering by th nuclei is obtained. Measurements were carried out simultaneously in two proton detection channels at scattering angles differing by 30 deg. For targets use was made of self-sustaining enriched foils (87% 50 Cr and 99% 52 Cr). The angular and energy dependences of the spin flip probabilities for 50 Cr and 52 Cr are shown to be rather different: for 52 Cr has a relatively slight energy dependence in the range of a resonance observed in the excitation function; for 50 Cr the behaviour of S(E) undergoes sharp changes. The experimetnal values of S(Q) for sup(50, 52)Cr differ rather strongly from the calculations made on the statistical model and depend considerably upon the scattering angle

Exciton Seebeck effect in molecular systems

Energy Technology Data Exchange (ETDEWEB)

Yan, Yun-An, E-mail: yunan@nano.gznc.edu.cn [Guizhou Provincial Key Laboratory of Computational Nanomaterial Science, Guizhou Normal College, Guiyang, Guizhou 550018 (China); Cai, Shaohong [Guizhou Key Laboratory of Economic System Simulation, Guizhou University of Finance and Economics, Guiyang 550004 (China)

2014-08-07

We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

The role of the quark-antiquark pairs in the spin-flip effects in QCD at large distances

International Nuclear Information System (INIS)

Goloskokov, S.V.

1989-01-01

In the model with taking account of the long-distance properties of QCD it is shown that the quark loops in the t-channel exchange and qq-bar sea contributions lead to the spin-flip amplitude growing as S at high energies and fixed momenta transfer. 15 refs.; 3 figs

Bose-Einstein condensation and indirect excitons: a review.

Science.gov (United States)

Combescot, Monique; Combescot, Roland; Dubin, François

2017-06-01

We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath

Determining the nature of excitonic dephasing in high-quality GaN/AlGaN quantum wells through time-resolved and spectrally resolved four-wave mixing spectroscopy

Science.gov (United States)

Gallart, M.; Ziegler, M.; Crégut, O.; Feltin, E.; Carlin, J.-F.; Butté, R.; Grandjean, N.; Hönerlage, B.; Gilliot, P.

2017-07-01

Applying four-wave mixing spectroscopy to a high-quality GaN/AlGaN single quantum well, we report on the experimental determination of excitonic dephasing times at different temperatures and exciton densities in III-nitride heterostructures. By comparing the evolution with the temperature of the dephasing and the spin-relaxation rate, we conclude that both processes are related to the rate of excitonic collisions. When spin relaxation occurs in the motional-narrowing regime, it remains constant over a large temperature range as the spin-precession frequency increases linearly with temperature, hence compensating for the observed decrease in the dephasing time. From those measurements, a value of the electron-hole exchange interaction strength of 0.45 meV at T =10 K is inferred.

Spin effects in high energy quark-quark scattering

International Nuclear Information System (INIS)

Goloskokov, S.V.; Selyugin, O.V.

1993-01-01

The spin amplitudes in high-energy quark-quark scattering at /t/>1 GeV 2 are analyzed. It is shown that the gluon contributions in the QCDα s 3 order lead to the spin-flip amplitude growing as s. This means the existence of the spin-flip part in pomeron exchange. The resulting T f is about few per cent of the spin-non-flip contribution. The factorization of the large-distance and high-energy effects in the spin-flip amplitude is obtained. 13 refs.; 2 figs.; 1 tab

Search for magnetic dipole strength and giant spin-flip resonances in heavy nuclei

Energy Technology Data Exchange (ETDEWEB)

Horen, D. J. [Oak Ridge National Lab., TN (USA); Ikegami, H.; Muraoka, M. [eds.; Osaka Univ., Suita (Japan). Research Center for Nuclear Physics

1980-01-01

A description is given of the use of high resolution (n, n) scattering and the (p, n) reaction as tools to investigate highly excited states with emphasis on information pertaining to magnetic dipole strength and giant spin-flip resonances in heavy nuclei. It is shown how the ability to uniquely determine the spins and parities of resonances observed in neutron scattering has been instrumental to an understanding of the distribution of M1 strength in sup(207,208)Pb. Some recent results of (p, n) studies with intermediate energy protons are discussed. Energy systematics of the giant Gamow-Teller (GT) resonance as well as a new ..delta..l = 1, ..delta..S = 1 resonance with J sup(..pi..) = (1,2)/sup -/ are presented. It is shown how the (p, n) reaction might be useful to locate M1 strength in heavy nuclei.

Anisotropy and Suppression of Spin-Orbit Interaction in a GaAs Double Quantum Dot

Science.gov (United States)

Hofmann, A.; Maisi, V. F.; Krähenmann, T.; Reichl, C.; Wegscheider, W.; Ensslin, K.; Ihn, T.

2017-10-01

The spin-flip tunneling rates are measured in GaAs-based double quantum dots by time-resolved charge detection. Such processes occur in the Pauli spin blockade regime with two electrons occupying the double quantum dot. Ways are presented for tuning the spin-flip tunneling rate, which on the one hand gives access to measuring the Rashba and Dresselhaus spin-orbit coefficients. On the other hand, they make it possible to turn on and off the effect of spin-orbit interaction with a high on/off ratio. The tuning is accomplished by choosing the alignment of the tunneling direction with respect to the crystallographic axes, as well as by choosing the orientation of the external magnetic field with respect to the spin-orbit magnetic field. Spin lifetimes of 10 s are achieved at a tunneling rate close to 1 kHz.

Anisotropy and Suppression of Spin-Orbit Interaction in a GaAs Double Quantum Dot.

Science.gov (United States)

Hofmann, A; Maisi, V F; Krähenmann, T; Reichl, C; Wegscheider, W; Ensslin, K; Ihn, T

2017-10-27

The spin-flip tunneling rates are measured in GaAs-based double quantum dots by time-resolved charge detection. Such processes occur in the Pauli spin blockade regime with two electrons occupying the double quantum dot. Ways are presented for tuning the spin-flip tunneling rate, which on the one hand gives access to measuring the Rashba and Dresselhaus spin-orbit coefficients. On the other hand, they make it possible to turn on and off the effect of spin-orbit interaction with a high on/off ratio. The tuning is accomplished by choosing the alignment of the tunneling direction with respect to the crystallographic axes, as well as by choosing the orientation of the external magnetic field with respect to the spin-orbit magnetic field. Spin lifetimes of 10 s are achieved at a tunneling rate close to 1 kHz.

Studies on 16 μm spin-flip raman lasers in Tottori University

International Nuclear Information System (INIS)

Miyazaki, Kazuhiko

1986-01-01

This report outlines the studies on 16 μm spin-flip Raman lasers which have been carried out in Tottori University, Japan. Following the introductory section, the second section of the report deals with performance of infrared lasers for molecular laser isotope separation of UF 6 . It is stressed that the wavelength of the oscillation line should be accurately controlled in the vicinity of 628 cm -1 . The third section addresses 16 μm infrared lasers. Semiconductor diode lasers of Pb 1-x Sn x Te or Pb 1-x Sn x Se are available for the infrared region around 16 μm. Though the wavelength resolution is high and the oscillation wavelength is adjustable in these lasers, their oscillation outputs are not sufficient for the purpose of uranium separation. On the other hand, there are active studies on light-excited infrared gas lasers. It seems very difficult, however, to adjust the wavelength of their oscillation lines to the infrared absorption wavelength of 235 U. Thus, attention is currently focused on Raman lasers in the region around 16 μm. The fourth section briefly summarizes studies conducted in the University during these ten-odd years and the research facilities currently used. In particular, the major part is devoted to research and development of infrared NH 3 lasers and studies on InSb SFR (spin-flip Raman) lasers pumped with an NH 3 laser. A 12.97 μm and a 13.27 μm oscillation line have been obtained with a high output, which is hoped to serve for increasing the output of the 15.9 μm line of InSb SFR lasers. (Nogami, K.)

Spin helical states and spin transport of the line defect in silicene lattice

Energy Technology Data Exchange (ETDEWEB)

Yang, Mou; Chen, Dong-Hai; Wang, Rui-Qiang [Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006 (China); Bai, Yan-Kui, E-mail: ykbai@semi.ac.cn [College of Physical Science and Information Engineering and Hebei Advance Thin Films Laboratory, Hebei Normal University, Shijiazhuang, Hebei 050024 (China)

2015-02-06

We investigated the electronic structure of a silicene-like lattice with a line defect under the consideration of spin–orbit coupling. In the bulk energy gap, there are defect related bands corresponding to spin helical states localized beside the defect line: spin-up electrons flow forward on one side near the line defect and move backward on the other side, and vice versa for spin-down electrons. When the system is subjected to random distribution of spin-flipping scatterers, electrons suffer much less spin-flipped scattering when they transport along the line defect than in the bulk. An electric gate above the line defect can tune the spin-flipped transmission, which makes the line defect as a spin-controllable waveguide. - Highlights: • Band structure of silicene with a line defect. • Spin helical states around the line defect and their probability distribution features. • Spin transport along the line defect and that in the bulk silicene.

Robust adiabatic approach to optical spin entangling in coupled quantum dots

International Nuclear Information System (INIS)

Gauger, Erik M; Benjamin, Simon C; Lovett, Brendon W; Nazir, Ahsan; Stace, Thomas M

2008-01-01

Excitonic transitions offer a possible route to ultrafast optical spin manipulation in coupled nanostructures. We perform here a detailed study of the three principal exciton-mediated decoherence channels for optically controlled electron spin qubits in coupled quantum dots: radiative decay of the excitonic state, exciton-phonon interactions, and Landau-Zener transitions between laser-dressed states. We consider a scheme for producing an entangling controlled-phase gate on a pair of coupled spins which, in its simplest dynamic form, renders the system subject to fast decoherence rates associated with exciton creation during the gating operation. In contrast, we show that an adiabatic approach employing off-resonant laser excitation allows us to suppress all sources of decoherence simultaneously, significantly increasing the fidelity of operations at only a relatively small gating time cost. We find that controlled-phase gates accurate to one part in 10 2 can realistically be achieved with the adiabatic approach, whereas the conventional dynamic approach does not appear to support a fidelity suitable for scalable quantum computation. Our predictions could be demonstrated experimentally in the near future

Spin-dependent quantum transport in nanoscaled geometries

Science.gov (United States)

Heremans, Jean J.

2011-10-01

We discuss experiments where the spin degree of freedom leads to quantum interference phenomena in the solid-state. Under spin-orbit interactions (SOI), spin rotation modifies weak-localization to weak anti-localization (WAL). WAL's sensitivity to spin- and phase coherence leads to its use in determining the spin coherence lengths Ls in materials, of importance moreover in spintronics. Using WAL we measure the dependence of Ls on the wire width w in narrow nanolithographic ballistic InSb wires, ballistic InAs wires, and diffusive Bi wires with surface states with Rashba-like SOI. In all three systems we find that Ls increases with decreasing w. While theory predicts the increase for diffusive wires with linear (Rashba) SOI, we experimentally conclude that the increase in Ls under dimensional confinement may be more universal, with consequences for various applications. Further, in mesoscopic ring geometries on an InAs/AlGaSb 2D electron system (2DES) we observe both Aharonov-Bohm oscillations due to spatial quantum interference, and Altshuler-Aronov-Spivak oscillations due to time-reversed paths. A transport formalism describing quantum coherent networks including ballistic transport and SOI allows a comparison of spin- and phase coherence lengths extracted for such spatial- and temporal-loop quantum interference phenomena. We further applied WAL to study the magnetic interactions between a 2DES at the surface of InAs and local magnetic moments on the surface from rare earth (RE) ions (Gd3+, Ho3+, and Sm3+). The magnetic spin-flip rate carries information about magnetic interactions. Results indicate that the heavy RE ions increase the SOI scattering rate and the spin-flip rate, the latter indicating magnetic interactions. Moreover Ho3+ on InAs yields a spin-flip rate with an unusual power 1/2 temperature dependence, possibly characteristic of a Kondo system. We acknowledge funding from DOE (DE-FG02-08ER46532).

Rates and energetics of tyrosine ring flips in yeast iso-2-cytochrome c

International Nuclear Information System (INIS)

Nall, B.T.; Zuniga, E.H.

1990-01-01

Isotope-edited nuclear magnetic resonance spectroscopy is used to monitor ring flip motion of the five tyrosine side chains in the oxidized and reduced forms of yeast iso-2-cytochrome c. With specifically labeled protein purified from yeast grown on media containing [3,5- 13 C]tyrosine, isotope-edited one-dimensional proton spectra have been collected over a 5-55 degree C temperature range. The spectra allow selective observation of the 10 3,5 tyrosine ring proton resonances and, using a two-site exchange model, allow estimation of the temperature dependence of ring flip rates from motion-induced changes in proton line shapes. For the reduced protein, tyrosines II and IV are in fast exchange throughout the temperature range investigated, or lack resolvable differences in static chemical shifts for the 3,5 ring protons. Tyrosines I, III, and V are in sloe exchange at low temperatures and in fast exchange at high temperatures. Spectral simulations give flip rates for individual tyrosines in a range of one flip per second at low temperatures to thousands of flips per second at high temperatures. Eyring plots show that two of the tyrosines (I and III) have essentially the same activation parameters. Tentative sequence-specific assignments for the tyrosines in reduced iso-2 are suggested by comparison to horse cytochrome c. For oxidized iso-2, five resonances are observed at high temperatures, suggesting flip rates for all five tyrosines sufficient to average static chemical shift differences. At lower temperatures, there is evidence of intermediate and slow flipping for some of the rings

Electron--noble-gas spin-flip scattering at low energy

International Nuclear Information System (INIS)

Walker, T.G.; Bonin, K.; Happer, W.

1987-01-01

The spin-exchange rates and spin-relaxation rates for thermal electrons colliding with noble-gas atoms are calculated using the orthogonalized-plane-wave approximation and via partial-wave analysis. The two techniques give similar results and are in order-of-magnitude agreement with the experimental rate in Ar

Spin-orbit coupling and electric-dipole spin resonance in a nanowire double quantum dot.

Science.gov (United States)

Liu, Zhi-Hai; Li, Rui; Hu, Xuedong; You, J Q

2018-02-02

We study the electric-dipole transitions for a single electron in a double quantum dot located in a semiconductor nanowire. Enabled by spin-orbit coupling (SOC), electric-dipole spin resonance (EDSR) for such an electron can be generated via two mechanisms: the SOC-induced intradot pseudospin states mixing and the interdot spin-flipped tunneling. The EDSR frequency and strength are determined by these mechanisms together. For both mechanisms the electric-dipole transition rates are strongly dependent on the external magnetic field. Their competition can be revealed by increasing the magnetic field and/or the interdot distance for the double dot. To clarify whether the strong SOC significantly impact the electron state coherence, we also calculate relaxations from excited levels via phonon emission. We show that spin-flip relaxations can be effectively suppressed by the phonon bottleneck effect even at relatively low magnetic fields because of the very large g-factor of strong SOC materials such as InSb.

Large deviation principle for one-dimensional random walk in dynamic random environment: attractive spin-flips and simple symmetric exclusion

NARCIS (Netherlands)

Avena, L.; Hollander, den W.Th.F.; Redig, F.H.J.

2010-01-01

Consider a one-dimensional shift-invariant attractive spin-flip system in equilibrium, constituting a dynamic random environment, together with a nearest-neighbor random walk that on occupied sites has a local drift to the right but on vacant sites has a local drift to the left. In previous work we

Bright triplet excitons in caesium lead halide perovskites

Science.gov (United States)

Becker, Michael A.; Vaxenburg, Roman; Nedelcu, Georgian; Sercel, Peter C.; Shabaev, Andrew; Mehl, Michael J.; Michopoulos, John G.; Lambrakos, Samuel G.; Bernstein, Noam; Lyons, John L.; Stöferle, Thilo; Mahrt, Rainer F.; Kovalenko, Maksym V.; Norris, David J.; Rainò, Gabriele; Efros, Alexander L.

2018-01-01

Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Controlling T2 blurring in 3D RARE arterial spin labeling acquisition through optimal combination of variable flip angles and k-space filtering.

Science.gov (United States)

Zhao, Li; Chang, Ching-Di; Alsop, David C

2018-02-09

To improve the SNR efficiency and reduce the T 2 blurring of 3D rapid acquisition with relaxation enhancement stack-of-spiral arterial spin labeling imaging by using variable refocusing flip angles and k-space filtering. An algorithm for determining the optimal combination of variable flip angles and filtering correction is proposed. The flip angles are designed using extended phase graph physical simulations in an analytical and global optimization framework, with an optional constraint on deposited power. Optimal designs for correcting to Hann and Fermi window functions were compared with conventional constant amplitude or variable flip angle only designs on 6 volunteers. With the Fermi window correction, the proposed optimal designs provided 39.8 and 27.3% higher SNR (P variable flip angle designs. Even when power deposition was limited to 50% of the constant amplitude design, the proposed method outperformed the SNR (P variable flip angles can be derived as the output of an optimization problem. The combined design of variable flip angle and k-space filtering provided superior SNR to designs primarily emphasizing either approach singly. © 2018 International Society for Magnetic Resonance in Medicine.

Design of a spin-flip cavity for the measurement of the antihydrogen hyperfine structure

CERN Document Server

Kroyer, T

2008-01-01

In the framework of the ASACUSA collaboration at the CERN Antiproton Decelerator an experiment for precisely testing the CPT invariance of the hydrogen hyperfine structure is currently being designed. An integral part of the set-up is the 1.42 GHz spin-flipping cavity, which should have a good field homogeneity over the large aperture of the antihydrogen beam. After the evaluation of various approaches a structure based on a resonant stripline is proposed as a concrete cavity design. For this structure the field homogeneity, undesired modes, coupling and power issues are discussed in detail.

Colloquium: Excitons in atomically thin transition metal dichalcogenides

Science.gov (United States)

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; Heinz, Tony F.; Marie, Xavier; Amand, Thierry; Urbaszek, Bernhard

2018-04-01

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Exciton-relaxation dynamics in lead halides

International Nuclear Information System (INIS)

Iwanaga, Masanobu; Hayashi, Tetsusuke

2003-01-01

We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide

Excitonic Effects in Methylammonium Lead Halide Perovskites

Energy Technology Data Exchange (ETDEWEB)

Beard, Matthew C [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Chen, Xihan [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Lu, Haipeng [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Yang, Ye [National Renewable Energy Laboratory (NREL), Golden, CO (United States)

2018-05-01

The exciton binding energy in methylammonium lead iodide (MAPbI3) is about 10 meV, around 1/3 of the available thermal energy (kBT ~ 26 meV) at room temperature. Thus, exciton populations are not stable at room temperature at moderate photoexcited carrier densities. However, excitonic resonances dominate the absorption onset. Furthermore, these resonances determine the transient absorbance and transient reflectance spectra. The exciton binding energy is a reflection of the Coulomb interaction energy between photoexcited electrons and holes. As such, it serves as a marker for the strength of electron/hole interactions and impacts a variety of phenomena, such as, absorption, radiative recombination, and Auger recombination. In this Perspective, we discuss the role of excitons and excitonic resonances in the optical properties of lead-halide perovskite semiconductors. Finally, we discuss how the strong light-matter interactions induce an optical stark effect splitting the doubly spin degenerate ground exciton states and are easily observed at room temperature.

Stimulated polarization wave process in spin 3/2 chains

International Nuclear Information System (INIS)

Furman, G. B.

2007-01-01

Stimulated wave of polarization, triggered by a flip of a single spin, presents a simple model of quantum amplification. Recently, it has been demonstrated that, in an idealized one-dimensional Ising spin 1/2 chain with nearest-neighbor interactions and realistic spin 1/2 chain including the natural dipole-dipole interactions, irradiated by a weak resonant transverse field, a wave of flipped spins can be triggered by a single spin flip. Here we focuse on control of polarization wave in chain of spin 3/2, where the nuclear quadrupole interaction is dominant. Results of simulations for 1D spin chains and rings with up to five spins are presented.

Hanle model of a spin-orbit coupled Bose-Einstein condensate of excitons in semiconductor quantum wells

Science.gov (United States)

Andreev, S. V.; Nalitov, A. V.

2018-04-01

We present a theoretical model of a driven-dissipative spin-orbit coupled Bose-Einstein condensate of indirect excitons in semiconductor quantum wells (QW's). Our steady-state solution of the problem shares analogies with the Hanle effect in an optical orientation experiment. The role of the spin pump in our case is played by Bose-stimulated scattering into a linearly-polarized ground state and the depolarization occurs as a result of exchange interaction between electrons and holes. Our theory agrees with the recent experiment [A. A. High et al., Phys. Rev. Lett. 110, 246403 (2013), 10.1103/PhysRevLett.110.246403], where spontaneous emergence of spatial coherence and polarization textures have been observed. As a complementary test, we discuss a configuration where an external magnetic field is applied in the structure plane.

Distance measurements across randomly distributed nitroxide probes from the temperature dependence of the electron spin phase memory time at 240 GHz

Science.gov (United States)

Edwards, Devin T.; Takahashi, Susumu; Sherwin, Mark S.; Han, Songi

2012-10-01

At 8.5 T, the polarization of an ensemble of electron spins is essentially 100% at 2 K, and decreases to 30% at 20 K. The strong temperature dependence of the electron spin polarization between 2 and 20 K leads to the phenomenon of spin bath quenching: temporal fluctuations of the dipolar magnetic fields associated with the energy-conserving spin "flip-flop" process are quenched as the temperature of the spin bath is lowered to the point of nearly complete spin polarization. This work uses pulsed electron paramagnetic resonance (EPR) at 240 GHz to investigate the effects of spin bath quenching on the phase memory times (TM) of randomly-distributed ensembles of nitroxide molecules below 20 K at 8.5 T. For a given electron spin concentration, a characteristic, dipolar flip-flop rate (W) is extracted by fitting the temperature dependence of TM to a simple model of decoherence driven by the spin flip-flop process. In frozen solutions of 4-Amino-TEMPO, a stable nitroxide radical in a deuterated water-glass, a calibration is used to quantify average spin-spin distances as large as r¯=6.6 nm from the dipolar flip-flop rate. For longer distances, nuclear spin fluctuations, which are not frozen out, begin to dominate over the electron spin flip-flop processes, placing an effective ceiling on this method for nitroxide molecules. For a bulk solution with a three-dimensional distribution of nitroxide molecules at concentration n, we find W∝n∝1/r, which is consistent with magnetic dipolar spin interactions. Alternatively, we observe W∝n for nitroxides tethered to a quasi two-dimensional surface of large (Ø ˜ 200 nm), unilamellar, lipid vesicles, demonstrating that the quantification of spin bath quenching can also be used to discern the geometry of molecular assembly or organization.

Gigantic spin splitting of exciton states in CdSe:Mn hexagonal crystal

International Nuclear Information System (INIS)

Komarov, A.V.; Ryabchenko, S.M.; Semenov, Yu.G.; Shanina, B.D.; Vitrikhovskij, N.I.; AN Ukrainskoj SSR, Kiev. Inst. Poluprovodnikov)

1980-01-01

Gigantic spin splitting of exciton states in magneto-doped semiconductors is observed for the first time in the CdSe: Mn hexagonal crystal. A theoretical interpretation of some features of the effect due to the anisotropy of the crystal is presented. The parameters of the band structure are determined by comparing with the experiments: Δ 1 =46+-3, Δ 2 =137+-1, Δ 3 =140.6+-0.3 meV. It is shown that in CdSe:Mn just as in cubic semiconductors, exchange interaction with magnetic impurities is ferromagnetic for electrons of the conductivity band and antiferromagnetic for electrons of the valence band. The exchange constants are of the same order of magnetude as those for the CdTe:Mn, ZnTe:Mn and ZnSe:Mn crystals

A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes

Science.gov (United States)

Sahin Tiras, Kevser; Wang, Yifei; Harmon, Nicholas J.; Wohlgenannt, Markus; Flatte, Michael E.

In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.

Electron Spin Optical Orientation in Charged Quantum Dots

Science.gov (United States)

Shabaev, A.; Gershoni, D.; Korenev, V. L.

2005-03-01

We present a theory of nonresonant optical orientation of electron spins localized in quantum dots. This theory explains the negative circularly polarized photoluminescence of singlet trions localized in quantum dots previously observed in experiments where trion polarization changed to negative with time and where the degree of the negative polarization increased with intensity of pumping light. We have shown that this effect can be explained by the accumulation of dark excitons that occurs due to the spin blocking of the singlet trion formation - the major mechanism of dark exciton recombination. The accumulation of dark excitons results from a lack of electrons with a spin matching the exciton polarization. The electron spin lifetime is shortened by a transverse magnetic field or a temperature increase. This takes the block off the dark exciton recombination and restores the positive degree of trion polarization. The presented theory gives good agreement with experimental data.

Frequency Fine-tuning of a Spin-flip Cavity for Antihydrogen Atoms

CERN Document Server

Federmann, S; Mahner, E; Juhasz, B; Widmann, E

2012-01-01

As part of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) physics program a spin-flip cavity, for measurements of the ground-state hyperfine transition frequency of antihydrogen atoms, is needed. The purpose of the cavity is to excite antihydrogen atoms depending on their polarisation by a microwave field operating at 1.42 GHz. The delicacy of designing such a cavity lies in achieving and maintaining the required properties of this field over a large aperture of 10 cm and for a long period of time (required amplitude stability is 1% over 12 h). This paper presents the frequency fine tuning techniques developed to obtain the desired centre frequency of 1.42GHz with a Q value below 500 as well as the circuit used for the frequency sweep over a bandwidth of 6MHz.

Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport

Science.gov (United States)

Seward, Kenton; Lin, Zhibin; Lusk, Mark

2012-02-01

The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained.

Observation of surface excitons in rare gas solids

International Nuclear Information System (INIS)

Saile, V.; Skibowski, M.; Steinmann, W.; Guertler, P.; Koch, E.E.; Kozevnikov, A.

1976-04-01

Evidence is obtained for the excitation of surface excitons in solid Ar, Kr and Xe in optical transmission and reflection experiments using synchrotron radiation. They are located at photon energies ranging from 0.6 eV for Ar to 0.1 eV for Xe below the corresponding bulk excitons excited from the valence bands. Their halfwidths (20-50 MeV) is less than half the values found for the bulk excitons. Some are split by an amount considerably smaller than the spin orbit splitting of the valence bands. (orig.) [de

Molecular weight dependence of exciton diffusion in poly(3-hexylthiophene)

DEFF Research Database (Denmark)

Masri, Zarifi; Ruseckas, Arvydas; Emelianova, Evguenia V.

2013-01-01

A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial π–π aggregates of polymer chromophores and about 100...... times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient...

Mechanisms of spin-flipping and metal-insulator transition in nano-Fe3O4

Science.gov (United States)

Dito Fauzi, Angga; Aziz Majidi, Muhammad; Rusydi, Andrivo

2017-04-01

Fe3O4 is a half-metallic ferrimagnet with {{T}\\text{C}}˜ 860 K exhibiting metal-insulator transition (MIT) at  ˜120 K. In bulk form, the saturation magnetization is 0.6 Tesla (˜471 emu cm-3). A recent experimental study has shown that the saturation magnetization of nano-Fe3O4 thin films can achieve up to  ˜760 emu cm-3, attributed to spin-flipping of Fe ions at tetrahedral sites assisted by oxygen vacancies (V O). Such a system has shown to have higher MIT temperature (˜150 K). The spin-flipping is a new phenomenon in Fe3O4, while the MIT is a long-standing one. Here, we propose a model and calculations to investigate the mechanisms of both phenomena. Our results show that, for the system without V O, the ferrimagnetic configuration is energetically favorable. Remakably, upon inclusion of V O, the ground-state configuration switches into ferromagnetic. As for the MIT, by proposing temperature dependences of some hopping integrals in the model, we demonstrate that the system without and with V O undergo the MIT in slightly different ways, leading to higher MIT temperature for the system with V O, in agreement with the experimental data. Our results also show that the MIT in both systems occur concomitantly with the redistribution of electrons among the three Fe ions in each Fe3O4 formula unit. As such temperature dependences of hopping integrals may arise due to dynamic Jahn-Teller effects, our phenomenological theory may provide a way to reconcile existing theories relating the MIT to the structural transition and the charge ordering.

Exciton lifetime and spin dynamics in type-I In1−xAlxAs/Ga0.67Al0.33As quantum dots: Photoluminescence and pump-probe experiments

International Nuclear Information System (INIS)

Ben Daly, A.; Bernardot, F.; Barisien, T.; Galopin, E.; Lemaître, A.; Maaref, M.A.; Testelin, C.

2015-01-01

The exciton lifetime and spin relaxation have been studied in self-assembled In 1−x Al x As/Ga 0.67 Al 0.33 As quantum dots (QDs). Time-resolved photoluminescence and resonant pump-probe measurements were performed, at variable temperature and for different QD aluminium compositions. At low temperature, a long exciton-spin relaxation time has been measured, in agreement with the QD zero-dimensional confinement and the quenching of the relaxation mechanisms. The existence of a quasi-2D regime, in sample with a high QD density, has been observed. The importance of thermally-activated processes toward excited states is also evidenced, for QDs with different compositions and sizes. - Highlights: • The exciton lifetime and spin relaxation have been studied in In 1−x Al x As/Ga 0.67 Al 0.33 As quantum dot (QD). • Time-resolved photoluminescence (TRPL) and pump-probe measurements were performed, at variable temperature and for different QD aluminium compositions. • From the PL decay time, several thermal activation processes, related to 0D or 2D regime, or dependending on collective mechanisms. • The importance of thermal activated processes toward excited states is also evidenced, for QD with different compositions and sizes

Nuclear spin cooling by electric dipole spin resonance and coherent population trapping

Science.gov (United States)

Li, Ai-Xian; Duan, Su-Qing; Zhang, Wei

2017-09-01

Nuclear spin fluctuation suppression is a key issue in preserving electron coherence for quantum information/computation. We propose an efficient way of nuclear spin cooling in semiconductor quantum dots (QDs) by the coherent population trapping (CPT) and the electric dipole spin resonance (EDSR) induced by optical fields and ac electric fields. The EDSR can enhance the spin flip-flop rate and may bring out bistability under certain conditions. By tuning the optical fields, we can avoid the EDSR induced bistability and obtain highly polarized nuclear spin state, which results in long electron coherence time. With the help of CPT and EDSR, an enhancement of 1500 times of the electron coherence time can been obtained after a 500 ns preparation time.

Differential cross sections and spin flip for inelastic scattering of 15.0-18.25 MeV neutrons on carbon 12

International Nuclear Information System (INIS)

Thumm, M.

1976-01-01

The angular distribution of the spin-flip analysing power is stronly energy-dependent, supporting the assumption of structure effects. Elastic scattering data were also measured and analysed together with results of other authors in the frame work of the optical model. An interpretation of the inelastic scattering data was only possible by the assumption of a strong, energy-dependent deformation of the spin-orbit potential. Therefore the results of the inelastic channel were also compared with a microscopic DWBA theory. In the framework of this formalism, the energy dependence could be reproduced quite well. (BJ) [de

Propagation effect on photoluminescence of spin-aligned high-density exciton magnetic polarons in Cd{sub 0.8}Mn{sub 0.2}Te

Energy Technology Data Exchange (ETDEWEB)

Nagata, M.; Hirase, T.; Miyajima, K., E-mail: miyajima@rs.tus.ac.jp

2017-04-15

Characteristics of photoluminescence (PL) originating from high-density exciton magnetic polarons (HD-EMPs) for Cd{sub 0.8}Mn{sub 0.2}Te were investigated. The PL appeared only under selective excitation of the localized excitons, and the intensity increased superlinearly with the excitation density. Directivity of the PL was revealed. Therefore, it is concluded that the superlinear increase in the PL intensity resulted from a light amplification process owing to the stimulated emission. In addition, the existence of birefringence that originates from a uniaxial gradation of the Mn ion concentrations was revealed. The degree of circular polarization (DOCP) of the PL is important to obtain the spin alignment state of the HD-EMPs. The initial DOCPs of the PL were examined by removing a variation of the polarization during propagation inside the sample. As a result, it was found that the initial DOCPs of the PL were almost constant for the photon energy. The obtained initial DOCPs exhibited different values for right- and left-circularly polarized excitations, which resulted from different mechanisms of the spin alignment of the HD-EMPs.

Spin currents from Helium in intense-field photo-ionization

International Nuclear Information System (INIS)

Bhattacharyya, S; Mukherjee, Mahua; Chakrabarti, J; Faisal, F H M

2007-01-01

Spin dynamics is studied by computing spin-dependent ionization current of He in intense laser field in relativistic field theoretic method. Spin-flip and spin-asymmetry in current generation is obtained with circularly polarized light. The spin-flip is a dynamical effect of intense laser field on an ionized spinning electron. Transformation properties of the up and down spin ionization amplitudes show that the sign of spin can be controlled by a change of helicity of the laser photons from outside

Population decay time and distribution of exciton states analyzed by rate equations based on theoretical phononic and electron-collisional rate coefficients

Science.gov (United States)

Oki, Kensuke; Ma, Bei; Ishitani, Yoshihiro

2017-11-01

Population distributions and transition fluxes of the A exciton in bulk GaN are theoretically analyzed using rate equations of states of the principal quantum number n up to 5 and the continuum. These rate equations consist of the terms of radiative, electron-collisional, and phononic processes. The dependence of the rate coefficients on temperature is revealed on the basis of the collisional-radiative model of hydrogen plasma for the electron-collisional processes and theoretical formulation using Fermi's "golden rule" for the phononic processes. The respective effects of the variations in electron, exciton, and lattice temperatures are exhibited. This analysis is a base of the discussion on nonthermal equilibrium states of carrier-exciton-phonon dynamics. It is found that the exciton dissociation is enhanced even below 150 K mainly by the increase in the lattice temperature. When the thermal-equilibrium temperature increases, the population fluxes between the states of n >1 and the continuum become more dominant. Below 20 K, the severe deviation from the Saha-Boltzmann distribution occurs owing to the interband excitation flux being higher than the excitation flux from the 1 S state. The population decay time of the 1 S state at 300 K is more than ten times longer than the recombination lifetime of excitons with kinetic energy but without the upper levels (n >1 and the continuum). This phenomenon is caused by a shift of population distribution to the upper levels. This phonon-exciton-radiation model gives insights into the limitations of conventional analyses such as the ABC model, the Arrhenius plot, the two-level model (n =1 and the continuum), and the neglect of the upper levels.

Spin-flip dynamics of the Curie-Weiss model Loss of Gibbsianness with possibly broken symmetry.

CERN Document Server

Külske, C

2005-01-01

We study the conditional probabilities of the Curie-Weiss Ising model in vanishing external field under a symmetric independent stochastic spin-flip dynamics and discuss their set of bad configurations (points of discontinuity). We exhibit a complete analysis of the transition between Gibbsian and non-Gibbsian behavior as a function of time, extending the results for the corresponding lattice model, where only partial answers can be obtained. For initial inverse temperature $\\b \\leq 1$, we prove that the time-evolved measure is always Gibbsian. For $1 \\frac{3}{2}$, we observe the new phenomenon of symmetry-breaking of bad configurations: The time-evolved measure loses its Gibbsian character at a sharp transition time, and bad configurations with non-zero spin-average appear. These bad configurations merge into a neutral configuration at a later transition time, while the measure stays non-Gibbs. In our proof we give a detailed analysis of the phase-diagram of a Curie-Weiss random field Ising model with possi...

Flipped neutrino emissivity from strange matter

International Nuclear Information System (INIS)

Goyal, A.; Dutta, S.

1994-01-01

Energy loss due to wrong helicity sterile neutrinos through spin flip processes leads to rapid cooling of nascent neutron stars. The observed cooling of neutron stars associated with SN 1987A seems to preclude the existence of Dirac neutrinos with a mass in excess of 20 keV. Assuming that nuclear matter in the core of the neutron star undergoes a phase transition to quark matter leading to a strange star or a neutron star with a strange matter core, we examine the emission of flipped Dirac neutrinos for two dominant processes: quark-neutrino scattering [q+ν - (bar ν + )→q+ν + (bar ν - )] and the quark neutrino pair bremsstrahlung process [q+q→q+q+ν - bar ν - (ν+bar ν + )]. We determine the composition of quark matter just after core bounce and examine the effect of neutrino degeneracy on the emission rate and mean free path of the wrong helicity neutrinos

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

Science.gov (United States)

Ahmad, Shahab; Baumberg, Jeremy J.; Vijaya Prakash, G.

2013-12-01

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H25NH3)2PbI4(1-y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices.

Hyperfine interaction mediated electric-dipole spin resonance: the role of frequency modulation

International Nuclear Information System (INIS)

Li, Rui

2016-01-01

The electron spin in a semiconductor quantum dot can be coherently controlled by an external electric field, an effect called electric-dipole spin resonance (EDSR). Several mechanisms can give rise to the EDSR effect, among which there is a hyperfine mechanism, where the spin-electric coupling is mediated by the electron–nucleus hyperfine interaction. Here, we investigate the influence of frequency modulation (FM) on the spin-flip efficiency. Our results reveal that FM plays an important role in the hyperfine mechanism. Without FM, the electric field almost cannot flip the electron spin; the spin-flip probability is only about 20%. While under FM, the spin-flip probability can be improved to approximately 70%. In particular, we find that the modulation amplitude has a lower bound, which is related to the width of the fluctuated hyperfine field. (paper)

Simultaneous monitoring of singlet and triplet exciton variations in solid organic semiconductors driven by an external static magnetic field

Energy Technology Data Exchange (ETDEWEB)

Ding, Baofu, E-mail: b.ding@ecu.edu.au; Alameh, Kamal, E-mail: k.alameh@ecu.edu.au [Electron Science Research Institute, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027 (Australia)

2014-07-07

The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.

Simultaneous monitoring of singlet and triplet exciton variations in solid organic semiconductors driven by an external static magnetic field

International Nuclear Information System (INIS)

Ding, Baofu; Alameh, Kamal

2014-01-01

The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.

Clinical utility of partial flip angle T2-weighted spin-echo imaging of the brain

International Nuclear Information System (INIS)

Chang, K.H.; Yi, J.G.; Han, M.H.; Han, M.C.; Kim, C.W.; Cho, M.H.; Cho, Z.H.

1990-01-01

To assess the clinical usefulness of partial flip angle (PFA) spin-echo (SE) brain imaging, a total of eighty patients were examined with both conventional double echo T2-weighted SE (2500/30, 80/90deg/one excitation) and PFA double echo SE (1200/30, 70/45deg/two excitations) on 2.0T system. Two comparative studies were performed: (1) In 65 patients PFA SE technique was compared with conventional SE without flow compensating gradients, and (2) in 15 patients the former was compared with the latter with flow compensating gradients. Imaging time was nearly identical in each sequence. In both studies we found that PFA T2-weighted SE images were almost identical to those obtained with the conventional SE technique in the contrast characteristics and the detection rate of the abnormalities (100%, 85/85 lesions), and more importantly, PFA SE revealed few flow artifacts in the brain stem, temporal lobes and basal ganglia which were frequently seen on conventional SE without flow compensating gradients. Additionally, PFA SE images demonstrated no suppression of CSF flow void in the aqueduct which was commonly seen on conventional SE with flow compensating gradients. In overall image quality, the PFA SE images, particularly the second echo images, were almost comparable with those of conventional SE with flow compensating gradients. A flip angle of 45deg seems to be close to Ernst angle, the angle at which maximum signal occurs, for a given TR of 1200 msec for CSF and most of the abnormalities containing higher water content. In conclusion, PFA SE sequence (i.e. 1200/30, 70/45deg/2) appears to be useful as a primary or an adjunctive technique in certain clinical circumstances, particularly in imaging of hydrocephalic patients for assessing aqueductal patency. (orig.)

Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides

KAUST Repository

Moody, Galan

2016-03-14

Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe2), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides

KAUST Repository

Moody, Galan; Hao, Kai; Dass, Chandriker Kavir; Singh, Akshay; Xu, Lixiang; Tran, Kha; Chen, Chang-Hsiao; Li, Ming-yang; Li, Lain-Jong; Clark, Genevieve; Bergh ä user, Gunnar; Malic, Ermin; Knorr, Andreas; Xu, Xiaodong; Li, Xiaoqin

2016-01-01

Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe2), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Magnetic excitation and local magnetic susceptibility of the excitonic insulator Ta2NiSe5 investigated by 77Se NMR

Science.gov (United States)

Li, Shang; Kawai, Shunsuke; Kobayashi, Yoshiaki; Itoh, Masayuki

2018-04-01

77Se NMR measurements were made on polycrystalline and single-crystalline samples to elucidate local magnetic susceptibility and magnetic excitation of Ta2NiSe5 , which is proposed to undergo an exciton condensation accompanied by a structural transition at Tc=328 K . We determine the 77Se Knight shift tensors for the three Se sites and analyze their anisotropy based on the site symmetry. The temperature dependence of the Knight shift is discussed on the basis of spin and orbital susceptibilities calculated for two-chain and two-dimensional three-band models. The large fraction of the Se 4 p orbital polarization due to the mixing between Ni 3 d and Se 4 p orbitals is estimated from the analysis of the transferred hyperfine coupling constant. Also the nuclear spin-lattice relaxation rate 1 /T1 is found not to show a coherent peak just below Tc and to obey the thermally activated temperature dependence with a spin gap energy of 1770 ±40 K . This behavior of 1 /T1 monitors the exciton condensation as proposed by the theoretical study of 1 /T1 based on the three-chain Hubbard model for the excitonic insulator.

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

International Nuclear Information System (INIS)

Ahmad, Shahab; Vijaya Prakash, G.; Baumberg, Jeremy J.

2013-01-01

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C 12 H 25 NH 3 ) 2 PbI 4(1−y) Br 4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices

The influence of flip angle on the magic angle effect

International Nuclear Information System (INIS)

Zurlo, J.V.; Blacksin, M.F.; Karimi, S.

2000-01-01

Objective. To assess the impact of flip angle with gradient sequences on the ''magic angle effect''. We characterized the magic angle effect in various gradient echo sequences and compared the signal- to-noise ratios present on these sequences with the signal-to-noise ratios of spin echo sequences.Design. Ten normal healthy volunteers were positioned such that the flexor hallucis longus tendon remained at approximately at 55 to the main magnetic field (the magic angle). The tendon was imaged by a conventional spin echo T1- and T2-weighted techniques and by a series of gradient techniques. Gradient sequences were altered by both TE and flip angle. Signal-to-noise measurements were obtained at segments of the flexor hallucis longus tendon demonstrating the magic angle effect to quantify the artifact. Signal-to-noise measurements were compared and statistical analysis performed. Similar measurements were taken of the anterior tibialis tendon as an internal control.Results and conclusions. We demonstrated the magic angle effect on all the gradient sequences. The intensity of the artifact was affected by both the TE and flip angle. Low TE values and a high flip angle demonstrated the greatest magic angle effect. At TE values less than 30 ms, a high flip angle will markedly increase the magic angle effect. (orig.)

Spin-flip measurements in the proton inelastic scattering on 12C and giant resonance effects

International Nuclear Information System (INIS)

De Leo, R.; D'Erasmo, G.; Ferrero, F.; Pantaleo, A.; Pignanelli, M.

1975-01-01

Differential cross sections and spin-flip probabilities (SFP) for the inelastic scattering of protons, exciting the 2 + state at 4.43 MeV in 12 C, have been measured at several incident energies between 15.9 and 37.6 MeV. The changes in the shape of the SFP angular distributions are rather limited, while the absolute values show a pronounced increase, resonant like, in two energy regions centered at about 20 and 29 MeV. The second resonance reproduces very closely the energy dependence of the E2 giant quadrupole strength found in a previous experiment. The resonance at 20 MeV should correspond to a substructure of the E1 giant dipole resonance. (Auth.)

Spin-flip transition and Faraday effect in antiferromagnet KMnF3 in megagauss magnetic field

International Nuclear Information System (INIS)

Mukhin, A.A.; Plis, V.I.; Popov, A.I.; Zvezdin, A.K.; Platonov, V.; Tatsenko, O.M.

1998-01-01

Faraday effect in the antiferromagnet KMnF 3 has been investigated in pulse explosive fields up to 500 T at T=78 K. The laser wavelength 0.63 μm was used in the experiment. The magnetic field dependence of Faraday rotation in this antiferromagnet shows a unique feature of a lack of saturation effect in the fields up to 500 T whereas critical field of spin-flip transition is about 120 T. The theoretical analysis of microscopic nature of Faraday rotation, including the diamagnetic, magneto-dipole and paramagnetic mechanisms has been performed. The strong competition of these mechanisms is important to explain the extremely small value of the effect and its unusual magnetic field dependence

Application of the Landau-Zener-Stückelberg-Majorana dynamics to the electrically driven flip of a hole spin

Science.gov (United States)

Pasek, W. J.; Maialle, M. Z.; Degani, M. H.

2018-03-01

An idea of employing the Landau-Zener-Stückelberg-Majorana dynamics to flip a spin of a single ground state hole is introduced and explored by a time-dependent simulation. This configuration interaction study considers a hole confined in a quantum molecule formed in an InSb 〈111 〉 quantum wire by application of an electrostatic potential. An up-down spin-mixing avoided crossing is formed by nonaxial terms in the Kohn-Luttinger Hamiltonian and the Dresselhaus spin-orbit one. Manipulation of the system is possible by the dynamic change of an external vertical electric field, which enables the consecutive driving of the hole through two anticrossings. Moreover, a simple model of the power-law-type noise that impedes precise electric control of the system is included in the form of random telegraph noise to estimate the limitations of the working conditions. We show that in principle the process is possible, but it requires precise control of the parameters of the driving impulse.

Current-induced spin transfer torque in ferromagnet-marginal Fermi liquid double tunnel junctions

International Nuclear Information System (INIS)

Mu Haifeng; Zheng Qingrong; Jin Biao; Su Gang

2005-01-01

Current-induced spin transfer torque through a marginal Fermi liquid (MFL) which is connected to two noncollinearly aligned ferromagnets via tunnel junctions is discussed in terms of the nonequilibrium Green function method. It is found that in the absence of the spin-flip scattering, the magnitude of the torque increases with the polarization and the coupling constant λ of the MFL, whose maximum increases with λ linearly, showing that the interactions between electrons tend to enhance the spin torque. When the spin-flip scattering is included, an additional spin torque is induced. It is found that the spin-flip scattering enhances the spin torque and gives rise to a nonlinear angular shift

Flipped neutrino emissivity from strange matter

Energy Technology Data Exchange (ETDEWEB)

Goyal, A.; Dutta, S. (Department of Physics and Astrophysics, University of Delhi, Delhi 110007 (India))

1994-04-15

Energy loss due to wrong helicity sterile neutrinos through spin flip processes leads to rapid cooling of nascent neutron stars. The observed cooling of neutron stars associated with SN 1987A seems to preclude the existence of Dirac neutrinos with a mass in excess of 20 keV. Assuming that nuclear matter in the core of the neutron star undergoes a phase transition to quark matter leading to a strange star or a neutron star with a strange matter core, we examine the emission of flipped Dirac neutrinos for two dominant processes: quark-neutrino scattering [[ital q]+[nu][sub [minus

Variational study of the stability of the Nagaoka state against single-spin flips in the two-dimensional t-t#prime# Hubbard model

International Nuclear Information System (INIS)

Bajdich, M.; Hlubina, R.

2001-01-01

Making use of variational wave functions of the Basile-Elser type we study the stability of the Nagaoka state against single-spin flips in the two-dimensional t-t#prime# Hubbard model for t#prime#/t∼0.5. In the low-density limit the variational estimate of the stability region of the Nagaoka state is in qualitative agreement with the predictions of the T-matrix approximation

The effect of inquiry-flipped classroom model toward students' achievement on chemical reaction rate

Science.gov (United States)

Paristiowati, Maria; Fitriani, Ella; Aldi, Nurul Hanifah

2017-08-01

The aim of this research is to find out the effect of Inquiry-Flipped Classroom Models toward Students' Achievement on Chemical Reaction Rate topic. This study was conducted at SMA Negeri 3 Tangerang in Eleventh Graders. The Quasi Experimental Method with Non-equivalent Control Group design was implemented in this study. 72 students as the sample was selected by purposive sampling. Students in experimental group were learned through inquiry-flipped classroom model. Meanwhile, in control group, students were learned through guided inquiry learning model. Based on the data analysis, it can be seen that there is significant difference in the result of the average achievement of the students. The average achievement of the students in inquiry-flipped classroom model was 83,44 and the average achievement of the students in guided inquiry learning model was 74,06. It can be concluded that the students' achievement with inquiry-flipped classroom better than guided inquiry. The difference of students' achievement were significant through t-test which is tobs 3.056 > ttable 1.994 (α = 0.005).

Coherent and correlated spin transport in nanoscale superconductors

Energy Technology Data Exchange (ETDEWEB)

Morten, Jan Petter

2008-03-15

Motivated by the desire for better understanding of nano electronic systems, we theoretically study the conductance and noise characteristics of current flow between superconductors, ferromagnets, and normal-metals. Such nano structures can reveal information about superconductor proximity effects, spin-relaxation processes, and spintronic effects with potential applications for different areas of mesoscopic physics. We employ the quasiclassical theory of superconductivity in the Keldysh formalism, and calculate the nonequilibrium transport of spin and charge using various approaches like the circuit theory of quantum transport and full counting statistics. For two of the studied structures, we have been able to compare our theory to experimental data and obtain good agreement. Transport and relaxation of spin polarized current in superconductors is governed by energy-dependent transport coefficients and spin-flip rates which are determined by quantum interference effects. We calculate the resulting temperature-dependent spin flow in ferromagnet-superconductor devices. Experimental data for spin accumulation and spin relaxation in a superconducting nano wire is in agreement with the theory, and allows for a spin-flip spectroscopy that determines the dominant mechanism for spin-flip relaxation in the studied samples. A ferromagnet precessing under resonance conditions can give rise to pure spin current injection into superconductors. We find that the absorbed spin current is measurable as a temperature dependent Gilbert damping, which we calculate and compare to experimental data. Crossed Andreev reflection denotes superconducting pairing of electrons flowing from different normal-metal or ferromagnet terminals into a superconductor. We calculate the nonlocal currents resulting from this process in competition with direct electron transport between the normal-metal terminals. We take dephasing into account, and study the nonlocal current when the types of contact in

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.

Electrical control of charged carriers and excitons in atomically thin materials

Science.gov (United States)

Wang, Ke; De Greve, Kristiaan; Jauregui, Luis A.; Sushko, Andrey; High, Alexander; Zhou, You; Scuri, Giovanni; Taniguchi, Takashi; Watanabe, Kenji; Lukin, Mikhail D.; Park, Hongkun; Kim, Philip

2018-02-01

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices1-3. The unique band structure4-7 of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley-spin qubits8. 2D TMDs also provide a platform for novel quantum optoelectronic devices9-11 due to their large exciton binding energy12,13. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings14-16. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin-valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons.

New predictions for generalized spin polarizabilities from heavy baryon chiral perturbation theory

International Nuclear Information System (INIS)

Chung-Wen Kao; Barbara Pasquini; Marc Vanderhaeghen

2004-01-01

We extract the next-to-next-to-leading order results for spin-flip generalized polarizabilities (GPs) of the nucleon from the spin-dependent amplitudes for virtual Compton scattering (VCS) at Ο(p 4 ) in heavy baryon chiral perturbation theory. At this order, no unknown low energy constants enter the theory, allowing us to make absolute predictions for all spin-flip GPs. Furthermore, by using constraint equations between the GPs due to nucleon crossing combined with charge conjugation symmetry of the VCS amplitudes, we get a next-to-next-to-next-to-leading order prediction for one of the GPs. We provide estimates for forthcoming double polarization experiments which allow to access these spin-flip GPs of the nucleon

Donor-driven spin relaxation in multivalley semiconductors.

Science.gov (United States)

Song, Yang; Chalaev, Oleg; Dery, Hanan

2014-10-17

The observed dependence of spin relaxation on the identity of the donor atom in n-type silicon has remained without explanation for decades and poses a long-standing open question with important consequences for modern spintronics. Taking into account the multivalley nature of the conduction band in silicon and germanium, we show that the spin-flip amplitude is dominated by short-range scattering off the central-cell potential of impurities after which the electron is transferred to a valley on a different axis in k space. Through symmetry arguments, we show that this spin-flip process can strongly affect the spin relaxation in all multivalley materials in which time-reversal cannot connect distinct valleys. From the physical insights gained from the theory, we provide guidelines to significantly enhance the spin lifetime in semiconductor spintronics devices.

Spin current through quantum-dot spin valves

International Nuclear Information System (INIS)

Wang, J; Xing, D Y

2006-01-01

We report a theoretical study of the influence of the Coulomb interaction on the equilibrium spin current in a quantum-dot spin valve, in which the quantum dot described by the Anderson impurity model is coupled to two ferromagnetic leads with noncollinear magnetizations. In the Kondo regime, electrons transmit through the quantum dot via higher-order virtual processes, in which the spin of either lead electrons or a localized electron on the quantum dot may reverse. It is found that the magnitude of the spin current decreases with increasing Coulomb interactions due to spin flip effects on the dot. However, the spatial direction of the spin current remains unchanged; it is determined only by the exchange coupling between two noncollinear magnetizations

Control of electron spin decoherence in nuclear spin baths

Science.gov (United States)

Liu, Ren-Bao

2011-03-01

Nuclear spin baths are a main mechanism of decoherence of spin qubits in solid-state systems, such as quantum dots and nitrogen-vacancy (NV) centers of diamond. The decoherence results from entanglement between the electron and nuclear spins, established by quantum evolution of the bath conditioned on the electron spin state. When the electron spin is flipped, the conditional bath evolution is manipulated. Such manipulation of bath through control of the electron spin not only leads to preservation of the center spin coherence but also demonstrates quantum nature of the bath. In an NV center system, the electron spin effectively interacts with hundreds of 13 C nuclear spins. Under repeated flip control (dynamical decoupling), the electron spin coherence can be preserved for a long time (> 1 ms) . Thereforesomecharacteristicoscillations , duetocouplingtoabonded 13 C nuclear spin pair (a dimer), are imprinted on the electron spin coherence profile, which are very sensitive to the position and orientation of the dimer. With such finger-print oscillations, a dimer can be uniquely identified. Thus, we propose magnetometry with single-nucleus sensitivity and atomic resolution, using NV center spin coherence to identify single molecules. Through the center spin coherence, we could also explore the many-body physics in an interacting spin bath. The information of elementary excitations and many-body correlations can be extracted from the center spin coherence under many-pulse dynamical decoupling control. Another application of the preserved spin coherence is identifying quantumness of a spin bath through the back-action of the electron spin to the bath. We show that the multiple transition of an NV center in a nuclear spin bath can have longer coherence time than the single transition does, when the classical noises due to inhomogeneous broadening is removed by spin echo. This counter-intuitive result unambiguously demonstrates the quantumness of the nuclear spin bath

Unexpected enhancements and reductions of rf spin resonance strengths

Directory of Open Access Journals (Sweden)

M. A. Leonova

2006-05-01

Full Text Available We recently analyzed all available data on spin-flipping stored beams of polarized protons, electrons, and deuterons. Fitting the modified Froissart-Stora equation to the measured polarization data after crossing an rf-induced spin resonance, we found 10–20-fold deviations from the depolarizing resonance strength equations used for many years. The polarization was typically manipulated by linearly sweeping the frequency of an rf dipole or rf solenoid through an rf-induced spin resonance; spin-flip efficiencies of up to 99.9% were obtained. The Lorentz invariance of an rf dipole’s transverse ∫Bdl and the weak energy dependence of its spin resonance strength E together imply that even a small rf dipole should allow efficient spin flipping in 100 GeV or even TeV storage rings; thus, it is important to understand these large deviations. Therefore, we recently studied the resonance strength deviations experimentally by varying the size and vertical betatron tune of a 2.1  GeV/c polarized proton beam stored in COSY. We found no dependence of E on beam size, but we did find almost 100-fold enhancements when the rf spin resonance was near an intrinsic spin resonance.

Spin-exchange and spin-destruction rates for the 3He-Na system

International Nuclear Information System (INIS)

Borel, P.I.; Soegaard, L.V.; Svendsen, W.E.; Andersen, N.

2003-01-01

Optically pumped Na is used as a spin-exchange partner to polarize 3 He. Polarizations around 20% have routinely been achieved in sealed spherical glass cells containing 3 He, N 2 , and a few droplets of Na. An optical technique has been developed to determine the Na- 3 He spin-exchange rate coefficient. By monitoring the Na spin relaxation ''in the dark,'' the average Na-Na spin-destruction cross section at 330 degree sign C is estimated to be around 5x10 -19 cm 2 . This value is 2-5 (15-30) times smaller than the previously reported values for the K-K (Rb-Rb) spin-relaxation cross section. In the temperature range 310-355 degree sign C the spin-exchange rate coefficient is found to be (6.1±0.6)x10 -20 cm 3 /s with no detectable temperature dependence. This value is in good agreement with a previous theoretical estimate reported by Walker and it is only slightly lower than the corresponding Rb- 3 He spin-exchange rate coefficient. The total Na- 3 He spin-destruction rate coefficient is, within errors, found to be the same as the Na- 3 He spin-exchange rate coefficient, thereby indicating that the maximum possible photon efficiency may approach unity for the Na- 3 He system. A technique, in which a charge-coupled device camera is used to take images of faint unquenched fluorescence light, has been utilized to allow for an instantaneous determination of the sodium number densities during the rate coefficient measurements

Electrons, holes, and excitons in GaAs polytype quantum dots

Energy Technology Data Exchange (ETDEWEB)

Climente, Juan I.; Segarra, Carlos; Rajadell, Fernando; Planelles, Josep, E-mail: josep.planelles@uji.es [Departament de Química Física i Analítica, Universitat Jaume I, E-12080 Castelló (Spain)

2016-03-28

Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.

Electrical control of optical orientation of neutral and negatively charged excitons in an n -type semiconductor quantum well

Science.gov (United States)

Dzhioev, R. I.; Korenev, V. L.; Lazarev, M. V.; Sapega, V. F.; Gammon, D.; Bracker, A. S.

2007-01-01

We report electric field induced increase of spin orientation of negatively charged excitons (trions) localized in n -type GaAs/AlGaAs quantum well. Under resonant excitation of free neutral heavy-hole excitons, the polarization of trions increases dramatically with electrical injection of electrons. The polarization enhancement correlates strongly with trion/exciton luminescence intensity ratio. This effect results from a very efficient trapping of free neutral excitons by the quantum well interfacial fluctuations (“natural” quantum dots) containing resident electrons.

Magnetic resonance imaging of lumbar spine. Comparison of multiple spin echo and low flip angle gradient echo imaging

Energy Technology Data Exchange (ETDEWEB)

Murakami, Takamichi; Fujita, Norihiko; Harada, Koushi; Kozuka, Takahiro (Osaka Univ. (Japan). Faculty of Medicine)

1989-07-01

Sixteen patients including 13 cases with disk herniation and 3 cases with spondylosis of lumbar spine were examined on a resistive MRI system operating at 0.1 T. All lesions were studied with both multiple spin echo (MSE) and low flip angle gradient echo (LF) techniques to evaluate which technique is more effective in detecting the disk degeneration and the indentation on subarachnoid space. MSE images were obtained with repetition time (TR) of 1100-1500 ms or cardiac gating, an echo time (TE) of 30, 60, 90, 120, 150, and 180 ms symmetrical 6 echoes, and total acquisition time of more than 281 sec. LF images were obtained with TR of 500, 250, and 100 ms, TE of 18 ms, a flip angle of 30 degree, and total acquisition time of 128 sec. Eleven lesions of spinal disk degeneration and 12 of indentation on subarachnoid space were detected with LF. On the other hand, 26 lesions of spinal disk degeneration and 38 of indentation on subarachnoid space were detected with MSE. Although the parameters of LF employed in this study were relatively effective to emphasize T2{sup *}-based contrast, the ability of LF in detection of spinal disk degeneration and indentation on subarachnoid space is less than that of MSE. Signal contrast to noise ratios for normal disk and degenerative disk, epidural-fat and disk herniated material, CSF and disk herniated material, and epidural-fat and CSF were less than 4 with LF, but more than 4 with MSE. This difference of contrast to noise ratio between MSE and LF was one of the main causes of the difference of the detection rate of spinal disk degeneration and indentation on subarachnoid space. (author).

Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites

KAUST Repository

Yin, Jun; Li, Hong; Cortecchia, Daniele; Soci, Cesare; Bredas, Jean-Luc

2017-01-01

calculations including corrections due to spin orbit couplings and electron hole interactions, a computationally intensive molecular cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level

Spin excitations in 48Ca and 90Zr with 319 MeV protons

International Nuclear Information System (INIS)

Nanda, S.K.

1985-05-01

Cross sections, analyzing powers, and spin-flip probabilities have been measured in the low momentum transfer region in the 90 Zr(p vector, p' vector) 90 Zr* reaction at 319 MeV. A rich fine structure is observed for the first time in inelastic proton scattering in the previously proposed M1 giant resonance region. Angular distribution of most of these states are consistent with M1 excitation. The excitation energies of the fine structure states are in good agreement with similar structure seen in electron scattering; however, discrepancies in spin assignments remain. The measured cross section for the entire bump is about 37 +- 10% of the Distorted Wave Impulse Approximation (DWIA) prediction for the M1 strength in 90 Zr with simple wave functions. However, an analysis of the fine structure states reveals about 15% of the strength in the M1 region to be due to narrow El states; another 8% is attributed to M2 strength. The spin-flip measurements for 90 Zr reveal a large spin-flip probability value for the M1 region; good agreement is obtained with DWIA calculations. However, a large cross section for spin excitations distributed uniformly over the excitation energy region from about 7 to 25 MeV is observed for the first time. The spin excitation strength in this giant resonance continuum is found to about 0.80 mb/sr/MeV. Angular distributions for the spin-flip cross sections from 7 to 18 MeV in steps of 2 MeV have been analyzed with low multipole spin excitation calculations in the DWIA framework; the observed spin-flip strength in this region is found to be consistent with spin excitation involving angular momentum transfer of up to two. Finally, cross section, analyzing power, and spin-flip probability data have also been obtained for the 10.23 MeV M1 transition in the 48 Ca(p vector, p' vector) 48 Ca* reaction at 319 MeV. The quenching of M1 strength in 48 Ca relative to theoretical predictions is found to be consistent with previous work

Nuclear spin response studies in inelastic polarized proton scattering

International Nuclear Information System (INIS)

Jones, K.W.

1988-01-01

Spin-flip probabilities S/sub nn/ have been measured for inelastic proton scattering at incident proton energies around 300 MeV from a number of nuclei. At low excitation energies S/sub nn/ is below the free value. For excitation energies above about 30 MeV for momentum transfers between about 0.35 fm/sup /minus/1/ and 0.65 fm/sup / minus/1/ S/sub nn/ exceeds free values significantly. These results suggest that the relative ΔS = 1(ΔS = 0 + ΔS = 1) nuclear spin response approaches about 90% in the region of the enhancement. Comparison of the data with slab response calculations are presented. Decomposition of the measured cross sections into σ(ΔS = 0) and σ(ΔS = 1) permit extraction of nonspin-flip and spin-flip dipole and quadrupole strengths. 29 refs., 11 figs

Flipping-shuttle oscillations of bright one- and two-dimensional solitons in spin-orbit-coupled Bose-Einstein condensates with Rabi mixing

Science.gov (United States)

Sakaguchi, Hidetsugu; Malomed, Boris A.

2017-10-01

We analyze the possibility of macroscopic quantum effects in the form of coupled structural oscillations and shuttle motion of bright two-component spin-orbit-coupled striped (one-dimensional, 1D) and semivortex (two-dimensional, 2D) matter-wave solitons, under the action of linear mixing (Rabi coupling) between the components. In 1D, the intrinsic oscillations manifest themselves as flippings between spatially even and odd components of striped solitons, while in 2D the system features periodic transitions between zero-vorticity and vortical components of semivortex solitons. The consideration is performed by means of a combination of analytical and numerical methods.

Measurement of the depolarization of the reaction 27Al(p vector,p vector.)27Al for the study of the spin-spin potential

International Nuclear Information System (INIS)

Loeh, H.

1981-01-01

For the study of the spin-spin interaction in the optical potential the depolarisation in the elastic scattering of polarized protons was measured. The double-scattering experiments were performed in the angular range 40 0 -110 0 at an incident energy of 10.35 MeV at the Erlangen QD-magnetic spectrometer. The determination of the optical model parameters independent from the spin-spin potentials was performed by the fitting of these to the observables and sigmasup(di). These were obtained from a measurement of the angular distribution of the analyzing power and the differential cross section in the 4π-scattering chamber for the reaction 27 Al(p vector,p 0 ) at the same energy. The compound contributions present at this energy, which can also influence the depolarization, were regarded by the calculation of the compound-elastic non-spin-flip respectively spin-flip subcross sections by means of the formalism of Hofmann, Richert, Tepel and Weidenmueller. Because the target nucleus 27 Al possesses in the ground state a spin I=5/2, also the possible quadrupole spin flip had to be included. This was performed by coupled-channel calculations. The respecting compound contributions and quadrupole effects corrected depolarization data could by used for the study of the spin-spin potentials by means of DWBA calculations. As result it was shown that for the description of the experimental data a spherical spin-spin potential of the strength Vsub(SS)=1.5+-0.3 MeV had to be assumed. (orig.) [de

Excitons in Core-Shell Nanowires with Polygonal Cross Sections.

Science.gov (United States)

Sitek, Anna; Urbaneja Torres, Miguel; Torfason, Kristinn; Gudmundsson, Vidar; Bertoni, Andrea; Manolescu, Andrei

2018-04-11

The distinctive prismatic geometry of semiconductor core-shell nanowires leads to complex localization patterns of carriers. Here, we describe the formation of optically active in-gap excitonic states induced by the interplay between localization of carriers in the corners and their mutual Coulomb interaction. To compute the energy spectra and configurations of excitons created in the conductive shell, we use a multielectron numerical approach based on the exact solution of the multiparticle Hamiltonian for electrons in the valence and conduction bands, which includes the Coulomb interaction in a nonperturbative manner. We expose the formation of well-separated quasidegenerate levels, and focus on the implications of the electron localization in the corners or on the sides of triangular, square, and hexagonal cross sections. We obtain excitonic in-gap states associated with symmetrically distributed electrons in the spin singlet configuration. They acquire large contributions due to Coulomb interaction, and thus are shifted to much higher energies than other states corresponding to the conduction electron and the vacancy localized in the same corner. We compare the results of the multielectron method with those of an electron-hole model, and we show that the latter does not reproduce the singlet excitonic states. We also obtain the exciton lifetime and explain selection rules which govern the recombination process.

The Nuclear Spin Nanomagnet

OpenAIRE

Korenev, V. L.

2007-01-01

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei brings the optical transition energy into resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of...

Exciton fine structure in CdSe nanoclusters

International Nuclear Information System (INIS)

Leung, K.; Pokrant, S.; Whaley, K.B.

1998-01-01

The fine structure in the CdSe nanocrystal absorption spectrum is computed by incorporating two-particle electron-hole interactions and spin-orbit coupling into a tight-binding model, with an expansion in electron-hole single-particle states. The exchange interaction and spin-orbit coupling give rise to dark, low-lying states that are predominantly triplet in character, as well as to a manifold of exciton states that are sensitive to the nanocrystal shape. Near the band gap, the exciton degeneracies are in qualitative agreement with the effective mass approximation (EMA). However, instead of the infinite lifetimes for dark states characteristic of the EMA, we obtain finite radiative lifetimes for the dark states. In particular, for the lowest, predominantly triplet, states we obtain radiative lifetimes of microseconds, in qualitative agreement with the experimental measured lifetimes. The resonant Stokes shifts obtained from the splitting between the lowest dark and bright states are also in good agreement with experimental values for larger crystallites. Higher-lying states exhibit significantly more complex behavior than predicted by EMA, due to extensive mixing of electron-hole pair states. copyright 1998 The American Physical Society

Thermal spin pumping mediated by magnons in the semiclassical regime

International Nuclear Information System (INIS)

Nakata, Kouki

2012-01-01

We microscopically analyze thermal spin pumping mediated by magnons, at the interface between a ferromagnetic insulator and a non-magnetic metal, in the semiclassical regime. The generation of a spin current is discussed by calculating the thermal spin transfer torque, which breaks the spin conservation law for conduction electrons and operates the coherent magnon state. Inhomogeneous thermal fluctuations between conduction electrons and magnons induce a net spin current, which is pumped into the adjacent non-magnetic metal. The pumped spin current is proportional to the temperature difference. When the effective temperature of magnons is lower than that of conduction electrons, localized spins lose spin angular momentum by emitting magnons and conduction electrons flip from down to up by absorbing all the emitted momentum, and vice versa. Magnons at the zero mode cannot contribute to thermal spin pumping because they are eliminated by the spin-flip condition. Consequently thermal spin pumping does not cost any kind of applied magnetic fields

H-point exciton transitions in bulk MoS2

International Nuclear Information System (INIS)

Saigal, Nihit; Ghosh, Sandip

2015-01-01

Reflectance and photoreflectance spectrum of bulk MoS 2 around its direct bandgap energy have been measured at 12 K. Apart from spectral features due to the A and B ground state exciton transitions at the K-point of the Brillouin zone, one observes additional features at nearby energies. Through lineshape analysis the character of two prominent additional features are shown to be quite different from that of A and B. By comparing with reported electronic band structure calculations, these two additional features are identified as ground state exciton transitions at the H-point of the Brillouin zone involving two spin-orbit split valance bands. The excitonic energy gap at the H-point is 1.965 eV with a valance bands splitting of 185 meV. While at the K-point, the corresponding values are 1.920 eV and 205 meV, respectively

Relaxation of electron–hole spins in strained graphene nanoribbons

International Nuclear Information System (INIS)

Prabhakar, Sanjay; Melnik, Roderick

2015-01-01

We investigate the influence of magnetic field originating from the electromechanical effect on the spin-flip behaviors caused by electromagnetic field radiation in the strained graphene nanoribbons (GNRs). We show that the spin splitting energy difference (≈10 meV) due to pseudospin is much larger than the spin-orbit coupling effect (Balakrishnan et al 2013 Nat. Phys. 9 284) that might provide an evidence of broken symmetry of degeneracy. The induced spin splitting energy due to ripple waves can be further enhanced with increasing values of applied tensile edge stress for potential applications in straintronic devices. In particular, we show that the enhancement in the magnitude of the ripple waves due to externally applied tensile edge stress extends the tuning of spin-flip behaviors to larger widths of GNRs. (paper)

Partial flip angle spin-echo imaging to obtain T1 weighted images with electrocardiographic gating

International Nuclear Information System (INIS)

Kawamitsu, Hideaki; Sugimura, Kazuro; Kasai, Toshifumi; Kimino, Katsuji

1993-01-01

ECG-gated spin-echo (SE) imaging can reduce physiologic motion artifact. However, it does not provide strong T 1 -weighted images, because the repetition time (TR) depends on heart rate (HR). For odd-echo SE imaging, T 1 contrast can be maximized by using a smaller flip angle (FA) of initial excitation RF pulses. We investigated the usefulness of partial FA SE imaging in order to obtain more T 1 -dependent contrast with ECG gating and determined the optimal FA at each heart rate. In computer simulation and phantom study, the predicted image contrast and signal-to-noise ratio (SNR) obtained for each FA (0∼180deg) and each HR (55∼90 beats per minute (bpm)) were compared with those obtained with conventional T 1 -weighted SE imaging (TR=500 ms, TE=20 ms, FA=90deg). The optimal FA was decreased by reducing HR. The FA needed to obtain T 1 -dependent contrast identical to that with T 1 -weighted SE imaging was 43deg at a HR of 65 bpm, 53deg at 70 bpm, 60deg at 75 bpm. This predicted FA were in excellent agreement with that obtained with clinical evaluation. The predicted SNR was decreased by reducing FA. The SNR of partial FA SE imaging at HR of 65 bpm (FA=43deg) was 80% of that with conventional T 1 -weighted SE imaging. However, this imaging method presented no marked clinical problem. ECG-gated partial FA SE imaging provides better T 1 -dependent contrast than conventional ECG-gated SE imaging, especially for Gd-DTPA enhanced imaging. (author)

SPINS OF LARGE ASTEROIDS: A HINT OF A PRIMORDIAL DISTRIBUTION IN THEIR SPIN RATES

Energy Technology Data Exchange (ETDEWEB)

Steinberg, Elad; Sari, Re’em [The Hebrew University of Jerusalem, Jerusalem (Israel)

2015-04-15

The Asteroid Belt and the Kuiper Belt are relics from the formation of our solar system. Understanding the size and spin distribution of the two belts is crucial for a deeper understanding of the formation of our solar system and the dynamical processes that govern it. In this paper, we investigate the effect of collisions on the evolution of the spin distribution of asteroids and KBOs. We find that the power law nature of the impactors’ size distribution leads to a Lévy distribution of the spin rates. This results in a power law tail in the spin distribution, in stark contrast to the usually quoted Maxwellian distribution. We show that for bodies larger than 10 km, collisions alone lead to spin rates peaking at 0.15–0.5 revolutions per day. Comparing that to the observed spin rates of large asteroids (R > 50 km), we find that the spins of large asteroids, peaking at ∼1–2 revolutions per day, are dominated by a primordial component that reflects the formation mechanism of the asteroids. Similarly, the Kuiper Belt has undergone virtually no collisional spin evolution, assuming current densities. Collisions contribute a spin rate of ∼0.01 revolutions per day, thus the observed fast spin rates of KBOs are also primordial in nature.

Radiative recombination of excitons in amorphous semiconductors

Energy Technology Data Exchange (ETDEWEB)

Singh, Jai [School of Engineering and Logistics, Faculty Technology, B-41, Charles Darwin University, Darwin, NT 0909 (Australia)]. E-mail: jai.singh@cdu.edu.au

2005-04-15

A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments.

Radiative recombination of excitons in amorphous semiconductors

International Nuclear Information System (INIS)

Singh, Jai

2005-01-01

A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments

Shorter Exciton Lifetimes via an External Heavy-Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light-Emitting Diodes.

Science.gov (United States)

Einzinger, Markus; Zhu, Tianyu; de Silva, Piotr; Belger, Christian; Swager, Timothy M; Van Voorhis, Troy; Baldo, Marc A

2017-10-01

Multiexcited-state phenomena are believed to be the root cause of two exigent challenges in organic light-emitting diodes; namely, efficiency roll-off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed-fluorescence-emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy-atom effect of brominated host molecules leads to increased spin-orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll-off at high excitation densities. Efficient organic light-emitting diodes with better roll-off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real-world applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Measurement of the depolarization of the reaction 27Al (p vector, p vector.) 27Al for the study of the spin-spin potential

International Nuclear Information System (INIS)

Loeh, H.

1981-01-01

For the study of the spin-spin interactions in the optical potential the depolarization in the elastic scattering of polarized protons was measured. The double-scattering experiments were performed in the angular range 40 0 -110 0 at an incident energy of 10.35 MeV at the Erlangen QD magnetic spectrometer. The determination of the optical model parameters independent from the spin-spin potentials resulted by the fit of those to the observables and sigmasup(di). These were obtained from a measurement of the angular distribution of the analyzing power and the differential cross section in the 4π-scattering chamber for the reaction 27 Al (p vector,psub(o)) at the same energy. The compound contributions present at this energy, which can also influence the depolarization, were regarded by the calculation of the compound-elastic non-spin-flip respectively spin-flip subcross sections by means of the formalism of Hofmann, Richard, Tepel, and Weidenmueller. Because of the target nucleus 27 Al posesses in the ground state a spin I=5/2 also the possible quadrupole spin flip had to be included in the analysis. This was performed by coupled channel calculations. The depolarization data corrected according to compound contributions and quadrupole effects could now be applied to the study of the spin-spin potentials by means of DWBA calculations. As result it turned out that for the description of the experimental data a spherical spin-spin potential of the strength Vsub(SS)=1.5+-0.3 MeV had to be assumed. For the addition of a tensor term however no necessity resulted. (orig.) [de

Spin-flip isovector giant resonances from the 90Zr (n,p) 90Y reaction at 200 MeV

International Nuclear Information System (INIS)

Raywood, K.J.; Spicer, B.M.

1989-01-01

Doubly differential cross sections of the reaction 90 Zr(n,p) 90 Y have been measured at 200 MeV for excitations up to 38 MeV in the residual nucleus. An overall resolution of 1.3 MeV was achieved. The spectra show qualitative agreement in shape and magnitude with recent RPA calculations; however all of the calculations underestimate the high excitation region of the spectra. A multipole decomposition of the data has been performed using differential cross sections calculated in the DWIA. An estimate of the Gamow-Teller strength in the reaction is given. The isovector spin-flip dipole giant resonance has been identified and there is also an indication of isovector monopole strength. 39 refs., 16 figs., 1 tab

Deuteron spin-flip reactions and supermultiplet potential model of interaction of the lightest clusters

CERN Document Server

Lebedev, V M; Struzhko, B G

2002-01-01

Heterogeneous data on the double and triple differential cross sections of d + p -> np + p and d + t(h) -> np + t(h) or d + t -> nn + h nuclear reactions are reduced by Migdal-Watson approximation to the unified shape of the differential cross section angular dependence having in mind just singlet nucleon-nucleon pair formation. The results are compared with the supermultiplet potential model of the lightest nuclei interaction. The d + t(h) collision is characterized by the fact that the power of V sup [ sup 4 sup 1 sup ] (r) potential is 50% higher than that of the V sup [ sup 3 sup 2 sup ] (r) one ([f] = [41] and [f] = [32] are the orbital Young patterns. This is why the theory is able to describe quantitatively both the above experiment and the elastic scattering one. However, for d + p collision the difference of potential powers for the [f] = [3] and [f] = [21] patterns equals 20% only and the agreement of theory with experiment on deuteron spin-flip is merely qualitative

Numerical modeling of exciton-polariton Bose-Einstein condensate in a microcavity

Science.gov (United States)

Voronych, Oksana; Buraczewski, Adam; Matuszewski, Michał; Stobińska, Magdalena

2017-06-01

A novel, optimized numerical method of modeling of an exciton-polariton superfluid in a semiconductor microcavity was proposed. Exciton-polaritons are spin-carrying quasiparticles formed from photons strongly coupled to excitons. They possess unique properties, interesting from the point of view of fundamental research as well as numerous potential applications. However, their numerical modeling is challenging due to the structure of nonlinear differential equations describing their evolution. In this paper, we propose to solve the equations with a modified Runge-Kutta method of 4th order, further optimized for efficient computations. The algorithms were implemented in form of C++ programs fitted for parallel environments and utilizing vector instructions. The programs form the EPCGP suite which has been used for theoretical investigation of exciton-polaritons. Catalogue identifier: AFBQ_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFBQ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: BSD-3 No. of lines in distributed program, including test data, etc.: 2157 No. of bytes in distributed program, including test data, etc.: 498994 Distribution format: tar.gz Programming language: C++ with OpenMP extensions (main numerical program), Python (helper scripts). Computer: Modern PC (tested on AMD and Intel processors), HP BL2x220. Operating system: Unix/Linux and Windows. Has the code been vectorized or parallelized?: Yes (OpenMP) RAM: 200 MB for single run Classification: 7, 7.7. Nature of problem: An exciton-polariton superfluid is a novel, interesting physical system allowing investigation of high temperature Bose-Einstein condensation of exciton-polaritons-quasiparticles carrying spin. They have brought a lot of attention due to their unique properties and potential applications in polariton-based optoelectronic integrated circuits. This is an out-of-equilibrium quantum system confined

Well separated trion and neutral excitons on superacid treated MoS{sub 2} monolayers

Energy Technology Data Exchange (ETDEWEB)

Cadiz, Fabian, E-mail: cadiz@insa-toulouse.fr; Tricard, Simon; Gay, Maxime; Lagarde, Delphine; Wang, Gang; Robert, Cedric; Renucci, Pierre; Urbaszek, Bernhard; Marie, Xavier [Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Av. Rangueil, 31077 Toulouse (France)

2016-06-20

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS{sub 2} MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS{sub 2} MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

Excitons in insulators

International Nuclear Information System (INIS)

Grasser, R.; Scharmann, A.

1983-01-01

This chapter investigates absorption, reflectivity, and intrinsic luminescence spectra of free and/or self-trapped (localized) excitons in alkali halides and rare gas solids. Introduces the concepts underlying the Wannier-Mott and Frenkel exciton models, two extreme pictures of an exciton in crystalline materials. Discusses the theoretical and experimental background; excitons in alkali halides; and excitons in rare gas solids. Shows that the intrinsic optical behavior of wide gap insulators in the range of the fundamental absorption edge is controlled by modified Wannier-Mott excitons. Finds that while that alkali halides only show free and relaxed molecular-like exciton emission, in rare gas crystals luminescence due to free, single and double centered localized excitons is observed. Indicates that the simultaneous existence of free and self-trapped excitons in these solid requires an energy barrier for self-trapping

Radiative and non-radiative relaxation of excitons in strain-compensated quantum dots

International Nuclear Information System (INIS)

Kujiraoka, M.; Ishi-Hayase, J.; Akahane, K.; Yamamoto, Y.; Ema, K.; Sasaki, M.

2008-01-01

We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump-probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2 ns for orthogonally polarized exciton ground states

Spin-dependent transport in ferromagnet/semiconductor/ferromagnet junctions: a fully relativistic approach

International Nuclear Information System (INIS)

Popescu, Voicu; Ebert, Hubert; Papanikolaou, Nikolaos; Zeller, Rudolf; Dederichs, Peter H

2004-01-01

We present a fully relativistic generalization of the Landauer-Buettiker formalism that has been implemented within the framework of the spin-polarized relativistic screened Korringa-Kohn-Rostoker Green function method. This approach, going beyond the two-current model, supplies a more general description of the electronic transport. It is shown that the relativistic conductance can be split in terms of individual spin-diagonal and spin-off-diagonal (spin-flip) components, which allows a detailed analysis of the influence of spin-orbit-coupling-induced spin-flip processes on the spin-dependent transport. We apply our method to calculate the ballistic conductance in Fe/GaAs/Fe magnetic tunnel junctions. We find that, by removing the spin selection rules, the spin-orbit coupling strongly influences the conductance, not only qualitatively but also quantitatively, especially in the anti-parallel alignment of the magnetization in the two Fe leads

On the possibility of excitonic magnetism in Ir double perovskites

Czech Academy of Sciences Publication Activity Database

Pajskr, K.; Novák, Pavel; Pokorný, Vladislav; Kolorenč, Jindřich; Arita, R.; Kuneš, Jan

2016-01-01

Roč. 93, č. 3 (2016), 1-6, č. článku 035129. ISSN 1098-0121 R&D Projects: GA ČR GA13-25251S Institutional support: RVO:68378271 Keywords : spin-orbit coupling * double perovskite * excitonic magnetism Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.736, year: 2014

Giant spin rotation under quasiparticle-photoelectron conversion: Joint effect of sublattice interference and spin-orbit coupling

DEFF Research Database (Denmark)

Kuemmeth, Ferdinand; Rashba, E I

2009-01-01

Spin- and angular-resolved photoemission spectroscopy is a basic experimental tool for unveiling spin polarization of electron eigenstates in crystals. We prove, by using spin-orbit coupled graphene as a model, that photoconversion of a quasiparticle inside a crystal into a photoelectron can...... be accompanied with a dramatic change in its spin polarization, up to a total spin flip. This phenomenon is typical of quasiparticles residing away from the Brillouin-zone center and described by higher rank spinors and results in exotic patterns in the angular distribution of photoelectrons....

Attenuation of spin resonance signals in media with the multi-component system of collectivized electrons

International Nuclear Information System (INIS)

Vojtenko, V.A.

1995-01-01

Universal relaxation theory of spectral line form at electron scattering light with spin flip at scattering of neutrons and at electron paramagnetic resonance, is plotted. Signals of spin resonances are shown to be subjected to strong attenuation caused by mutual transformations of various current carriers in multicomponent spin systems contained in intermetallic actinides with heavy fermions, in HTSC-crystals, in indirect highly alloyed semiconductors, solid solutions and superlattices. Physical reasons of observation of light strong scattering with spin flip in intermetallic actinides with semi-width independent of the wave vector are discussed. 19 refs

Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect

Science.gov (United States)

Park, Kyoung-Duck; Jiang, Tao; Clark, Genevieve; Xu, Xiaodong; Raschke, Markus B.

2018-01-01

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (XD) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of XD states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the XD emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe2 on a gold substrate, we demonstrate 6 × 105-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 103 of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

Normal Spin Asymmetries in Elastic Electron-Proton Scattering

International Nuclear Information System (INIS)

M. Gorchtein; P.A.M. Guichon; M. Vanderhaeghen

2004-01-01

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions. We further discuss this observable in the quasi-RCS kinematics which may be dominant at certain kinematical conditions and find it to be governed by the photon helicity-flip RCS amplitudes

Normal Spin Asymmetries in Elastic Electron-Proton Scattering

International Nuclear Information System (INIS)

Gorchtein, M.; Guichon, P.A.M.; Vanderhaeghen, M.

2005-01-01

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions. We further discuss this observable in the quasi-RCS kinematics which may be dominant at certain kinematical conditions and find it to be governed by the photon helicity-flip RCS amplitudes

Energy transfer of excitons between quantum wells separated by a wide barrier

International Nuclear Information System (INIS)

Lyo, S. K.

2000-01-01

We present a microscopic theory of the excitonic Stokes and anti-Stokes energy-transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch (Δ) at low temperatures (T). Several important intrinsic energy-transfer mechanisms have been examined, including dipolar coupling, real and virtual photon-exchange coupling, and over-barrier ionization of the excitons via exciton-exciton Auger processes. The transfer rate is calculated as a function of T and the center-to-center distance d between the wells. The rates depend sensitively on T for plane-wave excitons. For localized excitons, the rates depend on T only through the T dependence of the exciton localization radius. For Stokes energy transfer, the dominant energy transfer occurs through a photon-exchange interaction, which enables the excitons from the higher-energy wells to decay into free electrons and holes in the lower-energy wells. The rate has a slow dependence on d, yielding reasonable agreement with recent data from GaAs/Al x Ga 1-x As quantum wells. The dipolar rate is about an order of magnitude smaller for large d (e.g., d=175Aa) with a stronger range dependence proportional to d -4 . However, the latter can be comparable to the radiative rate for small d (e.g., d≤80Aa). For anti-Stokes transfer through exchange-type (e.g., dipolar and photon-exchange) interactions, we show that thermal activation proportional to exp(-Δ/k B T) is essential for the transfer, contradicting a recent nonactivated result based on the Fo''rster-Dexter's spectral-overlap theory. Phonon-assisted transfer yields a negligibly small rate. On the other hand, energy transfer through over-barrier ionization of excitons via Auger processes yields a significantly larger nonactivated rate which is independent of d. The result is compared with recent data

Competition of edge effects on the electronic properties and excitonic effects in short graphene nanoribbons

International Nuclear Information System (INIS)

Lu, Yan; Wei, Sheng; Jin, Jing; Wang, Li; Lu, Wengang

2016-01-01

We explore the electronic properties and exciton effects in short graphene nanoribbons (SGNRs), which have two armchair edges and two zigzag edges. Our results show that both of these two types of edges have profound effects on the electronic properties and exciton effects. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states are alternatively changed between the bulk and the edge states as the lengths of the zigzag edges increase, due to the competition between the states of the two types of edges. The energy gaps, as a function of the lengths of the armchair edges, will then induce two kinds of trends. Furthermore, two kinds of exciton energies and exciton binding energies are found, which can be understood through the two kinds of HOMO and LUMO states in SGNRs. In addition, we find that the three triplet exciton states are not totally energy degenerate in SGNRs due to the spin-polarized states on the zigzag edges. (paper)

DomFLIP++

International Nuclear Information System (INIS)

Hendrysiak, W.; Raggl, A.; Slany, W.

1996-01-01

DomFLIP++ is the knowledge engineering module of the *FLIP++ project. *FLIP++ is a tool for optimizing multiple criteria problems. It uses fuzzy constraints to model optimizing criteria and applies algorithms such as Tabu search or genetic algorithms to the problems. DomFLIP++ is a C++ library. It allows the definition of new optimization problems. It helps a domain engineer to design the structure of a new problem. However, there is a domain independent interface to other *FLIP++ modules such as OptiFLIP++, DynaFLIP++, and InterFLIP++. After each iteration in the optimization process, the considered instantiations of the problem are evaluated. Each evaluation produces a list of violated constraints. For each constraint in further iterations of the optimization. A domain can be fine-tuned through modifications of constraints, through editing their repair lists, and through change in the optimizing parameters. A well-tuned domain can be successfully applied for optimization. Object-oriented design and implementation makes this module easy to modify and to reuse. Definition of new domains, system extensions with new optimizing algorithms, and definition of specific domain-dependent repair steps can be done efficiently. DomFLIP++ is tested on real-world example, namely scheduling the steel plant LD3 in Linz, Austria

Counting and tensorial properties of twist-two helicity-flip nucleon form factors

International Nuclear Information System (INIS)

Chen Zhang; Ji Xiangdong

2005-01-01

We perform a systematic analysis on the off-forward matrix elements of the twist-two quark and gluon helicity-flip operators. By matching the allowed quantum numbers and their crossing channel counterparts (a method developed by Ji and Lebed), we systematically count the number of independent nucleon form factors in off-forward scattering of matrix elements of these quark and gluon spin-flip operators. In particular, we find that the numbers of independent nucleon form factors of twist-two, helicity-flip quark (gluon) operators are 2n-1 (2n-5) if n is odd, and 2n-2 (2n-6) if n is even, with n≥2 (n≥4). We also analyze and write down the tensorial/Lorentz structure and kinematic factors of the expansion of these operators' matrix elements in terms of the independent form factors. These generalized form factors define the off-forward quark and gluon helicity-flip distributions in the literature

Fingerprints of entangled spin and orbital physics in itinerant ferromagnets via angle-resolved resonant photoemission

Science.gov (United States)

Da Pieve, F.

2016-01-01

A method for mapping the local spin and orbital nature of the ground state of a system via corresponding flip excitations is proposed based on angle-resolved resonant photoemission and related diffraction patterns, obtained here via an ab initio modified one-step theory of photoemission. The analysis is done on the paradigmatic weak itinerant ferromagnet bcc Fe, whose magnetism, a correlation phenomenon given by the coexistence of localized moments and itinerant electrons, and the observed non-Fermi-Liquid behavior at extreme conditions both remain unclear. The combined analysis of energy spectra and diffraction patterns offers a mapping of local pure spin-flip, entangled spin-flip-orbital-flip excitations and chiral transitions with vortexlike wave fronts of photoelectrons, depending on the valence orbital symmetry and the direction of the local magnetic moment. Such effects, mediated by the hole polarization, make resonant photoemission a promising tool to perform a full tomography of the local magnetic properties even in itinerant ferromagnets or macroscopically nonmagnetic systems.

Exciton correlations and input–output relations in non-equilibrium exciton superfluids

International Nuclear Information System (INIS)

Ye, Jinwu; Sun, Fadi; Yu, Yi-Xiang; Liu, Wuming

2013-01-01

The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron–hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg–Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input–output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton–polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron–electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor ν T =1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: ► Establish the relations between photoluminescence and transport measurements. ► Stress the

SU(5): to flip or not to flip

International Nuclear Information System (INIS)

Nanopoulos, D.V.; Wisconsin Univ., Madison,

1988-01-01

Flipped SU(5) possesses some unique features as a grand unified theory: elegant missing partner mechanism, see-saw neutrino masses, no Higgs adjoints. This last property makes flipped SU(5) the only known grand unified theory derivable from 4-dimensional superstrings. When derived from the superstrings, flipped SU(5) possesses, in addition a hierarchical fermion mass spectrum. All these recent developments involving flipped SU(5) are discussed in a detailed but simple way, including phenomenological consequences at low energies

Driven spin transitions in fluorinated single- and bilayer-graphene quantum dots

Science.gov (United States)

Żebrowski, D. P.; Peeters, F. M.; Szafran, B.

2017-06-01

Spin transitions driven by a periodically varying electric potential in dilute fluorinated graphene quantum dots are investigated. Flakes of monolayer graphene as well as electrostatic electron traps induced in bilayer graphene are considered. The stationary states obtained within the tight-binding approach are used as the basis for description of the system dynamics. The dilute fluorination of the top layer lifts the valley degeneracy of the confined states and attenuates the orbital magnetic dipole moments due to current circulation within the flake. The spin-orbit coupling introduced by the surface deformation of the top layer induced by the adatoms allows the spin flips to be driven by the AC electric field. For the bilayer quantum dots the spin flip times is substantially shorter than the spin relaxation. Dynamical effects including many-photon and multilevel transitions are also discussed.

Calculation of exchange coupling constants in triply-bridged dinuclear Cu(II) compounds based on spin-flip constricted variational density functional theory.

Science.gov (United States)

Seidu, Issaka; Zhekova, Hristina R; Seth, Michael; Ziegler, Tom

2012-03-08

The performance of the second-order spin-flip constricted variational density functional theory (SF-CV(2)-DFT) for the calculation of the exchange coupling constant (J) is assessed by application to a series of triply bridged Cu(II) dinuclear complexes. A comparison of the J values based on SF-CV(2)-DFT with those obtained by the broken symmetry (BS) DFT method and experiment is provided. It is demonstrated that our methodology constitutes a viable alternative to the BS-DFT method. The strong dependence of the calculated exchange coupling constants on the applied functionals is demonstrated. Both SF-CV(2)-DFT and BS-DFT affords the best agreement with experiment for hybrid functionals.

Chiral tunneling of topological states: towards the efficient generation of spin current using spin-momentum locking.

Science.gov (United States)

Habib, K M Masum; Sajjad, Redwan N; Ghosh, Avik W

2015-05-01

We show that the interplay between chiral tunneling and spin-momentum locking of helical surface states leads to spin amplification and filtering in a 3D topological insulator (TI). Our calculations show that the chiral tunneling across a TI pn junction allows normally incident electrons to transmit, while the rest are reflected with their spins flipped due to spin-momentum locking. The net result is that the spin current is enhanced while the dissipative charge current is simultaneously suppressed, leading to an extremely large, gate-tunable spin-to-charge current ratio (∼20) at the reflected end. At the transmitted end, the ratio stays close to 1 and the electrons are completely spin polarized.

A general explanation on the correlation of dark matter halo spin with the large-scale environment

Science.gov (United States)

Wang, Peng; Kang, Xi

2017-06-01

Both simulations and observations have found that the spin of halo/galaxy is correlated with the large-scale environment, and particularly the spin of halo flips in filament. A consistent picture of halo spin evolution in different environments is still lacked. Using N-body simulation, we find that halo spin with its environment evolves continuously from sheet to cluster, and the flip of halo spin happens both in filament and nodes. The flip in filament can be explained by halo formation time and migrating time when its environment changes from sheet to filament. For low-mass haloes, they form first in sheets and migrate into filaments later, so their mass and spin growth inside filament are lower, and the original spin is still parallel to filament. For high-mass haloes, they migrate into filaments first, and most of their mass and spin growth are obtained in filaments, so the resulted spin is perpendicular to filament. Our results well explain the overall evolution of cosmic web in the cold dark matter model and can be tested using high-redshift data. The scenario can also be tested against alternative models of dark matter, such as warm/hot dark matter, where the structure formation will proceed in a different way.

Intracranial cerebrospinal fluid spaces imaging using a pulse-triggered three-dimensional turbo spin echo MR sequence with variable flip-angle distribution

International Nuclear Information System (INIS)

Hodel, Jerome; Silvera, Jonathan; Bekaert, Olivier; Decq, Philippe; Rahmouni, Alain; Bastuji-Garin, Sylvie; Vignaud, Alexandre; Petit, Eric; Durning, Bruno

2011-01-01

To assess the three-dimensional turbo spin echo with variable flip-angle distribution magnetic resonance sequence (SPACE: Sampling Perfection with Application optimised Contrast using different flip-angle Evolution) for the imaging of intracranial cerebrospinal fluid (CSF) spaces. We prospectively investigated 18 healthy volunteers and 25 patients, 20 with communicating hydrocephalus (CH), five with non-communicating hydrocephalus (NCH), using the SPACE sequence at 1.5T. Volume rendering views of both intracranial and ventricular CSF were obtained for all patients and volunteers. The subarachnoid CSF distribution was qualitatively evaluated on volume rendering views using a four-point scale. The CSF volumes within total, ventricular and subarachnoid spaces were calculated as well as the ratio between ventricular and subarachnoid CSF volumes. Three different patterns of subarachnoid CSF distribution were observed. In healthy volunteers we found narrowed CSF spaces within the occipital aera. A diffuse narrowing of the subarachnoid CSF spaces was observed in patients with NCH whereas patients with CH exhibited narrowed CSF spaces within the high midline convexity. The ratios between ventricular and subarachnoid CSF volumes were significantly different among the volunteers, patients with CH and patients with NCH. The assessment of CSF spaces volume and distribution may help to characterise hydrocephalus. (orig.)

Intracranial cerebrospinal fluid spaces imaging using a pulse-triggered three-dimensional turbo spin echo MR sequence with variable flip-angle distribution

Energy Technology Data Exchange (ETDEWEB)

Hodel, Jerome [Unite Analyse et Restauration du Mouvement, UMR-CNRS, 8005 LBM ParisTech Ensam, Paris (France); University Paris Est Creteil (UPEC), Creteil (France); Assistance Publique-Hopitaux de Paris, Paris (France); Hopital Henri Mondor, Department of Neuroradiology, Creteil (France); Hopital Henri Mondor, Creteil (France); Silvera, Jonathan [University Paris Est Creteil (UPEC), Creteil (France); Assistance Publique-Hopitaux de Paris, Paris (France); Hopital Henri Mondor, Department of Neuroradiology, Creteil (France); Bekaert, Olivier; Decq, Philippe [Unite Analyse et Restauration du Mouvement, UMR-CNRS, 8005 LBM ParisTech Ensam, Paris (France); University Paris Est Creteil (UPEC), Creteil (France); Assistance Publique-Hopitaux de Paris, Paris (France); Hopital Henri Mondor, Department of Neurosurgery, Creteil (France); Rahmouni, Alain [University Paris Est Creteil (UPEC), Creteil (France); Assistance Publique-Hopitaux de Paris, Paris (France); Hopital Henri Mondor, Department of Radiology, Creteil (France); Bastuji-Garin, Sylvie [University Paris Est Creteil (UPEC), Creteil (France); Assistance Publique-Hopitaux de Paris, Paris (France); Hopital Henri Mondor, Department of Public Health, Creteil (France); Vignaud, Alexandre [Siemens Healthcare, Saint Denis (France); Petit, Eric; Durning, Bruno [Laboratoire Images Signaux et Systemes Intelligents, UPEC, Creteil (France)

2011-02-15

To assess the three-dimensional turbo spin echo with variable flip-angle distribution magnetic resonance sequence (SPACE: Sampling Perfection with Application optimised Contrast using different flip-angle Evolution) for the imaging of intracranial cerebrospinal fluid (CSF) spaces. We prospectively investigated 18 healthy volunteers and 25 patients, 20 with communicating hydrocephalus (CH), five with non-communicating hydrocephalus (NCH), using the SPACE sequence at 1.5T. Volume rendering views of both intracranial and ventricular CSF were obtained for all patients and volunteers. The subarachnoid CSF distribution was qualitatively evaluated on volume rendering views using a four-point scale. The CSF volumes within total, ventricular and subarachnoid spaces were calculated as well as the ratio between ventricular and subarachnoid CSF volumes. Three different patterns of subarachnoid CSF distribution were observed. In healthy volunteers we found narrowed CSF spaces within the occipital aera. A diffuse narrowing of the subarachnoid CSF spaces was observed in patients with NCH whereas patients with CH exhibited narrowed CSF spaces within the high midline convexity. The ratios between ventricular and subarachnoid CSF volumes were significantly different among the volunteers, patients with CH and patients with NCH. The assessment of CSF spaces volume and distribution may help to characterise hydrocephalus. (orig.)

Negative circular polarization as a universal property of quantum dots

International Nuclear Information System (INIS)

Taylor, Matthew W.; Spencer, Peter; Murray, Ray

2015-01-01

This paper shows that negative circular polarization, a spin flip of polarized carriers resulting in emission of opposite helicity, can be observed in undoped, n-doped, and p-doped InAs/GaAs quantum dots. These results contradict the usual interpretation of the effect. We show using power dependent and time resolved spectroscopy that the generation of negative circular polarization correlates with excited state emission. Furthermore, a longer spin lifetime of negatively polarized excitons is observed where emission is largely ground state in character

Variable flip angle excitation for reduced acquisition time magnetic resonance imaging

International Nuclear Information System (INIS)

Mills, T.C.; Ortendahl, D.A.; Hylton, N.M.; Carlson, J.W.; Crooks, L.E.; Kaufman, L.

1987-01-01

This paper describes an MRI technique which can be used to acquire images at short TR values while maintaining the sensitivity to disease found in longer TR images. For spin echo imaging there are three acquisition parameters that can be set in the imaging protocol; TR, the repetition interval; TE, the time of echo and Θ, the excitation flip angle. Standard imaging techniques set Θ to 90 degrees regardless of the TR value. With Θ fixed, imaging systems have been optimized by varying the value for TE and TR with the results in general indicating the need for long TR values. However, if the flip angle is included as a variable acquisition parameter the optimal operating point can be changed. The solution to the Bloch equation shows a functional relationship between the flip angle and the ratio TR/T1. This functionality was first observed by Ernst and Anderson as a method to increase the signal generated in fourier transform magnetic resonance spectroscopy. When TR/T1<1 the optimum flip angle for producing maximum magnetization in the transverse plane is less then 90 degrees. Therefore, by reducing both TR and flip angle it is possible to maintain signal intensity while reducing the time of data acquisition

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants.

Science.gov (United States)

Pinchetti, Valerio; Di, Qiumei; Lorenzon, Monica; Camellini, Andrea; Fasoli, Mauro; Zavelani-Rossi, Margherita; Meinardi, Francesco; Zhang, Jiatao; Crooker, Scott A; Brovelli, Sergio

2018-02-01

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag + is an emerging electronic dopant in III-V and II-VI nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag + is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag + . This optical activation process and the associated modification of the electronic configuration of Ag + remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag + to paramagnetic Ag 2+ . The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Spin dynamics in relativistic ionization with highly charged ions in super-strong laser fields

International Nuclear Information System (INIS)

Klaiber, Michael; Yakaboylu, Enderalp; Bauke, Heiko; Hatsagortsyan, Karen Z; Müller, Carsten; Paulus, Gerhard G

2014-01-01

Spin dynamics and induced spin effects in above-threshold ionization of hydrogenlike highly charged ions in super-strong laser fields are investigated. Spin-resolved ionization rates in the tunnelling regime are calculated by employing two versions of a relativistic Coulomb-corrected strong-field approximation (SFA). An intuitive simpleman model is developed which explains the derived scaling laws for spin flip and spin asymmetry effects. The intuitive model as well as our ab initio numerical simulations support the analytical results for the spin effects obtained in the dressed SFA where the impact of the laser field on the electron spin evolution in the bound state is taken into account. In contrast, the standard SFA is shown to fail in reproducing spin effects in ionization even at a qualitative level. The anticipated spin-effects are expected to be measurable with modern laser techniques combined with an ion storage facility. (paper)

The Flipped Journal Club.

Science.gov (United States)

Bounds, Richard; Boone, Stephen

2018-01-01

Educators struggle to develop a journal club format that promotes active participation from all levels of trainees. The explosion of social media compels residencies to incorporate the evaluation and application of these resources into evidence-based practice. We sought to design an innovative "flipped journal club" to achieve greater effectiveness in meeting goals and objectives among residents and faculty. Each journal club is focused on a specific clinical question based on a landmark article, a background article, and a podcast or blog post. With the "flipped" model, residents are assigned to prepare an in-depth discussion of one of these works based on their level of training. At journal club, trainees break into small groups and discuss their assigned readings with faculty facilitation. Following the small-group discussions, all participants convene to summarize key points. In redesigning our journal club, we sought to achieve specific educational outcomes, and improve participant engagement and overall impressions. Sixty-one residents at our emergency medicine program participated in the flipped journal club during the 2015-2016 academic year, with supervision by core faculty. Program evaluation for the flipped journal club was performed using an anonymous survey, with response rates of 70% and 56% for residents and faculty, respectively. Overall, 95% of resident respondents and 100% of faculty respondents preferred the flipped format. The "flipped journal club" hinges upon well-selected articles, incorporation of social media, and small-group discussions. This format engages all residents, holds learners accountable, and encourages greater participation among residents and faculty.

Optical orientation of the homogeneous non-equilibrium Bose-Einstein condensate of bright excitons (polaritons)

OpenAIRE

Korenev, V. L.

2011-01-01

A simple model, describing the dynamics of the non-equilibrium pseudospin of a homogeneous Bose-Einstein condensate of exciton polaritons, has been formulated. It explains the suppression of spin splitting of a non-equilibrium polariton condensate in an external magnetic field, the optical alignment, and the conversion of alignment into orientation of polaritons. It has been shown that inverse effects are possible, to wit, the spontaneous circular polarization and the enhancement of spin spli...

Spin relaxation rates in quantum dots: Role of the phonon modulated spin orbit interaction

Science.gov (United States)

Alcalde, A. M.; Romano, C. L.; Marques, G. E.

2008-11-01

We calculate the spin relaxation rates in InAs and GaAs parabolic quantum dots due to the interaction of spin carriers with acoustical phonons. We consider a spin relaxation mechanism completely intrinsic to the system, since it is based on the modulation of the spin-orbit interaction by the acoustic phonon potential, which is independent of any structural properties of the confinement potential. The electron-phonon deformation potential and the piezoelectric interaction are described by the Pavlov-Firsov spin-phonon Hamiltonian. Our results demonstrate that, for narrow-gap semiconductors, the deformation potential interaction becomes dominant. This behavior is not observed for wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the relaxation processes. We also demonstrate that the spin relaxation rates are particularly sensitive to values of the Landé g-factor, which depend strongly on the spatial shape of the confinement.

On flipping first-semester calculus: a case study

Science.gov (United States)

Petrillo, Joseph

2016-05-01

High failure rates in calculus have plagued students, teachers, and administrators for decades, while science, technology, engineering, and mathematics programmes continue to suffer from low enrollments and high attrition. In an effort to affect this reality, some educators are 'flipping' (or inverting) their classrooms. By flipping, we mean administering course content outside of the classroom and replacing the traditional in-class lectures with discussion, practice, group work, and other elements of active learning. This paper presents the major results from a three-year study of a flipped, first-semester calculus course at a small, comprehensive, American university with a well-known engineering programme. The data we have collected help quantify the positive and substantial effects of our flipped calculus course on failure rates, scores on the common final exam, student opinion of calculus, teacher impact on measurable outcomes, and success in second-semester calculus. While flipping may not be suitable for every teacher, every student, and in every situation, this report provides some evidence that it may be a viable option for those seeking an alternative to the traditional lecture model.

Faster pediatric 3-T abdominal magnetic resonance imaging: comparison between conventional and variable refocusing flip-angle single-shot fast spin-echo sequences

Energy Technology Data Exchange (ETDEWEB)

Ruangwattanapaisarn, Nichanan [Mahidol University, Department of Diagnostic and Therapeutic Radiology, Ramathibodi Hospital, Bangkok (Thailand); Stanford University, LPCH Department of Radiology, Stanford, CA (United States); Loening, Andreas M.; Saranathan, Manojkumar; Vasanawala, Shreyas S. [Stanford University, LPCH Department of Radiology, Stanford, CA (United States); Litwiller, Daniel V. [GE Healthcare, Rochester, MN (United States)

2015-06-15

Single-shot fast spin echo (SSFSE) is particularly appealing in pediatric patients because of its motion robustness. However radiofrequency energy deposition at 3 tesla forces long pauses between slices, leading to longer scans, longer breath-holds and more between-slice motion. We sought to learn whether modulation of the SSFSE refocusing flip-angle train could reduce radiofrequency energy deposition without degrading image quality, thereby reducing inter-slice pauses and overall scan times. We modulated the refocusing flip-angle train for SSFSE to minimize energy deposition while minimizing blurring and motion-related signal loss. In a cohort of 50 consecutive patients (25 boys, mean age 5.5 years, range 1 month to 17 years) referred for abdominal MRI we obtained standard SSFSE and variable refocusing flip-angle (vrfSSFSE) images and recorded sequence scan times. Two readers independently scored the images in blinded, randomized order for noise, tissue contrast, sharpness, artifacts and left lobe hepatic signal uniformity on a four-point scale. The null hypothesis of no difference between SSFSE and vrfSSFSE image-quality was assessed with a Mann-Whitney U test, and the null hypothesis of no scan time difference was assessed with the paired t-test. SSFSE and vrfSSFSE mean acquisition times were 54.3 and 26.2 s, respectively (P-value <0.0001). For each reader, SSFSE and vrfSSFSE noise, tissue contrast, sharpness and artifacts were not significantly different (P-values 0.18-0.86). However, SSFSE had better left lobe hepatic signal uniformity (P < 0.01, both readers). vrfSSFSE is twice as fast as SSFSE, with equivalent image quality with the exception of left hepatic lobe signal heterogeneity. (orig.)

Influence of soliton distributions on the spin-dependent electronic ...

Indian Academy of Sciences (India)

interactions, so that spin memory can only be as long as a few seconds [6]. Therefore, spin-flip .... In addition, the term −σ · hβ is the internal exchange energy with hβ .... electrons density of states for short chains containing 100 carbon atoms.

Effects on spin asymmetries of special effects at 900

International Nuclear Information System (INIS)

Lipkin, H.J.

1986-01-01

Hadron and quark exchange contributions to spin symmetries at 90 0 in hadron elastic scattering are investigated. The angular distribution of scattering cross sections is considered for incident protons with contributions from nonflip and double-flip amplitudes for states of parallel and antiparallel spins. 5 refs

Calculation of the exchange coupling constants of copper binuclear systems based on spin-flip constricted variational density functional theory.

Science.gov (United States)

Zhekova, Hristina R; Seth, Michael; Ziegler, Tom

2011-11-14

We have recently developed a methodology for the calculation of exchange coupling constants J in weakly interacting polynuclear metal clusters. The method is based on unrestricted and restricted second order spin-flip constricted variational density functional theory (SF-CV(2)-DFT) and is here applied to eight binuclear copper systems. Comparison of the SF-CV(2)-DFT results with experiment and with results obtained from other DFT and wave function based methods has been made. Restricted SF-CV(2)-DFT with the BH&HLYP functional yields consistently J values in excellent agreement with experiment. The results acquired from this scheme are comparable in quality to those obtained by accurate multi-reference wave function methodologies such as difference dedicated configuration interaction and the complete active space with second-order perturbation theory. © 2011 American Institute of Physics

Magnetization reversal through soliton flip in biquadratic ferromagnet with varying exchange interactions

CERN Document Server

Daniel, M

2002-01-01

We study the phenomenon of magnetization reversal in the form of a soliton flip in a biquadratic ferromagnetic spin chain induced by varying bilinear and biquadratic exchange interactions. This is carried out by analysing the evolution of the velocity and amplitude of the soliton using a perturbation analysis.

Optical orientation of the homogeneous nonequilibrium Bose-Einstein condensate of exciton polaritons

Science.gov (United States)

Korenev, V. L.

2012-07-01

A simple model, describing the steady state of the nonequilibrium polarization of a homogeneous Bose-Einstein condensate of exciton polaritons, is considered. It explains the suppression of spin splitting of a nonequilibrium polariton condensate in an external magnetic field, the linear polarization, the linear-to-circular polarization conversion, and the unexpected sign of the circular polarization of the condensate all on equal footing. It is shown that inverse effects are possible, to wit, spontaneous circular polarization and the enhancement of spin splitting of a nonequilibrium condensate of polaritons.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

Directory of Open Access Journals (Sweden)

Yuichi Tsuchiya

2015-12-01

Full Text Available cFLIP (cellular FLICE-like inhibitory protein is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIPL, cFLIPS, and cFLIPR. cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Theory of Spin States of Quantum Dot Molecules

Science.gov (United States)

Ponomarev, I. V.; Reinecke, T. L.; Scheibner, M.; Stinaff, E. A.; Bracker, A. S.; Doty, M. F.; Gammon, D.; Korenev, V. L.

2007-04-01

The photoluminescence spectrum of an asymmetric pair of coupled InAs quantum dots in an applied electric field shows a rich pattern of level anticrossings, crossings and fine structure that can be understood as a superposition of charge and spin configurations. We present a theoretical model that provides a description of the energy positions and intensities of the optical transitions in exciton, biexciton and charged exciton states of coupled quantum dots molecules.

Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites

Science.gov (United States)

Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.

2016-03-01

UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.

CNI polarimetry and the hadronic spin dependence of pp scattering

International Nuclear Information System (INIS)

Trueman, T.L.

1996-01-01

Methods for limiting the size of hadronic spin-flip in the Coulomb- Nuclear Interference. region are critically assessed. This work was presented at the High Energy Polarimetry Workshop in Amsterdam, Sept. 9, 1996 and the RHIC Spin Collaboration meeting in Marseille, Sept. 17, 1996

Spontaneous and superfluid chiral edge states in exciton-polariton condensates

Science.gov (United States)

Sigurdsson, H.; Li, G.; Liew, T. C. H.

2017-09-01

We present a scheme of interaction-induced topological band structures based on the spin anisotropy of exciton-polaritons in semiconductor microcavities. We predict theoretically that this scheme allows the engineering of topological gaps, without requiring a magnetic field or strong spin-orbit interaction (transverse electric-transverse magnetic splitting). Under nonresonant pumping we find that an initially topologically trivial system undergoes a topological transition upon the spontaneous breaking of phase symmetry associated with polariton condensation. Under either nonresonant or resonant coherent pumping we find that it is also possible to engineer a topological dispersion that is linear in wave vector—a property associated with polariton superfluidity.

Flipped Learning

DEFF Research Database (Denmark)

Holmboe, Peter; Hachmann, Roland

I FLIPPED LEARNING – FLIP MED VIDEO kan du læse om, hvordan du som underviser kommer godt i gang med at implementere video i undervisning, der har afsæt i tankerne omkring flipped learning. Bogen indeholder fire dele: I Del 1 fokuserer vi på det metarefleksive i at tænke video ind i undervisningen...

Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

Science.gov (United States)

Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.

2016-06-01

We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

Test of a two-dimensional neutron spin analyzer

International Nuclear Information System (INIS)

Falus, Peter; Vorobiev, Alexei; Krist, Thomas

2006-01-01

The aim of this measurement was to test the new large-area spin polarization analyzer for the EVA-SERGIS beamline at Institute Laue Langevin (ILL). The spin analyzer, which was built in Berlin selects one of the two spin states of a neutron beam of wavelength 5.5 A impinging on a horizontal sample and reflected or scattered from the sample. The spin is analyzed for all neutrons scattered into a detector with an area of 190 mmx190 mm positioned 2.7 m behind the sample, thus covering an angular interval of 4 o x4 o . The tests were done at the HMI V14 beamline followed by tests at the EVA beamline at ILL. The transmission for the two spin components, the flipping ratio and small angle scattering were recorded while scanning the incoming beam on the analyzer. It was clearly visible, that due to the stacked construction the intensity is blocked at regular intervals. Careful inspection shows that the transmission of the good spin component is more than 0.72 for 60% of the detector area and the corrected flipping ratio is more than 47 for 60% of the detector area. Although some small-angle scattering is visible, it is notable that this analyzer design has small scattering intensities

Test of a two-dimensional neutron spin analyzer

Science.gov (United States)

Falus, Péter; Vorobiev, Alexei; Krist, Thomas

2006-11-01

The aim of this measurement was to test the new large-area spin polarization analyzer for the EVA-SERGIS beamline at Institute Laue Langevin (ILL). The spin analyzer, which was built in Berlin selects one of the two spin states of a neutron beam of wavelength 5.5 Å impinging on a horizontal sample and reflected or scattered from the sample. The spin is analyzed for all neutrons scattered into a detector with an area of 190 mm×190 mm positioned 2.7 m behind the sample, thus covering an angular interval of 4°×4°. The tests were done at the HMI V14 beamline followed by tests at the EVA beamline at ILL. The transmission for the two spin components, the flipping ratio and small angle scattering were recorded while scanning the incoming beam on the analyzer. It was clearly visible, that due to the stacked construction the intensity is blocked at regular intervals. Careful inspection shows that the transmission of the good spin component is more than 0.72 for 60% of the detector area and the corrected flipping ratio is more than 47 for 60% of the detector area. Although some small-angle scattering is visible, it is notable that this analyzer design has small scattering intensities.

Composition-Dependent Energy Splitting between Bright and Dark Excitons in Lead Halide Perovskite Nanocrystals.

Science.gov (United States)

Chen, Lan; Li, Bin; Zhang, Chunfeng; Huang, Xinyu; Wang, Xiaoyong; Xiao, Min

2018-03-14

Perovskite semiconductor nanocrystals with different compositions have shown promise for applications in light-emitting devices. Dark excitonic states may suppress light emission from such nanocrystals by providing an additional nonradiative recombination channel. Here, we study the composition dependence of dark exciton dynamics in nanocrystals of lead halides by time-resolved photoluminescence spectroscopy at cryogenic temperatures. The presence of a spin-related dark state is revealed by magneto-optical spectroscopy. The energy splitting between bright and dark states is found to be highly sensitive to both halide elements and organic cations, which is explained by considering the effects of size confinement and charge screening, respectively, on the exchange interaction. These findings suggest the possibility of manipulating dark exciton dynamics in perovskite semiconductor nanocrystals by composition engineering, which will be instrumental in the design of highly efficient light-emitting devices.

Spin precession and spin waves in a chiral electron gas: Beyond Larmor's theorem

Science.gov (United States)

Karimi, Shahrzad; Baboux, Florent; Perez, Florent; Ullrich, Carsten A.; Karczewski, Grzegorz; Wojtowicz, Tomasz

2017-07-01

Larmor's theorem holds for magnetic systems that are invariant under spin rotation. In the presence of spin-orbit coupling this invariance is lost and Larmor's theorem is broken: for systems of interacting electrons, this gives rise to a subtle interplay between the spin-orbit coupling acting on individual single-particle states and Coulomb many-body effects. We consider a quasi-two-dimensional, partially spin-polarized electron gas in a semiconductor quantum well in the presence of Rashba and Dresselhaus spin-orbit coupling. Using a linear-response approach based on time-dependent density-functional theory, we calculate the dispersions of spin-flip waves. We obtain analytic results for small wave vectors and up to second order in the Rashba and Dresselhaus coupling strengths α and β . Comparison with experimental data from inelastic light scattering allows us to extract α and β as well as the spin-wave stiffness very accurately. We find significant deviations from the local density approximation for spin-dependent electron systems.

Perovskite Excitonics : Primary Exciton Creation and Crossover from Free Carriers to a Secondary Exciton Phase

NARCIS (Netherlands)

Sarritzu, Valerio; Sestu, Nicola; Marongiu, Daniela; Chang, Xueqing; Wang, Qingqian; Loi, Maria Antonietta; Quochi, Francesco; Saba, Michele; Mura, Andrea; Bongiovanni, Giovanni

2018-01-01

Understanding exciton formation is of fundamental importance for emerging optoelectronic materials, like hybrid organic-inorganic perovskites, as excitons are the lowest-energy photoexcitations in semiconductors, are electrically neutral, and do not directly contribute to charge transport, but can

Excitonic processes at organic heterojunctions

Science.gov (United States)

He, ShouJie; Lu, ZhengHong

2018-02-01

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.

On Flipping First-Semester Calculus: A Case Study

Science.gov (United States)

Petrillo, Joseph

2016-01-01

High failure rates in calculus have plagued students, teachers, and administrators for decades, while science, technology, engineering, and mathematics programmes continue to suffer from low enrollments and high attrition. In an effort to affect this reality, some educators are "flipping" (or inverting) their classrooms. By flipping, we…

Spin-polaron theory of high-Tc superconductivity: I, spin polarons and high-Tc pairing

International Nuclear Information System (INIS)

Wood, R.F.

1993-06-01

The concept of a spin polaron is introduced and contrasted with the more familiar ionic polaron picture. A brief review of aspects of ionic bipolaronic superconductivity is given with particular emphasis on the real-space pairing and true Bose condensation characteristics. The formation energy of spin polarons is then calculated in analogy with ionic polarons. The spin-flip energy of a Cu spin in an antiferromagnetically aligned CuO 2 plane is discussed. It is shown that the introduction of holes into the CuO 2 planes will always lead to the destruction of long-range AF ordering due to the formation of spin polarons. The pairing of two spin polarons can be expected because of the reestablishment of local (short-range) AF ordering; the magnitude of the pairing energy is estimated using a simplified model. The paper closes with a brief discussion of the formal theory of spin polarons

Nonvolatile flip-flop based on pseudo-spin-transistor architecture and its nonvolatile power-gating applications for low-power CMOS logic

Science.gov (United States)

Yamamoto, Shuu'ichirou; Shuto, Yusuke; Sugahara, Satoshi

2013-07-01

We computationally analyzed performance and power-gating (PG) ability of a new nonvolatile delay flip-flop (NV-DFF) based on pseudo-spin-MOSFET (PS-MOSFET) architecture using spin-transfer-torque magnetic tunnel junctions (STT-MTJs). The high-performance energy-efficient PG operations of the NV-DFF can be achieved owing to its cell structure employing PS-MOSFETs that can electrically separate the STT-MTJs from the ordinary DFF part of the NV-DFF. This separation also makes it possible that the break-even time (BET) of the NV-DFF is designed by the size of the PS-MOSFETs without performance degradation of the normal DFF operations. The effect of the area occupation ratio of the NV-DFFs to a CMOS logic system on the BET was also analyzed. Although the optimized BET was varied depending on the area occupation ratio, energy-efficient fine-grained PG with a BET of several sub-microseconds was revealed to be achieved. We also proposed microprocessors and system-on-chip (SoC) devices using nonvolatile hierarchical-memory systems wherein NV-DFF and nonvolatile static random access memory (NV-SRAM) circuits are used as fundamental building blocks. Contribution to the Topical Issue “International Semiconductor Conference Dresden-Grenoble - ISCDG 2012”, Edited by Gérard Ghibaudo, Francis Balestra and Simon Deleonibus.

The Flipped Journal Club

Directory of Open Access Journals (Sweden)

Richard Bounds

2017-12-01

Full Text Available Introduction Educators struggle to develop a journal club format that promotes active participation from all levels of trainees. The explosion of social media compels residencies to incorporate the evaluation and application of these resources into evidence-based practice. We sought to design an innovative “flipped journal club” to achieve greater effectiveness in meeting goals and objectives among residents and faculty. Methods Each journal club is focused on a specific clinical question based on a landmark article, a background article, and a podcast or blog post. With the “flipped” model, residents are assigned to prepare an in-depth discussion of one of these works based on their level of training. At journal club, trainees break into small groups and discuss their assigned readings with faculty facilitation. Following the small-group discussions, all participants convene to summarize key points. In redesigning our journal club, we sought to achieve specific educational outcomes, and improve participant engagement and overall impressions. Results Sixty-one residents at our emergency medicine program participated in the flipped journal club during the 2015–2016 academic year, with supervision by core faculty. Program evaluation for the flipped journal club was performed using an anonymous survey, with response rates of 70% and 56% for residents and faculty, respectively. Overall, 95% of resident respondents and 100% of faculty respondents preferred the flipped format. Conclusion The “flipped journal club” hinges upon well-selected articles, incorporation of social media, and small-group discussions. This format engages all residents, holds learners accountable, and encourages greater participation among residents and faculty.

Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

Energy Technology Data Exchange (ETDEWEB)

Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B., E-mail: benito.alen@csic.es [IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid (Spain); Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A. [Institute for Systems based on Optoelectronics and Microtechnology (ISOM), Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid (Spain)

2015-11-02

External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

Confined exciton spectroscopy

International Nuclear Information System (INIS)

Torres, Clivia M.S.

1998-01-01

Full text: In this work, the exciton is considered as a sensor of the electronic and optical properties of materials such as semiconductors, which have size compared to the exciton De Broglie wavelength, approximately 20 nm, depending on the semiconductor. Examples of electron-phonon, electron-electron, photon-electron, exciton-polariton, phonon-plasmon, are presented, under different confinement conditions such as quantum wells, superlattices

Frenkel-Charge-Transfer exciton intermixing theory for molecular crystals with two isolated Frenkel exciton states.

Science.gov (United States)

Bondarev, Igor; Popescu, Adrian

We develop an analytical theory for the intra-intermolecular exciton intermixing in periodic 1D chains of planar organic molecules with two isolated low-lying Frenkel exciton states, typical of copper phthalocyanine (CuPc) and other transition metal phthalocyanine molecules. We formulate the Hamiltonian and use the exact Bogoliubov diagonalization procedure to derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer (CT) exciton state. By comparing our theoretical spectrum with available experimental CuPc absorption data, we obtain the parameters of the Frenkel-CT exciton intermixing in CuPc thin films. The two Frenkel exciton states here are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the CT exciton, showing the coupling constant 0.17 eV in agreement with earlier electron transport experiments. Our results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines. DOE-DE-SC0007117 (I.B.), UNC-GA ROI Grant (A.P.).

Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

International Nuclear Information System (INIS)

Tran, Vy; Fei, Ruixiang; Yang, Li

2015-01-01

We report first-principles GW–Bethe–Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G 0 W 0 ) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet–triplet splitting, and electron–hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron–hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges. (paper)

Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers

Science.gov (United States)

Kim, Kwiseon; Erkan Kose, Muhammet; Graf, Peter; Kopidakis, Nikos; Rumbles, Garry; Shaheen, Sean E.

2009-03-01

Phenyl cored 3-arm and 4-arm thiophene dendrimers are promising materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules to guide the synthesis of novel dendrimers with improved efficiency. A method is developed to estimate the exciton diffusion lengths for the dendrimers and similar chromophores in amorphous films. The approach exploits Fermi's Golden Rule to estimate the energy transfer rates for an ensemble of bimolecular complexes in random orientations. Using Poisson's equation to evaluate Coulomb integrals led to efficient calculation of excitonic couplings between the transition densities. Monte-Carlo simulations revealed the dynamics of energy transport in the dendrimers. Experimental exciton diffusion lengths of the dendrimers range 10 ˜ 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with experiments. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors that determine the exciton diffusion length in amorphous films.

Effects of surface and interface traps on exciton and multi-exciton dynamics in core/shell quantum dots

Science.gov (United States)

Bozio, Renato; Righetto, Marcello; Minotto, Alessandro

2017-08-01

Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.

Ultrafast spin injection from Cd1-x Mn x Te magnetic barriers into a CdTe quantum well studied by pump-probe spectroscopy

International Nuclear Information System (INIS)

Aoshima, I.; Nishibayashi, K.; Souma, I.; Murayama, A.; Oka, Y.

2006-01-01

Spin injection from diluted magnetic semiconductor (DMS) barriers of Cd 1- x Mn x Te into a quantum well (QW) of CdTe is studied, by means of pump-probe absorption spectroscopy in magnetic fields. Fast decay characteristics of circularly polarized differential absorbances of spin-polarized excitons in the DMS barrier show the exciton injection time of 6 ps from the barriers into the QW. In accordance with the fast relaxation of the spin-polarized excitons from the barrier, we observe the rise of circular polarization degree for the differential absorption of the CdTe QW in magnetic fields, evidently indicating the spin injection. In addition, the circular polarization degree up to 0.3 is developed in the well immediately after pumping, originating from the fast relaxation of a heavy hole (hh) spin less than 0.2 ps, due to the giant Zeeman effect caused by the penetration of the hh wave function into the DMS barriers

Spin-resolved electron waiting times in a quantum-dot spin valve

Science.gov (United States)

Tang, Gaomin; Xu, Fuming; Mi, Shuo; Wang, Jian

2018-04-01

We study the electronic waiting-time distributions (WTDs) in a noninteracting quantum-dot spin valve by varying spin polarization and the noncollinear angle between the magnetizations of the leads using the scattering matrix approach. Since the quantum-dot spin valve involves two channels (spin up and down) in both the incoming and outgoing channels, we study three different kinds of WTDs, which are two-channel WTD, spin-resolved single-channel WTD, and cross-channel WTD. We analyze the behaviors of WTDs in short times, correlated with the current behaviors for different spin polarizations and noncollinear angles. Cross-channel WTD reflects the correlation between two spin channels and can be used to characterize the spin-transfer torque process. We study the influence of the earlier detection on the subsequent detection from the perspective of cross-channel WTD, and define the influence degree quantity as the cumulative absolute difference between cross-channel WTDs and first-passage time distributions to quantitatively characterize the spin-flip process. We observe that influence degree versus spin-transfer torque for different noncollinear angles as well as different polarizations collapse into a single curve showing universal behaviors. This demonstrates that cross-channel WTDs can be a pathway to characterize spin correlation in spintronics system.

Anisotropic optical absorption induced by Rashba spin-orbit coupling in monolayer phosphorene

Science.gov (United States)

Li, Yuan; Li, Xin; Wan, Qi; Bai, R.; Wen, Z. C.

2018-04-01

We obtain the effective Hamiltonian of the phosphorene including the effect of Rashba spin-orbit coupling in the frame work of the low-energy theory. The spin-splitting energy bands show an anisotropy feature for the wave vectors along kx and ky directions, where kx orients to ΓX direction in the k space. We numerically study the optical absorption of the electrons for different wave vectors with Rashba spin-orbit coupling. We find that the spin-flip transition from the valence band to the conduction band induced by the circular polarized light closes to zero with increasing the x-component wave vector when ky equals to zero, while it can be significantly increased to a large value when ky gets a small value. When the wave vector varies along the ky direction, the spin-flip transition can also increase to a large value, however, which shows an anisotropy feature for the optical absorption. Especially, the spin-conserved transitions keep unchanged and have similar varying trends for different wave vectors. This phenomenon provides a novel route for the manipulation of the spin-dependent property of the fermions in the monolayer phosphorene.

Flipped dark matter

Energy Technology Data Exchange (ETDEWEB)

Ellis, J.; Hagelin, J.S.; Kelley, S.; Nanopoulos, D.V.; Olive, K.A.

1988-08-04

We study candidates for dark matter in a minimal flipped SU(5) x U(1) supersymmetric GUT. Since the model has no R-parity, spin-1/2 supersymmetric partners of conventional particles mix with other neutral fermions including neutrinos, and can decay into them. The lighest particle which is predominantly a gaugino/higgsino mixture decays with a lifetime tau/sub chi/ approx. = 1-10/sup 9/ s. The model contains a scalar 'flaton' field whose coherent oscillations decay before cosmological nucleosynthesis, and whose pseudoscalar partner contributes negligibly to ..cap omega.. if it is light enough to survive to the present epoch. The fermionic 'flatino' partner of the flaton has a lifetime tau/sub PHI/ approx. = 10/sup 28/-10/sup 34/ yr and is a viable candiate for metastable dark matter with ..cap omega.. < or approx. 1.

Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd1-xMnxTe: Froehlich and deformation potential exciton-phonon interaction

International Nuclear Information System (INIS)

Riera, R; Rosas, R; Marin, J L; Bergues, J M; Campoy, G

2003-01-01

A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd 1-x Mn x Te as an example. The incident radiation frequency ω l is taken above the fundamental absorption region. The photoexcited electron and hole make real transitions through the LO phonon, when one considers Froehlich (F) and deformation potential (DP) interactions. The strong exchange interaction, typical of these materials, leads to a large spin splitting of the exciton states in the magnetic field. Neglecting Landau quantization, this Zeeman splitting gives rise to the formation of eight bands (two conduction and six valence ones) and ten different exciton states according to the polarization of the incident light. Explicit expressions for the MPRRS intensity of second and third order, the indirect creation and annihilation probabilities, the exciton lifetime, and the probabilities of transition between different exciton states and different types of exciton as a function of ω l and the external magnetic field are presented. The selection rules for all hot exciton transitions via exciton-photon interaction and F and DP exciton-phonon interactions are investigated. The exciton energies, as a function of B, the Mn concentration x, and the temperature T, are compared to a theoretical expression. Graphics for creation and annihilation probabilities, lifetime, and Raman intensity of second and third order are discussed

Field-induced negative differential spin lifetime in silicon.

Science.gov (United States)

Li, Jing; Qing, Lan; Dery, Hanan; Appelbaum, Ian

2012-04-13

We show that the electric-field-induced thermal asymmetry between the electron and lattice systems in pure silicon substantially impacts the identity of the dominant spin relaxation mechanism. Comparison of empirical results from long-distance spin transport devices with detailed Monte Carlo simulations confirms a strong spin depolarization beyond what is expected from the standard Elliott-Yafet theory even at low temperatures. The enhanced spin-flip mechanism is attributed to phonon emission processes during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where (beyond a critical field) reduction of the transit time between spin-injector and spin-detector is accompanied by a counterintuitive reduction in spin polarization and an apparent negative spin lifetime.

Self-trapped excitons in LH2 bacteriochlorophyll-protein complexes under high pressure

International Nuclear Information System (INIS)

Timpmann, K.; Ellervee, Aleksandr; Kuznetsov, Anatoli; Laisaar, Arlentin; Trinkunas, Gediminas; Freiberg, Arvi

2003-01-01

The absorption and emission spectra of excitons in LH2 antenna complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been studied under hydrostatic pressure. The measurements made between ambient pressure and 6 kbar over a broad temperature range reveal largely different rates of the pressure-induced shifts for the absorption and emission bands. Numerical calculations based on exciton polaron model provide evidence for the exciton self-trapping at ambient pressure as well as for the pressure stabilization of the self-trapped exciton states responsible for the emission, whereas the light absorbing states belong to nearly free excitons over the whole pressure and temperature ranges studied

Electrical control of truly two-dimensional neutral and charged excitons in monolayer MoSe2

Science.gov (United States)

Ross, Jason; Wu, Sanfeng; Yu, Hongyi; Ghimire, Nirmal; Jones, Aaron; Aivazian, Grant; Yan, Jiaqiang; Mandrus, David; Xiao, Di; Xiao, Di; Xu, Xiaodong

2013-03-01

Monolayer transition metal dichalcogenides (TMDs) have emerged as ideal 2D semiconductors with valley and spin polarized excitations expected to enable true valley-tronics. Here we investigate MoSe2, a TMD which has yet to be characterized in the monolayer limit. Specifically, we examine excitons and trions (their singly charged counterparts) in the ultimate 2D limit. Utilizing high quality exfoliated MoSe2 monolayers, we report the observation and electrostatic tunability of positively charged (X +) , neutral (Xo), and negatively charged (X-) excitons via photoluminescence in FETs. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, while the linewidth is narrow (5 meV) at temperatures below 55 K. This is greater spectral contrast than in any known quasi-2D system. Further, the charging energies for X + and X- to are nearly identical implying the same effective mass for electrons and holes, which supports their recent description as massive Dirac fermions. This work demonstrates that monolayer MoSe2 is an ultimate 2D semiconductor opening the door for the investigation of truly 2D exciton physics while laying the ground work necessary to begin valley-spin polarization studies. Support: US DoE, BES, Division of MSE. HY and WY supported by Research Grant Council of Hong Kong

MR imaging of the temporomandibular joint. Part 2. Effect of flip angle on MR imaging with FLASH sequence

International Nuclear Information System (INIS)

Sakamoto, Maya; Sasano, Takashi; Higano, Shuichi; Takahashi, Shoki; Kurihara, Noriko

1998-01-01

In our previous study on MR imaging of the temporomandibular joint (TMJ), fast low angle shot (FLASH) showed the highest image contrast between disc and surrounding TMJ tissues compared with those of 4 other sequences (i,e., fast imaging with steady precession (FISP), conventional T1-weighted spin echo (SE) and fast spin echo (FSE, TR/TE/ETL: 1100/12/3, 3000/15/7)). Furthermore, FLASH also received a high score on visual evaluation including the position and contour of the disc, and the border between the disc and surrounding tissues. Therefore, we concluded that FLASH was the most suitable sequence for evaluating the TMJ disc. However, the image contrast and signal intensity on MR imaging with gradient echo pulse sequence are affected by flip angle. Consequently, in this report, to find the most suitable flip angle for MR scanning of the TMJ using a FLASH sequence (TR/TE: 450/11), ten TMJs of 5 volunteers were experimentally imaged with various flip angles from 10 degrees to 70 degrees at an interval of 10 degrees between 10 to 70. The image contrast and contrast-to-noise ratio (CNR) between the disc and surrounding tissues were compared. In addition, signal-to-noise ratio (SNR) of phantoms was also calculated using the same imaging parameters. Visual evaluation including position and contour of the disc, and the border between the disc and surrounding tissues, was also performed by 4 radiologists. As the flip angle increased, imaging contrast decreased while SNR increased. Images with flip angles between 30 and 60 degrees demonstrated high CNR. On visual evaluation, images using flip angles between 30 and 50 degrees received high scores. In conclusion, FLASH sequence with a flip angle between 30 and 50 degrees was considered most suitable for evaluating the TMJ disc based on the results of visual assessment and analysis of three major components of image diagnostic quality: image contrast, CNR and SNR. (author)

Some comments on flipped SU(5) times U(1) and flipped unification in general

Energy Technology Data Exchange (ETDEWEB)

Barr, S.M. (Bartol Research Institute, University of Delaware, Newark, Delaware 19716 (US))

1989-10-01

A general group-theoretical discussion of flipped embeddings is given. In addition to the well-known flipped SU(5) and flipped SO(10), the existence of flipped E{sub 6} and E{sub 7} is shown, as well as several families and special cases of flipped embeddings. A possible physical reason, essentially based on the group theory of flipped embeddings, why nature prefers the low-energy group SU(3){times}SU(2){times}U(1) to alternatives such as SU(4){times}U(1) and SU(5) is pointed out.

Some comments on flipped SU(5)xU(1) and flipped unification in general

International Nuclear Information System (INIS)

Barr, S.M.

1989-01-01

A general group-theoretical discussion of flipped embeddings is given. In addition to the well-known flipped SU(5) and flipped SO(10), the existence of flipped E 6 and E 7 is shown, as well as several families and special cases of flipped embeddings. A possible physical reason, essentially based on the group theory of flipped embeddings, why nature prefers the low-energy group SU(3)xSU(2)xU(1) to alternatives such as SU(4)xU(1) and SU(5) is pointed out

Non-contrast-enhanced 4D MR angiography with STAR spin labeling and variable flip angle sampling: a feasibility study for the assessment of Dural Arteriovenous Fistula

Energy Technology Data Exchange (ETDEWEB)

Jang, Jinhee; Kim, Bom-yi; Choi, Hyun Seok; Jung, So-Lyung; Ahn, Kook-Jin; Kim, Bum-soo [The Catholic University of Korea, Department of Radiology, Seoul St. Mary' s Hospital, College of Medicine, Seoul (Korea, Republic of); Schmitt, Peter [Siemens AG, Healthcare Sector, Erlangen (Germany); Kim, Inseong; Paek, Munyoung [Siemens AG, Healthcare, Seoul (Korea, Republic of)

2014-04-15

This study aimed to evaluate the feasibility of non-contrast-enhanced 4D magnetic resonance angiography (NCE 4D MRA) with signal targeting with alternative radiofrequency (STAR) spin labeling and variable flip angle (VFA) sampling in the assessment of dural arteriovenous fistula (DAVF) in the transverse sinus. Nine patients underwent NCE 4D MRA for the evaluation of DAVF in the transverse sinus at 3 T. One patient was examined twice, once before and once after the interventional treatment. All patients also underwent digital subtraction angiography (DSA) and/or contrast-enhanced magnetic resonance angiography (CEMRA). For the acquisition of NCE 4D MRA, a STAR spin tagging method was used, and a VFA sampling was applied in the data readout module instead of a constant flip angle. Two readers evaluated the NCE 4D MRA data for the diagnosis of DAVF and its type with consensus. The results were compared with those from DSA and/or CEMRA. All patients underwent NCE 4D MRA without any difficulty. Among seven patients with patent DAVFs, all cases showed an early visualization of the transverse sinus on NCE 4D MRA. Except for one case, the type of DAVF of NCE 4D MRA was agreed with that of reference standard study. Cortical venous reflux (CVR) was demonstrated in two cases out of three patients with CVR. NCE 4D MRA with STAR tagging and VFA sampling is technically and clinically feasible and represents a promising technique for assessment of DAVF in the transverse sinus. Further technical developments should aim at improvements of spatial and temporal coverage. (orig.)

Non-contrast-enhanced 4D MR angiography with STAR spin labeling and variable flip angle sampling: a feasibility study for the assessment of Dural Arteriovenous Fistula

International Nuclear Information System (INIS)

Jang, Jinhee; Kim, Bom-yi; Choi, Hyun Seok; Jung, So-Lyung; Ahn, Kook-Jin; Kim, Bum-soo; Schmitt, Peter; Kim, Inseong; Paek, Munyoung

2014-01-01

This study aimed to evaluate the feasibility of non-contrast-enhanced 4D magnetic resonance angiography (NCE 4D MRA) with signal targeting with alternative radiofrequency (STAR) spin labeling and variable flip angle (VFA) sampling in the assessment of dural arteriovenous fistula (DAVF) in the transverse sinus. Nine patients underwent NCE 4D MRA for the evaluation of DAVF in the transverse sinus at 3 T. One patient was examined twice, once before and once after the interventional treatment. All patients also underwent digital subtraction angiography (DSA) and/or contrast-enhanced magnetic resonance angiography (CEMRA). For the acquisition of NCE 4D MRA, a STAR spin tagging method was used, and a VFA sampling was applied in the data readout module instead of a constant flip angle. Two readers evaluated the NCE 4D MRA data for the diagnosis of DAVF and its type with consensus. The results were compared with those from DSA and/or CEMRA. All patients underwent NCE 4D MRA without any difficulty. Among seven patients with patent DAVFs, all cases showed an early visualization of the transverse sinus on NCE 4D MRA. Except for one case, the type of DAVF of NCE 4D MRA was agreed with that of reference standard study. Cortical venous reflux (CVR) was demonstrated in two cases out of three patients with CVR. NCE 4D MRA with STAR tagging and VFA sampling is technically and clinically feasible and represents a promising technique for assessment of DAVF in the transverse sinus. Further technical developments should aim at improvements of spatial and temporal coverage. (orig.)

Spin Transport in Mesoscopic Superconducting-Ferromagnetic Hybrid Conductor

Directory of Open Access Journals (Sweden)

Zein W. A.

2008-01-01

Full Text Available The spin polarization and the corresponding tunneling magnetoresistance (TMR for a hybrid ferromagnetic / superconductor junction are calculated. The results show that these parameters are strongly depends on the exchange field energy and the bias voltage. The dependence of the polarization on the angle of precession is due to the spin flip through tunneling process. Our results could be interpreted as due to spin imbalance of carriers resulting in suppression of gap energy of the superconductor. The present investigation is valuable for manufacturing magnetic recording devices and nonvolatile memories which imply a very high spin coherent transport for such junction.

Spin Transport in Mesoscopic Superconducting-Ferromagnetic Hybrid Conductor

Directory of Open Access Journals (Sweden)

Zein W. A.

2008-01-01

Full Text Available The spin polarization and the corresponding tunneling magnetoresistance (TMR for a hybrid ferromagnetic/superconductor junction are calculated. The results show that these parameters are strongly depends on the exchange field energy and the bias voltage. The dependence of the polarization on the angle of precession is due to the spin flip through tunneling process. Our results could be interpreted as due to spin imbalance of carriers resulting in suppression of gap energy of the superconductor. The present investigation is valuable for manufacturing magnetic recording devices and nonvolatile memories which imply a very high spin coherent transport for such junction.

Inverse engineering for fast transport and spin control of spin-orbit-coupled Bose-Einstein condensates in moving harmonic traps

Science.gov (United States)

Chen, Xi; Jiang, Ruan-Lei; Li, Jing; Ban, Yue; Sherman, E. Ya.

2018-01-01

We investigate fast transport and spin manipulation of tunable spin-orbit-coupled Bose-Einstein condensates in a moving harmonic trap. Motivated by the concept of shortcuts to adiabaticity, we design inversely the time-dependent trap position and spin-orbit-coupling strength. By choosing appropriate boundary conditions we obtain fast transport and spin flip simultaneously. The nonadiabatic transport and relevant spin dynamics are illustrated with numerical examples and compared with the adiabatic transport with constant spin-orbit-coupling strength and velocity. Moreover, the influence of nonlinearity induced by interatomic interaction is discussed in terms of the Gross-Pitaevskii approach, showing the robustness of the proposed protocols. With the state-of-the-art experiments, such an inverse engineering technique paves the way for coherent control of spin-orbit-coupled Bose-Einstein condensates in harmonic traps.

Probing quantum coherence in single-atom electron spin resonance

Science.gov (United States)

Willke, Philip; Paul, William; Natterer, Fabian D.; Yang, Kai; Bae, Yujeong; Choi, Taeyoung; Fernández-Rossier, Joaquin; Heinrich, Andreas J.; Lutz, Christoper P.

2018-01-01

Spin resonance of individual spin centers allows applications ranging from quantum information technology to atomic-scale magnetometry. To protect the quantum properties of a spin, control over its local environment, including energy relaxation and decoherence processes, is crucial. However, in most existing architectures, the environment remains fixed by the crystal structure and electrical contacts. Recently, spin-polarized scanning tunneling microscopy (STM), in combination with electron spin resonance (ESR), allowed the study of single adatoms and inter-atomic coupling with an unprecedented combination of spatial and energy resolution. We elucidate and control the interplay of an Fe single spin with its atomic-scale environment by precisely tuning the phase coherence time T2 using the STM tip as a variable electrode. We find that the decoherence rate is the sum of two main contributions. The first scales linearly with tunnel current and shows that, on average, every tunneling electron causes one dephasing event. The second, effective even without current, arises from thermally activated spin-flip processes of tip spins. Understanding these interactions allows us to maximize T2 and improve the energy resolution. It also allows us to maximize the amplitude of the ESR signal, which supports measurements even at elevated temperatures as high as 4 K. Thus, ESR-STM allows control of quantum coherence in individual, electrically accessible spins. PMID:29464211

Calculation of nuclear-spin-relaxation rate for spin-polarized atomic hydrogen

International Nuclear Information System (INIS)

Ahn, R.M.C.; Eijnde, J.P.H.W.V.; Verhaar, B.J.

1983-01-01

Approximations introduced in previous calculations of spin relaxation for spin-polarized atomic hydrogen are investigated by carrying out a more exact coupled-channel calculation. With the exception of the high-temperature approximation, the approximations turn out to be justified up to the 10 -3 level of accuracy. It is shown that at the lowest temperatures for which experimental data are available, the high-temperature limit underestimates relaxation rates by a factor of up to 2. For a comparison with experimental data it is also of interest to pay attention to the expression for the atomic hydrogen relaxation rates in terms of transition amplitudes for two-particle collisions. Discrepancies by a factor of 2 among previous derivations of relaxation rates are pointed out. To shed light on these discrepancies we present two alternative derivations in which special attention is paid to identical-particle aspects. Comparing with experiment, we find our theoretical volume relaxation rate to be in better agreement with measured values than that obtained by other groups. The theoretical surface relaxation rate, however, still shows a discrepancy with experiment by a factor of order 50

Spin current induced by a charged tip in a quantum point contact

Energy Technology Data Exchange (ETDEWEB)

Shchamkhalova, B.S., E-mail: s.bagun@gmail.com

2017-03-15

We show that the charged tip of the probe microscope, which is widely used in studying the electron transport in low-dimensional systems, induces a spin current. The effect is caused by the spin–orbit interaction arising due to an electric field produced by the charged tip. The tip acts as a spin-flip scatterer giving rise to the spin polarization of the net current and the occurrence of a spin density in the system.

Energy Migration in Organic Thin Films--From Excitons to Polarons

Science.gov (United States)

Mullenbach, Tyler K.

The rise of organic photovoltaic devices (OPVs) and organic light-emitting devices has generated interest in the physics governing exciton and polaron dynamics in thin films. Energy transfer has been well studied in dilute solutions, but there are emergent properties in thin films and greater complications due to complex morphologies which must be better understood. Despite the intense interest in energy transport in thin films, experimental limitations have slowed discoveries. Here, a new perspective of OPV operation is presented where photovoltage, instead of photocurrent, plays the fundamental role. By exploiting this new vantage point the first method of measuring the diffusion length (LD) of dark (non-luminescent) excitons is developed, a novel photodetector is invented, and the ability to watch exciton arrival, in real-time, at the donor-acceptor heterojunction is presented. Using an enhanced understanding of exciton migration in thin films, paradigms for enhancing LD by molecular modifications are discovered, and the first exciton gate is experimentally and theoretically demonstrated. Generation of polarons from exciton dissociation represents a second phase of energy migration in OPVs that remains understudied. Current approaches are capable of measuring the rate of charge carrier recombination only at open-circuit. To enable a better understanding of polaron dynamics in thin films, two new approaches are presented which are capable of measuring both the charge carrier recombination and transit rates at any OPV operating voltage. These techniques pave the way for a more complete understanding of charge carrier kinetics in molecular thin films.

Flipped Learning, Flipped Satisfaction, Getting the Balance Right

Science.gov (United States)

Swinburne, Rosemary Fisher; Ross, Bella; LaFerriere, Richard; Maritz, Alex

2017-01-01

This paper explores students' perceptions of their learning outcomes, engagement, and satisfaction with a technology-facilitated flipped approach in a third-year core subject at an Australian university during 2014. In this pilot study, findings reveal that students preferred the flipped approach to the traditional face-to-face delivery and…

Current-induced damping of nanosized quantum moments in the presence of spin-orbit interaction

Science.gov (United States)

Mahfouzi, Farzad; Kioussis, Nicholas

2017-05-01

Motivated by the need to understand current-induced magnetization dynamics at the nanoscale, we have developed a formalism, within the framework of Keldysh Green function approach, to study the current-induced dynamics of a ferromagnetic (FM) nanoisland overlayer on a spin-orbit-coupling (SOC) Rashba plane. In contrast to the commonly employed classical micromagnetic LLG simulations the magnetic moments of the FM are treated quantum mechanically. We obtain the density matrix of the whole system consisting of conduction electrons entangled with the local magnetic moments and calculate the effective damping rate of the FM. We investigate two opposite limiting regimes of FM dynamics: (1) The precessional regime where the magnetic anisotropy energy (MAE) and precessional frequency are smaller than the exchange interactions and (2) the local spin-flip regime where the MAE and precessional frequency are comparable to the exchange interactions. In the former case, we show that due to the finite size of the FM domain, the "Gilbert damping" does not diverge in the ballistic electron transport regime, in sharp contrast to Kambersky's breathing Fermi surface theory for damping in metallic FMs. In the latter case, we show that above a critical bias the excited conduction electrons can switch the local spin moments resulting in demagnetization and reversal of the magnetization. Furthermore, our calculations show that the bias-induced antidamping efficiency in the local spin-flip regime is much higher than that in the rotational excitation regime.

Dephasing due to Nuclear Spins in Large-Amplitude Electric Dipole Spin Resonance.

Science.gov (United States)

Chesi, Stefano; Yang, Li-Ping; Loss, Daniel

2016-02-12

We analyze effects of the hyperfine interaction on electric dipole spin resonance when the amplitude of the quantum-dot motion becomes comparable or larger than the quantum dot's size. Away from the well-known small-drive regime, the important role played by transverse nuclear fluctuations leads to a Gaussian decay with characteristic dependence on drive strength and detuning. A characterization of spin-flip gate fidelity, in the presence of such additional drive-dependent dephasing, shows that vanishingly small errors can still be achieved at sufficiently large amplitudes. Based on our theory, we analyze recent electric dipole spin resonance experiments relying on spin-orbit interactions or the slanting field of a micromagnet. We find that such experiments are already in a regime with significant effects of transverse nuclear fluctuations and the form of decay of the Rabi oscillations can be reproduced well by our theory.

The flipped classroom

DEFF Research Database (Denmark)

Triantafyllou, Evangelia

2015-01-01

One of the novel ideas in teaching that heavily relies on current technology is the “flipped classroom” approach. In a flipped classroom the traditional lecture and homework sessions are inverted. Students are provided with online material in order to gain necessary knowledge before class, while...... class time is devoted to clarifications and application of this knowledge. The hypothesis is that there could be deep and creative discussions when teacher and students physically meet. This paper presents design considerations for flipped classrooms, and discusses how Moodle can facilitate...... with a discussion of the opportunities and challenges when implementing the flipped model in a virtual learning environment (VLE) like Moodle....

Flipped Classroom Approach

Directory of Open Access Journals (Sweden)

Fezile Ozdamli

2016-07-01

Full Text Available Flipped classroom is an active, student-centered approach that was formed to increase the quality of period within class. Generally this approach whose applications are done mostly in Physical Sciences, also attracts the attention of educators and researchers in different disciplines recently. Flipped classroom learning which wide-spreads rapidly in the world, is not well recognized in our country. That is why the aim of study is to attract attention to its potential in education field and provide to make it recognize more by educators and researchers. With this aim, in the study what flipped classroom approach is, flipped classroom technology models, its advantages and limitations were explained.

Nuclear Spin Nanomagnet in an Optically Excited Quantum Dot

Science.gov (United States)

Korenev, V. L.

2007-12-01

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins—the nuclear spin nanomagnet.

Detection of trans–cis flips and peptide-plane flips in protein structures

Energy Technology Data Exchange (ETDEWEB)

Touw, Wouter G., E-mail: wouter.touw@radboudumc.nl [Radboud University Medical Center, Geert Grooteplein-Zuid 26-28, 6525 GA Nijmegen (Netherlands); Joosten, Robbie P. [Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam (Netherlands); Vriend, Gert, E-mail: wouter.touw@radboudumc.nl [Radboud University Medical Center, Geert Grooteplein-Zuid 26-28, 6525 GA Nijmegen (Netherlands)

2015-07-28

A method is presented to detect peptide bonds that need either a trans–cis flip or a peptide-plane flip. A coordinate-based method is presented to detect peptide bonds that need correction either by a peptide-plane flip or by a trans–cis inversion of the peptide bond. When applied to the whole Protein Data Bank, the method predicts 4617 trans–cis flips and many thousands of hitherto unknown peptide-plane flips. A few examples are highlighted for which a correction of the peptide-plane geometry leads to a correction of the understanding of the structure–function relation. All data, including 1088 manually validated cases, are freely available and the method is available from a web server, a web-service interface and through WHAT-CHECK.

Detection of trans–cis flips and peptide-plane flips in protein structures

International Nuclear Information System (INIS)

Touw, Wouter G.; Joosten, Robbie P.; Vriend, Gert

2015-01-01

A method is presented to detect peptide bonds that need either a trans–cis flip or a peptide-plane flip. A coordinate-based method is presented to detect peptide bonds that need correction either by a peptide-plane flip or by a trans–cis inversion of the peptide bond. When applied to the whole Protein Data Bank, the method predicts 4617 trans–cis flips and many thousands of hitherto unknown peptide-plane flips. A few examples are highlighted for which a correction of the peptide-plane geometry leads to a correction of the understanding of the structure–function relation. All data, including 1088 manually validated cases, are freely available and the method is available from a web server, a web-service interface and through WHAT-CHECK

Direct measurement of exciton valley coherence in monolayer WSe2

KAUST Repository

Hao, Kai

2016-02-29

In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time—a crucial quantity for valley pseudospin manipulation—is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron–hole pairs) in monolayer WSe₂ (refs ,). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron–hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

Delocalization of Coherent Triplet Excitons in Linear Rigid Rod Conjugated Oligomers.

Science.gov (United States)

Hintze, Christian; Korf, Patrick; Degen, Frank; Schütze, Friederike; Mecking, Stefan; Steiner, Ulrich E; Drescher, Malte

2017-02-02

In this work, the triplet state delocalization in a series of monodisperse oligo(p-phenyleneethynylene)s (OPEs) is studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR) determining zero-field splitting, optical spin polarization, and proton hyperfine couplings. Neither the zero-field splitting parameters nor the optical spin polarization change significantly with OPE chain length, in contrast to the hyperfine coupling constants, which showed a systematic decrease with chain length n according to a 2/(1 + n) decay law. The results provide striking evidence for the Frenkel-type nature of the triplet excitons exhibiting full coherent delocalization in the OPEs under investigation with up to five OPE repeat units and with a spin density distribution described by a nodeless particle in the box wave function. The same model is successfully applied to recently published data on π-conjugated porphyrin oligomers.

Generation of pure spin currents via spin Seebeck effect in self-biased hexagonal ferrite thin films

Energy Technology Data Exchange (ETDEWEB)

Li, Peng; Ellsworth, David; Chang, Houchen; Janantha, Praveen; Richardson, Daniel; Phillips, Preston; Vijayasarathy, Tarah; Wu, Mingzhong, E-mail: mwu@lamar.colostate.edu [Department of Physics, Colorado State University, Fort Collins, Colorado 80523 (United States); Shah, Faisal [Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556 (United States)

2014-12-15

Light-induced generation of pure spin currents in a Pt(2.5 nm)/BaFe{sub 12}O{sub 19}(1.2 μm)/sapphire(0.5 mm) structure is reported. The BaFe{sub 12}O{sub 19} film had strong in-plane uniaxial anisotropy and was therefore self-biased. Upon exposure to light, a temperature difference (ΔT) was established across the BaFe{sub 12}O{sub 19} thickness that gave rise to a pure spin current in the Pt via the spin Seebeck effect. Via the inverse spin Hall effect, the spin current produced an electric voltage across one of the Pt lateral dimensions. The voltage varied with time in the same manner as ΔT and flipped its sign when the magnetization in BaFe{sub 12}O{sub 19} was reversed.

Exciton-dominant Electroluminescence from a Diode of Monolayer MoS2

Science.gov (United States)

2014-05-14

injected electrons and holes, is a reliable technique to study exciton recombination processes in monolayer MoS2, including val- ley and spin excitation...temperature. After superimposing a white light scattering image of the de - vice, we find that the electroluminescence is localized at the edge of the...We find the emerged feature (labeled NX) peaks at 550 nm with energy of 2.255 eV. In low dimensional system, like monolayer MoS2, Coulomb interactions

Flip-J: Development of the System for Flipped Jigsaw Supported Language Learning

Science.gov (United States)

Yamada, Masanori; Goda, Yoshiko; Hata, Kojiro; Matsukawa, Hideya; Yasunami, Seisuke

2016-01-01

This study aims to develop and evaluate a language learning system supported by the "flipped jigsaw" technique, called "Flip-J". This system mainly consists of three functions: (1) the creation of a learning material database, (2) allocation of learning materials, and (3) formation of an expert and jigsaw group. Flip-J was…

Exploring Flipped Classroom Effects on Second Language Learners' Cognitive Processing

Science.gov (United States)

Kim, Jeong-eun; Park, Hyunjin; Jang, Mijung; Nam, Hosung

2017-01-01

This study investigated the cognitive effects of the flipped classroom approach in a content-based instructional context by comparing second language learners' discourse in flipped vs. traditional classrooms in terms of (1) participation rate, (2) content of comments, (3) reasoning skills, and (4) interactional patterns. Learners in two intact…

Flipped Learning

DEFF Research Database (Denmark)

Hachmann, Roland; Holmboe, Peter

arbejde med faglige problemstillinger gennem problembaserede og undersøgende didaktiske designs. Flipped Learning er dermed andet og mere end at distribuere digitale materialer til eleverne forud for undervisning. Flipped Learning er i lige så høj grad et syn på, hvordan undervisning med digitale medier...

Fine structure of an exciton coupled to a single Fe2 + ion in a CdSe/ZnSe quantum dot

Science.gov (United States)

Smoleński, T.; Kazimierczuk, T.; Goryca, M.; Pacuski, W.; Kossacki, P.

2017-10-01

We present a polarization-resolved photoluminescence study of the exchange interaction effects in a prototype system consisting of an individual Fe2 + ion and a single neutral exciton confined in a CdSe/ZnSe quantum dot. A maximal possible number of eight fully linearly polarized lines in the bright exciton emission spectrum is observed, evidencing complete degeneracy lifting in the investigated system. We discuss the conditions required for such a scenario to take place: anisotropy of the electron-hole interaction and the zero-field splitting of the Fe2 + ion spin states. Neglecting either of these components is shown to restore partial degeneracy of the transitions, making the excitonic spectrum similar to those previously reported for all other systems of quantum dots with single magnetic dopants.

Fine structure and optical pumping of spins in individual semiconductor quantum dots

Science.gov (United States)

Bracker, Allan S.; Gammon, Daniel; Korenev, Vladimir L.

2008-11-01

We review spin properties of semiconductor quantum dots and their effect on optical spectra. Photoluminescence and other types of spectroscopy are used to probe neutral and charged excitons in individual quantum dots with high spectral and spatial resolution. Spectral fine structure and polarization reveal how quantum dot spins interact with each other and with their environment. By taking advantage of the selectivity of optical selection rules and spin relaxation, optical spin pumping of the ground state electron and nuclear spins is achieved. Through such mechanisms, light can be used to process spins for use as a carrier of information.

Fine structure and optical pumping of spins in individual semiconductor quantum dots

International Nuclear Information System (INIS)

Bracker, Allan S; Gammon, Daniel; Korenev, Vladimir L

2008-01-01

We review spin properties of semiconductor quantum dots and their effect on optical spectra. Photoluminescence and other types of spectroscopy are used to probe neutral and charged excitons in individual quantum dots with high spectral and spatial resolution. Spectral fine structure and polarization reveal how quantum dot spins interact with each other and with their environment. By taking advantage of the selectivity of optical selection rules and spin relaxation, optical spin pumping of the ground state electron and nuclear spins is achieved. Through such mechanisms, light can be used to process spins for use as a carrier of information

Rate equation modelling of the optically pumped spin-exchange source

International Nuclear Information System (INIS)

Stenger, J.; Rith, K.

1995-01-01

Sources for spin polarized hydrogen or deuterium, polarized via spin-exchange of a laser optically pumped alkali metal, can be modelled by rate equations. The rate equations for this type of source, operated either with hydrogen or deuterium, are given explicitly with the intention of providing a useful tool for further source optimization and understanding. Laser optical pumping of alkali metal, spin-exchange collisions of hydrogen or deuterium atoms with each other and with alkali metal atoms are included, as well as depolarization due to flow and wall collisions. (orig.)

Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells

KAUST Repository

Dimitrov, Stoichko

2016-01-13

The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

Flipped Classroom Experiences: Student Preferences and Flip Strategy in a Higher Education Context

Science.gov (United States)

McNally, Brenton; Chipperfield, Janine; Dorsett, Pat; Del Fabbro, Letitia; Frommolt, Valda; Goetz, Sandra; Lewohl, Joanne; Molineux, Matthew; Pearson, Andrew; Reddan, Gregory; Roiko, Anne; Rung, Andrea

2017-01-01

Despite the popularity of the flipped classroom, its effectiveness in achieving greater engagement and learning outcomes is currently lacking substantial empirical evidence. This study surveyed 563 undergraduate and postgraduate students (61% female) participating in flipped teaching environments and ten convenors of the flipped courses in which…

How to Flip the Classroom--"Productive Failure or Traditional Flipped Classroom" Pedagogical Design?

Science.gov (United States)

Song, Yanjie; Kapur, Manu

2017-01-01

The paper reports a quasi-experimental study comparing the "traditional flipped classroom" pedagogical design with the "productive failure" (Kapur, 2016) pedagogical design in the flipped classroom for a 2-week curricular unit on polynomials in a Hong Kong Secondary school. Different from the flipped classroom where students…

Observation of Spin Flips with a Single Trapped Proton

CERN Document Server

Ulmer, S.; Blaum, K.; Kracke, H.; Mooser, A.; Quint, W.; Walz, J.

2011-01-01

Spin transitions of an isolated trapped proton are observed for the first time. The spin quantum jumps are detected via the continuous Stern-Gerlach effect which is used in an experiment with a single proton stored in a cryogenic Penning trap. This opens the way for a direct high-precision measurement of the magnetic moment of the proton and a new test of the matter-antimatter symmetry in the baryon sector. This method can also be applied to other light atomic nuclei.

Flipped Learning in the Workplace

Science.gov (United States)

Nederveld, Allison; Berge, Zane L.

2015-01-01

Purpose: The purpose of this paper is to serve as a summary of resources on flipped learning for workplace learning professionals. A recent buzzword in the training world is "flipped". Flipped learning and the flipped classroom are hot topics that have emerged in K-12 education, made their way to the university and are now being noticed…

Critical role of DNA intercalation in enzyme-catalyzed nucleotide flipping

Science.gov (United States)

Hendershot, Jenna M.; O'Brien, Patrick J.

2014-01-01

Nucleotide flipping is a common feature of DNA-modifying enzymes that allows access to target sites within duplex DNA. Structural studies have identified many intercalating amino acid side chains in a wide variety of enzymes, but the functional contribution of these intercalating residues is poorly understood. We used site-directed mutagenesis and transient kinetic approaches to dissect the energetic contribution of intercalation for human alkyladenine DNA glycosylase, an enzyme that initiates repair of alkylation damage. When AAG flips out a damaged nucleotide, the void in the duplex is filled by a conserved tyrosine (Y162). We find that tyrosine intercalation confers 140-fold stabilization of the extrahelical specific recognition complex, and that Y162 functions as a plug to slow the rate of unflipping by 6000-fold relative to the Y162A mutant. Surprisingly, mutation to the smaller alanine side chain increases the rate of nucleotide flipping by 50-fold relative to the wild-type enzyme. This provides evidence against the popular model that DNA intercalation accelerates nucleotide flipping. In the case of AAG, DNA intercalation contributes to the specific binding of a damaged nucleotide, but this enhanced specificity comes at the cost of reduced speed of nucleotide flipping. PMID:25324304

Flipping Excel

Science.gov (United States)

Frydenberg, Mark

2013-01-01

The "flipped classroom" model has become increasingly popular in recent years as faculty try new ways to engage students in the classroom. In a flipped classroom setting, students review the lecture online prior to the class session and spend time in class working on problems or exercises that would have been traditionally assigned as…

Flipped classroom

DEFF Research Database (Denmark)

Skov, Tobias Kidde; Jørgensen, Søren

2016-01-01

Artiklen beskriver Flipped Classroom som et didaktisk princip, der kan være med til at organisere og tilrettelægge en undervisning, med fokus på forskellige læringsformer. Det handler om at forstå Flipped Classroom som en opdeling i 2 faser og 3 led, som samlet set skaber en didaktisk organisering....

Optical nutation in the exciton range of spectrum

International Nuclear Information System (INIS)

Khadzhi, P. I.; Vasiliev, V. V.

2013-01-01

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level

Chromophore-Dependent Intramolecular Exciton-Vibrational Coupling in the FMO Complex: Quantification and Importance for Exciton Dynamics.

Science.gov (United States)

Padula, Daniele; Lee, Myeong H; Claridge, Kirsten; Troisi, Alessandro

2017-11-02

In this paper, we adopt an approach suitable for monitoring the time evolution of the intramolecular contribution to the spectral density of a set of identical chromophores embedded in their respective environments. We apply the proposed method to the Fenna-Matthews-Olson (FMO) complex, with the objective to quantify the differences among site-dependent spectral densities and the impact of such differences on the exciton dynamics of the system. Our approach takes advantage of the vertical gradient approximation to reduce the computational demands of the normal modes analysis. We show that the region of the spectral density that is believed to strongly influence the exciton dynamics changes significantly in the timescale of tens of nanoseconds. We then studied the impact of the intramolecular vibrations on the exciton dynamics by considering a model of FMO in a vibronic basis and neglecting the interaction with the environment to isolate the role of the intramolecular exciton-vibration coupling. In agreement with the assumptions in the literature, we demonstrate that high frequency modes at energy much larger than the excitonic energy splitting have negligible influence on exciton dynamics despite the large exciton-vibration coupling. We also find that the impact of including the site-dependent spectral densities on exciton dynamics is not very significant, indicating that it may be acceptable to apply the same spectral density on all sites. However, care needs to be taken for the description of the exciton-vibrational coupling in the low frequency part of intramolecular modes because exciton dynamics is more susceptible to low frequency modes despite their small Huang-Rhys factors.

Asteroid spin-rate studies using large sky-field surveys

Science.gov (United States)

Chang, Chan-Kao; Lin, Hsing-Wen; Ip, Wing-Huen; Prince, Thomas A.; Kulkarni, Shrinivas R.; Levitan, David; Laher, Russ; Surace, Jason

2017-12-01

Eight campaigns to survey asteroid rotation periods have been carried out using the intermediate Palomar Transient Factory in the past 3 years. 2780 reliable rotation periods were obtained, from which we identified two new super-fast rotators (SFRs), (335433) 2005 UW163 and (40511) 1999 RE88, and 23 candidate SFRs. Along with other three known super-fast rotators, there are five known SFRs so far. Contrary to the case of rubble-pile asteroids (i.e., bounded aggregations by gravity only), an internal cohesion, ranging from 100 to 1000 Pa, is required to prevent these five SFRs from flying apart because of their super-fast rotations. This cohesion range is comparable with that of lunar regolith. However, some candidates of several kilometers in size require unusually high cohesion (i.e., a few thousands of Pa). Therefore, the confirmation of these kilometer-sized candidates can provide important information about asteroid interior structure. From the rotation periods we collected, we also found that the spin-rate limit of C-type asteroids, which has a lower bulk density, is lower than for S-type asteroids. This result is in agreement with the general picture of rubble-pile asteroids (i.e., lower bulk density, lower spin-rate limit). Moreover, the spin-rate distributions of asteroids of 3 5 rev/day, regardless of the location in the main belt. The YORP effect is indicated to be less efficient in altering asteroid spin rates from our results when compared with the flat distribution found by Pravec et al. (Icarus 197:497-504, 2008. doi: 10.1016/j.icarus.2008.05.012). We also found a significant number drop at f = 5 rev/day in the spin-rate distributions of asteroids of D < 3 km.

To Flip or Not to Flip? An Exploratory Study Comparing Student Performance in Calculus I

Science.gov (United States)

Schroeder, Larissa B.; McGivney-Burelle, Jean; Xue, Fei

2015-01-01

The purpose of this exploratory, mixed-methods study was to compare student performance in flipped and non-flipped sections of Calculus I. The study also examined students' perceptions of the flipping pedagogy. Students in the flipped courses reported spending, on average, an additional 1-2 hours per week outside of class on course content.…

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 1. Optical spectroscopy of excitonic interactions involving the soret band

NARCIS (Netherlands)

Donker, H.; Koehorst, R.B.M.; Schaafsma, T.J.

2005-01-01

The photophysical properties of excited singlet states of zinc tetra-(p-octylphenyl)-porphyrin in 5-25-nm-thick films spin-coated onto quartz slides have been investigated by optical spectroscopy. Analysis of the polarized absorption spectra using a dipole-dipole exciton model with two mutually

Exciton emissions in alkali cyanides

International Nuclear Information System (INIS)

Weid, J.P. von der.

1979-10-01

The emissions of Alkali Cyanides X irradiated at low temperature were measured. In addition to the molecular (Frenkel Type) exciton emissions, another emitting centre was found and tentatively assigned to a charge transfer self trapped exciton. The nature of the molecular exciton emitting state is discussed. (Author) [pt

Spatially indirect excitons in coupled quantum wells

Energy Technology Data Exchange (ETDEWEB)

Lai, Chih-Wei Eddy [Univ. of California, Berkeley, CA (United States)

2004-03-01

Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer)² were

Flipping the Graduate Qualitative Research Methods Classroom: Did It Lead to Flipped Learning?

Science.gov (United States)

Earley, Mark

2016-01-01

The flipped, or inverted, classroom has gained popularity in a variety of fields and at a variety of educational levels, from K-12 through higher education. This paper describes the author's positive experience flipping a graduate qualitative research methods classroom. After a review of the current literature on flipped classrooms in higher…

Isoscalar spin excitation in 40Ca

International Nuclear Information System (INIS)

Morlet, M.; Willis, A.; Van de Wiele, J.; Marty, N.; Johnson, B.N.; Bimbot, L.; Guillot, J.; Jourdan, F.; Langevin-Joliot, H.; Rosier, L.; Glashausser, C.; Beatty, D.; Edwards, G.W.R.; Green, A.; Djalali, C.; Youn, M.Y.

1992-01-01

A signature S d y of isoscalar spin-transfer strength has been tested in the inelastic scattering of 400 MeV deuterons from 12 C. It was then applied to the study of 40 Ca over an angular range from 3 deg to 7 deg (momentum transfer range from 0.26 to 0.8 fm -1 ) and an excitation energy range from 6.25 to 42 MeV. This is the first study of isoscalar spin strength in the continuum. Spin excitations were found in the 9 MeV region, and over a broad range in the continuum with a cluster of strength around 15 MeV. The results are compared with spin-flip probability measurements in proton scattering. In contrast to the total relative spin response, which is strongly enhanced at high excitation, the isoscalar relative spin response is roughly consistent with non interacting Fermi gas values. (authors) 39 refs., 13 figs., 1 tab

Accurate adiabatic singlet-triplet gaps in atoms and molecules employing the third-order spin-flip algebraic diagrammatic construction scheme for the polarization propagator

Energy Technology Data Exchange (ETDEWEB)

Lefrancois, Daniel; Dreuw, Andreas, E-mail: dreuw@uni-heidelberg.de [Interdisciplinary Center for Scientific Computing, Ruprecht-Karls University, Im Neuenheimer Feld 205, 69120 Heidelberg (Germany); Rehn, Dirk R. [Departments of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping (Sweden)

2016-08-28

For the calculation of adiabatic singlet-triplet gaps (STG) in diradicaloid systems the spin-flip (SF) variant of the algebraic diagrammatic construction (ADC) scheme for the polarization propagator in third order perturbation theory (SF-ADC(3)) has been applied. Due to the methodology of the SF approach the singlet and triplet states are treated on an equal footing since they are part of the same determinant subspace. This leads to a systematically more accurate description of, e.g., diradicaloid systems than with the corresponding non-SF single-reference methods. Furthermore, using analytical excited state gradients at ADC(3) level, geometry optimizations of the singlet and triplet states were performed leading to a fully consistent description of the systems, leading to only small errors in the calculated STGs ranging between 0.6 and 2.4 kcal/mol with respect to experimental references.

Transmutation of skyrmions to half-solitons driven by the nonlinear optical spin Hall effect.

Science.gov (United States)

Flayac, H; Solnyshkov, D D; Shelykh, I A; Malpuech, G

2013-01-04

We show that the spin domains, generated in the linear optical spin Hall effect by the analog of spin-orbit interaction for exciton polaritons, are associated with the formation of a Skyrmion lattice. In the nonlinear regime, the spin anisotropy of the polariton-polariton interactions results in a spatial compression of the domains and in a transmutation of the Skyrmions into oblique half-solitons. This phase transition is associated with both the focusing of the spin currents and the emergence of a strongly anisotropic emission pattern.

Effect of localized surface-plasmon mode on exciton transport and radiation emission in carbon nanotubes.

Science.gov (United States)

Roslyak, Oleksiy; Cherqui, Charles; Dunlap, David H; Piryatinski, Andrei

2014-07-17

We report on a general theoretical approach to study exciton transport and emission in a single-walled carbon nanotube (SWNT) in the presence of a localized surface-plasmon (SP) mode within a metal nanoparticle interacting via near-field coupling. We derive a set of quantum mechanical equations of motion and approximate rate equations that account for the exciton, SP, and the environmental degrees of freedom. The material equations are complemented by an expression for the radiated power that depends on the exciton and SP populations and coherences, allowing for an examination of the angular distribution of the emitted radiation that would be measured in experiment. Numerical simulations for a (6,5) SWNT and cone-shaped Ag metal tip (MT) have been performed using this methodology. Comparison with physical parameters shows that the near-field interaction between the exciton-SP occurs in a weak coupling regime, with the diffusion processes being much faster than the exciton-SP population exchange. In such a case, the effect of the exciton population transfer to the MT with its subsequent dissipation (i.e., the Förster energy transfer) is to modify the exciton steady state distribution while reducing the equilibration time for excitons to reach a steady sate distribution. We find that the radiation distribution is dominated by SP emission for a SWNT-MT separation of a few tens of nanometers due to the fast SP emission rate, whereas the exciton-SP coherences can cause its rotation.

Detection of single electron spin resonance in a double quantum dota)

Science.gov (United States)

Koppens, F. H. L.; Buizert, C.; Vink, I. T.; Nowack, K. C.; Meunier, T.; Kouwenhoven, L. P.; Vandersypen, L. M. K.

2007-04-01

Spin-dependent transport measurements through a double quantum dot are a valuable tool for detecting both the coherent evolution of the spin state of a single electron, as well as the hybridization of two-electron spin states. In this article, we discuss a model that describes the transport cycle in this regime, including the effects of an oscillating magnetic field (causing electron spin resonance) and the effective nuclear fields on the spin states in the two dots. We numerically calculate the current flow due to the induced spin flips via electron spin resonance, and we study the detector efficiency for a range of parameters. The experimental data are compared with the model and we find a reasonable agreement.

Pump-probe spectroscopy of spin-injection dynamics in double quantum wells of diluted magnetic semiconductor

International Nuclear Information System (INIS)

Nishibayashi, K.; Aoshima, I.; Souma, I.; Murayama, A.; Oka, Y.

2006-01-01

Dynamics of spin injection has been investigated in a double quantum well (DQW) composed of a diluted magnetic semiconductor by the pump-probe transient absorption spectroscopy in magnetic field. The DQW consists of a non-magnetic well (NMW) of CdTe and a magnetic well (MW) of Cd 0.92 Mn 0.08 Te. The MW shows a transient absorption saturation in the exciton band for more than 200 ps after the optical pumping, while the exciton photoluminescence does not arise from the MW. In the NMW, the circular polarization degree of the transient absorption saturation shows an increase with increasing time. The results are interpreted by the individual tunneling of spin-polarized electrons and holes from the MW to the NMW with different tunneling times. Depolarization processes of the carrier spins in the MW and the NMW are also discussed

Investigation of spin-polarized transport in GaAs nanostructures

Energy Technology Data Exchange (ETDEWEB)

Tierney, B D; Day, T E; Goodnick, S M [Department of Electrical Engineering and Center for Solid State Electronics Research Arizona State University, Tempe, AZ 85287-5706 (United States)], E-mail: brian.tierney@asu.edu

2008-03-15

A spin field effect transistor (spin-FET) has been fabricated that employs nanomagnets as components of quantum point contact (QPC) structures to inject spin-polarized carriers into the high-mobility two-dimensional electron gas (2DEG) of a GaAs quantum well and to detect them. A centrally-placed non-magnetic Rashba gate controls both the density of electrons in the 2DEG and the electronic spin precession. Initial results are presented for comparable device structures modeled with an ensemble Monte Carlo (EMC) method. In the EMC the temporal and spatial evolution of the ensemble carrier spin polarization is governed by a spin density matrix formalism that incorporates the Dresselhaus and Rashba contributions to the D'yakanov-Perel spin-flip scattering mechanism, the predominant spin scattering mechanism in AlGaAs/GaAs heterostructures from 77-300K.

Equine First Aid Information Flip Booklet

OpenAIRE

Nay, Karah; Hoopes, Karl

2017-01-01

This is a flip chart type booklet with first aid information for horses, including checking vitals, pulse rate, respiration, mucus membrane color and capillary refill, signs of colic, deworming, vaccinations recommended for Utah, hoof care, and dental care.

Hydrogen-Bonding Interactions Trigger a Spin-Flip in Iron(III) Porphyrin Complexes**

Science.gov (United States)

Sahoo, Dipankar; Quesne, Matthew G; de Visser, Sam P; Rath, Sankar Prasad

2015-01-01

A key step in cytochrome P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen-bonding interactions on the electronic structure of a five-coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=5/2) and intermediate spin (S=3/2) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen-bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations. PMID:26109743

Spin fluctuations in iron based superconductors probed by NMR relaxation rate

Energy Technology Data Exchange (ETDEWEB)

Graefe, Uwe; Kuehne, Tim; Wurmehl, Sabine; Buechner, Bernd; Grafe, Hans-Joachim [IFW Dresden, Institute for Solid State Research, PF 270116, 01171 Dresden (Germany); Hammerath, Franziska [IFW Dresden, Institute for Solid State Research, PF 270116, 01171 Dresden (Germany); Department of Physics ' ' A. Volta' ' , University of Pavia-CNISM, I-27100 Pavia (Italy); Lang, Guillaume [3LPEM-UPR5, CNRS, ESPCI Paris Tech, 10 Rue Vauquelin, 75005 Paris (France)

2013-07-01

We present {sup 75}As nuclear magnetic resonance (NMR) results in F doped LaOFeAs iron pnictides. In the underdoped superconducting samples, pronounced spin fluctuations lead to a peak in the NMR spin lattice relaxation rate, (T{sub 1}T){sup -1}. The peak shows a typical field dependence that indicates a critical slowing of spin fluctuations: it is reduced in height and shifted to higher temperatures. In contrast, a similar peak in the underdoped magnetic samples at the ordering temperature of the spin density wave does not show such a field dependence. Furthermore, the peak is absent in optimally and overdoped samples, suggesting the absence of strong spin fluctuations. Our results indicate a glassy magnetic ordering in the underdoped samples that is in contrast to the often reported Curie Weiss like increase of spin fluctuations towards T{sub c}. Additional measurements of the linewidth and the spin spin relaxation rate are in agreement with such a glassy magnetic ordering that is most likely competing with superconductivity. Our results will be compared to Co doped BaFe{sub 2}As{sub 2}, where a similar peak in (T{sub 1}T){sup -1} has been observed.

Ultrafast optical control of individual quantum dot spin qubits.

Science.gov (United States)

De Greve, Kristiaan; Press, David; McMahon, Peter L; Yamamoto, Yoshihisa

2013-09-01

Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled

Inhomogeneous nuclear spin polarization induced by helicity-modulated optical excitation of fluorine-bound electron spins in ZnSe

Science.gov (United States)

Heisterkamp, F.; Greilich, A.; Zhukov, E. A.; Kirstein, E.; Kazimierczuk, T.; Korenev, V. L.; Yugova, I. A.; Yakovlev, D. R.; Pawlis, A.; Bayer, M.

2015-12-01

Optically induced nuclear spin polarization in a fluorine-doped ZnSe epilayer is studied by time-resolved Kerr rotation using resonant excitation of donor-bound excitons. Excitation with helicity-modulated laser pulses results in a transverse nuclear spin polarization, which is detected as a change of the Larmor precession frequency of the donor-bound electron spins. The frequency shift in dependence on the transverse magnetic field exhibits a pronounced dispersion-like shape with resonances at the fields of nuclear magnetic resonance of the constituent zinc and selenium isotopes. It is studied as a function of external parameters, particularly of constant and radio frequency external magnetic fields. The width of the resonance and its shape indicate a strong spatial inhomogeneity of the nuclear spin polarization in the vicinity of a fluorine donor. A mechanism of optically induced nuclear spin polarization is suggested based on the concept of resonant nuclear spin cooling driven by the inhomogeneous Knight field of the donor-bound electron.

Flipping qubits

International Nuclear Information System (INIS)

Martini De, F.; Sciarrino, F.; Sias, C.; Buzek, V.

2003-01-01

On a classical level the information can be represented by bits, each of which can be either 0 or 1. Quantum information, on the other hand, consists of qubits which can be represented as two-level quantum systems with one level labeled |0> and the other |1>. Unlike bits, qubits cannot only be in one of the two levels, but in any superposition of them as well. This superposition principle makes quantum information fundamentally different from its classical counterpart. One of the most striking difference between the classical and quantum information is as follows: it is not a problem to flip a classical bit, i.e., to change the value of a bit, a 0 to a 1 and vice versa. This is accomplished by a NOT gate. Flipping a qubit, however, is another matter: there exists the fundamental bound which prohibits to flip a qubit prepared in an arbitrary state |Ψ>=α|0> and to obtain the state |Ψ T >=β*|0>-α*|1> which is orthogonal to it, i.e., T |Ψ>=0. We experimentally realize the best possible approximation of the qubit flipping that achieves bounds imposed by complete positivity of quantum mechanics

Bounds on the maximum attainable equilibrium spin polarization of protons at high energy in HERA

International Nuclear Information System (INIS)

Vogt, M.

2000-12-01

For some years HERA has been supplying longitudinally spin polarised electron and positron (e ± ) beams to the HERMES experiment and in the future longitudinal polarisation will be supplied to the II1 and ZEUS experiments. As a result there has been a development of interest in complementing the polarised e ± beams with polarised protons. In contrast to the case of e ± where spin flip due to synchrotron radiation in the main bending dipoles leads to self polarisation owing to an up-down asymmetry in the spin flip rates (Sokolov-Ternov effect), there is no convincing self polarisation mechanism for protons at high energy. Therefore protons must be polarised almost at rest in a source and then accelerated to the working energy. At HERA, if no special measures are adopted, this means that the spins must cross several thousand ''spin-orbit resonances''. Resonance crossing can lead to loss of polarisation and at high energy such effects are potentially strong since spin precession is very pronounced in the very large magnetic fields needed to contain the proton beam in HERA-p. Moreover simple models which have been successfully used to describe spin motion at low and medium energies are no longer adequate. Instead, careful numerical spin-orbit tracking simulations are needed and a new, mathematically rigorous look at the theoretical concepts is required. This thesis describes the underlying theoretical concepts, the computational tools (SPRINT) and the results of such a study. In particular strong emphasis is put on the concept of the invariant spin field and its non-perturbative construction. The invariant spin field is then used to define the amplitude dependent spin tune and to obtain numerical non-perturbative estimates of the latter. By means of these two key concepts the nature of higher order resonances in the presence of snakes is clarified and their impact on the beam polarisation is analysed. We then go on to discuss the special aspects of the HERA-p ring

Advanced flip chip packaging

CERN Document Server

Lai, Yi-Shao; Wong, CP

2013-01-01

Advanced Flip Chip Packaging presents past, present and future advances and trends in areas such as substrate technology, material development, and assembly processes. Flip chip packaging is now in widespread use in computing, communications, consumer and automotive electronics, and the demand for flip chip technology is continuing to grow in order to meet the need for products that offer better performance, are smaller, and are environmentally sustainable. This book also: Offers broad-ranging chapters with a focus on IC-package-system integration Provides viewpoints from leading industry executives and experts Details state-of-the-art achievements in process technologies and scientific research Presents a clear development history and touches on trends in the industry while also discussing up-to-date technology information Advanced Flip Chip Packaging is an ideal book for engineers, researchers, and graduate students interested in the field of flip chip packaging.

Flip Video for Dummies

CERN Document Server

Hutsko, Joe

2010-01-01

The full-color guide to shooting great video with the Flip Video camera. The inexpensive Flip Video camera is currently one of the hottest must-have gadgets. It's portable and connects easily to any computer to transfer video you shoot onto your PC or Mac. Although the Flip Video camera comes with a quick-start guide, it lacks a how-to manual, and this full-color book fills that void! Packed with full-color screen shots throughout, Flip Video For Dummies shows you how to shoot the best possible footage in a variety of situations. You'll learn how to transfer video to your computer and then edi

To Flip or Not to Flip? Analysis of a Flipped Classroom Pedagogy in a General Biology Course

Science.gov (United States)

Heyborne, William H.; Perrett, Jamis J.

2016-01-01

In an attempt to better understand the flipped technique and evaluate its purported superiority in terms of student learning gains, the authors conducted an experiment comparing a flipped classroom to a traditional lecture classroom. Although the outcomes were mixed, regarding the superiority of either pedagogical approach, there does seem to be a…

Spin injection in self-assembled quantum dots coupled with a diluted magnetic quantum well

International Nuclear Information System (INIS)

Murayama, A.; Asahina, T.; Souma, I.; Koyama, T.; Hyomi, K.; Nishibayashi, K.; Oka, Y.

2007-01-01

Spin injection is studied in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of Zn 1- x - y Cd x Mn y Se, by means of time-resolved circularly polarized photoluminescence (PL). Excitonic PL from the CdSe QDs shows σ - -circular polarization in magnetic fields, mainly due to negative g-values of individual dots, when the energy difference of excitons between the QDs and DMS-QW is large as 300 meV. However, when such energy difference is comparable with LO-phonon energy in the QD, we observe an additional PL peak with the long lifetime as 3.5 ns and σ + -polarization in magnetic fields. It can be attributed to a type-II transition between the down-spin electron injected from the DMS-QW into the QDs, via LO-phonon-assisted resonant tunneling, and the down-spin heavy hole in the DMS-QW. In addition, the electron spin-injection is also evidenced by σ + -polarized PL with the fast rise-time of 20 ps in the QDs

Neutron spin optics: Fundamentals and verification

Energy Technology Data Exchange (ETDEWEB)

Pleshanov, N.K., E-mail: pleshanov_nk@pnpi.nrcki.ru

2017-05-01

Neutron spin optics (NSO) based on quantum aspects of the neutron interaction with magnetically anisotropic layers signifies transition in polarized neutron optics from 1D (spin selection) to 3D (spin manipulations). It may essentially widen the functionality of neutron optics. Among the advantages of NSO are compactness, zero-field option (guide fields are optional) and multi-functionality (beam spectrum, beam divergence and spin manipulations can be handled at the same time). Prospects in improving and developing neutron mirror spin turners (incl. flippers) are discussed. Two approaches to measurement of the efficiency of mirror flippers are introduced. The efficiency of a multilayer-backed neutron mirror flipper for monochromatic beams was found to be 97.5±0.5%. Such mirror flippers can combine monochromatization of a polarized beam with flipping spins of the monochromatized neutrons. To improve their performance, account of the spin-dependent refraction in the magnetic layer should be taken. For a monochromatic beam, supermirror-backed flippers are shown to be more advantageous, with a gain in intensity up to 4 times.

Comparing the Effectiveness of Blended, Semi-Flipped, and Flipped Formats in an Engineering Numerical Methods Course

Science.gov (United States)

Clark, Renee M.; Kaw, Autar; Besterfield-Sacre, Mary

2016-01-01

Blended, flipped, and semi-flipped instructional approaches were used in various sections of a numerical methods course for undergraduate mechanical engineers. During the spring of 2014, a blended approach was used; in the summer of 2014, a combination of blended and flipped instruction was used to deliver a semi-flipped course; and in the fall of…

Coherent Rabi Dynamics of a Superradiant Spin Ensemble in a Microwave Cavity

Science.gov (United States)

Rose, B. C.; Tyryshkin, A. M.; Riemann, H.; Abrosimov, N. V.; Becker, P.; Pohl, H.-J.; Thewalt, M. L. W.; Itoh, K. M.; Lyon, S. A.

2017-07-01

We achieve the strong-coupling regime between an ensemble of phosphorus donor spins in a highly enriched 28Si crystal and a 3D dielectric resonator. Spins are polarized beyond Boltzmann equilibrium using spin-selective optical excitation of the no-phonon bound exciton transition resulting in N =3.6 ×1 013 unpaired spins in the ensemble. We observe a normal mode splitting of the spin-ensemble-cavity polariton resonances of 2 g √{N }=580 kHz (where each spin is coupled with strength g ) in a cavity with a quality factor of 75 000 (γ ≪κ ≈60 kHz , where γ and κ are the spin dephasing and cavity loss rates, respectively). The spin ensemble has a long dephasing time (T2*=9 μ s ) providing a wide window for viewing the dynamics of the coupled spin-ensemble-cavity system. The free-induction decay shows up to a dozen collapses and revivals revealing a coherent exchange of excitations between the superradiant state of the spin ensemble and the cavity at the rate g √{N }. The ensemble is found to evolve as a single large pseudospin according to the Tavis-Cummings model due to minimal inhomogeneous broadening and uniform spin-cavity coupling. We demonstrate independent control of the total spin and the initial Z projection of the psuedospin using optical excitation and microwave manipulation, respectively. We vary the microwave excitation power to rotate the pseudospin on the Bloch sphere and observe a long delay in the onset of the superradiant emission as the pseudospin approaches full inversion. This delay is accompanied by an abrupt π -phase shift in the peusdospin microwave emission. The scaling of this delay with the initial angle and the sudden phase shift are explained by the Tavis-Cummings model.

Photoemission of Bi_{2}Se_{3} with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?

Directory of Open Access Journals (Sweden)

J. Sánchez-Barriga

2014-03-01

Full Text Available Topological insulators are characterized by Dirac-cone surface states with electron spins locked perpendicular to their linear momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50–70 eV with linear or circular polarization indeed probe the initial-state spin texture of Bi_{2}Se_{3} while circularly polarized 6-eV low-energy photons flip the electron spins out of plane and reverse their spin polarization, with its sign determined by the light helicity. Our photoemission calculations, taking into account the interplay between the varying probing depth, dipole-selection rules, and spin-dependent scattering effects involving initial and final states, explain these findings and reveal proper conditions for light-induced spin manipulation. Our results pave the way for future applications of topological insulators in optospintronic devices.

Optical and electronic properties of 2 H -Mo S2 under pressure: Revealing the spin-polarized nature of bulk electronic bands

Science.gov (United States)

Brotons-Gisbert, Mauro; Segura, Alfredo; Robles, Roberto; Canadell, Enric; Ordejón, Pablo; Sánchez-Royo, Juan F.

2018-05-01

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk 2 H -Mo S2 hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of A and B excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of 2 H -Mo S2 . Also, the pressure dependence of the indirect optical transitions of bulk 2 H -Mo S2 has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk Mo S2 may occur at pressures higher than 26 GPa.

Hydrogen-Bonding Interactions Trigger a Spin-Flip in Iron(III) Porphyrin Complexes**

OpenAIRE

Sahoo, Dipankar; Quesne, Matthew G; de?Visser, Sam P; Rath, Sankar Prasad

2015-01-01

A key step in cytochrome?P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen-bonding interactions on the electronic structure ...

Hydrogen-Bonding Interactions Trigger a Spin-Flip in Iron(III) Porphyrin Complexes**

OpenAIRE

Sahoo, Dipankar; Quesne, Matthew G; de Visser, Sam P; Rath, Sankar Prasad

2015-01-01

A key step in cytochrome P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen-bonding interactions on the electronic structure ...

Orbital and spin dynamics of intraband electrons in quantum rings driven by twisted light.

Science.gov (United States)

Quinteiro, G F; Tamborenea, P I; Berakdar, J

2011-12-19

We theoretically investigate the effect that twisted light has on the orbital and spin dynamics of electrons in quantum rings possessing sizable Rashba spin-orbit interaction. The system Hamiltonian for such a strongly inhomogeneous light field exhibits terms which induce both spin-conserving and spin-flip processes. We analyze the dynamics in terms of the perturbation introduced by a weak light field on the Rasha electronic states, and describe the effects that the orbital angular momentum as well as the inhomogeneous character of the beam have on the orbital and the spin dynamics.

Theory of Excitonic Delocalization for Robust Vibronic Dynamics in LH2.

Science.gov (United States)

Caycedo-Soler, Felipe; Lim, James; Oviedo-Casado, Santiago; van Hulst, Niek F; Huelga, Susana F; Plenio, Martin B

2018-06-11

Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of a variety of photosynthetic complexes. Different theoretical models that involve the coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom have been put forward to explain these observations. The ensuing debate concerning the relevance of either mechanism may have obscured their complementarity. To illustrate this balance, we quantify how the excitonic delocalization in the LH2 unit of Rhodopseudomonas acidophila purple bacterium leads to correlations of excitonic energy fluctuations, relevant coherent vibronic coupling, and importantly, a decrease in the excitonic dephasing rates. Combining these effects, we identify a feasible origin for the long-lasting oscillations observed in fluorescent traces from time-delayed two-pulse single-molecule experiments performed on this photosynthetic complex and use this approach to discuss the role of this complementarity in other photosynthetic systems.

Multi-Excitonic Quantum Dot Molecules

Science.gov (United States)

Scheibner, M.; Stinaff, E. A.; Doty, M. F.; Ware, M. E.; Bracker, A. S.; Gammon, D.; Ponomarev, I. V.; Reinecke, T. L.; Korenev, V. L.

2006-03-01

With the ability to create coupled pairs of quantum dots, the next step towards the realization of semiconductor based quantum information processing devices can be taken. However, so far little knowledge has been gained on these artificial molecules. Our photoluminescence experiments on single InAs/GaAs quantum dot molecules provide the systematics of coupled quantum dots by delineating the spectroscopic features of several key charge configurations in such quantum systems, including X, X^+,X^2+, XX, XX^+ (with X being the neutral exciton). We extract general rules which determine the formation of molecular states of coupled quantum dots. These include the fact that quantum dot molecules provide the possibility to realize various spin configurations and to switch the electron hole exchange interaction on and off by shifting charges inside the molecule. This knowledge will be valuable in developing implementations for quantum information processing.

Simplicity of state and overlap structure in finite-volume realistic spin glasses

International Nuclear Information System (INIS)

Newman, C.M.; Stein, D.L.

1998-01-01

We present a combination of heuristic and rigorous arguments indicating that both the pure state structure and the overlap structure of realistic spin glasses should be relatively simple: in a large finite volume with coupling-independent boundary conditions, such as periodic, at most a pair of flip-related (or the appropriate number of symmetry-related in the non-Ising case) states appear, and the Parisi overlap distribution correspondingly exhibits at most a pair of δ functions at ±q EA . This rules out the nonstandard mean-field picture introduced by us earlier, and when combined with our previous elimination of more standard versions of the mean-field picture, argues against the possibility of even limited versions of mean-field ordering in realistic spin glasses. If broken spin-flip symmetry should occur, this leaves open two main possibilities for ordering in the spin glass phase: the droplet-scaling two-state picture, and the chaotic pairs many-state picture introduced by us earlier. We present scaling arguments which provide a possible physical basis for the latter picture, and discuss possible reasons behind numerical observations of more complicated overlap structures in finite volumes. copyright 1998 The American Physical Society

Flipping for success: evaluating the effectiveness of a novel teaching approach in a graduate level setting

OpenAIRE

Moraros, John; Islam, Adiba; Yu, Stan; Banow, Ryan; Schindelka, Barbara

2015-01-01

Background Flipped Classroom is a model that?s quickly gaining recognition as a novel teaching approach among health science curricula. The purpose of this study was four-fold and aimed to compare Flipped Classroom effectiveness ratings with: 1) student socio-demographic characteristics, 2) student final grades, 3) student overall course satisfaction, and 4) course pre-Flipped Classroom effectiveness ratings. Methods The participants in the study consisted of 67 Masters-level graduate student...

Plasmon-exciton-polariton lasing

NARCIS (Netherlands)

Ramezani, M.; Halpin, A.; Fernández-Dominguez, A.I.; Feist, J.; Rodriguez, S.R.K.; Gómez-Rivas, J.; Garcia-Vidal, F.J.

2016-01-01

Strong coupling of Frenkel excitons with surface plasmons leads to the formation of bosonic quasi-particles known as plasmon-exciton-polaritons (PEPs).Localized surface plasmons in nanoparticles are lossy due to radiative and nonradiative decays, which has hampered the realization of polariton

Spin analysis and new effects in reflectivity measurements

International Nuclear Information System (INIS)

Fermon, C.

1996-01-01

We present two new effects in polarized neutron reflectivity. We show that we have a non symmetric spin-flip signal in reflectivity measurements on magnetic films when the external field is not negligible. This phenomenon is due to different Larmor precessions for the two spin states and has to be taken into account in some experiments. The second effect is still not understood but we present results indicating that the specular reflection on a non magnetic surface can induce a neutron beam depolarization or rotation. (authors)

An extended chain Ising model and its Glauber dynamics

International Nuclear Information System (INIS)

Zhao Xing-Yu; Fan Xiao-Hui; Huang Yi-Neng; Huang Xin-Ru

2012-01-01

It was first proposed that an extended chain Ising (ECI) model contains the Ising chain model, single spin double-well potentials and a pure phonon heat bath of a specific energy exchange with the spins. The extension method is easy to apply to high dimensional cases. Then the single spin-flip probability (rate) of the ECI model is deduced based on the Boltzmann principle and general statistical principles of independent events and the model is simplified to an extended chain Glauber—Ising (ECGI) model. Moreover, the relaxation dynamics of the ECGI model were simulated by the Monte Carlo method and a comparison with the predictions of the special chain Glauber—Ising (SCGI) model was presented. It was found that the results of the two models are consistent with each other when the Ising chain length is large enough and temperature is relative low, which is the most valuable case of the model applications. These show that the ECI model will provide a firm physical base for the widely used single spin-flip rate proposed by Glauber and a possible route to obtain the single spin-flip rate of other form and even the multi-spin-flip rate. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

Comparison of Flipped Model to Traditional Classroom Learning in a Professional Pharmacy Course

Directory of Open Access Journals (Sweden)

Colleen McCabe

2017-09-01

Full Text Available The flipped classroom is an approach to incorporate active learning that is being used in secondary education, higher education, and professional schools. This study investigates its impact on student learning and confidence in a professional degree program course. A quasi-experimental study was conducted to evaluate pharmacy students enrolled in a semester-long didactic traditional classroom course compared to students learning the same material using a flipped model through online self-study modules in a hands-on experiential learning course. Before and after each learning experience, students of each group completed a 16-item knowledge assessment on four topic areas and rated their level of confidence with each topic area on a Likert scale. There was a significant difference in knowledge with students in the traditional course scoring higher than students using flipped approach in the experiential course. Furthermore, the flipped experiential course students did not improve assessment scores from pre-test to post-test. For confidence rating, the traditional course group ranked confidence higher than the flipped experiential group for all topics. These findings challenge the notion that the flipped model using self-study in an experiential setting can be a substitution for didactic delivery of pharmacy education.

Flipped Cryptons and the UHECRs

CERN Document Server

Ellis, Jonathan Richard; Nanopoulos, Dimitri V; Ellis, John

2004-01-01

Cryptons are metastable bound states of fractionally-charged particles that arise generically in the hidden sectors of models derived from heterotic string. We study their properties and decay modes in a specific flipped SU(5) model with long-lived four-particle spin-zero bound states called {\\it tetrons}. We show that the neutral tetrons are metastable, and exhibit the ninth- and tenth-order non-renormalizable superpotential operators responsible for their dominant decays. By analogy with QCD, we expect charged tetrons to be somewhat heavier, and to decay relatively rapidly via lower-order interactions that we also exhibit. The expected masses and lifetimes of the neutral tetrons make them good candidates for cold dark matter (CDM), and a potential source of the ultra-high energy cosmic rays (UHECRs) which have been observed, whereas the charged tetrons would have decayed in the early Universe.

Flipped Classroom Approach

Science.gov (United States)

Ozdamli, Fezile; Asiksoy, Gulsum

2016-01-01

Flipped classroom is an active, student-centered approach that was formed to increase the quality of period within class. Generally this approach whose applications are done mostly in Physical Sciences, also attracts the attention of educators and researchers in different disciplines recently. Flipped classroom learning which wide-spreads rapidly…

Selectively Modulating Triplet Exciton Formation in Host Materials for Highly Efficient Blue Electrophosphorescence.

Science.gov (United States)

Li, Huanhuan; Bi, Ran; Chen, Ting; Yuan, Kai; Chen, Runfeng; Tao, Ye; Zhang, Hongmei; Zheng, Chao; Huang, Wei

2016-03-23

The concept of limiting the triplet exciton formation to fundamentally alleviate triplet-involved quenching effects is introduced to construct host materials for highly efficient and stable blue phosphorescent organic light-emitting diodes (PhOLEDs). The low triplet exciton formation is realized by small triplet exciton formation fraction and rate with high binding energy and high reorganization energy of triplet exciton. Demonstrated in two analogue molecules in conventional donor-acceptor molecule structure for bipolar charge injection and transport with nearly the same frontier orbital energy levels and triplet excited energies, the new concept host material shows significantly suppressed triplet exciton formation in the host to avoid quenching effects, leading to much improved device efficiencies and stabilities. The low-voltage-driving blue PhOLED devices exhibit maximum efficiencies of 43.7 cd A(-1) for current efficiency, 32.7 lm W(-1) for power efficiency, and 20.7% for external quantum efficiency with low roll-off and remarkable relative quenching effect reduction ratio up to 41%. Our fundamental solution for preventing quenching effects of long-lived triplet excitons provides exciting opportunities for fabricating high-performance devices using the advanced host materials with intrinsically small triplet exciton formation cross section.

Correlation effect of Rabi oscillations of excitons in quantum dots

International Nuclear Information System (INIS)

Ishi-Hayase, J.; Akahane, K.; Yamamoto, Y.; Kujiraoka, M.; Ema, K.; Sasaki, M.

2008-01-01

We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree-Fock-type Coulomb interactions between excitons

Flipping Freshman Mathematics

Science.gov (United States)

Zack, Laurie; Fuselier, Jenny; Graham-Squire, Adam; Lamb, Ron; O'Hara, Karen

2015-01-01

Our study compared a flipped class with a standard lecture class in four introductory courses: finite mathematics, precalculus, business calculus, and calculus 1. The flipped sections watched video lectures outside of class and spent time in class actively working on problems. The traditional sections had lectures in class and did homework outside…

Signatures of Förster and Dexter transfer processes in coupled nanostructures for linear and two-dimensional coherent optical spectroscopy

Science.gov (United States)

Specht, Judith F.; Richter, Marten

2015-03-01

In this manuscript, we study the impact of the two Coulomb induced resonance energy transfer processes, Förster and Dexter coupling, on the spectral signatures obtained by double quantum coherence spectroscopy. We show that the specific coupling characteristics allow us to identify the underlying excitation transfer mechanism by means of specific signatures in coherent spectroscopy. Therefore, we control the microscopic calculated coupling strength of spin preserving and spin flipping Förster transfer processes by varying the mutual orientation of the two quantum emitters. The calculated spectra reveal the optical selection rules altered by Förster and Dexter coupling between two semiconductor quantum dots. We show that Dexter coupling between bright and dark two-exciton states occurs.

Flipped Library Instruction Does Not Lead to Learning Gains for First-Year English Students

Directory of Open Access Journals (Sweden)

Kimberly Miller

2017-09-01

Full Text Available A Review of: Rivera, E. (2017. Flipping the classroom in freshman English library instruction: A comparison study of a flipped class versus a traditional lecture method. New Review of Academic Librarianship, 23(1, 18-27. http://dx.doi.org/10.1080/13614533.2016.1244770 Abstract Objective – To determine whether a flipped classroom approach to freshman English information literacy instruction improves student learning outcomes. Design – Quasi-experimental. Setting – Private suburban university with 7,000 graduate and undergraduate students. Subjects – First-year English students. Methods – Students in six sections of first-year “English 2” received library instruction; three sections received flipped library instruction and three sections received traditional library instruction. Students in the flipped classroom sections were assigned two videos to watch before class, as an introduction to searching the Library’s catalog and key academic databases. These students were also expected to complete pre-class exercises that allowed them to practice what they learned through the videos. The face-to-face classes involved a review of the flipped materials alongside additional activities. Works cited pages from the students’ final papers were collected from all six sections, 31 from the flipped sections and 34 from the non-flipped sections. A rubric was used to rate the works cited pages. The rubric was based on the Association of College and Research Libraries’ Information Literacy Competency Standards for Higher Education (ACRL, 2000, Standard Two, Outcome 3a, and included three criteria: “authority,” “timeliness,” and “variety.” Each criterion was rated at one of three levels: “exemplary,” “competent,” or “developing.” Main Results – Works cited pages from the students who received non-flipped instruction were more likely to score “exemplary” for at least one of the three criteria when compared to works

Order and chaos in the nonlinear response of driven nuclear spin systems

Energy Technology Data Exchange (ETDEWEB)

Brun, E; Derighetti, B; Holzner, R; Ravani, M [Zurich Univ. (Switzerland). Inst. fuer Physik

1984-01-01

The authors report on observations of ordered and chaotic behavior of a nonlinear system of strongly polarized nuclear spins inside the tuning coil of an NMR detector. The combined system: spins plus LC-circuit, may act as a nonlinear bistable absorber or a spin-flip laser, depending on the sign of the nuclear spin polarization. For the NMR laser experimental evidence is presented for limit-cycle behavior, sequences of bifurcations which lead to chaos, intermittency, multistability, and pronounced hysteresis effects. The experimental facts are compared with computer solutions of appropriate Bloch equations for the macroscopic order parameters.

Isoscalar spin response in 40Ca and 12C

International Nuclear Information System (INIS)

Tomasi-Gustafsson, E.; Morlet, M.; Bimbot, L.; Guillot, J.; Jourdan, F.; Langevin-Joliot, H.; Marty, N.; Rosier, L.; Van de Wiele, J.; Willis, A.; Johnson, B.N.; Glashausser, C.; Djalali, C.

1994-01-01

A method founded on the measure of an approximated spin-flip probability, in the inelastic diffusion (d,d') at 400 MeV (incident energy) has been applied to the research of isoscalar spin strengths in calcium 40 and carbon 12. In calcium 40 the spin excitations have been revealed towards an excitation energy of 9 MeV and in the continuum a strength concentration appears about 15 MeV. In carbon 12 spin structures appear up to an excitation energy of 30 MeV; beyond 35 MeV the isoscalar spin response, as in calcium 40, is compatible with the expected value for a Fermi gas of particles without interactions. Microscopic calculations DWIA are in good agreement with the data of carbon 12. (O.L.). 30 refs., 5 figs

Excitons in the rare gas solids

International Nuclear Information System (INIS)

1988-01-01

Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs

Flipping Radiology Education Right Side Up.

Science.gov (United States)

O'Connor, Erin E; Fried, Jessica; McNulty, Nancy; Shah, Pallav; Hogg, Jeffery P; Lewis, Petra; Zeffiro, Thomas; Agarwal, Vikas; Reddy, Sravanthi

2016-07-01

In flipped learning, medical students independently learn facts and concepts outside the classroom, and then participate in interactive classes to learn to apply these facts. Although there are recent calls for medical education reform using flipped learning, little has been published on its effectiveness. Our study compares the effects of flipped learning to traditional didactic instruction on students' academic achievement, task value, and achievement emotions. At three institutions, we alternated flipped learning with traditional didactic lectures during radiology clerkships, with 175 medical students completing a pretest on general diagnostic imaging knowledge to assess baseline cohort comparability. Following instruction, posttests and survey examinations of task value and achievement emotions were administered. Linear mixed effects analysis was used to examine the relationship between test scores and instruction type. Survey responses were modeled using ordinal category logistic regression. Instructor surveys were also collected. There were no baseline differences in test scores. Mean posttest minus pretest scores were 10.5% higher in the flipped learning group than in the didactic instruction group (P = 0.013). Assessment of task value and achievement emotions showed greater task value, increased enjoyment, and decreased boredom with flipped learning (all P flipped learning condition. Flipped learning was associated with increased academic achievement, greater task value, and more positive achievement emotions when compared to traditional didactic instruction. Further investigation of flipped learning methods in radiology education is needed to determine whether flipped learning improves long-term retention of knowledge, academic success, and patient care. Copyright © 2016 The Association of University Radiologists. Published by Elsevier Inc. All rights reserved.

Localization of excitons by molecular layer formation in a polymer film

International Nuclear Information System (INIS)

Chattopadhyay, S.; Datta, A.

2005-01-01

Spin coated films of atactic polystyrene of two different molecular weights have been studied with uv spectroscopy and x-ray reflectivity, the film thickness (d) varying from ∼2R g to ∼12R g where R g is the unperturbed radius of gyration of the polymer. uv extinction due to the pure electronic singlet 1 A 1g → 1 E 1u is seen to increase with d -1 for 4R g ≤d≤12R g (region 1). This suggests excitonic interaction along d. The variation of total exciton energy (E) of the A 1g →E 1u singlet with d in region 1 can be well explained by formation of linear J-aggregates of polystyrene molecules, in a lattice with spacing 'a' (in A) R g g , along d. Atomic force microscopic images of the films show the presence of 'spheres' distributed randomly on film surfaces with in-plane dimensions matching a. From the variation of E with d -2 the effective mass (m eff ) of the exciton is also determined. For R g g (region 2) the extinction and E become essentially independent of d, indicating exciton localization along d, and the value of m eff becomes very large. This enhancement in the effective mass maybe used to quantify localization. The variations of electron density (ρ) with d, i.e., the electron density profiles (EDPs) of the films extracted from x-ray reflectivity studies, indicate formation of layers with period 'b' (in A), R g g parallel to substrate surface in region 2 and a constant ρ film in region 1. On raising the temperature of a typical film to 60 deg. C, the layering was seen to almost vanish, as obtained from both the EDP and the Patterson function of the reflectivity profile. The close correspondence between 'a' and 'b' indicates that the molecules forming the J-aggregates form the layers, too. The average difference in ρ between successive extrema in the EDPs in region 2, denoted by δ, can be used as the order parameter for the layering transition. For PS-5, δ>0 at d≅4R g , where the exciton is still delocalized. Layering reduces the Hamaker

The evidence for 'flipping out': A systematic review of the flipped classroom in nursing education.

Science.gov (United States)

Betihavas, Vasiliki; Bridgman, Heather; Kornhaber, Rachel; Cross, Merylin

2016-03-01

The flipped classroom has generated interest in higher education providing a student-centred approach to learning. This has the potential to engage nursing students in ways that address the needs of today's students and the complexity of contemporary healthcare. Calls for educational reform, particularly in healthcare programs such as nursing, highlight the need for students to problem-solve, reason and apply theory into practice. The drivers towards student-based learning have manifested in team, problem and case-based learning models. Though there has been a shift towards the flipped classroom, comparatively little is known about how it is used in nursing curricula. The aims of this systematic review were to examine how the flipped classroom has been applied in nursing education and outcomes associated with this style of teaching. Five databases were searched and resulted in the retrieval of 21 papers: PubMed, CINAHL, EMBASE, Scopus and ERIC. After screening for inclusion/exclusion criteria, each paper was evaluated using a critical appraisal tool. Data extraction and analysis were completed on all included studies. This systematic review screened 21 titles and abstracts resulting in nine included studies. All authors critically appraised the quality of the included studies. Five studies were identified and themes identified were: academic performance outcomes, and student satisfaction implementing the flipped classroom. Use of the flipped classroom in higher education nursing programmes yielded neutral or positive academic outcomes and mixed results for satisfaction. Engagement of students in the flipped classroom model was achieved when academics informed and rationalised the purpose of the flipped classroom model to students. However, no studies in this review identified the evaluation of the process of implementing the flipped classroom. Studies examining the process and ongoing evaluation and refinement of the flipped classroom in higher education nursing

Optical manipulation of electron spin in quantum dot systems

Science.gov (United States)

Villas-Boas, Jose; Ulloa, Sergio; Govorov, Alexander

2006-03-01

Self-assembled quantum dots (QDs) are of particular interest for fundamental physics because of their similarity with atoms. Coupling two of such dots and addressing them with polarized laser light pulses is perhaps even more interesting. In this paper we use a multi-exciton density matrix formalism to model the spin dynamics of a system with single or double layers of QDs. Our model includes the anisotropic electron-hole exchange in the dots, the presence of wetting layer states, and interdot tunneling [1]. Our results show that it is possible to switch the spin polarization of a single self-assembled quantum dot under elliptically polarized light by increasing the laser intensity. In the nonlinear mechanism described here, intense elliptically polarized light creates an effective exchange channel between the exciton spin states through biexciton states, as we demonstrate by numerical and analytical methods. We further show that the effect persists in realistic ensembles of dots, and we propose alternative ways to detect it. We also extend our study to a double layer of quantum dots, where we find a competition between Rabi frequency and tunneling oscillations. [1] J. M. Villas-Boas, S. E. Ulloa, and A. O. Govorov, Phys. Rev. Lett. 94, 057404 (2005); Phys. Rev. B 69, 125342 (2004).

Flipping for success: evaluating the effectiveness of a novel teaching approach in a graduate level setting.

Science.gov (United States)

Moraros, John; Islam, Adiba; Yu, Stan; Banow, Ryan; Schindelka, Barbara

2015-02-28

Flipped Classroom is a model that's quickly gaining recognition as a novel teaching approach among health science curricula. The purpose of this study was four-fold and aimed to compare Flipped Classroom effectiveness ratings with: 1) student socio-demographic characteristics, 2) student final grades, 3) student overall course satisfaction, and 4) course pre-Flipped Classroom effectiveness ratings. The participants in the study consisted of 67 Masters-level graduate students in an introductory epidemiology class. Data was collected from students who completed surveys during three time points (beginning, middle and end) in each term. The Flipped Classroom was employed for the academic year 2012-2013 (two terms) using both pre-class activities and in-class activities. Among the 67 Masters-level graduate students, 80% found the Flipped Classroom model to be either somewhat effective or very effective (M = 4.1/5.0). International students rated the Flipped Classroom to be significantly more effective when compared to North American students (X(2) = 11.35, p Students' perceived effectiveness of the Flipped Classroom had no significant association to their academic performance in the course as measured by their final grades (r s = 0.70). However, students who found the Flipped Classroom to be effective were also more likely to be satisfied with their course experience. Additionally, it was found that the SEEQ variable scores for students enrolled in the Flipped Classroom were significantly higher than the ones for students enrolled prior to the implementation of the Flipped Classroom (p = 0.003). Overall, the format of the Flipped Classroom provided more opportunities for students to engage in critical thinking, independently facilitate their own learning, and more effectively interact with and learn from their peers. Additionally, the instructor was given more flexibility to cover a wider range and depth of material, provide in-class applied learning

Excitons

Energy Technology Data Exchange (ETDEWEB)

Kozhushner, M

1975-06-01

The theory of quasi particles is explained to layman readers and the significance of the discovery of excitons is pointed out. New possibilities of the study of electron-hole interactions and of superconductivity are indicated.

The Partially Flipped Classroom: The Effects of Flipping a Module on "Junk Science" in a Large Methods Course

Science.gov (United States)

Burgoyne, Stephanie; Eaton, Judy

2018-01-01

Flipped classrooms are gaining popularity, especially in psychology statistics courses. However, not all courses lend themselves to a fully flipped design, and some instructors might not want to commit to flipping every class. We tested the effectiveness of flipping just one component (a module on junk science) of a large methods course. We…

Probing exciton density of states through phonon-assisted emission in GaN epilayers: A and B exciton contributions

Science.gov (United States)

Cavigli, Lucia; Gabrieli, Riccardo; Gurioli, Massimo; Bogani, Franco; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas; Vinattieri, Anna

2010-09-01

A detailed experimental investigation of the phonon-assisted emission in a high-quality c -plane GaN epilayer is presented up to 200 K. By performing photoluminescence and reflectivity measurements, we find important etaloning effects in the phonon-replica spectra, which have to be corrected before addressing the lineshape analysis. Direct experimental evidence for free exciton thermalization is found for the whole temperature range investigated. A close comparison with existing models for phonon replicas originating from a thermalized free exciton distribution shows that the simplified and commonly adopted description of the exciton-phonon interaction with a single excitonic band leads to a large discrepancy with experimental data. Only the consideration of the complex nature of the excitonic band in GaN, including A and B exciton contributions, allows accounting for the temperature dependence of the peak energy, intensity, and lineshape of the phonon replicas.

Anomalous scattering of neutrons in spin-polarized media

International Nuclear Information System (INIS)

Bashkin, E.P.

1989-01-01

A new exchange mechanism of inelastic scattering with spin flip for slow neutrons propagating through a spin-polarized medium is studied. The scattering is accompanied by emission or absorption of thermal fluctuations of the transverse magnetization of the medium; the weakly damped Larmor precession of nuclear spins in the external magnetic field plays the main role in these fluctuations. Under the conditions of giant opalescence the effect is enormous and the corresponding cross sections are significantly greater than the standard elastic scattering cross sections. Thus in the case of 29 Si↑ and 3 He↑ under typical experimental conditions the cross sections of these inelastic processes are of the order of 10 5 -10 6 b

Scaling laws of Rydberg excitons

Science.gov (United States)

Heckötter, J.; Freitag, M.; Fröhlich, D.; Aßmann, M.; Bayer, M.; Semina, M. A.; Glazov, M. M.

2017-09-01

Rydberg atoms have attracted considerable interest due to their huge interaction among each other and with external fields. They demonstrate characteristic scaling laws in dependence on the principal quantum number n for features such as the magnetic field for level crossing or the electric field of dissociation. Recently, the observation of excitons in highly excited states has allowed studying Rydberg physics in cuprous oxide crystals. Fundamentally different insights may be expected for Rydberg excitons, as the crystal environment and associated symmetry reduction compared to vacuum give not only optical access to many more states within an exciton multiplet but also extend the Hamiltonian for describing the exciton beyond the hydrogen model. Here we study experimentally and theoretically the scaling of several parameters of Rydberg excitons with n , for some of which we indeed find laws different from those of atoms. For others we find identical scaling laws with n , even though their origin may be distinctly different from the atomic case. At zero field the energy splitting of a particular multiplet n scales as n-3 due to crystal-specific terms in the Hamiltonian, e.g., from the valence band structure. From absorption spectra in magnetic field we find for the first crossing of levels with adjacent principal quantum numbers a Br∝n-4 dependence of the resonance field strength, Br, due to the dominant paramagnetic term unlike for atoms for which the diamagnetic contribution is decisive, resulting in a Br∝n-6 dependence. By contrast, the resonance electric field strength shows a scaling as Er∝n-5 as for Rydberg atoms. Also similar to atoms with the exception of hydrogen we observe anticrossings between states belonging to multiplets with different principal quantum numbers at these resonances. The energy splittings at the avoided crossings scale roughly as n-4, again due to crystal specific features in the exciton Hamiltonian. The data also allow us to

Generation and decay dynamics of triplet excitons in Alq3 thin films under high-density excitation conditions.

Science.gov (United States)

Watanabe, Sadayuki; Furube, Akihiro; Katoh, Ryuzi

2006-08-31

We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.

Confchem Conference on Flipped Classroom: Student Engagement with Flipped Chemistry Lectures

Science.gov (United States)

Seery, Michael K.

2015-01-01

This project introduces the idea of "flipped lecturing" to a group of second-year undergraduate students. The aim of flipped lecturing is to provide much of the "content delivery" of the lecture in advance, so that the lecture hour can be devoted to more in-depth discussion, problem solving, and so on. As well as development of…

Mapping the Local Density of Optical States of a Photonic Crystal with Single Quantum Dots

DEFF Research Database (Denmark)

Wang, Qin; Stobbe, Søren; Lodahl, Peter

2011-01-01

We use single self-assembled InGaAs quantum dots as internal probes to map the local density of optical states of photonic crystal membranes. The employed technique separates contributions from nonradiative recombination and spin-flip processes by properly accounting for the role of the exciton...... fine structure. We observe inhibition factors as high as 70 and compare our results to local density of optical states calculations available from the literature, thereby establishing a quantitative understanding of photon emission in photonic crystal membranes. © 2011 American Physical Society....

Excitons

International Nuclear Information System (INIS)

Kozhushner, M.

1975-01-01

The theory of quasi particles is explained to layman readers and the significance of the discovery of excitons is pointed out. New possibilities of the study of electron-hole interactions and of superconductivity are indicated. (L.O.)

Electron spin dynamics and optical orientation of Mn2+ ions in GaAs

Science.gov (United States)

Akimov, I. A.; Dzhioev, R. I.; Korenev, V. L.; Kusrayev, Yu. G.; Sapega, V. F.; Yakovlev, D. R.; Bayer, M.

2013-04-01

We present an overview of spin-related phenomena in GaAs doped with low concentration of Mn-acceptors (below 1018 cm-3). We use the combination of different experimental techniques such as spin-flip Raman scattering and time-resolved photoluminescence. This allows to evaluate the time evolution of both electron and Mn spins. We show that optical orientation of Mn ions is possible under application of weak magnetic field, which is required to suppress the manganese spin relaxation. The optically oriented Mn2+ ions maintain the spin and return part of the polarization back to the electron spin system providing a long-lived electron spin memory. This leads to a bunch of spectacular effects such as non-exponential electron spin decay and spin precession in the effective exchange fields.

Single electron-spin memory with a semiconductor quantum dot

International Nuclear Information System (INIS)

Young, Robert J; Dewhurst, Samuel J; Stevenson, R Mark; Atkinson, Paola; Bennett, Anthony J; Ward, Martin B; Cooper, Ken; Ritchie, David A; Shields, Andrew J

2007-01-01

We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime

Entangled exciton states in quantum dot molecules

Science.gov (United States)

Bayer, Manfred

2002-03-01

Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For

Single-spin addressing in an atomic Mott insulator

DEFF Research Database (Denmark)

Weitenberg, Christof; Endres, Manuel; Sherson, Jacob

2011-01-01

directly monitored the tunnelling quantum dynamics of single atoms in the lattice prepared along a single line, and observed that our addressing scheme leaves the atoms in the motional ground state. The results should enable studies of entropy transport and the quantum dynamics of spin impurities...... and quantum spin dynamics. Here we demonstrate how such control can be implemented at the most fundamental level of a single spin at a specific site of an optical lattice. Using a tightly focused laser beam together with a microwave field, we were able to flip the spin of individual atoms in a Mott insulator...... with sub-diffraction-limited resolution, well below the lattice spacing. The Mott insulator provided us with a large two-dimensional array of perfectly arranged atoms, in which we created arbitrary spin patterns by sequentially addressing selected lattice sites after freezing out the atom distribution. We...

Excitons in Single-Walled Carbon Nanotubes and Their Dynamics

Science.gov (United States)

Amori, Amanda R.; Hou, Zhentao; Krauss, Todd D.

2018-04-01

Understanding exciton dynamics in single-walled carbon nanotubes (SWCNTs) is essential to unlocking the many potential applications of these materials. This review summarizes recent progress in understanding exciton photophysics and, in particular, exciton dynamics in SWCNTs. We outline the basic physical and electronic properties of SWCNTs, as well as bright and dark transitions within the framework of a strongly bound one-dimensional excitonic model. We discuss the many facets of ultrafast carrier dynamics in SWCNTs, including both single-exciton states (bright and dark) and multiple-exciton states. Photophysical properties that directly relate to excitons and their dynamics, including exciton diffusion lengths, chemical and structural defects, environmental effects, and photoluminescence photon statistics as observed through photon antibunching measurements, are also discussed. Finally, we identify a few key areas for advancing further research in the field of SWCNT excitons and photonics.

Uncertainty in T1 mapping using the variable flip angle method with two flip angles

International Nuclear Information System (INIS)

Schabel, Matthias C; Morrell, Glen R

2009-01-01

Propagation of errors, in conjunction with the theoretical signal equation for spoiled gradient echo pulse sequences, is used to derive a theoretical expression for uncertainty in quantitative variable flip angle T 1 mapping using two flip angles. This expression is then minimized to derive a rigorous expression for optimal flip angles that elucidates a commonly used empirical result. The theoretical expressions for uncertainty and optimal flip angles are combined to derive a lower bound on the achievable uncertainty for a given set of pulse sequence parameters and signal-to-noise ratio (SNR). These results provide a means of quantitatively determining the effect of changing acquisition parameters on T 1 uncertainty. (note)

Models of coherent exciton condensation

International Nuclear Information System (INIS)

Littlewood, P B; Eastham, P R; Keeling, J M J; Marchetti, F M; Simons, B D; Szymanska, M H

2004-01-01

That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focusing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers

Models of coherent exciton condensation

Energy Technology Data Exchange (ETDEWEB)

Littlewood, P B [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Eastham, P R [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Keeling, J M J [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Marchetti, F M [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Simons, B D [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Szymanska, M H [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom)

2004-09-08

That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focusing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.

Detection of trans-cis flips and peptide-plane flips in protein structures

NARCIS (Netherlands)

Touw, W.G.; Joosten, R.P.; Vriend, G.

2015-01-01

A coordinate-based method is presented to detect peptide bonds that need correction either by a peptide-plane flip or by a trans-cis inversion of the peptide bond. When applied to the whole Protein Data Bank, the method predicts 4617 trans-cis flips and many thousands of hitherto unknown

Vibration characteristics of an inclined flip-flow screen panel in banana flip-flow screens

Science.gov (United States)

Xiong, Xiaoyan; Niu, Linkai; Gu, Chengxiang; Wang, Yinhua

2017-12-01

A banana flip-flow screen is an effective solution for the screening of high-viscosity, high-water and fine materials. As one of the key components, the vibration characteristics of the inclined flip-flow screen panel largely affects the screen performance and the processing capacity. In this paper, a mathematical model for the vibration characteristic of the inclined flip-flow screen panel is proposed based on Catenary theory. The reasonability of Catenary theory in analyzing the vibration characteristic of flip-flow screen panels is verified by a published experiment. Moreover, the effects of the rotation speed of exciters, the incline angle, the slack length and the characteristics of the screen on the vertical deflection, the vertical velocity and the vertical acceleration of the screen panel are investigated parametrically. The results show that the rotation speed of exciters, the incline angle, the slack length and the characteristics of the screen have significant effects on the vibrations of an inclined flip-flow screen panel, and these parameters should be optimized.

Flip-Flop of Steroids in Phospholipid Bilayers: Effects of the Chemical Structure on Transbilayer Diffusion

DEFF Research Database (Denmark)

Parisio, Giulia; Sperotto, Maria Maddalena; Ferrarini, Alberta

2012-01-01

is still missing. The scarce knowledge derives from the difficulty of experimental determination. Because of its slow rate on the molecular time scale, flip-flop is challenging also for computational techniques. Here, we report a study of the passive transbilayer diffusion of steroids, based on a kinetic......, dielectric permittivity, acyl chain order parameters, and lateral pressure. The flip-flop rates are determined by solving the Master Equation that governs the time evolution of the system, with transition rates between free energy minima evaluated according to the Kramers theory. Considering various steroids...... (cholesterol, lanosterol, ketosterone, 5-cholestene, 25-hydroxycholesterol, and testosterone), we can discuss how differences in molecular shape and polarity affect the pathway and the rate of flip-flop in a liquid crystalline 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) bilayer, at low steroid...

Density-dependent squeezing of excitons in highly excited semiconductors

International Nuclear Information System (INIS)

Nguyen Hong Quang.

1995-07-01

The time evolution from coherent states to squeezed states of high density excitons is studied theoretically based on the boson formalism and within the Random Phase Approximation. Both the mutual interaction between excitons and the anharmonic exciton-photon interaction due to phase-space filling of excitons are taken into account. It is shown that the exciton squeezing depends strongly on the exciton density in semiconductors and becomes smaller with increasing the latter. (author). 16 refs, 2 figs

The Flipped Classroom in Counselor Education

Science.gov (United States)

Moran, Kristen; Milsom, Amy

2015-01-01

The flipped classroom is proposed as an effective instructional approach in counselor education. An overview of the flipped-classroom approach, including advantages and disadvantages, is provided. A case example illustrates how the flipped classroom can be applied in counselor education. Recommendations for implementing or researching flipped…

Spin Depolarization due to Beam-Beam Interaction in NLC

Energy Technology Data Exchange (ETDEWEB)

Thompson, Kathleen A

2001-01-04

Calculations of spin depolarization effects due to the beam-beam interaction are presented for several NLC designs. The depolarization comes from both classical (Bargmann-Michel-Telegdi precession) and quantum (Sokolov-Ternov spin-flip) effects. It is anticipated that some physics experiments at future colliders will require a knowledge of the polarization to better than 0.5% precision. We compare the results of CAIN simulations with the analytic estimates of Yokoya and Chen for head-on collisions. We also study the effects of transverse offsets and beamstrahlung-induced energy spread.

Towards a coherent picture of excitonic coherence in the Fenna–Matthews–Olson complex

International Nuclear Information System (INIS)

Fidler, Andrew F; Caram, Justin R; Hayes, Dugan; Engel, Gregory S

2012-01-01

Observations of long-lived coherence between excited states in several photosynthetic antenna complexes has motivated interest in developing a more detailed understanding of the role of the protein matrix in guiding the underlying dynamics of the system. These experiments suggest that classical rate laws may not provide an adequate description of the energy transfer process and that quantum effects must be taken into account to describe the near unity transfer efficiency in these systems. Recently, it has been shown that coherences between different pairs of excitons dephase at different rates. These details should provide some insight about the underlying electronic structure of the complex and its coupling to the protein bath. Here we show that a simple model can account for the different dephasing rates as well as the most current available experimental evidence of excitonic coherences in the Fenna–Matthews–Olson complex. The differences in dephasing rates can be understood as arising largely from differences in the delocalization and shared character between the underlying electronic states. We also suggest that the anomalously low dephasing rate of the exciton 1–2 coherence is enhanced by non-secular effects. (paper)

Flipped Classroom, active Learning?

DEFF Research Database (Denmark)

Andersen, Thomas Dyreborg; Levinsen, Henrik; Philipps, Morten

2015-01-01

Action research is conducted in three physics classes over a period of eighteen weeks with the aim of studying the effect of flipped classroom on the pupils agency and learning processes. The hypothesis is that flipped classroom teaching will potentially allocate more time to work actively...

Domain Walls and Macroscopic Spin-Flip-Like States in GdxCo1-x/GdyCo1-y Bilayers

Science.gov (United States)

Martin, Jose I.

2005-03-01

Exchange coupled double layers (ECDL) made of rare earth -- transition metal amorphous alloys are of basic and technological interest, as they present different magnetization configurations when the composition is changed or when the temperature is varied crossing the compensation temperatures (Tcomp) of both ferrimagnetic alloys. In this work, amorphous GdxCo1-x(100 nm)/GdyCo1-y(100 nm) ECDL have been prepared to investigate the magnetization reversal and the stable magnetic configurations when the compositions of both layers are similar: x = 0.22, y = 0.24. The samples have been grown by co-sputtering on corning glass substrates, which has allowed to analyze the behaviour within each layer by transverse Kerr effect measurements. A rich variety of behaviours has been found in the temperature range between the Tcomp of both layers, including magnetization reversal by annihilation/creation of a Bloch wall across the sample thickness, and a macroscopic spin-flip-like metamagnetic state where the magnetic moments form a double antiferromagnetic state with the presence of a N'eel-like wall when the magnetizations of both layers are similar [1]. The whole observed behavior can be understood in terms of a deduced general magnetic field -- temperature phase diagram. [1] R. Morales et al. Phys. Rev. B 70, 174440 (2004). Work supported by Spanish CICYT.

Exciton-dopant and exciton-charge interactions in electronically doped OLEDs

International Nuclear Information System (INIS)

Williams, Christopher; Lee, Sergey; Ferraris, John; Zakhidov, A. Anvar

2004-01-01

The electronic dopants, like tetrafluorocyanoquinodimethane (F 4 -TCNQ) molecules, used for p-doping of hole transport layers in organic light-emitting diodes (OLEDs) are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We observed EL quenching in OLED with F 4 -TCNQ doped N,N'-diphenyl-N'N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine hole transport layer at large dopant concentrations, >5%. To separate the effects of exciton-dopant quenching, from exciton-polaron quenching we have intentionally doped the emissive layer of (8-tris-hydroxyquinoline) with three acceptors (A) of different electron affinities: F 4 -TCNQ, TCNQ, and C 60 , and found that C 60 is the strongest EL-quencher, while F 4 -TCNQ is the weakest, contrary to intuitive expectations. The new effects of charge transfer and usually considered energy transfer from exciton to neutral (A) and charged acceptors (A - ) are compared as channels for non-radiative Ex-A decay. At high current loads the EL quenching is observed, which is due to decay of Ex on free charge carriers, hole polarons P + . We consider contributions to Ex-P + interaction by short-range charge transfer and describe the structure of microscopic charge transfer (CT)-processes responsible for it. The formation of metastable states of 'charged excitons' (predicted and studied by Agranovich et al. Chem. Phys. 272 (2001) 159) by electron transfer from a P to an Ex is pointed out, and ways to suppress non-radiative Ex-P decay are suggested

Exciton Formation in Disordered Semiconductors

DEFF Research Database (Denmark)

Klochikhin, A.; Reznitsky, A.; Permogorov, S.

1999-01-01

Stationary luminescence spectra of disordered solid solutions can be accounted by the model of localized excitons. Detailed analysis of the long time decay kinetics of luminescence shows that exciton formation in these systems is in great extent due to the bimolecular reaction of separated carrie...

A High-Spin Rate Measurement Method for Projectiles Using a Magnetoresistive Sensor Based on Time-Frequency Domain Analysis.

Science.gov (United States)

Shang, Jianyu; Deng, Zhihong; Fu, Mengyin; Wang, Shunting

2016-06-16

Traditional artillery guidance can significantly improve the attack accuracy and overall combat efficiency of projectiles, which makes it more adaptable to the information warfare of the future. Obviously, the accurate measurement of artillery spin rate, which has long been regarded as a daunting task, is the basis of precise guidance and control. Magnetoresistive (MR) sensors can be applied to spin rate measurement, especially in the high-spin and high-g projectile launch environment. In this paper, based on the theory of a MR sensor measuring spin rate, the mathematical relationship model between the frequency of MR sensor output and projectile spin rate was established through a fundamental derivation. By analyzing the characteristics of MR sensor output whose frequency varies with time, this paper proposed the Chirp z-Transform (CZT) time-frequency (TF) domain analysis method based on the rolling window of a Blackman window function (BCZT) which can accurately extract the projectile spin rate. To put it into practice, BCZT was applied to measure the spin rate of 155 mm artillery projectile. After extracting the spin rate, the impact that launch rotational angular velocity and aspect angle have on the extraction accuracy of the spin rate was analyzed. Simulation results show that the BCZT TF domain analysis method can effectively and accurately measure the projectile spin rate, especially in a high-spin and high-g projectile launch environment.

Response functions of cold neutron matter: density, spin and current fluctuations

Energy Technology Data Exchange (ETDEWEB)

Keller, Jochen; Sedrakian, Armen [Institut fuer Theoretische Physik, Goethe-Universitaet, Frankfurt am Main (Germany)

2014-07-01

We study the response of a single-component pair-correlated baryonic Fermi-liquid to density, spin, and their current perturbations. A complete set of response functions is calculated in the low-temperature regime. We derive the spectral functions of collective excitations associated with the density, density-current, spin, and spin-current perturbations. The dispersion relations of density and spin fluctuations are determined and it is shown that the density fluctuations lead to exciton-like undamped bound states, whereas the spin excitations correspond to diffusive modes above the pair-breaking threshold. The contribution of the collective pair-breaking modes to the specific heat of neutron matter at subnuclear densities is computed and is shown to be comparable to that of the degenerate electron gas at not too low temperatures.

Fractional Solitons in Excitonic Josephson Junctions

OpenAIRE

Hsu, Ya-Fen; Su, Jung-Jung

2015-01-01

The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ? 0 applied. The system is mapped into a pseudospin ferromagnet then described numeric...

Spin-relaxation time in the impurity band of wurtzite semiconductors

Science.gov (United States)

Tamborenea, Pablo I.; Wellens, Thomas; Weinmann, Dietmar; Jalabert, Rodolfo A.

2017-09-01

The spin-relaxation time for electrons in the impurity band of semiconductors with wurtzite crystal structure is determined. The effective Dresselhaus spin-orbit interaction Hamiltonian is taken as the source of the spin relaxation at low temperature and for doping densities corresponding to the metallic side of the metal-insulator transition. The spin-flip hopping matrix elements between impurity states are calculated and used to set up a tight-binding Hamiltonian that incorporates the symmetries of wurtzite semiconductors. The spin-relaxation time is obtained from a semiclassical model of spin diffusion, as well as from a microscopic self-consistent diagrammatic theory of spin and charge diffusion in doped semiconductors. Estimates are provided for particularly important materials. The theoretical spin-relaxation times compare favorably with the corresponding low-temperature measurements in GaN and ZnO. For InN and AlN we predict that tuning of the spin-orbit coupling constant induced by an external potential leads to a potentially dramatic increase of the spin-relaxation time related to the mechanism under study.

Slow Light Using Electromagnetically Induced Transparency from Spin Coherence in [110] Strained Quantum Wells

Science.gov (United States)

Chang, Shu-Wei; Chang-Hasnain, Connie J.; Wang, Hailin

2005-03-01

The electromagnetically induced transparency from spin coherence has been proposed in [001] quantum wells recently. [1] The spin coherence is a potential candidate to demonstrate semiconductor-based slow light at room temperature. However, the spin coherence time is not long enough to demonstrate a significant slowdown factor in [001] quantum wells. Further, the required transition of light-hole excitons lies in the absorption of heavy-hole continuum states. The extra dephasing and absorption from these continuum states are drawbacks for slow light. Here, we propose to use [110] strained quantum wells instead of [001] quantum wells. The long spin relaxation time in [110] quantum wells at room temperature, and thus more robust spin coherence, [2] as well as the strain-induced separation [3, 4] of the light-hole exciton transition from the heavy-hole continuum absorption can help to slow down light in quantum wells. [1] T. Li, H. Wang, N. H. Kwong, and R. Binder, Opt. Express 11, 3298 (2003). [2] Y. Ohno, R. Terauchi, T. Adachi, F. Matsukura, and H. Ohno, Phys. Rev. Lett. 83, 4196 (1999). [3] C. Y. P. Chao and S. L. Chuang, Phys. Rev. B 46, 4110 (1992). [4] C. Jagannath, E. S. Koteles, J. Lee, Y. J. Chen, B. S. Elman, and J. Y. Chi, Phys. Rev. B 34, 7027 (1986).

Nanoscale Morphology of Doctor Bladed versus Spin-Coated Organic Photovoltaic Films

KAUST Repository

Pokuri, Balaji Sesha Sarath

2017-08-17

Recent advances in efficiency of organic photovoltaics are driven by judicious selection of processing conditions that result in a “desired” morphology. An important theme of morphology research is quantifying the effect of processing conditions on morphology and relating it to device efficiency. State-of-the-art morphology quantification methods provide film-averaged or 2D-projected features that only indirectly correlate with performance, making causal reasoning nontrivial. Accessing the 3D distribution of material, however, provides a means of directly mapping processing to performance. In this paper, two recently developed techniques are integrated—reconstruction of 3D morphology and subsequent conversion into intuitive morphology descriptors —to comprehensively image and quantify morphology. These techniques are applied on films generated by doctor blading and spin coating, additionally investigating the effect of thermal annealing. It is found that morphology of all samples exhibits very high connectivity to electrodes. Not surprisingly, thermal annealing consistently increases the average domain size in the samples, aiding exciton generation. Furthermore, annealing also improves the balance of interfaces, enhancing exciton dissociation. A comparison of morphology descriptors impacting each stage of photophysics (exciton generation, dissociation, and charge transport) reveals that spin-annealed sample exhibits superior morphology-based performance indicators. This suggests substantial room for improvement of blade-based methods (process optimization) for morphology tuning to enhance performance of large area devices.

Flipped cryptons and ultrahigh energy cosmic rays

CERN Document Server

Ellis, Jonathan Richard; Nanopoulos, D V

2004-01-01

Cryptons are metastable bound states of fractionally-charged particles that arise generically in the hidden sectors of models derived from heterotic string. We study their properties and decay modes in a specific flipped SU(5) model with long-lived four-particle spin-zero bound states called tetrons. We show that the neutral tetrons are metastable, and exhibit the tenth order nonrenormalizable superpotential operators responsible for their dominant decays. By analogy with QCD, we expect charged tetrons to be somewhat heavier, and to decay relatively rapidly via lower-order interactions that we also exhibit. The expected masses and lifetimes of the neutral tetrons make them good candidates for cold dark matter, and a potential source of the ultrahigh energy cosmic rays which have been observed, whereas the charged tetrons would have decayed in the early Universe.

Exciton management in organic photovoltaic multidonor energy cascades.

Science.gov (United States)

Griffith, Olga L; Forrest, Stephen R

2014-05-14

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

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.

Spin-dependent recombination processes in wide band gap II-Mn-VI compounds

International Nuclear Information System (INIS)

Godlewski, M.; Yatsunenko, S.; Khachapuridze, A.; Ivanov, V.Yu.

2004-01-01

Mechanisms of optical detection of magnetic resonance in wide band gap II-Mn-VI diluted magnetic semiconductor (DMS) are discussed based on the results of photoluminescence (PL), PL kinetics, electron spin resonance (ESR) and optically detected magnetic resonance (ODMR) and optically detected cyclotron resonance (ODCR) investigations. Spin-dependent interactions between localized spins of Mn 2+ ions and spins/magnetic moments of free, localized or bound carriers are responsible for the observed ODMR signals. We conclude that these interactions are responsible for the observed rapid shortening of the PL decay time of 4 T 1 → 6 A 1 intra-shell emission of Mn 2+ ions and also for the observed delocalization of excitons in low dimensional structures

Chiral topological excitons in a Chern band insulator

Science.gov (United States)

Chen, Ke; Shindou, Ryuichi

2017-10-01

A family of semiconductors called Chern band insulators are shown to host exciton bands with nonzero topological Chern integers and chiral exciton edge modes. Using a prototypical two-band Chern insulator model, we calculate a cross-correlation function to obtain the exciton bands and their Chern integers. The lowest exciton band acquires Chern integers such as ±1 and ±2 in the electronic Chern insulator phase. The nontrivial topology can be experimentally observed both by a nonlocal optoelectronic response of exciton edge modes and by a phase shift in the cross-correlation response due to the bulk mode. Our result suggests that magnetically doped HgTe, InAs/GaSb quantum wells, and (Bi,Sb)2Te3 thin films are promising candidates for a platform of topological excitonics.

A brief description of the biomechanics and physiology of a strongman event: the tire flip.

Science.gov (United States)

Keogh, Justin W L; Payne, Amenda L; Anderson, Brad B; Atkins, Paul J

2010-05-01

The purpose of this study was to (a) characterize the temporal aspects of a popular strongman event, the tire flip; (b) gain some insight into the temporal factors that could distinguish the slowest and fastest flips; and (c) obtain preliminary data on the physiological stress of this exercise. Five resistance-trained subjects with experience in performing the tire flip gave informed consent to participate in this study. Each subject performed 2 sets of 6 tire flips with a 232-kg tire with 3 minutes of rest between sets. Temporal variables were obtained from video cameras positioned 10 m from the tire, perpendicular to the intended direction of the tire flip. Using the "stopwatch" function in Silicon Coach, the duration of each tire flip and that of the first pull, second pull, transition, and push phases were recorded. Physiological stress was estimated via heart rate and finger-prick blood lactate response. Independent T-tests revealed that the 2 faster subjects (0.38 +/- 0.17 s) had significantly (p tire flip performance and that this exercise provides relatively high degrees of physiological stress.

Isoscalar spin response in {sup 40}Ca and {sup 12}C

Energy Technology Data Exchange (ETDEWEB)

Tomasi-Gustafsson, E [Laboratoire National Saturne, Centre d` Etudes de Saclay, 91 - Gif-sur-Yvette (France); Morlet, M; Bimbot, L; Guillot, J; Jourdan, F; Langevin-Joliot, H; Marty, N; Rosier, L; Van de Wiele, J; Willis, A [Paris-11 Univ., 91 - Orsay (France). Inst. de Physique Nucleaire; Baker, F T [Georgia Univ., Athens, GA (United States); Johnson, B N [South Carolina Univ., Columbia, SC (United States); Glashausser, C [Rutgers--the State Univ., New Brunswick, NJ (United States); Djalali, C [Paris-11 Univ., 91 - Orsay (France). Inst. de Physique Nucleaire; [South Carolina Univ., Columbia, SC (United States)

1994-12-31

A method founded on the measure of an approximated spin-flip probability, in the inelastic diffusion (d,d`) at 400 MeV (incident energy) has been applied to the research of isoscalar spin strengths in calcium 40 and carbon 12. In calcium 40 the spin excitations have been revealed towards an excitation energy of 9 MeV and in the continuum a strength concentration appears about 15 MeV. In carbon 12 spin structures appear up to an excitation energy of 30 MeV; beyond 35 MeV the isoscalar spin response, as in calcium 40, is compatible with the expected value for a Fermi gas of particles without interactions. Microscopic calculations DWIA are in good agreement with the data of carbon 12. (O.L.). 30 refs., 5 figs.

Organic Spin-Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering.

Science.gov (United States)

Jang, Hyuk-Jae; Richter, Curt A

2017-01-01

Since the first observation of the spin-valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin-polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin-flip mechanisms in organic semiconductors and the role of hybrid metal-organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin-transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal-organic interface by interface engineering can greatly impact the efficiency of spin-polarized carrier injection. Here, progress on efficient spin-polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self-assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single-crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin-polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fine structure of the exciton electroabsorption in semiconductor superlattices

Energy Technology Data Exchange (ETDEWEB)

Monozon, B.S., E-mail: borismonozon@mail.ru [Physics Department, Marine Technical University, 3 Lotsmanskaya Str., 190008 St.Petersburg (Russian Federation); Schmelcher, P. [Zentrum für Optische Quantentechnologien, The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

2017-02-15

Wannier-Mott excitons in a semiconductor layered superlattice (SL) are investigated analytically for the case that the period of the superlattice is much smaller than the 2D exciton Bohr radius. Additionally we assume the presence of a longitudinal external static electric field directed parallel to the SL axis. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the electric field strength are presented in an explicit form. We focus on the fine structure of the ground quasi-2D exciton level formed by the series of closely spaced energy levels adjacent from the high frequencies. These levels are related to the adiabatically slow relative exciton longitudinal motion governed by the potential formed by the in-plane exciton state. It is shown that the external electric fields compress the fine structure energy levels, decrease the intensities of the corresponding optical peaks and increase the exciton binding energy. A possible experimental study of the fine structure of the exciton electroabsorption is discussed.

A novel integration of online and flipped classroom instructional models in public health higher education.

Science.gov (United States)

Galway, Lindsay P; Corbett, Kitty K; Takaro, Timothy K; Tairyan, Kate; Frank, Erica

2014-08-29

In 2013, a cohort of public health students participated in a 'flipped' Environmental and Occupational Health course. Content for the course was delivered through NextGenU.org and active learning activities were carried out during in-class time. This paper reports on the design, implementation, and evaluation of this novel approach. Using mixed-methods, we examined learning experiences and perceptions of the flipped classroom model and assessed changes in students' self-perceived knowledge after participation in the course. We used pre- and post-course surveys to measure changes in self-perceived knowledge. The post-course survey also included items regarding learning experiences and perceptions of the flipped classroom model. We also compared standard course review and examination scores for the 2013 NextGenU/Flipped Classroom students to previous years when the course was taught with a lecture-based model. We conducted a focus group session to gain more in-depth understanding of student learning experiences and perceptions. Students reported an increase in knowledge and survey and focus group data revealed positive learning experiences and perceptions of the flipped classroom model. Mean examination scores for the 2013 NextGenU/Flipped classroom students were 88.8% compared to 86.4% for traditional students (2011). On a scale of 1-5 (1 = lowest rank, 5 = highest rank), the mean overall rating for the 2013 NextGenU/Flipped classroom students was 4.7/5 compared to prior years' overall ratings of 3.7 (2012), 4.3 (2011), 4.1 (2010), and 3.9 (2009). Two key themes emerged from the focus group data: 1) factors influencing positive learning experience (e.g., interactions with students and instructor); and 2) changes in attitudes towards environmental and occupation health (e.g., deepened interest in the field). Our results show that integration of the flipped classroom model with online NextGenU courses can be an effective innovation in public health higher education

Instantaneous Rayleigh scattering from excitons localized in monolayer islands

DEFF Research Database (Denmark)

Langbein, Wolfgang; Leosson, Kristjan; Jensen, Jacob Riis

2000-01-01

We show that the initial dynamics of Rayleigh scattering from excitons in quantum wells can be either instantaneous or delayed, depending on the exciton ensemble studied. For excitation of the entire exciton resonance, a finite rise time given by the inverse inhomogeneous broadening: of the exciton...

Quantifying the Impact of Single Bit Flips on Floating Point Arithmetic

Energy Technology Data Exchange (ETDEWEB)

Elliott, James J [ORNL; Mueller, Frank [North Carolina State University; Stoyanov, Miroslav K [ORNL; Webster, Clayton G [ORNL

2013-08-01

In high-end computing, the collective surface area, smaller fabrication sizes, and increasing density of components have led to an increase in the number of observed bit flips. If mechanisms are not in place to detect them, such flips produce silent errors, i.e. the code returns a result that deviates from the desired solution by more than the allowed tolerance and the discrepancy cannot be distinguished from the standard numerical error associated with the algorithm. These phenomena are believed to occur more frequently in DRAM, but logic gates, arithmetic units, and other circuits are also susceptible to bit flips. Previous work has focused on algorithmic techniques for detecting and correcting bit flips in specific data structures, however, they suffer from lack of generality and often times cannot be implemented in heterogeneous computing environment. Our work takes a novel approach to this problem. We focus on quantifying the impact of a single bit flip on specific floating-point operations. We analyze the error induced by flipping specific bits in the most widely used IEEE floating-point representation in an architecture-agnostic manner, i.e., without requiring proprietary information such as bit flip rates and the vendor-specific circuit designs. We initially study dot products of vectors and demonstrate that not all bit flips create a large error and, more importantly, expected value of the relative magnitude of the error is very sensitive on the bit pattern of the binary representation of the exponent, which strongly depends on scaling. Our results are derived analytically and then verified experimentally with Monte Carlo sampling of random vectors. Furthermore, we consider the natural resilience properties of solvers based on the fixed point iteration and we demonstrate how the resilience of the Jacobi method for linear equations can be significantly improved by rescaling the associated matrix.

Spin-Related Micro-Photoluminescence in Fe3+ Doped ZnSe Nanoribbons

Directory of Open Access Journals (Sweden)

Lipeng Hou

2016-12-01

Full Text Available Spin-related emission properties have important applications in the future information technology; however, they involve microscopic ferromagnetic coupling, antiferromagnetic or ferrimagnetic coupling between transition metal ions and excitons, or d state coupling with phonons is not well understood in these diluted magnetic semiconductors (DMS. Fe3+ doped ZnSe nanoribbons, as a DMS example, have been successfully prepared by a thermal evaporation method. Their power-dependent micro-photoluminescence (PL spectra and temperature-dependent PL spectra of a single ZnSe:Fe nanoribbon have been obtained and demonstrated that alio-valence ion doping diminishes the exciton magnetic polaron (EMP effect by introducing exceeded charges. The d-d transition emission peaks of Fe3+ assigned to the 4T2 (G → 6A1 (S transition at 553 nm and 4T1 (G → 6A1 (S transition at 630 nm in the ZnSe lattice have been observed. The emission lifetimes and their temperature dependences have been obtained, which reflected different spin–phonon interactions. There exists a sharp decrease of PL lifetime at about 60 K, which hints at a magnetic phase transition. These spin–spin and spin–phonon interaction related PL phenomena are applicable in the future spin-related photonic nanodevices.

Collective State of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells under Pulse Resonance Excitation

DEFF Research Database (Denmark)

Larionov, A. V.; Timofeev, V. B.; Hvam, Jørn Märcher

2002-01-01

, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T properties of the collective phase of interwell excitons and experimental manifestations of this coherence...

Leptophobic Z{sup {prime}} in stringy flipped SU(5)

Energy Technology Data Exchange (ETDEWEB)

Lopez, J.L. [Bonner Nuclear Lab, Department of Physics, Rice University, 6100 Main Street, Houston, Texas 77005 (United States); Nanopoulos, D.V. [Center for Theoretical Physics, Department of Physics, Texas AM University, College Station, Texas 77843-4242 (United States)]|[Astroparticle Physics Group, Houston Advanced Research Center (HARC), The Mitchell Campus, The Woodlands, Texas 77381 (United States)

1997-01-01