Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W10: Focus Session: Spin Transport and Spin Hall Effect |
Hide Abstracts |
Sponsoring Units: DMP GMAG Chair: Allan MacDonald, University of Texas Room: LACC 153B |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W10.00001: Observation of spin Coulomb drag in a two-dimensional electron gas Invited Speaker: A electron propagating through a solid carries spin in addition to its mass and charge. However, unlike that latter two attributes, the spin of the electron is not a conserved quantity. Despite this fact, it has been widely assumed that for times short compared with the spin relaxation time the spin current obeys the same rules as the charge and mass current (momentum). Of these rules, one of the most fundamental is that the current is not affected by ``normal'' electron-electron (e-e) collisions, which conserve total momentum. In this talk we demonstrate that this assumption is invalid, and that over a broad temperature range the damping of spin transport in a two-dimensional electron gas (2DEG) is dominated by e-e collisions. In this work spin transport is characterized by the transient spin grating technique [1], which is based on optical injection of spin-polarized electrons. The 2DEG resides in a GaAs quantum well, into which electrons are donated by Si impurites in the GaAlAs barrier layers. Excitation of GaAs with two coherent, orthogonally polarized beams of light generates a wave of electron-spin polarization whose wavevector is controlled by the angle between the two beams. From the spin-density-wave decay rate as a function of wavevector we obtain the spin diffusion coefficient, D$_{s}$, in the range T= 5K-295 K. We obtain the charge diffusion coefficient, D$_{c}$ from 4-contact transport measurements. We find that D$_{s }$is substantially suppressed compared with D$_{c}$. Their ratio yields directly the ``spin Coulomb drag'' resistivity, $\rho _{\uparrow \downarrow } $, which is proportional to the rate at which e-e collisions transfer momentum between up and down-spin populations [2]. The measured $\rho _{\uparrow \downarrow } $ is in quantitative agreement with theoretical predictions. These results indicate that modeling of spintronic devices in high-mobility semiconductors must include a strong suppression of spin diffusion originating from e-e interactions. In collaboration with C. Weber, N. Gedik, J.E. Moore, J. Stephens, and D.D. Awschalom, and supported by DOE. [1] A.R. Cameron et al., Phys. Rev. Lett. 76, 4793 (1996). [2] I. D'Amico and G. Vignale, Phys. Rev. B 62, 4853 (2000). [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W10.00002: Spin diffusion in GaAs quantum wells C.P. Weber, N. Gedik, J.E. Moore, J. Orenstein , J. Stephens, D.D. Awschalom The diffusion coefficient of spin in semiconductors is often estimated using the Einstein relation,$D=\chi ^{-1}\mu $, relating diffusion and mobility. However, direct measurements of spin diffusion coefficient $D_{s}$ and the mobility \textit{$\mu $} in an n-GaAs quantum well sample with carrier density $n=7\times 10^{11}$cm$^{-2}$ have revealed that spin diffusion is substantially suppressed [1] relative to $\chi ^{-1}\mu $ because of the ``spin-Coulomb drag'' effect [2]. In this talk we present data that illustrate the behavior of D$_{s}$, \textit{$\mu $}, and the spin relaxation time as $n$ is lowered towards the metal to insulator transition. Spin transport is characterized by the transient spin grating technique [3], which is based on optical injection of an electron-spin polarization wave with variable wavevector. Spin relaxation is measured by polarization-resolved transient absorption and \textit{$\mu $} is obtained from 4-contact transport measurements. We discuss the evolution of spin Coulomb drag and D'yakanov-Perel spin relaxation from the degenerate to nondegenerate regimes. [1] J. Orenstein, APS invited talk, this session. [2] I. D'Amico and G. Vignale, Phys. Rev. B 62, 4853 (2000). [3] A.R. Cameron, et al., Phys. Rev. Lett., 4793 (1996). [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W10.00003: Charge-Hall effect driven by spin force: reciprocal of the spin-Hall effect Ping Zhang, Qian Niu A new kind of charge-Hall effect is shown in spin-orbit coupled semiconductor systems. Unlike in the ordinary Hall effect, the driving force in the longitudinal direction is a spin force, which may originate from the gradient of a Zeeman field or a spin-dependent chemical potential. A nontrivial exact Onsager relation is established between this effect and the electric field-induced spin-Hall effect. Surprisingly, we find that this Onsager relation cannot readily be established; only when the spin current is modified by including a torque dipole term, is the spin-Hall conductivity in response to an electric field found to correspond to our charge-Hall conductivity in response to a spin force. Remarkably, it is this modified spin current that is responsible for spin accumulation at a sample boundary as shown recently based on the spin continuity equation in a semiclassical theory. Our finding on the Onsager relation further points to the importance of this modified spin current. Spin-Hall current or accumulation can thus be tested through the Onsager relation by a measurement of our charge Hall effect in response to a spin force. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W10.00004: Pure and ``impure'' spin currents in mesoscopic four-probe semiconductor nanostructures with Rashba and Dresselhaus spin-orbit couplings Liviu Z\^arbo, Branislav Nikoli\'c The all-electrical induction of spin currents in semiconductor structures has emerged as a major goal of recent intense experimental and theoretical pursuits in spintronics. Here we propose a ballistic four-probe mesoscopic structure consisting of a finite-size two-dimensional electron gas in semiconductor heterostructures with both Rashba and (linear) Dresselhaus spin-orbit (SO) couplings, which can serve as a generator of pure (i.e., not accompanied by any net charge current) spin Hall current in the transverse voltage probes as well as longitudinal spin current in the current probes emerging in response to the injected unpolarized charge flow through the longitudinal leads. Since the Dresselhaus SO coupling is related via unitary transformation to the Rashba SO coupling, the presence of either one of them alone leads to the equivalent phenomenology where longitudinal current has only in-plane polarization in the transverse direction. However, when both couplings are present and not equal to each other, the longitudinal spin current that accompanies the charge transport through the same longitudinal leads also acquires a non-zero ouf-of-plane polarization. We also investigate the explicit dependence of the magnitude of the transverse pure spin Hall current on the interplay of the strengths of the two relevant SO couplings. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W10.00005: Quantized anomalous Hall effect and intrinsic spin transport in aRashba insulator Junren Shi, Qian Niu Reciprocal ($\mathbf{k}$-space) magnetic field (Berry curvature) is known to play an essential role in charge and spin transports in metallic systems with spin-orbit coupling, and can also lead to non-trivial transport properties in insulators. Here we propose such a band insulator constructed from a two-dimensional electron system with Rashba spin-orbit coupling and a periodic co-linear Zeeman exchange field. Such an insulator shows quantized anomalous charge Hall effect as well as a finite spin-hall effect. The spin hall coefficient is on the order of $e/4\pi$ , and is robust against the presence of disorder. Possible experimental realizations are discussed. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W10.00006: Gate-controlled spin-splitting in quantum dots with ferromagnetic leads in the Kondo regime Jan Martinek, Michael Sindel, Laszlo Borda, Jozef Barnas, Ralf Bulla, Juergen Koenig, Gerd Schoen, S. Maekawa, Jan von Delft The effect of a gate voltage on the spin-splitting of an electronic level in a quantum dot (QD) attached to ferromagnetic leads is studied in the Kondo regime using a generalized numerical renormalization group (NRG) technique. We find that the gate-voltage dependence of the QD level spin-splitting strongly depends on the shape of the density of states (DOS). For one class of DOS shapes there is nearly no gate-voltage dependence, for another, the gate voltage can be used to control the magnitude and sign of the spin-splitting, which can be interpreted as a local exchange magnetic field. We find that the spin-splitting acquires a new type of logarithmic divergence. We give an analytical explanation for our numerical results and explain how they arise due to spin-dependent charge fluctuations. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W10.00007: Hartree-Fock theory of a homogeneous 2D electron gas with Rashba spin-orbit Stefano Chesi, Gabriele Giuliani The interplay of electron-electron interactions and Rashba spin-orbit in a 2D electronic system is studied within the Hartree-Fock approximation. We consider homogeneous states parameterized by a "generalized chirality" and characterized by an arbitrary orientation of the local electron spin quantization axis in $\mathbf{k}$ space. We have studied the phase diagram in the space spanned by the electronic density and the strength of the spin-orbit coupling. The existence of paramagnetic chiral states with renormalized occupation is proved at both high and low density, while a spin polarized chiral state with nontrivial spin texture exists at intermediate densities. Interestingly the latter is constructed out of single particle wavefunctions that, at variance with the corresponding situation in the absence of spin-orbit coupling, are not solutions of the non interacting hamiltonian. A classic analog for this problem is provided by a system of interacting magnetic dipoles for which the spin-orbit coupling acts as an effective magnetic field. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W10.00008: Charge and spin transport in inhomogeneous semiconductor nanostructures using a Boltzmann equation approach Dan Csontos, Sergio E. Ulloa We report on a self-consistent computational approach based on the semiclassical, steady-state Boltzmann transport equation and the Poisson equation for the study of charge and spin transport in inhomogeneous semiconductor structures. We treat scattering within the relaxation time approximation, using both constant and realistic energy-dependent scattering rates. We solve the nonlinear, coupled Boltzmann-Poisson system of equations numerically, using finite difference and relaxation methods. In order to consider nonequilibrium spin transport we adopt a two-component model based on two electron distributions, one for spin-up, and one for spin-down electrons, satisfying two {\em coupled} Boltzmann equations in the presence of spin relaxation. We demonstrate our method by numerical calculations of the transport characteristics of model inhomogeneously doped semiconductor structures. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W10.00009: Conserved dissipationless spin current in a doped Mott insulator S.P. Kou, X.L. Qi, Z.Y. Weng The existence of conserved non-dissipative spin Hall currents has been shown in a strongly correlated system. The spin Hall conductance is determined by intrinsic bulk properties, which is independent of dissipation and remains finite even when the charge resistivity diverges at low temperature in strong magnetic fields, corresponding to a spin Hall insulator. Such a system is a doped Mott insulator described by the phase string theory and the spin Hall currents coexist with the Nernst effect in a low-temperature pseudogap phase. Possible applications in spintronics will be also discussed. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W10.00010: Non-equilibrium Green’s functions for the simulation of quantum spin transport Paul von Allmen, Fabiano Oyafuso, Seungwon Lee Spin transport in presence of a magnetic field and spin relaxation is computed within the non-equilibrium Green's function formalism. Spin transport, spin precession and spin relaxation are all described within the same theoretical framework. The full equations of motion for the time dependent Green's functions are solved numerically within the effective mass and the empirical tight-biding model. As an illustration of our methodology, we will present results on the spin transport across a ZnSe/GaAs interface and compare with time resolved Kerr rotation experiments. As observed in the experiment, the calculations show that the spin dynamics evolves from ZnSe-like to GaAs-like when the electric field across the heterojunction is increased. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W10.00011: Nonballistic two-channel Datta-Das spin field effect transistor Hyun-Woo Lee, Serkan Caliskan, Hyowon Park Scattering by impurities is believed to be harmful for a Datta- Das spin field effect transistor (SFET). We present a quantum mechanical analysis of a nonballistic SFET in the small width limit, where only two (two from spin) channels are allowed, and find that the SFET can show diverse behaviors depending on relative magnitudes of various important energy scales. We identify these scales and show that the nonballistic SFET can operate successfully in certain regimes of the energy scales. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W10.00012: DC spin current generation: adiabatic versus nonadiabatic regime L. Y. Wang, C. S. Tang, C. S. Chu We study a dc spin current (SC) generation in the adiabatic regime and compare it with the nonadiabatic result. The SC generation configuration is an AC-biased finger-gate (FG) that orients transversely to and locates atop of a Rashba-type narrow channel. The dynamical variation in the spin-orbit interaction (SOI) gives rise to two time-varying potentials that have different spatial profiles. It is the time variations and the spin dependencies of these two potentials that cause the generation of a dc SC. By comparing the results from adiabatic and nonadiabatic calculations, we aim to identify the respective characteristics and the condition of adiabaticity in such a SC generation configuration. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700