Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A19: Focus Session: Optical and Electrical Spin Generation in Semiconductors |
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Sponsoring Units: GMAG DMP Chair: David Awschalom, University of California, Santa Barbara Room: Baltimore Convention Center 316 |
Monday, March 13, 2006 8:00AM - 8:12AM |
A19.00001: Spatial Imaging and Mechanical Control of Spin Coherence in Strained GaAs Epilayers Hadrian Knotz, Vanessa Sih, Jason Stephens, David Awschalom Recent work has shown that electron spins in GaAs and related compounds respond to strain dramatically. In particular, the manipulation of the spin-orbit coupling in GaAs via strain may be used for the development of all-electrical spintronic devices. Here we have developed a mechanical vise to controllably and reproducibly tune the tensile strain in-situ over a typical range for strain engineered heterostructures, from 0.0 - 0.2\%\footnote{H. Knotz, A Holleitner, J. Stephens, R. C. Myers, and D. D. Awschalom, submitted (2005)}. The effect of uniaxial tensile strain on spin coherence and transport phenomena in n-type GaAs epilayers is probed using time-resolved Kerr rotation, photoluminescence, and optically-detected nuclear magnetic resonance spectroscopies. The bandgap, electron spin lifetime, electron g-factor, and nuclear quadrupole splitting are imaged over millimeter scale areas of the epilayers for continuously varying values of strain. Simple non-destructive techniques for characterizing strain dependent phenomena in semiconductor heterostructures will facilitate the development of strain engineered spintronic devices. [Preview Abstract] |
Monday, March 13, 2006 8:12AM - 8:24AM |
A19.00002: Generating spin currents mechanically in a semiconductor Prashant Sharma It is theoretically predicted that a traveling shear wave will create a spin current in certain direct-gap (for example III-V compound) semiconductors with contributions from both the valence bands and the conduction band (for $n$-doped semiconductors). We show that this spin-current is a property of the Fermi-Dirac sea, and is controlled by a geometric phase accumulated by the strain-induced Rashba parameters in a cycle. [Preview Abstract] |
Monday, March 13, 2006 8:24AM - 8:36AM |
A19.00003: Adiabatic quantum pumping of a desired ratio of spin current to charge current Sungjun Kim, Kunal Das, Ari Mizel We present a prescription for generating pure spin current or spin selective current, based on adiabatic quantum pumping in a tight-binding model of a one dimensional conductor. A formula for the instantaneous pumped current is derived without introducing the scattering matrix. Our calculations indicate that some pumping cycles produce the maximum value 2 of pumped spin while others reverse the direction of current as a result of small alterations of the pumping cycle. We find pumping cycles which produce essentially any ratio of spin current to charge current. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A19.00004: Theory of coherent optical generation of ballistic spin currents Invited Speaker: Ballistic spin polarized currents, and ballistic pure spin currents, can be directly injected in semiconductors by a variety of all-optical processes. No bias voltage is required. Carrier and spin distributions far from equilibrium can thus be produced in both doped and undoped semiconductors, and their evolution and the resulting transport studied. We review the calculations we have made of these processes, and highlight some of the experimental results of our colleagues. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A19.00005: Ballistic-diffusive crossover in spin propagation and precession Chris Weber, Joe Orenstein, Jason Stephens, David Awschalom In the transient spin grating (TSG) technique, electron spins are optically oriented in a standing wave (``grating'') of spin polarization with wavevector $q$. TSG measures spin propagation through the time-dependence of the grating amplitude. We have shown$^1$ that for GaAs 2DEGs diffusive spin motion at low mobility $\mu$ crosses over at high $\mu$ to motion that is ballistic on length-scale of the grating---the mean free path exceeds $q^{-1}$. The ballistic regime is characterized by oscillations in the spin-grating amplitude with frequency $\omega\approx v_Fq$. In this talk we present data from TSG and time-resolved Kerr rotation measurements on $n$-doped GaAs quantum wells. We explore the crossover between ballistic and diffusive behavior as a function of temperature and of disorder. We also explore a second, distinct crossover in the spin dynamics that occurs as $v_Fq$ is tuned through the spin-orbit precession frequency, $\Omega_{SO}\approx0.2$ THz. \newline [1] C. P. Weber \textit{et al. Nature} \textbf{437} p. 1330 (2005). [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A19.00006: Dissipation through spin Coulomb drag in electronic spin transport and optical excitations Irene D'Amico, Carsten A. Ullrich Spin Coulomb drag (SCD) constitutes an intrinsic source of dissipation for spin currents in metals and semiconductors. We discuss the power loss due to SCD in potential spintronics devices and analyze in detail the associated damping of collective spin-density excitations. It is found that SCD contributes substantially to the linewidth of intersubband spin plasmons in semiconductor quantum wells, which suggests the possibility of a purely optical quantitative measurement of the SCD effect in a parabolic well through inelastic light scattering. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A19.00007: Inhomogeneously-doped semiconductor junctions as a source of~ spontaneous spin polarization Yunong Qi, Michael Flatt\'{e} The dynamics of electric-field-driven packets of spin polarized~ carriers passing through an n+/n semiconductor junction have been~ studied. We find that spin packets that are initially very weakly~ spin polarized can have their polarization significantly amplified~ within the junction. The nonlinear spin transport and amplification~ is due to a spin-polarization dependent mobility originating from the Pauli exclusion principle that also~ gives rise to the spin Gunn effect. The spin polarization~ amplification we describe here is driven by the inhomogeneous doping~ density, and thus does not require the large applied electric field~ of the spin Gunn effect. Hence it may apply to situations at low~ electric fields and low temperatures in which spontaneous spin~ polarization has been seen experimentally. We further believe that~ these studies provide a direct way to study the spin-polarization~ dependent mobility in semiconductors. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A19.00008: Enhancement and suppression of spin density polarization due to inhomogeneous electric fields Dan Csontos, Sergio Ulloa We report on a theoretical and computational study of the spin polarization propagation in charge and spin inhomogeneous semiconductor structures. We use a self-consistent, semiclassical approach based on the Boltzmann transport equation to calculate the spin density imbalance, $\delta n_{s}$, defined as $\delta n_{s}=n_{\uparrow}-n_{\downarrow}$, and the spin density polarization, $P_{s}$, defined as the ratio $P_{s}=\delta n_{s}/n$, where $n$ is the total charge density, in the presence of inhomogeneous electric fields. We find that the spin-polarized transport can be drastically enhanced or suppressed by inhomogeneous electric fields, such as those arising at semiconductor interfaces. Furthermore, we find that the spin density imbalance, $\delta n_{s}$, and spin density polarization, $P_{s}$, have diametrally opposite dependence on doping concentrations and charge inhomogeneous distributions. This is in contrast to the common assumption in the literature that these two quantities essentially have the same spin relaxation lengths. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A19.00009: Spin polarization amplification within nonmagnetic semiconductors at room temperature Soon-Wook Jung, Hyun-Woo Lee We show theoretically that the spin polarization of current can be electrically controlled within nonmagnetic semiconductors by exploiting the fact the spin current, compared to the charge current, is weakly perturbed by electric driving forces. In particular, in a T-shaped current branching geometry made entirely of a nonmagnetic semiconductor, the spin polarization can be amplified to 100{\%} by tuning current branching ratios properly. The proposed amplification scheme does not use ferromagnets or magnetic field, and does not require low temperature operation, providing an efficient way to generate a highly spin polarized current in nonmagnetic semiconductors at room temperature. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A19.00010: Spin currents in the presence of non-uniform fields. Adnan Rebei A two dimensional electron gas with spin-orbit coupling and in the presence of an in-plane electric field give rise to a spin Hall current independent of the Rashba coupling. We show that this universality of the strength of the spin current survives even in the presence of non-uniform electric and magnetic fields. Gradients in magnetic fields are shown to couple charge and spin transport and hence provide a better way to detect spin currents. Our calculation parallels that of the derivation of the Chern-Simons effective action in a relativistic electron gas and hence avoids the use of the Kubo formula. The spin conductivity is also discussed in terms of spin accumulation in a non-uniformly magnetized square loop. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A19.00011: Magneto-Oscillations of Current-Induced Spin Polarization in 2DEG Maxim Vavilov We consider a disordered two-dimensional electron gas with spin-orbit coupling placed in a perpendicular magnetic field and calculate the magnitude and direction of the current-induced spin polarization. We find that in strong magnetic fields the polarization becomes an oscillatory function of the magnetic field and that the amplitude of these oscillations is parametrically larger than the polarization at zero magnetic field. We show that the enhanced amplitude of the polarization is a consequence of strong electron-hole asymmetry in a quantizing magnetic field. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A19.00012: Spontaneous Spin Polarization in Quantum Wires A. D. Klironomos, J. S. Meyer, K. A. Matveev A number of recent experiments report spin polarization in quantum wires in the absence of magnetic fields. These observations are in apparent contradiction with the Lieb-Mattis theorem, which forbids spontaneous spin polarization in one dimension. We show that sufficiently strong interactions between electrons induce deviations from the strictly one-dimensional geometry and indeed give rise to a ferromagnetic ground state in a certain range of electron densities. At higher densities, more complicated spin interactions lead to a possibly novel ground state. [Preview Abstract] |
Monday, March 13, 2006 10:48AM - 11:00AM |
A19.00013: Spin-polarized current created by quantum point contacts with spin-orbit interaction Mikio Eto We propose a new idea to create spin-polarized currents using quantum point contact (QPC) with spin-orbit interaction.\footnote{M.\ Eto, T.\ Hayashi, and Y.\ Kurotani, J.\ Phys.\ Soc.\ Jpn.\ {\bf 74}, 1934 (2005).} Neither magnetic fields nor magnetic materials are required. By numerical studies using Green function recursion method,\footnote{T.\ Ando, Phys.\ Rev.\ B {\bf 44}, 8017 (1991).} we show that (i) the conductance is quantized in units of $2e^2/h$ even in the presence of spin-orbit (SO) interaction, (ii) the current is spin-polarized in the transverse direction, and (iii) a spin polarization of more than 50\% can be realized with SO interaction strength in InGaAs heterostructures. The spin polarization stems from the transition between subbands of different spins during the transport through QPC. The spin-polarization ratio is determined by the adiabaticity of the transition, which is evaluated by the Landau-Zener theory. Since the two-terminal devices with QPC are easy to fabricate on semiconductors, they may be utilized for the spin injection in the spintronics. [Preview Abstract] |
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