Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T35: Focus Session: Spins in Semiconductors -Spin Hall Effect and Spin Currents |
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Sponsoring Units: GMAG DMP FIAP Chair: Michael Flatté, University of Iowa Room: E145 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T35.00001: An exact solution to the problem of spin edge states Samvel Badalyan, Vahram Grigoryan, Alex Matos Abiague We study the spin edge states, induced by the combined effect of spin-orbit interaction SOI and hard-wall confining potential, in a two-dimensional electron system, exposed to a perpendicular magnetic field. We find an exact solution of the problem and show that the spin-resolved edge states are separated in space. The SOI-generated rearrangement of the spectrum results in a peaked behavior of the net-spin current versus the Fermi energy. The predicted oscillations of the spin current with a period, determined by the SOI-renormalized cyclotron energy, can serve as an effective tool for controlling the spin motion in spintronic devices. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T35.00002: Effects of mechanical rotation on spin current Mamoru Matsuo, Jun'ichi Ieda, Sadamichi Maekawa In the frontier of spintronics, much attention is paid on the control of spin current. Due to the exciting progress of nanomechatrononics, the importance of mechanical manipulation of electron spin will increase. We discuss theoretically effects of mechanical rotation on spin current using generally covariant Dirac equation in the non-relativistic limit. Coupling between rotation and spin is represented by Levi-Civita connections and spinor connections. By taking the non-relativistic limit, we show these connections introduce SU(2) gauge potentials. We derive spin current and forces acting on electron spins in terms of the SU(2)xU(1) theory. The non-relativistic correspondences of the inertial effects, such as energy- momentum redshifts effects, Sagnac-type effect and spin-rotation coupling are discussed in the context of spintronics. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T35.00003: Electrical Measurement of the Direct Spin Hall Effect in Fe/GaAs Heterostructures E.S. Garlid, Q. Hu, M.K. Chan, C.J. Palmstr\O m, P.A. Crowell A handful of recent experiments, all of which have used optical techniques, have investigated the spin Hall effect in semiconductors. We report on an all-electrical measurement showing evidence of the spin Hall effect in epitaxial (100) Fe/GaAs heterostructures with a channel doping (Si) of $n =5\times 10^{16}$~cm$^{-3}$ and highly doped Schottky tunnel barriers ($n^+=5\times 10^{18}$~cm$^{-3}$). Multiple devices were fabricated on a single chip with the distance of the ferromagnetic (FM) electrodes from the channel edge varying between 2 and 10 $\mu$m. Devices were first characterized by performing non-local spin valve and Hanle measurements. Hall measurements were then performed with an unpolarized current flowing down the GaAs channel parallel to the magnetic easy axis of the FM electrodes. A magnetic field was applied along the FM hard axis to induce precession of the out-of-plane spins at the channel edges into the FM easy axis. We investigate the bias and temperature dependence of the resulting Hanle signal, and find that it is consistent with an extrinsic spin Hall effect. The data suggests that ionized impurity scattering is the dominant contribution to the spin Hall conductivity in these samples. Supported by ONR and NSF. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:42PM |
T35.00004: Electrically driven single spin resonance in double quantum dots Invited Speaker: We report on our recent progress in applying semiconductor quantum dots to quantum information processing with electron spin qubits. Mixing the electron's spin and charge degrees of freedom via a magnetic field gradient enables reasonably fast single spin rotations driven by electric fields. We generate sufficiently strong gradients on-chip using micron-size ferromagnets. Our method bypasses the need for localized and strong microwave magnetic fields, which in comparison are difficult to apply in quantum dots. In addition, micro-magnets facilitate the selective manipulation of electron spins. We demonstrate proper operation of our micro-magnet approach using GaAs double quantum dots where single spin resonances and coherent rotations are observed. Preliminary results on combining 1 and 2-qubits operations are also presented. The strong nuclear spin fluctuations in the GaAs lattice cause fast decoherence and limit the quality factor of electron spin qubits. On-going efforts to solve the decoherence problem use material free of nuclear spins (e.g. isotopically purified SiGe). Although the other leading electric coupling mechanism, the spin-orbit interaction, work well in GaAs, it might not be as efficient in these materials. Our micro-magnet method - which is applicable to any material - is therefore a crucial component for the further development of electron spin qubits in quantum dots. Moreover, the micro-magnet design we present has applications to other kind of spin qubits like paramagnetic defects in silicon. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T35.00005: Electrical detection of a hyperfine-induced spin-dependent Hall effect in ferromagnet-semiconductor heterostructures M.K. Chan, E.S. Garlid, P.A. Crowell, Q.O. Hu, C.J. Palmstr\O m We report electrical measurements of a spin-dependent Hall effect (SHE) in ferromagnet-semiconductor heterostructures. Steady state electron spin polarization is established in bulk n-GaAs by a forward biased Fe contact. We observe a Hanle effect in the Hall voltage measured across the spin-polarized region of the GaAs channel, consistent with spin dephasing. This signal changes sign under Fe contact magnetization reversal, indicating sensitivity to electron spin direction. The observed spin-dependent Hall signal is approximately two orders of magnitude larger than that expected from previous optical measurements of the SHE in n-GaAs, which was attributed to skew-scattering. This suggests that a different mechanism is active in our system. We demonstrate full suppression of the spin-dependent Hall signal by eliminating the nuclear spin polarization through a field cycling procedure. Additionally, while the electron spin accumulation, detected by a spin sensitive Fe contact, persists up to 200 K, the spin-dependent Hall signal is not observed above 120 K, in coincidence with the disappearance of the nuclear spin polarization due to delocalization of donor electrons. We conclude that the observed spin-dependent Hall signal is coupled to the nuclear spin polarization. NSF DMR 0804244. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T35.00006: Edge State Transport in a Quantum Spin Hall Insulator: Effects from Rashba Spin-Orbit Coupling Henrik Johannesson, Anders Str\"om, George I. Japaridze We analyze the dynamics of the helical edge modes of a quantum spin Hall insulator in the presence of a spatially non-uniform Rashba spin-orbit coupling. The Rashba coupling is found to open a scattering channel which causes localization of the edge modes when the electron-electron interaction or the spatial Rashba variation is sufficiently large. We discuss implications for experiments on edge state transport in HgTe quantum wells. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T35.00007: Ballistic spin dynamics in Rashba spin-orbit coupled systems Ryo Matsumoto, Shuichi Murakami We theoretically study a time evolution of a transient spin grating in a two-dimensional electron gas with a Rashba spin- orbit coupling in a ballistic regime. We calculate the out-of- plane spin Fourier components, which decays with a rapid oscillation. We investigate their lifetime and frequency and analyze their dependence on the grating vector $q$ and the spin- orbit coupling constant $\alpha$. The frequency is proportional to $q$ and $\alpha$, and we find that the lifetime of the spin polarization is proportional to the inverse square of $\alpha$ for the small $q$ limit. Finally we compare our calculation with the case for a diffusive regime. We show that the lifetime of the spin polarization is longer when the grating vector becomes smaller, which is in contrast with the diffusive case. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T35.00008: Thermoelectric effect by ``perfectly conducting'' edge current in the quantum spin Hall system Ryuji Takahashi, Shuichi Murakami We report thermoelectric transport in the quantum spin Hall (QSH) system. Because a two-dimensional QSH system has helical gapless edge states, which are stable against nonmagnetic impurities, it is expected that nonmagnetic impurities can suppress thermal conductivity without lowering ``perfectly conducting'' edge conductivity. We therefore calculate thermoelectric properties of the two-dimensional QSH system in narrow ribbon geometry and discuss how to optimize the figure of merit. We argue that inelastic scattering length of the edge states controls the thermoelectric properties. We show that the edge transport becomes dominant by lowering temperature, because of an increase of the inelastic scattering length. We also found that the contribution from edge and from the bulk compete each other. Correspondingly, by lowering temperature, the figure of merit will first decrease and then will increase again due to edge-state-dominant thermoelectric transport.\\[4pt] [1] R. Takahashi, S. Murakami, arXiv:0910.4827. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T35.00009: Intrinsic and extrinsic anomalous Hall effect as investigated by Lorenz number Yuki Shiomi, Yoshinori Onose, Yoshinori Tokura We have investigated thermal and electrical Hall conductivities ($\kappa _{xy} $, $\sigma _{xy} )$ for typical itinerant ferromagnets of Fe, Co, Ni, and their impurity-doped samples to examine the effect of scattering on the intrinsic (Berry-phase-induced) and the extrinsic (skew-scattering-induced) anomalous Hall current (AHC) in terms of the Lorenz number,$L_{xy} =\kappa _{xy} /\sigma _{xy} T$. The$L_{xy} $coincides with the free electron value ($L_0 )$ in the absence of inelastic scattering but it deviates from$L_0 $ if the Hall current is affected by the inelastic scattering. We showed that the Lorenz number for the intrinsic AHC is almost constant and coincides with$L_0 $ at finite temperature, which indicates the scattering-free nature of the intrinsic AHC. On the other hand, that for the extrinsic AHC steeply decreases from $L_0 $as temperature is increased from$T=0$. This clearly shows that the extrinsic AHC is certainly dependent on the scattering rate. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T35.00010: Electron-Magnon Scattering in Anomalous Hall Effect Shengyuan Yang, Hui Pan, Wong-Kong Tse, Qian Niu We study the role played by electron-magnon scattering in the anomalous Hall effect. We find that it has important contributions distinct from other scattering processes like impurities scattering and phonon scattering. As a demonstration, we calculate the Hall conductivity for a two dimensional Dirac model. The result indicates that as system control parameter varies, the competition between magnon scattering and other types of scattering changes the Hall conductivity drastically. In particular, the side jump contribution could acquire a strong temperature dependence. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T35.00011: ABSTRACT WITHDRAWN |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T35.00012: Three-dimensional topological insulators, superconductors and defects in the Dirac limit Pavan Hosur, Shinsei Ryu, Ashvin Vishwanath If the electronic energy dispersion in a system is Dirac-like at low energies, a number of proximate phases can be accessed by perturbations that open a gap in the spectrum. Using this approach in three-dimensions, we find, besides several well-known phases, what we call a Chiral Topological Insulator (cTI) and a Singlet Topological Superconductor (sTS). We construct explicit microscopic models on a simple cubic lattice for these phases. Also, a physical picture of constructing the cubic lattice by stacking weakly coupled layers makes the topological nature of the cTI and the sTS apparent. An analogous picture leads us to a model of the time-reversal-invariant $Z_2$ topological insulators on a simple cubic lattice. The Dirac model also allows a facile calculation of the quantum numbers of topological defects to reveal dualities between order parameters, and we find that a hedgehog in Neel order has quantum numbers corresponding to the cTI and sTS, and one in the cTI and the sTS carries spin 1/2. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T35.00013: Second-order optical susceptibility of a pure spin current Ren-Bao Liu, Jing Wang, Bang-Fen Zhu Spin currents are an essential element of spintronics. A pure spin current, formed by opposite spins moving in opposite directions with the same amplitude, bears neither net magnetization nor charge current and therefore is difficult to be detected by traditional electromagnetic induction method. While indirect measurement of a spin current via the inverse spin Hall effect or spin accumulation at stopping edges is possible, a fundamental question remains: How can a pure spin current be directly seen? By a systematic analysis of the peculiar symmetry properties of spin currents and microscopic calculations, we show that a pure spin current can have sizable second-order nonlinear optical susceptibility. This forms the basis of direct, non-destructive measurement of pure spin currents by standard nonlinear optical spectroscopy. [Preview Abstract] |
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