Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R19: Focus Session: Spin Hall Effect and Spin Transport |
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Sponsoring Units: GMAG DMP Chair: Allan MacDonald, University of Texas Room: Baltimore Convention Center 316 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R19.00001: Theory of Spin Hall Effect in GaAs Invited Speaker: In the spin Hall effect, an electric current in a system with spin-orbit coupling induces a transverse spin current which leads to non-equilibrium spin accumulation near sample boundaries. Generating and manipulating non-equilibrium spin magnetization by electric fields is one of the most desirable goals of semiconductor spintronics, because electric fields have potentialities for accessing individual spins at nanometer scales. In this talk, I review the different spin-orbit coupling mechanisms in direct gap semiconductors and the implications of these mechanisms for the spin Hall effect. In particular, we recently developed a theory that accounts for spin-orbit coupling at charged impurities. This coupling leads to \textit{extrinsic} spin currents that contain skew scattering and side jump contribution\ [1]. Applying our theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with recent experiments by Kato et al.\ [2]. Also, such contributions are important for p-doped GaAs. Furthermore, we analyzed the effect of \textit{intrinsic} spin-orbit coupling in the presence of anisotropic impurity scattering, and found that, somewhat surprisingly, an electrical field can lead to a bulk magnetization component \textit{perpendicular} to both the spin-orbit field and an external magnetic field. These works have been done in collaboration with B.I. Halperin, E.I. Rashba, and A.A. Burkov. \\{} [1] H.-A. Engel, B.I. Halperin, and E.I. Rashba, Phys.\ Rev.Lett. \textbf{95}, 166605 (2005). \\{} [2] Y.K. Kato, R.C. Myers, A.C. Gossard, and D.D. Awschalom, Science \textbf{306}, 1910 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R19.00002: Spin Hall Current Induced Edge-Spin Accumulation in Two-Dimensional Electron and Hole Systems Kentaro Nomura, Jairo Sinova, Nikolai Sinitsyn, Tomas Jungwirth, Joerg Wunderlich, Bernd Kaetsner, Allan MacDonald In spintronic devices, spin densities have traditionally been generated by external magnetic fields, circularly polarized light sources, or by spin injection from ferromagnets. Recently there has been considerable interest in a new strategy in which edge spin densities are generated electrically via the spin Hall effect. We have performed numerical studies on spin transport in two-dimensional systems with various spin-orbit interactions including both intrinsic and extrinsic effects. We find that the spin Hall current strongly depends on the character of the spin-orbit interactions. We address the relation between bulk spin currents and edge spin accumulations, and compare our results with recent experimental observations. K. Nomura, J. Sinova, N. A. Sinitsyn, A. H. MacDonald, Phys. Rev. B 72 165316 (2005). K. Nomura, J. Wunderlich, J. Sinova, B. Kaetsner, A. H. MacDonald, T. Jungwirth, to appear in Phys. Rev. B 72. J. Wunderlich, B. Kaetsner, J. Sinova, T. Jungwirth, Phys. Rev. Lett. 94, 047204 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R19.00003: Spin Hall effect in p-type semiconductors in magnetic fields Mehdi Zarea, Sergio Ulloa We calculate the spin Hall conductivity driven by Rashba spin-orbit interaction in $p$-type two-dimensional semiconductors in the presence of a perpendicular magnetic field. For a highly confined quantum well the eigenstates and eigenvalues of the system, described by a $k$-cubic Rashba term for heavy holes, can be described by Landau spinor states, as in the $k$-linear case [1]. The contribution of the interband transitions to the Kubo-Greenwood formula gives the density-dependent {\it conventional} spin Hall conductivity, which approaches its universal value $\sigma^z_{xy}=9e/8\pi$ for weak spin-orbit coupling and low Fermi energies, in agreement with previous work. However, two intraband contribution terms cancel this effect, resulting in {\it zero} conventional spin Hall conductivity. Adding the torque dipole contribution to the definition of spin current, we also study the {\it effective} spin conductivity. This is shown to be proportional to the total magnetization plus surface terms which exactly cancel it for small spin-orbit coupling. The fact that both effective and conventional spin Hall conductivities vanish is unexpected, especially as one expects the intraband transitions to evolve into vertex corrections for low magnetic fields. Supported by NSF-NIRT. [1] M. Zarea and S. E. Ulloa Phys. Rev. B 72, 085342 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R19.00004: Non-linear charge and spin Hall effect caused by insulating or charged disks in a two dimensional electron gas David Schmeltzer, Hsuan-Yeh Chang We show that a time depend electric field in the presence of static vortices caused by insulating regions induces a non-linear time average Hall effect without breaking time reversal symmetry. A time dependent electric field will induce a time dependent Lorentz force in the presence of static vortices of size $D$ giving rise to a Hall voltage in a two dimensional electronic gas. The origin of the vortices are insulating regions which cause the electronic wave function to vanish. This theory can explain the recent experiment in a two dimensional GaAs/AlGaAs dot with insulating disks. In this experiment, a driving microwave field in the y-direction has been applied and a DC voltage has been measured in the x-direction. When the insulating disks are replaced by charged disks, we suggest that the Aharonov-Casher effect will induce a spin-Hall effect. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R19.00005: The Helical Liquid and the Edge of Quantum Spin Hall Systems Congjun Wu, B. Andrei Bernevig, Shou-Cheng Zhang The edge states of the recently proposed quantum spin Hall systems constitute a new symmetry class of one-dimensional liquids dubbed the ``helical liquid,'' where the spin orientation is determined by the direction of electron motion. We prove a no-go theorem which states that a helical liquid with an odd number of components cannot be constructed in a purely 1D lattice system. In a helical liquid with an odd number of components, a uniform gap in the ground state can only appear when the time-reversal (TR) symmetry is spontaneously broken by interactions. On the other hand, a correlated two-particle backscattering term by an impurity can become relevant while keeping the TR invariance. We further study the Kondo effect in such a liquid which exhibits new features in the structure of the screening cloud. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R19.00006: ``Spin-orbit" susceptibility in the quantum spin Hall systems Shuichi Murakami There are two classes of insulators showing the spin Hall effect. One is a spin Hall insulator such as PbTe while the other is a quantum spin Hall system. They are distinguished by an absence or presence of edge states. To study such insulators showing the spin Hall effect, we construct a spin analog of the St\v{r}eda formula. We use the conserved spin current as proposed by Zhang et al.[cond-mat/0503505], thereby the resulting St\v{r}eda formula becomes quite simple (i.e. without any $\dot{{\bf s}}$ terms). As a result, the spin Hall conductivity for band insulators is proportional to a ``spin- orbit'' susceptibility, representing a response of the orbital magnetization to the Zeeman field (or equivalently a response of the spin magnetiation to the orbital magnetic field). We apply the result to real systems such as Bi$_{1-x}$Sb$_{x}$, because in insulating Bi$_{1-x}$Sb$_{x}$ the diamagnetic susceptibility is largely enhanced due to the spin-orbit coupling. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R19.00007: Stability of the quantum spin Hall effect with disorder and interactions Cenke Xu, Joel Moore The stability to interactions and disorder of the quantum spin Hall effect (QSHE) proposed for time-reversal-invariant 2D systems is discussed. The QSHE requires an energy gap in the bulk and gapless edge modes that conduct spin-up and spin-down excitations in opposite directions. When the number of Kramers pairs of edge modes is odd, certain one-particle scattering processes are forbidden due to a topological $Z_2$ index. However, two particle scattering processes can localize the edge modes and destroy the QSHE: the region of stability for both classes of models (even or odd number of Kramers pairs) is obtained explicitly. For a single Kramers pair the QSHE is stable to weak interactions and disorder, while for two Kramers pairs it is not; however, the two-pair case can be stabilized by {\it either} finite attractive or repulsive interactions. We also discuss a mechanism to generate 1D localized states in magnetic semiconductors. These states are localized by the domain wall between two opposite ferromagnetic orderings. The total number of these localized states can be expressed in terms of a real space topological number. The existence of these localized states can result in a quantum spin Hall effect as well as a quantum charge Hall effect. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R19.00008: Mesoscopic Spin Hall effect Philippe Jacquod, Inanc Adagideli, Jens Bardarson, Carlo Beenakker We focus on the mesoscopic spin Hall effect in ballistic microstructures. Using an extension of Landauer-Buttiker formalism, we calculate the average and variance of spin Hall conductance using the semiclassical approximation as well as Random Matrix theory. We compare these calculations to numerical simulations. We show in particular that the diagonal contribution corresponding to the classical (spin)conductance vanishes. However, interference corrections lead to a finite spin-Hall conductance. This conductance depends crucially on whether the system is regular or chaotic. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R19.00009: The pseudospin degree of freedom and dynamics in degenerate bands Qian Niu, Dimitrie Culcer Carriers in degenerate bands possess an additional degree of freedom, which may be regarded as a pseudospin. In a wave- packet picture, along with the center of mass motion in crystal momentum and real space, one must also take into account the probability amplitudes, which characterize the inter-band dynamics. The additional degree of freedom introduces nontrivial non-Abelian corrections to the carrier dynamics. These corrections include non-Abelian Berry curvature terms and a non-Abelian correction to the group velocity. For the four- band Luttinger model we will demonstrate that, under the action of an electric field, coherent wave-packet evolution leads to separation of up and down spins, discussing the role of the pseudospin in this process. We find that the helicity in the four band model can be expressed in terms of the pseudospin and is not conserved in an electric field. In addition, we will discuss the role of the pseudospin in determining the Landau levels in the Luttinger Hamiltonian by semiclassical quantization. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R19.00010: Transport Equations and Spin-Charge Propagating Mode in a Two Dimensional Hole Gas with Spin-Orbit Coupling Taylor Hughes, Andrei Bernevig, Yaroslaw Bazaliy, Shou-Cheng Zhang We find that the spin-charge motion in a strongly confined two-dimensional hole gas (2DHG) supports a propagating mode with cubic dispersion apart from the diffusive mode due to momentum scattering. Propagating modes seem to be a generic property of systems with spin-orbit coupling. Through a rigorous Keldysh approach, we obtain the transport equations for any system with spin-orbit coupling that can be represented using spin- 1/2 matrices. We specialize to the 2DHG and analyze the behavior of the hole spin relaxation time, diffusion coefficients, and spin-charge coupled motion. We also confirm the value of the spin Hall conductivity in the ballistic regime and discuss the viability of the propagating mode. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R19.00011: Spin-orbit coupling and Zeeman splitting of holes in GaAs nanostructures grown in [113] direction Taisuke Minagawa, Yuli Lyanda-Geller We have calculated the anisotropic spectrum, g-factor and spin- orbit interactions for hole carriers in GaAs quantum wells grown in direction [113], and quantum wires and quantum dots confined to such quantum wells. We have developed a simple analytical approach for calculations of spectra of holes confined to nanostructures within the Luttinger Hamiltonian scheme, which allows us to compute the wave functions and energies without invoking admixtures of ``light'' and ``heavy'' holes perturbatively. We have identified the dominant spin-orbit interaction terms and discuss their effects on dynamics of holes in external magnetic and electric fields. We have also calculated spin relaxation and dephasing times for 2D holes quantized along [113] direction. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R19.00012: Spin-orbit induced spin precession in 2D hole systems Dimitrie Culcer, Christian Lechner, Roland Winkler It is well known that two-dimensional hole systems are characterized by an effective spin 3/2. Recently it was shown that the spin density matrix for such spin-3/2 systems can be decomposed into a sequence of multipoles which has important contributions from higher-order multipoles beyond the ones known for electron systems [1]. We will show that these multipoles can precess even in the absence of a magnetic field if strong spin-orbit interactions are present. Particularly simple and enlightening expressions for the spin polarization and higher order multipoles emerge in certain geometries. We will discuss an experimental setup in which this precession can be observed and investigate promising spintronics applications. [1] R. Winkler, Phys. Rev. B 70, 125301 (2004) [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R19.00013: A Luttinger Hamiltonian is not enough M.C. Chang, C.P. Chuu, Q. Niu In the study of spintronics, it is important to understand carrier transport in multiple energy bands, such as in the conduction or valence bands in semiconductors. Such bands are often described by effective Luttinger Hamiltonians, derived from k.p perturbation and symmetry considerations. A closely related case is the Pauli Hamiltonian, which suppose to give an effective description for the upper Dirac bands in the non-relativistic limits. Here we show that such an effective Hamiltonian alone is not really sufficient. Various Berry phase type corrections must be supplemented to give proper description of electron charge and spin dynamics. [Preview Abstract] |
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