Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B15: Focus Session: Spins in Semiconductors - Spin Currents I |
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Sponsoring Units: DMP GMAG FIAP Chair: David Awschalom, University of California, Santa Barbara Room: D171 |
Monday, March 21, 2011 11:15AM - 11:51AM |
B15.00001: Optical effects of spin currents in semiconductors Invited Speaker: BANG-FEN ZHU, Department of Physics and Institute of Advanced Study, Tsinghua University, REN-BAO LIU, Department of Physics, The Chinese University of Hong Kong -- We predict the linear and second-order nonlinear optical effects of spin currents in semiconductors, based on systematic symmetry analysis and microscopic calculations with realistic models [1, 2]. By an analogue to the Ampere effect and Oersted effect, we conceived and verified that a spin current can be coupled to a ``photon spin curren'' carried by a polarized light beam, which causes sizeable Faraday rotation without involving net magnetization. Furthermore, a spin current can have a strong second-order nonlinear optical effect with unique polarization-dependence due to the special symmetry properties of the spin current. In particular, for a longitudinal spin current, in which the spins point parallel or anti-parallel to the current direction is a chiral quantity, a chiral sum-frequency effect will be induced. The second-order optical effects of spin currents have been experimentally verified immediately after the theoretical prediction [3]. These discoveries represent new phenomena in magneto-optics, with potential spin-photonic applications. They bring new opportunities to research of spintronics and may also facilitate research of topological insulators where the edge states form pure spin currents. References: \\[4pt] [1] J. Wang, B. F. Zhu, and R. B. Liu, \textit{Phys. Rev. Lett.} 100, 086603 (2008); see also Erratum, \textit{ibid }101, 069902 (2008) \\[0pt] [2] J. Wang, B. F. Zhu, and R. B. Liu, \textit{Phys. Rev. Lett.} 104, 256601 (2008). \\[0pt] [3] L. K. Werake and H. Zhao, \textit{Nature Physics} 6, 875 (2010). [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B15.00002: Magnetoelectric Photocurrent Generated by Direct Interband Transitions in InGaAs/InAlAs Two-Dimensional Electron Gas Junfeng Dai, Hai-Zhou Lu, Chunlei Yang, Shun-Qing Shen, Fu-Chun Zhang, Xiaodong Cui We report the observation of magnetoelectric photocurrent generated via direct interband transitions in an InGaAs/InAlAs two-dimensional electron gas by a linearly polarized incident light. The electric current is proportional to the in-plane magnetic field, which unbalances the velocities of the photoexcited carriers with opposite spins and consequently generates the electric current from a hidden spin photocurrent. The spin photocurrent can be evaluated from the measured electric current, and the conversion coefficient of spin photocurrent to electric current is self-consistently estimated to be 10$^{-3}$--10$^{-2}$ per Tesla. The observed light-polarization dependence of the electric current is well explained by a theoretical model which reveals the wave vector angle dependence of the photoexcited carrier density. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B15.00003: Anisotropic conductivity caused by spin-orbit interactions David H. Berman, Michael E. Flatt{\'e} Free propagation in a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit coupling shows strong anisotropy depending on the ratio of the coupling strength to the Fermi energy and on the ratio of the strengths of the Rashba and Dresselhaus interactions [1]. This spin-orbit induced anisotropy appears also in the local density of states near impurities. In addition the non-local conductivity, $\sigma_{i,j}({\bf r}, {\bf r'})$, computed in the absence of impurities is anisotropic. This is in contrast to the macroscopic conductivity in the presence of impurities which shows no anisotropy when only ladder diagrams are considered [2]. In all these instances, the degree of anisotropy can be controlled by application of electric fields perpendicular to the 2DEG. \\[4pt] [1] D. H. Berman and M. E. Flatt{\'e}, PRL {\bf 105}, 157202 (2010).\\[0pt] [2] O. Chalaev and D.Loss, Phys. Rev. B {\bf 71}, 245318 (2005). [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B15.00004: Drift and diffusion of spin and charge density waves in a two-dimensional electron gas Luyi Yang, J.D. Koralek, J. Orenstein, D.R. Tibbetts, J.L. Reno, M.P. Lilly We use transient grating spectroscopy (TGS) to study the persistent spin helix (PSH) state and electron-hole density wave (EHDW) in a 2D electron gas in the presence of an in-plane electric field parallel to the wavevector of the PSH or EHDW. By directly measuring the phase, we can measure the PSH and EHDW displacement with 10 nm spatial and sub-picosecond time resolution. We obtain both the spin diffusion and mobility and ambipolar diffusion and mobility from the TGS measurements of PSH and EHDW, respectively. The spin transresistivity extracted from the spin diffusion is in excellent agreement with the RPA theory of spin Coulomb drag (SCD). The spin mobility data indicate that SCD may also play a role in the spin wave drifting process. From the ambipolar diffusion and mobility, we obtain the transresistivity of electrons and holes in the same layer, which is much stronger than is typically seen in the conventional Coulomb drag experiments on coupled quantum wells. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B15.00005: Radial spin helix in two-dimensional electron systems with Rashba spin-orbit coupling Valeriy Slipko, Yuriy Pershin We suggest a new long-lived spin-polarization structure, a radial spin helix [1], and study its relaxation dynamics. For this purpose, starting with a system of equations for spin-polarization density, we find its general solution in the axially symmetric case. It is demonstrated that the radial spin helix of a certain period relaxes slower than homogeneous spin polarization and plain spin helix [2]. Importantly, the spin polarization at the center of the radial spin helix stays almost unchanged at short times. At longer times, when the initial nonexponential relaxation region ends, the relaxation of the radial spin helix occurs with the same time constant as that describing the relaxation of the plain spin helix. Experimentally, such a structure can be created using spin injection or extraction in a system with cylindrical electrodes or, possibly, by a modified spin gratings technique. \\[4pt] [1] Y. V. Pershin and V. A. Slipko, Phys. Rev. B 82, 125325 (2010).\\[0pt] [2] Y. V. Pershin, Phys. Rev. B 71, 155317 (2005). [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B15.00006: Anholonomic spin manipulation in drift transport in semiconductors Ben J. Moehlmann, Michael E. Flatt\'e We find that the electronic spin rotation induced by drift transport around a closed path in a wide variety of nonmagnetic semiconductors at zero magnetic field depends solely on the physical path taken. Physical paths that produce any possible spin rotation due to transport around a closed path are constructed for electrons experiencing strain or electric fields in (001), (110), or (111)-grown zincblende semiconductor quantum wells. Spin decoherence due to travel along the path is negligible compared to the background spin decoherence rate. The small size of the designed paths ($< 100$~nm scale in GaAs) may lead to applications in nanoscale spintronic circuits. This work was supported by an ONR MURI.\footnote{B. J. Moehlmann and M. E. Flatt\'e, arXiv:1007.0909} [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B15.00007: Mapping Spin-Orbit Splitting in Strained (In,Ga)As Epilayers B.M. Norman, C.J. Trowbridge, V. Sih, J. Stephens, A.C. Gossard, D.D. Awschalom Time-resolved and spatially resolved Faraday rotation spectroscopy is used to measure the magnitude and direction of the momentum-dependent spin splitting in strained InGaAs epilayers. The epilayers are lattice-matched to the GaAs substrate and designed to reduce inhomogeneous effects related to strain relaxation. Measurements of momentum-dependent spin splitting as a function of electron spin drift velocity along [100], [010], [110] and [1$\overline{1}$0] directions enable separation of isotropic and anisotropic effective magnetic fields that arise from uniaxial and biaxial strain along $\langle$110$\rangle$. Such electrically induced effective magnetic fields can be used for spin generation and manipulation in spintronics devices. We find that anisotropic and isotropic strain-induced effective magnetic fields are comparable in magnitude.~\footnote{B. M. Norman, C. J. Trowbridge, J. Stevens, A. C. Gossard, D. D. Awschalom, and V. Sih, Phys. Rev. B. {\bf 82}, 081304(R) (2010).} [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B15.00008: Spin-orbit interaction from low-symmetry localized defects in semiconductors Oleg Chalaev, G. Vignale, Michael Flatt\'e The presence of low-symmetry impurities or defect complexes in the zinc-blende direct-gap semiconductors (e.g. interstitials, Jahn-Teller distortions) results in a novel spin-orbit term in the effective Hamiltonian for the conduction band. The new spin-orbit interaction is proportional to the matrix element of the defect potential between the conduction and the valence bands. Because this interaction arises already in the first order of the expansion of the effective Hamiltonian in powers of $\Delta/E_g\ll1$ (where~$\Delta$ is the valence band spin-orbit splitting, and $E_g$ is the band gap), its contribution to the spin relaxation time may exceed that of previously studied contributions, such as the Rashba term, even for moderate concentrations of low-symmetry impurities. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B15.00009: Modeling of diffusion of injected electron spins in spin-orbit coupled microchannels Liviu P. Z\^arbo, Jairo Sinova, I. Knezevic, J. Wunderlich, T. Jungwirth Understanding of the collective electron spin dynamics under the influence of spin-orbit fields is a key requirement in the quest for all-electrical semiconductor spintronic devices. We investigate the spin dynamics of an ensemble of spin polarized electrons injected in the diffusive microchannel of a model device with linear Rashba and Dresselhaus spin-orbit coupling. Using a spin-dependent ensemble Monte Carlo method, we analyze the steady state spin density patterns dependence on channel dimension and orientation, spin-orbit coupling strengths and external magnetic fields. We show that in the persistent spin helix regime, the spin density patterns depend only on the system geometry and channel orientation. Magnetic fields of order of tesla are required to affect spin dynamics in the persistent spin helix regime. Our simulation results [PRB 82, 205320 (2010)] have been used to help understand the spin diffusion in the channel of the recently demonstrated spin Hall effect transistor [arXiv:1008.2844]. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B15.00010: Unitarity of scattering and edge spin accumulation in a ballistic and quasiballistic regimes Alexander Khaetskii, Eugene Sukhorukov We consider a 2D ballistic structure with spin-orbit-related splitting of the electron spectrum. We calculated the edge spin density which appears in the presence of a charge current through the structure. Combined effect of the boundary scattering and spin precession leads to oscillations of the edge polarization. The problem is solved with the use of the method of scattering states. We clarified the important role of the unitarity of scattering for the problem of edge spin accumulation. For Rashba Hamiltonian, which is linear in momentum, and in the case of a straight boundary it leads to exact cancellation of long-wave oscillations of the spin density with a period order of spin precession length. However, this appears to be rather exceptional case. In general, the smooth spin oscillations recover, as it happens, e.g., for the wiggly boundary. For qubic Hamiltonian (2D holes) the unitarity scattering conditions are different, as a result, even in the case of a straight boundary the cancellation of the smooth oscillations in spin density does not occur. Similar problem is considered for the case when the sample size is large compared to the mean free path which in its turn is much larger than the spin precession length. For example, for the cubic Hamiltonian the ``edge'' contribution to the spin density can be larger than the ``bulk'' one which appears as a result of the spin flux from the bulk. This demands the reinterpretation of the experimental results [1]. [1]. J. Wunderlich et al., PRL 94, 047204 (2005). [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B15.00011: Visualizing zitterbewegung in spin-orbit coupled semiconductor quantum wires Markku Jaaskelainen, Ulrich Zulicke We study a spin-orbit coupled parabolic quantum wire in the ballistic regime and develop a method for visualizing zitterbewegung in phase space. We introduce a Husimi distribution on the transverse coordinate and calculate the distribution of spin by a decomposition into Bloch vector components. The Husimi distribution corresponds to the simultaneous, unsharp measurement of the transverse position and velocity in accordance with the uncertainty principle. In phase space, the distribution exhibits a combination of spin precession and coherent oscillation along the longitudinal coordinate, i.e. zitterbewegung. This behavior closely matches the semiclassical dynamics for small values of the spin-orbit coupling. For increasing spin-orbit coupling strength, the oscillation amplitude initially increases, whereas for very large values the oscillation amplitude is quenched. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B15.00012: Theory of carrier dynamics in InSb parabolic quantum wells D. Saha, G.D. Sanders, C.J. Stanton, M. Bhowmick, T. Merritt, G.A. Khodaparast, T.D. Mishima, M.B. Santos InSb, with the narrowest gap among the III-V compound semiconductors, shows considerable promise as a quantum well material because its small conduction-band mass gives it a large room temperature electron mobility, and its large g-factor makes it attractive for spintronic devices. We present experiments and theoretical calculations for carrier dynamics in a strained 50-nm thick InSb/AlInSb parabolic quantum well. Our calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the parabolic confinement potential as well as pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with experiments. We model one and two color, time-resolved pump-probe differential transmission and reflectivity experiments. The change in the infrared probe pulse as a function of delay time provides information on carrier and spin relaxation dynamics. Both interband and intra-band dynamics are studied. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B15.00013: Imaging spin transport in a semiconductor with an iron-filled carbon nanotube Andrew Berger, Vidya Bhallamudi, Dominic Labanowski, Palash Banerjee, Camelia Marginean, Denis Pelekhov, David Stroud, P. Chris Hammel, Kathy McCreary, Roland Kawakami, Franziska Wolny, Thomas Muehl There has been much recent progress in the field of spintronic device fabrication, creating a need for characterization tools. We are developing a low-temperature scanned probe microscope with the ability to position, with high precision, a magnetized iron-filled carbon nanotube above a spin-injected semiconductor device [1]. The inhomogeneous field of this unique magnetic probe will be experienced by spins in the sample. We have developed a technique for simulating the effects of such an inhomogeneous field [2]. Crucially, we find that our scanned probe technique can create highly localized spin density features on a length scale comparable to the nanotube diameter. This will allow for spatial mapping of the spin density with high resolution -- a capability not possible in current electrical detection schemes. Such experiments may provide information about interface effects, scattering, and material properties which influence spin behavior. \\[4pt] [1] F. Wolny, et al. J. Appl. Phys. 104, 064908 (2008)\\[0pt] [2] V. Bhallamudi, et al. arXiv:1010.3747v1 [cond-mat.mes-hall] [Preview Abstract] |
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