Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D10: Focus Session: Spin Transport and Dynamics |
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Sponsoring Units: DMP GMAG Chair: David Awschalom, University of California-Santa Barbara Room: LACC 153B |
Monday, March 21, 2005 2:30PM - 3:06PM |
D10.00001: Current-Induced Polarization and the Spin-Hall effect in Semiconductors Invited Speaker: As a consequence of relativity, an electric field transforms into a magnetic field in the frame of a moving electron, and influences the spin of the electron. This is known as spin-orbit coupling, and it gives rise to interesting spin phenomena in non-magnetic semiconductors. Using Faraday and Kerr rotation spectroscopies with temporal and spatial resolution, we observe two such phenomena in III-V semiconductors: current-induced spin polarization\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{93}, 176601 (2004).} and the spin Hall effect\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, \textit{Science}, 11 November 2004 (10.1126/science.1105514) [http://dx.doi.org/10.1126/science.1105514].}. Strain-induced spin-orbit coupling gives rise to an internal magnetic field\footnote{Y. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, \textit{Nature} \textbf{427}, 50 (2004).}, which can be used to electrically polarize the spins, offering a pathway to electrically generate spin polarization within non-magnetic semiconductors. More recently, we have observed the spin Hall effect, which refers to an appearance of a pure spin current transverse to an applied electric field in the absence of applied magnetic fields. The spin Hall effect results in accumulation of spins at the edges of a sample, similar to charge accumulation in the conventional Hall effect. Such polarization is detected and imaged using Kerr rotation microscopy in both unstrained and strained samples. The polarization is out-of-plane and has opposite sign for the two edges, consistent with the predictions of the spin Hall effect. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D10.00002: Mechanical Control of Spin Coherence in Semiconductors H. Knotz, A.W. Holleitner, J. Stephens, R.C. Myers, D.D. Awschalom The in-plane strain in thin GaAs epilayers is probed using time-resolved Kerr rotation (TRKR), photoluminescence, and optically-detected nuclear magnetic resonance (ODNMR) spectroscopies. Strain is introduced into n-type GaAs epilayers using a novel mechanical deformation technique that allows tunable control of the strain levels in the epilayer. Strain induced shifts in the electron g-factor and photoluminescence peaks are observed and characterized from T=5K to room temperature. The magnitude of the in-plane strain can be determined from a control sample set of non strain-relaxed epilayers with in-plane strain determined with x-ray diffraction. Our results show promise for mechanical control of electron and nuclear spin in semiconductor nanostructures. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D10.00003: Imaging Electron Spin Flows in Semiconductors in the Presence of Electric, Magnetic, and Strain Fields Darryl Smith, Scott Crooker Using methods for scanning Kerr microscopy, we directly acquire two-dimensional images of spin-polarized electrons flowing laterally in bulk epilayers of n:GaAs [1]. Optical injection provides a local dc source of polarized electrons, whose subsequent drift and/or diffusion is controlled with electric, magnetic, and -in particular- strain fields. The acquired spatial maps directly reveal how flows of polarized electron spins respond to these three fields when applied individually or in combination. Spin precession induced by controlled uniaxial stress along the $\langle$110$\rangle$ axes demonstrates the direct $|\vec{k}|$-linear spin-orbit coupling of electron spin to the shear (off-diagonal) components of the strain tensor $\epsilon$, through a Hamiltonian of the form $c_3 (\sigma_x \epsilon_{xy}k_y - \sigma_y \epsilon_{yx}k_x)$. The $|\vec{k}|$- linear nature of the strain-induced ``internal" magnetic field ($\vec{B}_\epsilon$) leads to a spatial coherence of dc spin flows that is preserved over much greater distances as compared with spin flows manipulated by external magnetic fields. $\vec{B}_\epsilon$ is shown to be in-plane and orthogonal to $\vec{k}$, and therefore chiral for radially- diffusing spins. Lastly, the spatial period of strain-induced spin precession is explicitly shown to be independent of applied electrical bias, of possible benefit to future spintronic devices. [1] S. A. Crooker and D. L. Smith, cond-mat/0411461 [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D10.00004: Spin transfer and coherence in coupled quantum wells M. Poggio, G.M. Steeves, R.C. Myers, N.P. Stern, A.C. Gossard, D.D. Awschalom The possibility of developing spin-based electronic devices has focused recent interest on the study of carrier spin dynamics in semiconductor nanostructures. The accessibility of various excitonic states with the application of an external electric field make coupled quantum well (CQW) systems attractive for the study of both individual carrier and exciton spin dynamics. Spin dynamics of optically excited electrons confined in asymmetric Al$_{.33}$Ga$_{.67}$As/GaAs coupled quantum wells are investigated through time-resolved Faraday rotation experiments. The inter-well coupling is shown to depend on applied electric field and barrier thickness. We observe three coupling regimes: independent spin precession in isolated quantum wells, incoherent spin transfer between single-well states, and coherent spin transfer in a highly coupled system. Relative values of the inter-well tunneling time $\tau$, the inhomogeneous transverse electron spin lifetime $T_2^{\ast}$, and the Larmor precession period $ 1 / \nu_L$ appear to govern this behavior \footnote{M. Poggio, G. M. Steeves, R. C. Myers, N. P. Stern, A. C. Gossard, and D. D. Awschalom, Phys. Rev. B 70, 121305(R) (2004)}. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D10.00005: Spintronics without magnets: spin-optics. Maxim Khodas, Arcadi Shekhter, Alexander Finkel'stein We describe how to spin-polarize electrons in a two-dimensional semiconductor heterostructure with a field-effect gate control of the spin-orbit interaction. In the suggested scheme, a beam of electrons splits in two spin-polarized components propagating at different angles. The phenomenon is similar to the birefringence of light in crystal optics. We outline possible devices based on the spin-dependent refraction of the current carriers, including spin filter and spin switch. The proposed program aims to solve the goals of spintronics by using only nonmagnetic semiconductors. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D10.00006: Semiclassical Electron Transport in case of Spin-Orbit Interaction: a Way
to Spin-Polarize the Current Peter Silvestrov, E.G. Mishchenko, C.W.J. Beenakker Semiclassical solutions of a 2-dimensional Schr\"{o}dinger equation with the Rashba spin-orbit interaction and a smooth potential are considered. We argue that the electron motion in the semiclassical approximation follows the evolution of one of the spin-orbit-split energy subbands. Within a given subband the in-plane electron spin orientation is automatically adjusted to the momentum orientation. This allows to generate spin-polarized currents with the help of a Quantum Point Contact open for transmission only in a single subband. Out of plane spin polarization, proportional to the electric field, appears as a quantum correction. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D10.00007: Generation of spin currents via spin-flip Raman scattering Ali Najmaie, E. Ya. Sherman, J. E. Sipe We theoretically show that stimulated Raman scattering can be used to inject pure spin currents in doped non-centrosymmetric semiconductors. In the case of bulk n-doped GaAs, the spin current is caused by spin-flip Raman processes on electrons with opposite momenta and spins. The components of the injected spin current depend on the propagation direction and the polarization of the fields used in the Raman process. The estimated magnitude of the spin current shows that it can be detected experimentally. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D10.00008: Electron Spin Optical Orientation in Charged Quantum Dots A. Shabaev, Al. L. Efros, A.S. Bracker, E.A. Stinaff, D. Gammon, M.E. Ware, J.G. Tischler, D. Park, D. Gershoni, V.L. Korenev, I.A. Merkulov We present a theory of nonresonant optical orientation of electron spins localized in quantum dots. This theory explains the negative circularly polarized photoluminescence of singlet trions localized in quantum dots previously observed in experiments where trion polarization changed to negative with time and where the degree of the negative polarization increased with intensity of pumping light. We have shown that this effect can be explained by the accumulation of dark excitons that occurs due to the spin blocking of the singlet trion formation - the major mechanism of dark exciton recombination. The accumulation of dark excitons results from a lack of electrons with a spin matching the exciton polarization. The electron spin lifetime is shortened by a transverse magnetic field or a temperature increase. This takes the block off the dark exciton recombination and restores the positive degree of trion polarization. The presented theory gives good agreement with experimental data. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D10.00009: Optical pumping of electron and nuclear spin in a negatively-charged quantum dot Allan Bracker, Eric Stinaff, Morgan Ware, Dan Gammon, Andrew Shabaev, Joseph Tischler, Alexander Efros, David Gershoni, Vladimir Korenev, Igor Merkulov We report optical pumping of electron and nuclear spins in an individual negatively-charged quantum dot. With a bias-controlled heterostructure, we inject one electron into the quantum dot. Intense laser excitation produces negative photoluminescence polarization, which is easily erased by the Hanle effect, demonstrating optical pumping of a long-lived resident electron. The electron spin lifetime is consistent with the influence of nuclear spin fluctuations. Measuring the Overhauser effect in high magnetic fields, we observe a high degree of nuclear spin polarization, which is closely correlated to electron spin pumping. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D10.00010: Dynamical nuclear polarization and nuclear magnetic fields in semiconductor nanostructures Ionel Tifrea, Michael E. Flatt\'e We investigate the dynamical nuclear polarization effect due to the hyperfine interaction between electronic and nuclear spins in low dimensional semiconductor nanostructures. We derive the time and position dependence of the induced nuclear spin polarization and the resulting hyperfine and dipolar magnetic fields. The determining parameters are the local electronic density of states and the additional nuclear spin relaxation times due to iteractions other that the hyperfine interaction [1,2]. In GaAs/AlGaAs parabolic quantum wells the nuclear spin polarization can be as high as 80\% and the induced nuclear magnetic fields can approach a few kilogauss (the hyperfine field) and few gauss respectively (the dipolar field) when the electronic system is 100\% spin polarized. These fields and shifts can be tuned using small electric fields. We discuss the implications of such control for optical nuclear magnetic resonance experiments in low-dimensional semiconductors [3].\\ \noindent [1] I. \c{T}ifrea and M.E. Flatt\'{e}, Phys. Rev. Lett. {\bf 90}, 237601 (2003). \noindent [2] I. \c{T}ifrea and M.E. Flatt\'{e}, cond- mat/0411277. \noindent [3] M. Poggio, G.M. Steeves, R.C. Myers, Y. Kato, A.C. Gossard, and D.D. Awschalom, Phys. Rev. Lett. {\bf 91}, 207602 (2003). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D10.00011: Current-induced dynamic nuclear polarization in GaAs W.G. Moulton, Jun Lu, M.J.R. Hoch, P.L. Kuhns, A.P. Reyes Dynamic nuclear polarization (DNP) in GaAs is of interest because of possible applications in spintronics and quantum computing. Recent experiments by other workers (1,2) have demonstrated that nuclear spin polarization can be achieved in this system by optical pumping and spin injection through ferromagnetic contacts. We have found that DNP is induced in just-insulating bulk GaAs at low temperatures by applying a DC electric field with associated current. This Feher effect, previously observed in InSb by Clark et al, has been attributed to hot electrons. The behavior found in GaAs appears to be somewhat different to the InSb case. A model involving localized states interacting with carriers in the conduction band has been used to analyze the GaAs results. Possible mechanisms for the current induced polarization of the conduction electrons will be discussed. 1.Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Phys. Rev. Lett. 93, 176601 (2004). 2.J. Strand, B. D. Schultz, A. F. Isakovic, C. J. Palmstrøm, and P. A. Crowell, Phys. Rev. Lett. 91, 36602 (2003). Support by DARPA SPINS program gratefully acknowledged [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D10.00012: Dynamics of the electron--nuclear spin interaction in bulk GaAs Michael Oestreich, Stefanie D\"ohrmann, Daniel H\"agele We study systematically the interaction between electron spins and nuclear spins in bulk GaAs with $10^{15}$cm$^{-3}$ Si in dependence on excitation energy, excitation density, and sample temperature by time-- and polarization resolved spin quantum beat spectroscopy in an external magnetic field. The electron--nuclear spin interaction strongly depends on the energy of the optical excitation and yields an elaborated tool to distinguish directly between localized and free electrons. The detailed understanding of the nuclear spin polarization, which emerges also without optical excitation of spin polarized electrons, in combination with the ability to distinguish between free and localized electrons explain measured electron spin quantum beat frequencies (g--factors) which are in contrast to many published experiments. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D10.00013: STM NMR: local spectroscopy of a nuclear spin Alexander Balatsky Not all noise in experimental measurements is unwelcome. Certain types of fundamental noise contain valuable information about the system itself -- a notable example being the inherent voltage fluctuations that exist across the terminals of any resistor (Johnson noise), from which the electron temperature may be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. Felix Bloch noted in 1946 that statistical fluctuations of $N$ paramagnetic spins should give rise to measurable noise of order $\sqrt{N}$ spins, even in zero magnetic field. I will present the model of the noise in the STM spin polarized tunneling current that couples to the nuclear spin. Noise will have a feature that reflect the dynamics of a nuclear spin. This noise spectroscopy opens up a possibility to develop a single nulclear spin NMR spectroscopy. [Preview Abstract] |
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