Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P2: Controlling Electron Spins: Propagation and Dynamics |
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Sponsoring Units: DCMP Chair: Joseph Orenstein, University of California, Berkeley and Lawrence Berkeley National Laboratory Room: Morial Convention Center LaLouisiane C |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P2.00001: An Exact SU(2) Symmetry and Persistent Spin Helix in a Spin-Orbit Coupled System Invited Speaker: Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constant (the ReD model), and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength. It renders the spin lifetime infinite at this wavevector, giving rise to a Persistent Spin Helix (PSH). We obtain the spin fluctuation dynamics at, and away, from the symmetry point, and suggest experiments to observe the PSH. Recent experimental efforts have already discovered signs of the Persistent Spin Helix. Although reaching the Rashba equal to Dresselhauss point is difficult, the Persistent Spin Helix manifests itself in a long-lived spin-density wave even away from this point, making its discovery especially suitable for optical techniques such as transient spin-grating spectroscopy. The SU(2) symmetric point allows one to obtain the exact analytic transport equations from the diffusive to the ballistic regime, which has previously been accessible only through numerical work. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P2.00002: Evidence for a persistent spin helix in a 2-dimensional electron gas Invited Speaker: Using time-resolved transient spin-grating spectroscopy we uncover strong evidence for the existence of a persistent spin helix (PSH) in a 2-dimensional electron gas. The PSH is a collective helical spin excitation that persists far beyond the lifetime of its individual constituent spins when the Rashba ($\alpha )$ and Dresselhaus ($\beta )$ spin-orbit coupling terms are comparable. The helix is predicted to have an infinite lifetime when they are exactly equal. These effects have great potential for application to spintronics where they would allow rapid gate control of the spin lifetime over several orders of magnitude in systems with both high electron density and high mobility. The transient spin-grating technique is ideally suited for study of the PSH as it can directly measure the dynamics of optically generated spin excitations of variable spatial periodicity. This is accomplished by interfering two non-collinear, orthogonally polarized pulses from a Ti:Sapphire laser at the surface of the sample. Through the optical orientation effect in GaAs, this generates a spin excitation which varies periodically in space between up and down spins at a wavelength determined by the angle between the interfering pulses. We tune the spin-orbit coupling in our GaAs based quantum wells through asymmetric modulation doping, which has allowed us to increase $\alpha $ to be comparable to $\beta $. In these systems we find that spin-gratings with periodicity comparable to that of the PSH can live several orders of magnitude longer than the individual spin lifetime as measured by time-resolved Faraday rotation. We study this over a wide range of parameter space, systematically varying doping asymmetry, well width, and disorder. We find that the lifetime of the PSH in these samples is ultimately limited by the spatial disorder in the Rashba strength, and by a novel relaxation mechanism based on phonon-induced Rashba coupling. Supported by DMSE office of BES-DOE, NSF, MARCO, ASEE and CNID. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P2.00003: Imaging the Drift and Diffusion of Optically- and Electrically-Injected Spins in Semiconductors Invited Speaker: We use methods for low-temperature scanning Kerr-rotation microscopy to directly image the drift, diffusion, and precession of spin-polarized electrons flowing laterally in GaAs. The 2-D images are used to explore spin generation and propagation resulting from both optically- as well as electrically-injected electron spins. The focused probe laser also allows to locally probe and spatially resolve the depolarization of an electron spin distribution by a small applied transverse magnetic field. The shape of these ``local Hanle effect" curves reveals important spin transport properties including the spin lifetime, drift velocity, mobility, and diffusion length [1]. The data can be accurately modeled using numerical solutions to the spin-drift- diffusion equations. Spatially-dependent asymmetries in the local Hanle effect data directly reveal the presence of additional effective magnetic fields due to spin-orbit coupling, and the dependence of these spin-orbit fields on the in-plane electron momentum {\bf k}. Using spin imaging and local Hanle effect measurements, we measure the drift and diffusion of electrically-injected spins in lateral spin transport devices having biased Fe/GaAs tunnel-barrier contacts [1], both within the charge current path as well as outside of the charge current path where only a pure spin current exists. A bias-dependent reversal of the injected spin polarization is directly observed, and we discuss how optical pumping methods can be used to measure (and tune) the spin dependent sensitivity of the epitaxial Fe/GaAs contacts when used as electrical spin detectors. [1] M. Furis {\it et al.}, New J. Phys. {\bf 9}, 347 (2007); X. Lou {\it et al.}, Nature Physics {\bf 3}, 197 (2007) [Preview Abstract] |
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