Session J35: Focus Session: Spins in Semiconductors -- Spin Orbit Effects and Spin Relaxation

Spin dynamics of magnetic ions in semiconductor optical cavities

Magnetic atoms incorporated into semiconductor nanostructures are promising systems for studying local solid state environments due to the sensitivity of their electronic states to the local band structure. Additionally, coherence times for Mn atoms incorporated in Gallium Arsenide quantum wells are on the order of tens of nanoseconds, making them a candidate for quantum information and conventional magnetic storage applications. We extend previous studies to the time domain and find that in addition to the zero field Mn polarization and exchange splitting, the time-domain measurements reveal the presence of long electron spin dynamics and random strain fields. To elucidate the mechanisms involved, we utilize a combination of time resolved photoluminesence, time resolved Kerr rotation and optically detected magnetic resonance measurements. Furthermore, we incorporate these structures into solid state optical cavities to enhance optical coupling with the goal of isolating and manipulating a single magnetic atom in the solid state.   

Two-Dimensional Spin Transport with Two-Particle Spin-Orbit Coupling

We examine new spin-orbit effects on the quasi-particle properties in electron Fermi liquids in semiconductor quantum wells. These effects are independent of the structure asymmetry and come from the coupling of spins to their relative motion. We focus on the low-temperature, impurity-free and phonon scattering-free regime where the decoherence of spin transport is determined by the intrinsic spin-dependent interactions and Coulomb scattering between carriers. Quasiparticle parameters are obtained within the Random Phase Approximation for the Coulomb screening and by taking into account spin-dependent interactions to second perturbative order. The dependences of lifetime and effective mass on density and the quasiparticle energy are compared to previous results for Fermi Liquids obtained with only single-particle spin-orbit coupling (Rashba or Dresselhaus). We identify regimes where the spin-relative orbit coupling dominates, and we discuss cross-terms when both single-particle and spin-relative orbit interactions are present. These results are relevant for Spin-Hall and Spin-Coulomb Drag effects.   

Prediction of large linear-in-k spin splitting for holes in the 2D GaAs/AlAs system

The spin-orbit interaction generally leads to spin splitting (SS) of electron and hole energy states in solids, a splitting that is characterized by a scaling with the wavevector $\bf k$. Whereas for {\it 3D bulk zincblende} solids the electron (heavy hole) SS exhibits a cubic (linear) scaling with $k$, in {\it 2D quantum-wells} the electron (heavy hole) SS is currently believed to have a mostly linear (cubic) scaling. Such expectations are based on using a small 3D envelope function basis set to describe 2D physics. By treating instead the 2D system explicitly in a multi-band many-body approach we discover a large linear scaling of hole states in 2D. This scaling emerges from hole bands coupling that would be unsuspected by the standard model that judges coupling by energy proximity. This discovery of a linear Dresselhaus k-scaling for holes in 2D implies a different understanding of hole-physics in low-dimensions.   

Imprinting the momentum distribution anisotropy of photoexcited electrons in semiconductors into their spins by a ferromagnetic proximity effect

We calculate the spin polarization after the reflection off a ferromagnet while considering the anisotropic crystal momentum distribution of photoexcited electrons in semiconductors. The imprinted spin information persists much longer than the momentum relaxation time in subpicoseconds, and hence provides an enormous expansion of the time scale to observe the anisotropy. In steady state conditions, the scale may extend even longer to the spin-lattice relaxation time. In the case of cubic crystal symmetry, the initial momentum distribution of photoexcited electrons, and thus the imprinted spin vector, depends on the directions of the light polarization, the normal of the semiconductor/ferromagnet interface, and the magnetization. We show detailed results of various configurations for both linearly and circularly polarized light, and discuss the effects of the direct and mixing conductance of the interface. While mainly focusing on direct gap semiconductors, we also briefly mention the anisotropy signal due to indirect optical transitions in silicon.   

Diffusion of spins in GaAs in the presence of a strongly spatially varying local magnetic field

We report on the effect of a localized, strongly inhomogeneous magnetic field on the behavior of spins in GaAs. Such magnetic fields can provide spatially resolved information regarding spin diffusion, relaxation and precession. The sample in our experiments is a 2 micron thick n-GaAs (3e16 cm$^{-3}$, Si doped) epitaxial membrane. A circularly polarized pump beam injects the spins into the membrane. The local magnetic field is provided by a micro-magnetic particle mounted on a commercial AFM cantilever. Spin-polarized photoluminescence is then used to measure the Hanle effect. The changes in the Hanle response as a function of the magnetic tip position provide information regarding the microscopic environment experienced by the spins. We also present numerical analysis of the spin diffusion equation for our experimental conditions.   

Valence Band Spin-Splitting of GaAs in a Magnetic Field

We present calculations of the conduction and valence band Landau levels in bulk GaAs in a magnetic field. Our calculations are based on the 8-band Pidgeon-Brown model. Energy is accurate up to one meV. The wave functions for each Landau level and magnetic field are given in terms of linear combinations of the 8 basis states. We show that the conduction band and the spin-orbit split-off band can be treated as almost pure states without significant band mixing. However, the heavy-hole and light-hole bands are strongly mixed even for a small Landau level quantum number and at small magnetic fields. This mixing between heavy-hole and light-hole states gets stronger at higher magnetic fields. As a result, there are multiple optical transitions between a given valence band state and conduction band state even within a single Pidgeon-Brown manifold. We calculate the magneto absorption spectra, the magnetic circular dichroism and the optically pumped NMR signal as a function of magnetic field within Fermi's golden rule approximation. Comparison with experiment allows an accurate determination of the spin-split valence band structure.   

Emergence of the persistent spin helix in semiconductor quantum wells

The persistent spin helix is a collective spin excitation of 2D electron systems that emerges as a new conserved quantity when the spin-orbit interaction is tuned to recover SU(2) symmetry in the spin Hamiltonian.~~The spin helix has great potential for application to spintronics, where it would allow rapid gate control of the spin lifetime over several orders of magnitude in devices with both high electron density and high mobility.~~We observe the persistent spin helix in semiconductor quantum wells using transient grating spectroscopy.~~This technique uses femtosecond pulses of light to generate spatially non-uniform spin (or charge) patterns in the sample.~~Studying the decay of spin patterns of varying periodicity allows quantitative characterization of the diffusion properties of the material.~~Additionally, we have developed a phase-resolved version of the transient grating technique which has enabled us to observe the spin helix moving in an electric field with unprecedented spatial resolution.~~Supported by DMSE office of BES-DOE, NSF, MARCO, ASEE and CNID.   

Quasiparticle scattering in two dimensional helical liquid

We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed.   

Driving the persistent spin helix and electron-hole density waves with an electric field in n-GaAs quantum wells

We use transient spin grating spectroscopy to study the persistent spin helix (PSH) state of a 2D electron gas in the presence of an in-plane electric field parallel to the wavevector of the spin helix. By directly measuring its phase, we can measure the PSH displacement with 10 nm spatial and sub-picosecond time resolution. We demonstrate that the phase velocity of the PSH crosses zero at a nonzero wavevector. The data indicate that spin Coulomb drag may play a role in the spin wave drifting process. We also study the displacement of electron-hole density waves (EHDW's) as a function of electric field. Although charge neutral, the EHDW is found to drift, but with with a velocity that is much smaller than that of the surrounding electrons. We speculate that the drift is caused by a Coulomb drag interaction between the Fermi sea and the EHDW.   

Transient Exciton Spin Splitting in GaAs Quantum Wells under Near-Resonant Excitation

We investigated spin dependent exciton-exciton interaction and energy relaxation under near-resonant circularly polarized ps pulsed excitation in single, multiple, or double coupled GaAs/AlGaAs quantum wells. Transient exciton spin splitting and relaxation were determined from time-resolved photoluminescence (TRPL) spectroscopy and polarimetry with a streak camera system. In contrast to standard TRPL measurements based on up-conversion and pump-probe techniques, the streak-camera setup allows for speedy spectroscopy and Stokes polarimetry measurements as a function of the exciton density and magnetic/electric field under near -resonant excitation ($\sim $3 to 10 meV from the exciton resonance). For 6-nm and 14-nm GaAs/AlGaAs quantum wells at intermediate density (a few 10$^{10}$ cm$^{-2})$, a spin splitting of 2 and 1 meV appeared instantly within 10 ps after excitation and exhibited a decay time constant of $\sim $100 and 500 ps, respectively. In the presence of magnetic fields, the spin splitting and relaxation dynamics became non-exponential and exhibited asymmetric and nonlinear dependence on the direction and magnitude of the field up to 10 Tesla. We analyzed the spin splitting and relaxation dynamics in terms of inter-exciton and intra-exciton exchange interaction and exciton-carrier interaction.   

Zero Field Rashba Spin Splitting in InSb Hole Systems

We report weak anti-localization studies of 2D hole systems based in asymmetric Be-doped InAlSb/InSb quantum wells. In the valence band, spin-orbit effects are not only influenced by the structural asymmetry but also by the separation between the light and heavy hole bands. In these studies which varied not only carrier concentration but also well width, it was found that the Rashba interaction increases with increasing doping asymmetry and well width as expected, but was only in modest agreement with theoretical expectations for the Rashba interaction in the valence band.   

Spin-Orbit Coupling Effects in Strongly-Confined Electron Systems

We investigate theoretically spin-orbit coupling effects in low-dimensional electron systems. The consequences of introducing Rashba spin-orbit coupling into a two-dimensional electron system in the presence of additional strong lateral confinement and an external magnetic field are explored. The nature of the spin-orbital states in these systems and the resulting Berry phase effects are studied as a function of confinement geometry. The implications of these results are discussed in the context of recent experimental measurements of the properties of electrostatically-defined nanostructures in LaAlO$_{3}$/SrTiO$_{3}$ systems.   

Weak Antilocalization and Spin-Orbit Coupling in LaAlO$_{3}$/SrTiO$_{3}$ Nanostructures

Recently, nanoscale control of the metal-insulator transition at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ is demonstrated to be capable of making nanostructures exhibiting various functional electrical properties.\footnote{C. Cen, S. Thiel, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nature Materials \textbf{7}, 2136 (2008).}$^,$\footnote{C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science \textbf{323}, 1026 (2009)} We performed low temperature magneto-transport measurement on straight and L-shaped nanowire channels. Strong geometry dependence of weak antilocalization (WAL) due to spin-orbit (SO) coupling is observed. A sharp feature around $B $= 0 T of non-local voltage is measured 12 $\mu $m away from current carrying channel. Temperature and external magnetic field orientation dependences of WAL signatures are studied. The potential to explore spin degree of freedom with interfacial nanostructures opens new possibilities for novel oxide spintronic devices.