Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X35: Focus Session: Spins in Semiconductors -- Spin Dynamics |
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Sponsoring Units: GMAG DMP FIAP Chair: Roland Kawakami, University of California, Riverside Room: E145 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X35.00001: Theory of Electron Spin Relaxation in n-Doped Quantum Wells Nicholas Harmon, William Putikka, Robert Joynt Recent experiments have demonstrated long spin lifetimes in uniformly n-doped quantum wells. The spin dynamics of exciton, localized, and conduction spins are important for understanding these systems. We explain experimental behavior by invoking spin exchange between all spin species. By doing so we explain quantitatively and qualitatively the striking and unusual temperature dependence in (110)-GaAs quantum wells. We discuss possible future experiments to resolve the pertinent localized spin relaxation mechanisms. Our analysis allows us to suggest possible experimental scenarios that will optimize spin relaxation times in GaAs and CdTe quantum wells. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X35.00002: Measurement of anisotropic spin-orbit interaction in a two-dimensional electron gas using ballistic spin resonance Wing-Wa Yu, Sergey Frolov, Silvia Luescher, Yuan Ren, Joshua Folk, Werner Wegscheider We report the observation of strongly anisotropic spin-orbit interaction in a 2D electron gas (2DEG) along the [110] and [1$\bar{1}$0] crystal axes of a GaAs/AlGaAs triangular well, indicating that the Rashba and Dresselhaus contributions to the interaction are closely matched in this structure. Spin relaxation due to spin-orbit interaction is probed using ballistic spin resonance (BSR) [1] in a narrow 2DEG channel: an oscillating spin-orbit field is induced by high-frequency bouncing of free electrons between the channel walls, which leads to rapid relaxation when the bouncing frequency matches the Larmor frequency in a static external magnetic field. Drastically different BSR strengths are observed for channels fabricated along the two crystal axes, from which the magnitudes of Rashba and Dresselhaus contributions can be extracted. \newline [1] S. M. Frolov et al., Nature 458, 868-871(2009) [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X35.00003: Spin noise of electrons and holes in self-assembled (In,Ga)As quantum dots S.A. Crooker, J. Brandt, C. Sandfort, A. Greilich, D.R. Yakovlev, M. Bayer, D. Reuter, A.D. Wieck We measure the frequency spectra of random spin fluctuations, or ``spin noise", in ensembles of (In,Ga)As/GaAs quantum dots (QDs) at low temperatures [1]. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array, to measure and average noise spectra from 0-1 GHz continuously in real time (no experimental dead time) with sub-nanoradian/$\sqrt{\textrm{Hz}}$ sensitivity. Both electron \emph{and} hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their \emph{g}-factors and dephasing rates within the ensemble. A large, energy-dependent anisotropy of the in-plane hole \emph{g}-factor is clearly exposed, reflecting systematic variations in the average QD confinement potential. [1] arXiv:0909.1592 [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X35.00004: Spin noise spectroscopy from acoustic to GHz frequencies Invited Speaker: Performing perturbation free measurements on semiconductor quantum systems has long been banished to textbooks on quantum mechanics. The emergent technique of spin noise spectroscopy is challenging this restriction. Empowered only by the ever present intrinsic spin fluctuation dynamics in thermal equilibrium, spin noise spectroscopy is capable to directly deduce several physical properties of carriers spins in semiconductors from these fluctuations. Originating from spin noise measurements on alkali metal vapors in quantum optics [1] the method has become a powerful technique to unravel the intrinsic spin dynamics in semiconductors [2]. In this talk I will present the recent progress of spin noise spectroscopy and how it is used to monitor the spin dynamic in semiconductor quantum wells at thermal equilibrium and as a consequence thereof directly detect the spatial dynamics of the carriers being marked with their own spin on a microscopic scale [3]. Further I will present measurements of how the non-perturbative nature of spin noise spectroscopy gives valuable insight into the delicate dependence of the spin relaxation time of electrons on doping density and temperature in semiconductors n-doped in the vicinity of the metal-insulator transition where hyperfine and intra-band depolarization compete [4]. Also the measurement bandwidth can be extended to GHz frequencies by ultrafast optical probing [5] yielding in conjunction with depth resolved spin noise measurements insights into the origin of inhomogeneous spin dephasing effects at high magnetic fields [5]. Additionally I will present how spin noise spectroscopy can be employed to spatially depth resolve doping profiles with optical resolution [6] and give a summary on easy to implement techniques of spin noise spectroscopy at acoustic frequencies in alkali metal vapors. \\[4pt] \noindent [1] E.~Aleksandrov and V.~Zapassky, Zh. Eksp. Teor. Fiz. \textbf{81}, 132 (1981); S.~A. Crooker, D.~G. Rickel, A.~V. Balatsky, and D.~L. Smith, Nature \textbf{431}, 49 (2004).\\ \noindent [2] M. R\"omer, J. Hübner, and M. Oestreich, Rev. Sci. Instrum. 78, 103903 (2007).\\ \noindent [3] G. M\"uller, M. R\"omer, D. Schuh, W. Wegscheider, J. Hübner, and M. Oestreich, Phys. Rev. Lett., \textbf{101} 206601 (2008).\\ \noindent [4] M. R\"omer, H. Bernien, G. M\"uller, D. Schuh, J. H\"ubner, and M. Oestreich arXiv:0911.4084\\ \noindent [5] G. M. Müller, Michael R\"omer, Jens H\"ubner, M. Oestreich, arXiv:0909.3406\\ \noindent [6] M. R\"omer, J. Hübner, and M. Oestreich, Appl. Phys. Lett., \textbf{94} 112105 (2009). [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X35.00005: Measurement of Spin-dependent Conductivity in a GaAs/AlGaAs Two-dimensional Electron Gas Seyed Hadi Ebrahimnejad Rahbari, Yuan Ren, Sergey Frolov, Joshua Folk, Werner Wegscheider We describe a measurement of spin-dependent electrical conductivities of a partially polarized two-dimensional electron gas (2DEG), confined at the interface of GaAs/AlGaAs heterostructure and subject to an in-plane magnetic field. Our method uses polarized quantum point contacts to measure the nonequilibrium spin polarization that accompanies pure spin currents in a micron-wide channel of 2DEG. When the conductivities of spin-up and spin-down carriers are different, an unpolarized charge current that is injected into the center of the channel builds up a net spin accumulation near the injector, associated with an imbalance between the chemical potentials of the two spin populations. The chemical potential difference gives rise to a nonlocal voltage, which is then used to quantify the difference between spin-resolved conductivities. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X35.00006: Long range anisotropic RKKY interaction in a disordered two-dimensional electron gas with Rashba and Dresselhaus spin-orbit coupling Stefano Chesi, Daniel Loss We study the RKKY exchange interaction between localized magnetic moments in a disordered two-dimensional electron gas with Rashba and Dresselhaus spin-orbit coupling. The disorder-averaged magnetic interaction, due to its oscillatory character, is exponentially suppressed at a distance larger than the mean free path. However, the interaction is still long-ranged and decays in magnitude with the same power-law of the clean case, as revealed by a calculation of its variance. While an isotropic form of the Heisenberg type is valid at distance smaller than the spin-orbit length, the interaction becomes strongly anisotropic at larger distance. We study in detail the crossover to the asymptotic regime, realized when the impurities are a few spin-orbit lengths apart and the variance of the exchange energy is independent of the orientations of the two localized moments. Our results apply to nuclear moments embedded in III-V two-dimensional heterostructures or magnetic impurities deposited on metals or metal alloys, where the surface states display a sizable Rashba spin-orbit coupling. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X35.00007: Spin separation in cyclotron motion in anisotropic 2D systems T. Minagawa, Y. Lyanda-Geller Spin-orbit coupling in the two dimensional systems can lead to spatial separation of spins of charge carriers that experience cyclotron motion in an external magnetic field. In the experiment by Rokhinson et al, this effect was observed in p-GaAs grown along [113] direction. Such system, due to cubic anisotropy of the underlying GaAs crystal, possesses several remarkable properties. In particular, we investigate spin-orbit interactions of 2D holes quantized along [113] and demonstrate that the dominant contribution to spin-orbit coupling arises from the linear in the electron momentum terms. This is in contrast to hole system quantized along [001], in which the intrinsic spin-orbit interaction is cubic in the hole momentum. The linear in momentum terms in [113] system originate from both bulk GaAs cubic in momentum coupling and from asymmetry of the quantum well confining the 2D hole gas. We have analytically and numerically studied the spin separation of two-dimensional holes quantized along [113] direction and found strengths of spin-orbit coupling maximazing or eliminating the spatial separation of spins. We have also computed spin relaxation times for [113] 2D holes. We demonstrate that for spin-orbit interactions giving no spatial separation, the spin relaxation rate vanish. The same effect occurs in anisotropic electron systems, c.f., in the presence of both Rashba and Dresselhaus linear in momentum spin-orbit couplings. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X35.00008: Spin-orbit focusing of electrons in two dimensions David H. Berman, Michael E. Flatt\'e The Green function for an electron confined to two-dimensions and experiencing both Rashba and Dresselhaus spin-orbit interactions shows that an initially isotropic distribution of electrons injected at a point can form a narrow beam as electrons move away from the source. This effect depends critically on the ratio of the Rashba to Dresselhaus interaction strengths. Formation of anisotropic distributions can be explained semi-classically using a stationary phase analysis taking into account coalescing stationary points. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X35.00009: Magnetoelectric spin accumulation in mesoscopic conductors Philippe Jacquod We present a calculation of the electrically induced spin accumulation in mesoscopic systems in presence of both Rashba (with strength $\alpha)$ and Dresselhaus (with strength $\beta)$spin-orbit interactions. For diffusive systems, we follow a diffusion equation approach to show that magnetoelectric effects disappear and that there is thus no spin accumulation when $\alpha=\pm \beta$. We show however that the singularity is broadened and that the suppression of spin accumulation becomes physically relevant (i) in finite-sized systems, (ii) in the presence of a cubic Dresselhaus interaction, or (iii) for finite frequency measurements. For ballistic systems, we present a novel scattering approach to (global) spin accumulation where charge and spin-dependent chemical potentials are extracted by an external probe. In this way the total spin accumulation, including its average and mesoscopic fluctuations, as well as its correlations with spin currents can be calculated from the statistics of transmission probabilities. I will present such a calculation based on both random matrix theory and the trajectory-based semiclassical approach. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X35.00010: The Occurrence of Anomalous Conductance Plateaus and Spin Textures in Quantum Point Contacts J. Wan, M. Cahay, P. Debray, R. Newrock Recently, we used a NEGF formalism [1] to provide a theoretical explanation for the experimentally observed 0.5G$_{0}$ (G$_{0}$=2e$^{2}$/h) plateau in the conductance of side-gated quantum point contacts (QPCs) in the presence of lateral spin-orbit coupling (LSOC) [2]. We showed that the 0.5G$_{0}$ plateau appears in the QPCs without any external magnetic field as a result of three ingredients: an asymmetric lateral confinement, a LSOC, and a strong electron-electron (e-e) interaction. In this report, we present the results of simulations for a wide range of QPC dimensions and biasing parameters showing that the same physics predicts the appearance of other anomalous plateaus at non-integer values of G$_{0}$, including the well-known 0.7G$_{0}$ anomaly. These features are related to a plethora of spin textures in the QPC that depend sensitively on material, device, biasing parameters, temperature, and the strength of the e-e interaction. [1] J. Wan, M. Cahay, P. Debray, and R.S. Newrock, Phys. Rev. B 80, 155440 (2009). [2] P. Debray, S.M. Rahman, J. Wan, R.S. Newrock, M. Cahay, A.T. Ngo, S.E. Ulloa, S.T. Herbert, M. Muhammad, and M. Johnson, Nature Nanotech. 4, 759 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X35.00011: Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells Patrick Folkes, L.-C Tung, Godfrey Gumbs, Wen Xu, Y.-J Wang Magneto-infrared spectroscopy has been used to study the two-dimensional electron-hole system in a set of weakly-hybridized InAs-AlSb-GaSb coupled quantum well heterostructures using high magnetic fields up to 33 T and temperatures ranging from 4K to 45K in both Faraday and tilted geometries. We have observed a pair of cyclotron resonance (CR) absorption modes only at magnetic fields higher than 14 T which are insensitive to increasing temperature and parallel magnetic field. In addition, the dependence of the energy separation of the CR splittings on magnetic field is very different from that previously reported for weakly hybridized samples. The conduction-valence Landau level mixing effect or other known models for InAs/GaSb heterostructures cannot explain these unusual characteristics. We suggest that a spontaneous phase separation can account for most of the observed features. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X35.00012: Probing Excitonic Effects in Spin Relaxation Dynamics of Undoped InSb Multi Quantum Wells in the Mid-Infrared Mithun Bhowmick, M. Frazier, G.A. Khodaparast, T.D. Mishima, M.B. Santos The recent rapid progress in the field of spintronics involves extensive measurements of spin relaxation dynamics in semiconductors. In this work, we employed polarization- resolved differential transmission measurements in the mid-infrared, to probe spin relaxation times in undoped InSb multi quantum wells(QW). InSb offers several unique characteristics such as small effective mass, large g-factor, and a strong spin-orbit coupling. In addition, InSb is the narrowest band gap material for which room temperature excitons were reported. In undoped QWs, the electron-hole pair near the band gap forms an exciton. The exciton spin can relax via spin relaxation of either hole or electron, or spin flip of both entities simultaneously. The main mechanism, which simultaneously flips the electron and hole spins, is the exchange interaction. The complex nature of the valance band and the strong non-parabolicity in InSb can result in a short range exchange interaction in which the light hole and heavy hole mixing can flip the exciton spin. The goal of our studies is to understand and identify spin relaxation mechanisms close to several excitonic transitions. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X35.00013: Aharanov-Casher Effect for Spin Waves in a Ferromagnet Tianyu Liu, Giovanni Vignale Spin waves play a potentially important role in spintronics as means for modulating the magnetoresistance of devices. In this work we study how the propagation of spin waves can be controlled by electric fields, which couple to electrons via spin-orbit interaction. Starting from basic models of localized spins interacting via itinerant electrons (e.g. double exchange model, RKKY interaction) we study how spin-orbit coupling between the itinerant electrons and an electric field modifies the dynamics of spin waves. In particular, we provide a first-principle derivation of the Aharanov-Casher effect on the phase of spin waves in ferromagnetic rings. In a parallel study, we consider the propagation of spin waves on a textured magnetic background (e.g. a domain wall or a spiral magnetic structure). An analogy between the effect of a non-uniform magnetization background and that of spin-orbit coupling is developed. [Preview Abstract] |
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