Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin-Orbit Coupling in 1D and 2D Systems |
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Sponsoring Units: GMAG DMP FIAP Chair: Jean Heremans, Virginia Polytechnic Institute and State University Room: 104 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L8.00001: Spin transport on parallel coupled nanowires with Rashba spin-orbit interaction Mariama Rebello Sousa Dias, Victor Lopez-Richard, Gilmar Marques, Sergio Ulloa The nature of various electron and spin transport mechanisms can be unveiled by exploring the properties of parallel coupled nanowires with Rashba spin-orbit interaction (SOI). Studies of a directional coupler proved the modulation of quantum transport through the proximity of waveguides. The overall control of charge and even spin flux in this system appears promising for spintronics, as well as in hybrid devices that include superconducting or magnetic materials nearby. In this work, we have studied the spin transport properties of parallel coupled nanowires, with an electric field applied in the mixing region, using a transfer matrix formalism. In this configuration, a Rashba SOI is generated, which breaks the spin degeneracy. Moreover, various configurations of gate voltages, applied on the wire structure, are considered. Under this configuration we are able to analyze the modulation of the spin transport through the combination of SOI and system dimensions. The combination of SOI and gate voltages allows a modulation of the polarization, when the measured spin is projected along the direction of the Rashba spin-orbit field. We will discuss how this polarization depends on structure features and explain how to use this effect to control the spin flux. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L8.00002: Large spin-orbit quantum interference effects in a dual-gated InSb nanowire device Ilse van Weperen, Debbie Eeltink, Brian Tarasinski, Michael Wimmer, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven InSb nanowires are the material of choice for one-dimensional topological superconducting systems. One of their favorable properties is their strong spin-orbit interaction (SOI). Measurements of the SOI strength in InSb nanowires in an open system, relevant to topology experiments, are however lacking. We therefore study the SOI in InSb nanowires in a dual-gated InSb nanowire device by means of low field magnetoconductance measurements. At a temperature of 4 K we observe a large amplitude ($\sim$0.2 e$^{2}$/h) SOI quantum interference effect. The large quantum correction to the conductance indicates a strong SOI and a long phase coherence length. We observe a crossover between positive magnetoconductance at low conductance and negative magnetoconductance at larger conductance. We examine the tunability of SOI quantum interference effects at constant conductance. Surprisingly, SOI quantum interference effects do not depend on the orientation of the magnetic field w.r.t. the nanowire. We employ simulations of the coherent backscattering trajectories in a nanowire to elucidate this isotropic response. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L8.00003: Dynamics of a localized spin excitation close to the spin-helix regime Gian Salis, Matthias Walser, Patrick Altmann, Christian Reichl, Werner Wegscheider The time evolution of a local spin excitation in a (001)-confined two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit interactions of similar strength is investigated theoretically and compared with experimental data. Specifically, the consequences of a finite spatial extension of the initial spin polarization are studied for non-balanced Rashba and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction. We show that the initial out-of-plane spin polarization evolves into a helical spin pattern with a wave number that gradually approaches the value $q_0$ of the persistent spin helix mode. In addition to an exponential decay of the spin polarization that is proportional to both the spin-orbit imbalance and the cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the spin polarization by a factor that approaches $\exp(-q_0^2w^2/2)$ at longer times. This result bridges the gap between the formation of a long-lived helical spin mode and a spatially homogeneous spin decay described by the Dyakonov-Perel mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L8.00004: Determination of the Dresselhaus spin-orbit interaction in a (110)-oriented GaAs quantum well Yuansen Chen, Stefan Faelt, Werner Wegscheider, Gian Salis The Dresselhaus spin-orbit (SO) interaction is studied in a $(110)$-oriented and symmetrically doped GaAs quantum well (QW) by means of time-resolved Kerr rotation with a magnetic field applied along an oblique angle. The nonzero averaged SO field is obtained by introducing a DC current through the QW to shift the Fermi circle of the electron gas. By monitoring the change of the electron Larmor precession frequency induced by the current, we can determine both the magnitude and the direction of the Dresselhaus SO field. In agreement with the theoretical expectation, we find the SO field to be out-of-plane and to linearly increase with a current applied along the $[1\overline{1}0]$ direction. A negligible SO field is observed for a current along the $[001]$ direction. The vector sum of the SO field and the in-plane component of an external magnetic field leads to an observable tilting of the spin precession axis. The unidirectional SO field is expected to support a persistent spin helix state in the QW. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L8.00005: Ferroelectricity and Rashba-type band splittings in metal halides Minsung Kim, Jino Im, Arthur Freeman, Jisoon Ihm, Hosub Jin In this study, we investigate Rashba-type band splittings in metal halides. We use a minimal tight-binding model and first principles calculations based on density functional theory to understand the electronic structures of the materials. We find that different types of Rashba bands occur in the conduction and valence band edges in terms of the angular momentum textures. Also, the characteristics of the band splittings will be discussed in connection with the ferroelectric property. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L8.00006: Renormalization group study of interaction effects in Rashba-type tight-binding models Giulio Schober, Manfred Salmhofer The interplay between Rashba-type spin splitting and electron-electron interactions is studied using the functional renormalization group. Our starting point is an effective tight-binding model for the spin-split lowest conduction bands of BiTeI. The giant spin splitting of bulk energy states in this semiconductor arises as a consequence of strong atomic spin-orbit coupling and the noncentrosymmetric crystal structure. By successively integrating out high-energy degrees of freedom we investigate the competition between Fermi liquid instabilities focusing on unconventional superconductivity. The vertex function is efficiently parametrized in an N-patch scheme which takes into account the most relevant momenta on the Fermi surface and realizes a flow of effective interactions with broken SU(2) symmetry. Abstracting from the concrete material, we further study a class of minimal tight-binding models on the hexagonal Bravais lattice with Rashba-type dispersions near several time-reversal invariant momenta in the first Brioullin zone. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L8.00007: Influence of CoFe nanopillars on spin coherence in an InGaAs quantum well Yao Zhang, J.J. Heremans An array of ferromagnetic (Co$_{0.6}$Fe$_{0.4}$) nanopillars was fabricated on the surface of an InGaAs/InAlAs heterostructure. The interactions between the magnetic moments of the nanopillars and two-dimensional electrons in the In$ _{0.53} $Ga$ _{0.47} $As quantum well are experimentally studied by low-temperature antilocalization measurements. The presence of ferromagnetic nanopillars increases the spin-orbit scattering rate, interpreted as due to the spatially varying nanopillar magnetic field. At the quantum well, also an appreciable average fringing field $\sim$ 35 G normal to the surface is generated by the large saturation magnetization of the nanopillars. Numerical values show a good correspondence between the analysis of the experimental antilocalization data and calculations from a simple micromagnetic model. The measurements further show an increase in mobility due to surface metal coverage. Consistently, non-magnetic coverage is observed to decrease the spin-orbit scattering rate, as expected for increased Coulombic screening under the Elliott-Yafet spin-decoherence mechanism. The analysis also shows the inelastic scattering rate increasing as temperature increases, consistent with the Nyquist mechanism. The work is supported by DOE DE-FG02-08ER46532. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L8.00008: Interplay between spin-orbit coupling and many body effects in spin-flip waves in CdMnTe quantum wells Mehul Dixit, Carsten A. Ullrich We present a numerical study of spin-flip wave dispersions in a spin-polarized electron gas in an n-type dilute magnetic semiconductor heterostructure, using time-dependent density-functional response theory. The system under study is an asymmetrically modulation-doped CdMnTe quantum well with an in-plane magnetic field. Rashba and Dresselhaus spin-orbit coupling induces a wavevector-dependent spin splitting in the conduction bands. We demonstrate a striking organization and enhancement of the spin-orbit fields acting on the collective spin-flip wave due to an interplay with electronic many-body effects. Our results agree with recent inelastic light scattering experiments. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L8.00009: Current-induced spin polarization in anisotropic spin-orbit fields Benjamin Norman, Christopher Trowbridge, David Awschalom, Vanessa Sih Current-induced spin polarization is a phenomenon in which carrier spins are oriented when subjected to current flow. As an all-electrical means of aligning spins it has the potential to be useful for spintronics applications. However, the mechanism that produces this spin polarization remains an open question. Measurements are taken on strained In$_{0.04}$Ga$_{0.96}$As exhibiting a spin-orbit interaction that is anisotropic in momentum\footnote{B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, ``Current-induced spin polarization in anisotropic spin-orbit fields,'' submitted.}. We pattern contacts such that electrical conduction can be oriented along any in-plane direction. By varying the electron current direction we can continuously tune between the spin-orbit field extrema. We show that, contrary to expectation, the magnitude of the current-induced spin polarization decreases with increasing spin-orbit field. Furthermore, we find that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, except in the cases that the spin-orbit field is along an eigenvector of the spin relaxation tensor. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L8.00010: Inverse Edelstein Effect Ka Shen, Giovanni Vignale, Roberto Raimondi In this work, we provide a precise microscopic definition of the recently observed ``Inverse Edelstein Effect", in which a non-equilibrium spin accumulation in the plane of a two-dimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The drift-diffusion equations that govern the effect are presented and applied to the interpretation of the experiments. By taking into account the Rashba spin-orbit interaction, we show how the inverse Edelstein effect arises as a combination of the $z$-spin current flowing along the $y$-direction due to an non-equilibrium $S^y$ polarization and of the inverse spin Hall effect mechanism which yields, in turn, a $x$-flowing charge current. Explicit results have been shown in the diffusive regime where we have used the theoretical framework of the $SU(2)$ formulation for linear-in-momentum spin-orbit coupling. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L8.00011: Casimir effect in spin-orbit-coupled materials Andrew Allocca, Justin Wilson, Victor Galitski We propose the Casimir effect as a general method to observe Lifshitz transitions in two-dimensional electron systems. The concept is demonstrated with a planar semiconductor system with Rashba spin-orbit coupling and an applied Zeeman field. We calculate the Casimir force between the semiconductor and a plane parallel metallic plate at fixed separation as a function of the Zeeman splitting in the semiconductor. We find that as the Zeeman energy increases, the vanishing of a Fermi surface in either the upper or lower band of the Rashba system is indicated by a kink in the Casimir force. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L8.00012: Hidden spin polarization in inversion-symmetric bulk crystals Qihang Liu, Xiuwen Zhang, Jun-Wei Luo, Arthur Freeman, Alex Zunger Spin-orbit coupling (SOC) can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba (R-1) and Dresselhaus (D-1) SOC effects. Here we note that these spin polarization effects originate fundamentally from atomic site asymmetries, producing site dipole field (DF) or being site inversion asymmetry (IA), rather than from the well-known bulk space-group crystal asymmetry. In non-centrosymmetric crystals where bulk inversion symmetry is absent, the local atomic polarizations due to site DF and site IA add up to create the bulk R-1 and D-1 net polarization effects, respectively. On the other hand, in centrosymetric crystals with sectors $\alpha $ and $\beta $ that form together inversion partners, the\textbf{\textit{ total}} spin is zero in every twofold-degenerate energy bands (termed spin degenerate). Yet we find that \textbf{\textit{local spin polarization}} can exist on each asymmetric sector, leading to ``R-2'' or ``D-2'' spin polarizations respectively. These local spin polarizations from asymmetric sectors $\alpha $and \quad $\beta $ compensated each other, forced by the bulk inversion symmetry. We demonstrate such remarkable R-2 and D-2 polarizations in some specific centrosymmetric crystals by first-principles calculations. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in a much broader class of 3D bulk solids that own global inversion symmetry, and thus open the possibility to provide new routines for manipulating electron spins. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L8.00013: Spin-Orbit Effect on Dirac Cone in Selenium and Tellurium under Pressure Motoaki Hirayama, Shoji Ishibashi, Takashi Miyake We study the electronic structures of the group-VI elements, Se and Te, from first-principles in the local spin density approximation with the GW self-energy correction included. Both Se and Te are gapful at the ambient pressure. We use the Quantum MAterials Simulator (QMAS) package for the calculation with the spin-orbit interaction [1], and the full-potential linear muffin-tin orbitals method for the calculation of the non-relativistic self-energy. The calculated band gap is in excellent agreement with experiments, where the spin-orbit interaction substantially reduces the gap. The materials undergo an insulator-to-metal transition under pressure. In the metallic phase, at a certain pressure, two conducting states appear at around the H point, and they cross each other near the Fermi level. If the spin-orbit interaction is neglected, the states have linear dispersion in the vicinity of the crossing point, forming Dirac cone. The band crossing is protected even in the presence of spin-orbit interaction by the helical structure with the threefold symmetry. The spin structure at the H-point is peculiar: The spins for all non-degenerate states are confined in the ab plane, and points to the radial direction. [1] http://www.qmas.jp/ [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L8.00014: Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts Jan Heyder, Florian Bauer, Jan von Delft Quantum point contacts (QPCs) are short one-dimensional constrictions, usually patterned in a two-dimensional electron system, e.g. by applying voltages to local gates. The linear conductance of a point contact is quantized in units of $G_Q = 2e^2/h$. In addition, measured conductance curves exhibit an unexpected shoulder around $0.7 G_Q$. In this regime quantities like electrical and thermal conductance, noise and thermo-power have anomalous behavior. These phenomena are collectively known as the ``0.7-anomaly'' in QPCs, and their origin is still subject to controversial discussions. We offer a detailed microscopic explanation for the 0.7-anomaly. Its origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest one-dimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, magnetic spin susceptibility and inelastic scattering rate. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L8.00015: The 0.7-anomaly in quantum point contacts with spin-orbit coupling Olga Goulko, Florian Bauer, Jan Heyder, Jan von Delft In addition to plateaus at integer values of $G_0 = 2e^2/h$, the linear conductance of a quantum point contact shows an anomalous shoulder at around $0.7G_0$. We study how the shape of this 0.7-anomaly is influenced by spin-orbit effects. We discuss both non-interacting and interacting systems at zero temperature, the latter of which we study using a functional renormalization group approach. In the presence of an external magnetic field, spin-orbit effects can significantly influence the shape of the 0.7-anomaly by mimicking and enhancing the effect of Coulomb interactions via changing the relative heights of the peaks of the local densities of states in the two spin states. We provide a detailed microscopic explanation of this effect and propose a setup for an experimental realization. [Preview Abstract] |
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