Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W33: Focus Session: Spins in Narrow Gap Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Berry Jonker, Naval Research Laboratory Room: Morial Convention Center 224 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W33.00001: Spin Injection, Manipulation, and Detection, in InAs Nanodevices G.M. Jones, B.T. Jonker, B.R. Bennett, J.R. Meyer, M.E. Twigg, T.L. Reinecke, D. Park, S.V. Pereverzev, C.S. Badescu, C.H. Li, A.T. Hanbicki, O. Van'terve, I. Vurgaftman In this talk the authors will discuss their progress using InAs heterostructures to produce spin-polarized injection and detection, as well as manipulation of coherent spin-polarized electrons for a spin-based FET (SpinFET). High-quality n-type InAs heterostructures demonstrate many favorable characteristics necessary to the study of spin dynamics, including 2DEG's with small effective mass (m* = 0.023) and large g-factor (g = -15). Previously, high-mobility InAs heterostructures have been demonstrated in which electrons pass ballistically over hundreds of nanometers up to room temperature. Our devices seek to exploit the strong Spin-Orbit effect present in InAs to manipulate coherent spin-polarized electrons during transport, by producing perpendicular electric field using isolated top-gates fabricated over the electron transport region. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W33.00002: Electrical Spin Injection into InAs Wetting Layer Connie H. Li, George Kioseoglou, Aubrey T. Hanbicki, Ramasis Goswami, Steve Hellberg, Berry T. Jonker, Mesut Yasar, Athos Petrou InAs is an attractive material for optoelectronic and high-speed transistor devices due to its small bandgap and high electron mobility. Owing to its large Rashba spin-orbit coupling, the 2DEG formed in InAs-based heterostructures has also been proposed for spin transport within a spin FET$^{1}$. Here we demonstrate efficient spin injection from Fe into a thin ($\sim $3ML) InAs wetting layer (WL) that forms on GaAs before the formation of QDs. Cross sectional TEM shows that the WL is continuous laterally over many microns, and transport measurements reveal 2DEG-like behavior. The WL electroluminescence is readily distinguished from that of the QDs, and dominates emission at higher biases over a wide temperature range up to RT. We measure an optical circular polarization of 26{\%} at 5K due to the injection of spin-polarized electrons from a reverse-biased Fe Schottky contact, which corresponds to an electron spin polarization $>$50{\%} after lifetime corrections, demonstrating that even this remarkably thin layer supports high spin polarization. This polarization stayed relatively constant up to 60K, and decreased to $\sim $6{\%} at RT, consistent with D'yakonov-Perel spin relaxation mechanism. Supported by ONR, NRL core funds, and NSF. $^{1}$ S. Datta and B. Das, Appl. Phys. Lett. \textbf{56}, 665 (1990). [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W33.00003: Spin polarized current in an InSb film Matthew Frazier, J.J. Heremans, Giti A. Khodaparast Recently, there has been much interest in developing and exploring spin based semiconductor devices and phenomena. One of the key challenges in developing spin based devices is to generate, control, and measure spin currents directly. In this talk, we report interband circular photogalvanic (CPG) effects using pulsed near-infrared radiations in an InSb film grown by the MOCVD technique. The film is n-type Te-doped with electron density of $\sim $ 6.0 x10$^{15}$ cm$^{-3}$ and mobility of 58,500 cm$^{2}$/Vs at 100 K. We observe a CPG current whose direction and magnitude depend on the helicity of the incident light, the angle of the incidence, and temperature. Our observation is important to understand zero-field spin splitting mechanisms in a system with strong-spin orbit interaction. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W33.00004: Spin coherence times in n-type InSb thin films R.L. Kallaher, J.J. Heremans In order to investigate the spin coherence times in the narrow gap semiconductor InSb, low temperature magnetoresistance measurements were performed on Te-doped thin films of InSb in weak perpendicularly applied magnetic fields. The measured changes in the resistance, as a function of applied field, in these n-type films show anti-localization phenomena that occur as a consequence of the strong spin-orbit interaction present in InSb. Hence, the magnitude of both the spin and the phase coherence times of the electrons in InSb can be determined by fitting the measured magnetoresistance curves to a localization theory that includes the effects of spin-orbit scattering. Such fits reveal that for the Te-doping levels investigated, the spin coherence times vary from approximately 20 ps to 200 ps at low temperatures, with very weak or no temperature dependence below 10 K. Furthermore, by analyzing the spin coherence times in films with different Te doping densities, it is shown that the Elliot-Yafet mechanism is responsible for the spin decoherence in doped InSb at low temperatures. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W33.00005: Control and probe of carrier and spin relaxations in InSb based quantum wells Kanokwan Nontapot, R.N. Kini, G.A. Khodaparast, N. Goel, T.D. Mishima, M.B. Santos The growing interest in spin-related phenomena and devices has prompted intense activity in the science and engineering of narrow gap semiconductors (NGS). NGS offer several scientifically unique features such as small effective masses, large g-factors, high intrinsic mobilities, and large spin- orbit coupling effects. In this work we report the dynamics of photo-excited carrier/spin in several InSb based quantum wells (QWs) using standard pump-probe spectroscopy and magneto- optical Kerr (MOKE) effect. Our InSb QWs are grown on GaAs (001) substrates with the Al$_{x}$In$_{1-x}$Sb barrier layers. We studied one undoped and five doped QWs with the electron concentrations in the wells ranging from ${\sim}1-4.4{\times}10^ {11}cm^{-2}$, where only the ground-state subband is occupied. The electron mobility in the samples are ranging from ${\sim} 70,000-100,000 cm^{2}/Vs$ at 4.2 K. We observed strong dependence of the dynamics to the density of photo-excited carriers and the pump photon energy and only weak variation with changing the samples' temperature. Our results are important to understand different relaxation mechanisms in NGS with strong-spin orbit interactions. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W33.00006: Temperature dependence of spin-resolved transverse magnetic focusing in InSb- and InAs heterostructures J.J. Heremans, R.L. Kallaher, R.B. Lillianfeld, Hong Chen, N. Goel, S.J. Chung, M.B. Santos, W. Van Roy, G. Borghs Spin-orbit interaction in InSb/InAlSb and InAs/AlGaSb heterostructures leads to spin-split orbital motion. The spatial separation of semiclassical cyclotron orbits is experimentally detected by measuring the magnetoresistance of lithographically patterned mesoscopic ballistic transverse magnetic focusing geometries. The 1st and 2nd focusing maxima show multiplet structure, consistent with spin-split ballistic orbits and spin-dependent reflection off the focusing barrier. In InSb, the transverse magnetic focusing effect is observable up to 150 K, while the multiplet structure in the maxima shows a more pronounced temperature dependence and fades by 80 K. In InAs, the transverse magnetic focusing effect persists to 60 K, and the multiplet structure is visible up to 20 K. The difference in temperature dependence between maxima and multiplet structure indicates that a length scale separate from the mean-free-path governs the observation of splitting, pointing to the use of transverse magnetic focusing for quantifying the temperature dependence of spin coherence. (NSF DMR-0618235, DMR-0080054, DMR-0209371) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W33.00007: External electric field effects on AAS oscillations in narrow gap semiconductors R. B. Lillianfeld, R. L. Kallaher, D. E. Davis, J. J. Heremans, Hong Chen, N. Goel, S. J. Chung, M. B. Santos, W. Van Roy, G. Borghs We present experiments on quantum interference phenomena in semiconductors with strong spin-orbit interaction, using mesoscopic parallel ring arrays fabricated on InSb/InAlSb and InAs/AlGaSb heterostructures. A front gate modulates the spin-orbit interaction, which in turn affects the oscillatory interference phenomena. The experiments investigate the low temperature resistance of the ring arrays as a function of weak perpendicularly applied magnetic fields as well as applied gate voltage. The low field magnetoresistance in the arrays has the h/2e periodicity characteristic of Altshuler-Aronov-Spivak (AAS) oscillations. Despite reduced gate action typical of narrow-gap heterostructures (characterized by Hall measurements), we note an effect on the oscillatory magnetoresistance. The AAS oscillation magnitudes acquire a quasi-periodic modulation as function of gate voltage, and the localization background broadens at higher electron densities. The nature of these influences is examined. (NSF DMR-0618235, DMR-0080054, DMR-0209371) [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W33.00008: Measurements of spin polarization and spin relaxation in 2D electron gases S.M Frolov, A. Venkatesan, W. Yu, J.A. Folk, W. Wegscheider Pure spin currents are generated and detected using quantum point contacts in narrow channels of a GaAs/AlGaAs 2D electron gas. A spin relaxation length of 50 microns is achieved due to a cancellation of Rashba and Dresselhaus spin-orbit interactions along 110 crystal direction. Spin currents observed at zero magnetic field are correlated with the 0.7 conductance feature of quantum point contacts, suggesting static spontaneous spin polarization. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W33.00009: Separation of the Rashba and Dresselhaus terms using the Square and Rectangular Loop Arrays in InGaAs/InAlAs Quantum Wells Takaaki Koga$^{1}$, Minu Kim, Yoshiaki Sekine The spin interference effect [1,2] was investigated for square and rectangular loop arrays that were nanolithographically defined in InGaAs/InAlAs quantum wells both theoretically and experimentally. In theory, interference between the following spin wave functions were considered : $\Psi ^{CW}$=\textbf{R}$_{-x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{-y}$(\textit{$\theta $}$_{h})\Psi _{i}$ (wave function after the clockwise path in a rectangular loop) and $\Psi ^{CCW}$=\textbf{R}$_{y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{-x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{-y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{x}$(\textit{$\theta $}$_{v})\Psi _{i}$ (wave function after the counter-clockwise path), where the spin rotation operators \textbf{R}$_{\xi }$(\textit{$\theta $}$_{v,h})$ were obtained from solving the Poisson and Schrdinger equations self-consistently including the Rashba and Dresselhaus Hamiltonians. Then, the gate-dependence of the norm $\vert \Psi ^{CW}+\Psi ^{CCW}\vert ^{2}$, averaged over all directions for the initial spin ($\Psi _{i})$, were compared to the gate-dependence of the AAS oscillation amplitude in the experiment. We propose that the measurement of the spin interference effect is a reliable method for the simultaneous determination of the Rashba and Dresselhaus terms quantitatively. [1] Koga \textit{et al.}, PRB \textbf{70}, 161302(R) (2004);\textit{ ibid.} \textbf{74}, 041302(R) (2006). [2] Koga \textit{et al.}, phys. stat. sol. (C) \textbf{3}, 4220 (2006). [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W33.00010: Aharonov-Casher effect in hole ring with spin-orbit interaction Mario Borunda, Alexey Kovalev, Tomas Jungwirth, Laurens Molenkamp, Jairo Sinova We study the quantum interference effects induced by the Aharonov-Casher phase in a ring structure two-dimensional heavy-hole (HH) system with spin-orbit interaction. The influence of the spin-orbit interaction on the transport causes interference effects which are a signature of the topological Aharonov-Casher phase. We present numerical calculations of the magnetoconductance and spin dependent transport in realistic semiconductor ring structures. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W33.00011: Plasmons of a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit coupling. Jesus A Maytorena, Catalina Lopez-Bastidas, Elmer Cruz We calculate the dielectric response of a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit (SO) coupling within the self-consistent-field approach. We obtain the dispersion relations of the collective modes and the regions of single-particle excitations, related with intra- and inter-spin-split transitions. The interplay of both types of SO couplings give rise to an angular anisotropy of the spin-splitting energy. As consequence, the plasmon spectrum and the Landau damping regions show a dependence on the direction of the wave vector transfer. This response is in contrast to that of vanishing Rashba or Dresselhaus case. We also discuss the dependence of this spectral characteristics on the electron density and SO coupling strengths, and derive expressions for the intra- and inter- SO plasmons in the long-wavelength limit. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W33.00012: Magnetoplasmon excitations in Rashba spintronic quantum wires: Maxons, rotons, and negative-B dispersion Manvir Kushwaha We report on the theoretical investigation of plasmon excitations in a quasi-two-dimensional electron gas in the presence of a harmonic potential, a perpendicular magnetic field, and the spin-orbit interaction (SOI) induced by the Rashba effect. The resultant system is a Q1D quantum wire with free propagation in the y direction and magnetoelectric quantization along the x. The problem involves three length scales: ${\it l}_0=\sqrt{\hbar/m^*\omega_0}$, ${\it l}_c=\sqrt{\hbar/m^*\omega_c}$, and ${\it l}_{\alpha}=\hbar^2/(2m^*\alpha)$, which characterize the relative strengths in the interplay of confinement, the magnetic field, and the Rashba SOI. The resulting Schr\"odinger-like equations are two coupled equations, which cannot be solved in an explicit form. However, invoking the limits of a strong magnetic field, ${\it l}_c \ll {\it l}_0$, and $k_y{\it l}_0\ll 1$ allow us to solve this set of coupled equations exactly. We then derive and discuss the dispersion relations for collective excitations within the framework of Bohm-Pines' RPA. The intrasubband and intersubband magnetoplasmons in a Q1DEG are characterized, respectively, by the negative-B dispersion and the magnetoroton excitation. Here we scrutinize the effect of the Rashba SOI on these characteristics in depth. We observe that the SOI modifies drastically the behavior of both magnetoplasmons in the long wavelength limit and may render them relatively more susceptible to the Landau damping in the short wavelength limit. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W33.00013: Resonant spin polarization and Hall conductances in a two-dimensional electron gas Degang Zhang, Yao-Ming Mu, Chin-Sen Ting We have studied transport properties in a two-dimensional electron gas with equal Rashba and Dresselhaus spin-orbit interactions under a perpendicular magnetic field. By employing the exact solution for this system, we found resonant charge and spin Hall conductances at a certain magnetic field, where all the nearest-neighboring Landau levels cross. Near the magnetic field, there exists a resonant spin polarization, which can also induce resonant charge and spin Hall effects. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W33.00014: Spin signal recovery in a two-dimensional electron gas with a quantum point contact Jae-Seung Jeong, Hyun-Woo Lee We study transport properties of the spin-polarized current in a two-dimensional electron gas(2DEG) including a quantum point contact(QPC) in the presence of Rashba spin-orbit(RSO) coupling. Spin-resolved conductance is investigated numerically using a recursive Green function method, with special attention to the quantum effects of spin-charge transport channels. It is found that when the conductance is examined as a function of the RSO coupling strength, the conductance modulation ratio, defined as the ratio between the maximum and minimum conductances, can be enhanced by the QPC in the ballistic and weakly diffusive regime as well. Decaying rate of the spin-polarization can be also reduced. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W33.00015: Semiclassical Equations of Motion for Bloch Electrons in External Fields, Including Spin-Orbit Interaction. W.C. Kerr This talk considers an electron moving in a periodic potential with spin-orbit interaction and perturbed by external slowly varying electric field and uniform magnetic field. Superposition of the time-independent Bloch spinor states of the unperturbed Hamiltonian gives a wave packet state with both wavevector space and spin-orientation amplitude factors. The time-dependent variational principle produces equations of motion for the centers of the wave packet in both configuration and wavevector space and for the spin-orientation factors. For spinless electrons this procedure yields the familiar semiclassical equations of motion augmented by an orbital magnetic moment contribution to the Bloch band energy and an ``anomalous velocity'' proportional to a Berry curvature.\footnote{M. C. Chang \& Q. Niu, Phys. Rev. B {\bf{53}}, 7010 (1996)} The inclusion of spin-orbit interaction gives additional contributions to the velocity involving different Berry curvatures. One is a spin-dependent contribution to the magnetic moment, and another is an electric-dipole-like contribution that is also proportional to the spin operator. [Preview Abstract] |
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