Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q35: Focus Session: Spins in Semiconductors -- Spin Device Physics |
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Sponsoring Units: GMAG DMP FIAP Chair: Hanan Dery, University of Rochester Room: E145 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q35.00001: Spin dynamics for wave packets in Rashba systems Bailey C. Hsu, Jean-Francois S. Van Huele We explore the spin dynamics of wave packets inside Rashba systems using a spin propagator approach. A spin propagator gives conditional probability amplitude between two points in position and spin space at a time interval [1]. We derive and apply the Rashba spin propagator to localized spin wave packets. We observe several interesting features, such as\textit{spin separation}, \textit{persistent spin helix}, \textit{bamboo-shooting structure, ripple formation structure}, and others [2]. We discuss these features which depend critically on the choice of both the width of the wavepacket and the Rashba coupling strength quantitatively with realistic experimental data. [1].B. C. Hsu and J.-F. S. Van Huele, J. Phys. A: Math. Theor., \textbf{42}, 475304 (2009). [2]B. C. Hsu and J.-F. S. Van Huele, Phys. Rev. B, \textbf{80} , 19XXXX (2009). [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q35.00002: D'yakonov-Perel' spin relaxation in the interacting electron gas in doped semiconductors Matthew Mower, Giovanni Vignale D'yakonov-Perel' spin relaxation describes spin precession limited by collisions, an effect studied by many authors. It is of fundamental importance in spintronics as it controls spin polarization decay times. The spin relaxation rate is expressed as $1/\tau_s=\langle\Omega_{\mathbf{k}}^2\tau_k^*\rangle$, where $\Omega_{\mathbf{k}}$ is the spin precession frequency due to spin-orbit interaction, and $\tau_k^*$ is an effective momentum relaxation time quite different from that in electrical conductivity or the quasiparticle lifetime ($\langle\,\rangle$ denotes an average over the classical or quantum momentum distribution). An explicit quantitative study of $\tau_k^*$ in three dimensional degenerate electron liquids has not yet been provided. In this study we adapt the classic Abrikosov-Khalatnikov theory of transport in Fermi liquids to the calculation of $\tau_k^*$ in the degenerate electron gas. This approach enables us to include both direct and exchange scattering processes. We also introduce an effective electron-electron interaction to include correlation corrections to $\tau_k^*$. Results are presented for different densities and sub-Fermi temperatures. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q35.00003: New Paradigms for Spintronics: Spin-Valve Effect in Single-Crystal Ruthenates M. Ge, S. Chikara, O.B. Korneta, T.F. Qi, G. Cao The spin valve effect is thought to be a delicate quantum phenomenon that depends upon precision deposition and nanoscale patterning of artificial thin-film heterostructures whose quality and performance are difficult to control. Here we demonstrate that a novel, strong spin valve effect exists in \textit{bulk} single-crystal ruthenates having an anisotropic, layered crystal structure [1]. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices. \\[4pt] [1] G. Caoet al., \textit{PRL}\textbf{100}, 016604 (2008) [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q35.00004: Modeling spin transport with current-sensing spin detectors Jing Li, Ian Appelbaum The impulse response (or ``Green's function'') of a current-sensing spin detector is derived analytically by incorporating the proper boundary conditions. This result is also compared to a Monte Carlo simulation (which automatically takes the proper boundary conditions into account) and an empirical spin transit time distribution obtained from experimental spin precession measurements. In the strong drift-dominated transport regime, this spin \emph{current} impulse response can be approximated by multiplying the spin \emph{density} impulse response by the average drift velocity. However, in weak drift fields, large modeling errors up to a factor of 3 in most-probable spin transit time can be incurred unless the full spin current Green's function is used. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q35.00005: A general dynamical model for spin transprot in a non-degenerte semiconductor with non-collinear ferromagnet terminals Yang Song, Hanan Dery We present a comprehensive model of spin transport in a non-degenerate semiconductor in contact with multiple ferromagnetic terminals. The model is generalized to include non-collinear magnetization configurations as well as the dynamic response following a perturbation of one of the magnetization directions. We reexamine the quasi-neutrality approximation and in addition to the WKB calculation of the coherent FM/SC interface transmission, we include a non-coherent transport mechanism due to the inhomogeneous doping profile at the interface region. The non-coherent effect is relevant in forward bias conditions where we find a nearly opposite average spin polarization rule in the semiconductor channel. The validity of the employed boundary conditions at the reverse biased interface are checked by calculating the Dyakonov-Perel spin relaxation of the injected hot electrons. We study transient currents and other dynamical signals of a three terminal device with 16 distinguishable states. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q35.00006: Conductance asymmetry of a slot gate Si-MOSFET in a strong parallel magnetic field Issai Shlimak, D. I. Golosov, A. Butenko, K.-J. Friedland, S. V. Kravchenko We report measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities $n_{1,2}$ across the slot. The dynamic longitudinal resistance was measured by the standard lock-in technique, while maintaining a large DC current through the source-drain channel. We find that the conductance of the sample in a strong parallel magnetic field is asymmetric with respect to the DC current direction. This asymmetry increases with magnetic field. The results are interpreted in terms of electron spin accumulation or depletion near the slot. Preprint arXiv:0909.1491 [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q35.00007: Billiard simulation and FFT analysis of AAS oscillations in nanofabricated InGaAs Takaaki Koga, Sebastien Faniel, Shunsuke Mineshige, Toru Matsuura, Yoshiaki Sekine Gate-voltage-dependent amplitude of magneto-conductance oscillation was analyzed using FFT method. The obtained FFT spectrum was compared with the areal dependence of the occurrence and spin interferece amplitude, calculated for Altshuler-Aronov-Spivak (AAS) type time-reversal pairs of the interference paths on all possible classical trajectroies that were obtained by extensive billiard simulations within the given structures. We have calcuated generic spin interference (SI) curves as a function of the Rashba parameter $\alpha$, for various values of the Dresselhaus parameter ${\rm b}_{41}^{6c6c} $ [eV\AA$^3$]. The comparison between theory and experiment suggested that the value of ${\rm b}_{41}^{6c6c}$ should be considerably reduced from 27 eV\AA$^3$, the generally known value from the ${\bf k\cdot p}$ theory. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q35.00008: Controlling the spin-orbit amplitude in a non-flat quantum well Oleg Shalaev, Giovanni Vignale Using the inverse-scattering theory, we adjust the wave functions in a quantum well so that electrons occupying the lowest subband conserve their spin projection, while electrons occupying the higher subband experience Rashba spin-orbit interaction. Shifting the Fermi level in the well with an external gate, one can drastically change the strength of the spin-orbit interaction felt by electrons. Such system can work as a spin-orbit trigger which has two states: (i) when the spin projection $s_z$ is a constant and (ii) when the spin precesses due to the spin-orbit interaction. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q35.00009: Influence of magnetic field orientation on the Zeeman spin-splitting in InGaAs quantum point contacts Theodore Martin, Alex Szorkovszky, Adam Micolich, Alex Hamilton, Colleen Marlow, Richard Taylor, Heiner Linke, Hongqi Xu We present measurements of the Zeeman spin-splitting in a quantum point contact (QPC) etched into an InGaAs/InP heterostructure [1], comparing magnetic field orientations in the plane and perpendicular to the InGaAs quantum well. We observe an isotropic Zeeman splitting for fields oriented in the plane of the quantum well, with a magnitude that is enhanced by up to a factor of two compared to two-dimensional electron systems in InGaAs/InP [2]. The Zeeman splitting is much larger when the magnetic field is perpendicular to the quantum well, resulting in a g-factor of 15.7 in the one dimensional limit. \\[4pt] [1] T. P. Martin, \emph{et al.}, \emph{Appl. Phys. Lett.} \textbf{93}, 012105 (2008).\\[0pt] [2] B. Kowalski, \emph{et al.}, \emph{Phys. Rev. B} \textbf{49}, 14786 (1994). [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q35.00010: Exchange coupling in permalloy/BiFeO$_3$ heterostructures John Heron, Chen Wang, David Carlton, Mark Nowakowski, Martin Gajek, David Awschalom, Jeff Bokor, Dan Ralph, R. Ramesh BiFeO$_3$ is a ferroelectric and antiferromagnetic multiferroic with the ferroelectric and antiferromagnetic order parameters coupled at room temperature. This coupling results in the reorientation of the ferroelectric and magnetic domains as applied voltages switch the electric polarization. Previous studies using ferromagnet/BiFeO$_3$ heterostructures have shown that the anisotropy of the ferromagnetic layer can be tuned by the ferroelectric domain structure of the BiFeO$_3$ film [1, 2]. The physical mechanism driving this exchange bias with BiFeO$_3$ is still under investigation. We use patterned permalloy structures, with varying aspect ratios, on BiFeO$_3$ thin films to investigate the physics of this interaction. The results of our studies using MFM, PEEM, and MOKE to understand this mechanism as a means to electric field control of magnetic structures will be presented. \\[4pt] [1] H. Bea et al., Physical Review Letters 100, 017204 (2008).\\[0pt] [2] L.W. Martin et al., Nanoletters 8, 2050 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q35.00011: Tunable g-factors and spin orbit coupling strength in SiGe single-hole transistors G. Katsaros, P. Spathis, M. Stoffel, F. Fournel, M. Mongillo, V. Bouchiat, F. Lefloch, A. Rastelli, O. Schmidt, S. De Franceschi A prominent branch of spintronics aims at exploiting the electronic spin degree of freedom either for encoding and manipulating quantum information or for switching the state of transistors in a more efficient way. While ground-breaking achievements could be made mainly on GaAs-based heterostructures, the importance of exploring alternative material systems with favourable properties such as long spin coherence is now widely recognized. Here we report for the first time the realisation of single-hole transistors based on individual self-assembled SiGe quantum dots. We observe a variety of low-temperature transport regimes depending on the strength of quantum confinement and the tunnel coupling to the leads. Transport spectroscopy reveals largely anisotropic hole g factors with pronounced dependence on gate voltage and magnetic field. Quantitative evidence of a strong spin-orbit coupling is obtained from the observation of field-induced avoided crossings between energy levels with different spin quantum number. Interestingly, the coupling strength is found to vary (and even vanish) with the field direction, in line with recent theoretical predictions. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q35.00012: Selection rules of intervalley spin scattering in silicon and germanium Jian-Ming Tang, Michael E. Flatte, Brian T. Collins Manipulation of carrier spins in semiconductor devices requires long spin transport lengths and coherence times. The spin coherence times in silicon are known to be long at low temperature. Near room temperature, the intervalley spin scattering due to spin-orbit interaction becomes important relative to the intravalley acoustic scattering. We study spin flip processes in silicon and also in germanium. The selection rules for various intervalley scattering processes are analyzed. The spin-flip rate from the f processes is an order of magnitude larger than from the g processes. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q35.00013: Spin relaxation in SiGe islands Hans Malissa, Wolfgang Jantsch, Gang Chen, Thomas Fromherz, Friedrich Schaffler, Guenther Bauer, Alexei Tyryshkin, Stephen Lyon, Zbyslaw Wilamowski We investigate the spin properties of electrons confined in MBE grown SiGe islands using photoluminescence and electron spin resonance (ESR), the latter both in continuous wave and in pulsed mode. Three dimensional Ge islands are grown on unstructured and structured Si(100) substrates, which leads to strain in the Si layer that is deposited on top, giving rise to electron confinement inside the Si layer above the Ge islands. This growth sequence is repeated up to 12 times in order to obtain a total of more than $10^{10}$ quantum dots. Under illumination with sub band gap light, we observe a g-factor, ESR line width, and spin lifetimes that correspond to Si conduction band electrons with an additional inhomogeneous broadening. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q35.00014: Ab-initio calculation of spin relaxation times Oscar D. Restrepo, Wolfgang Windl One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.\\[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. \textbf{94}, 212103 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q35.00015: Electric field control of spin precession in KTaO$_3$ field-effect transistor Hiroyuki Nakamura, Tsuyoshi Kimura Field-effect transistors (FET) with KTaO$_3$ single crystal channel were fabricated to study spin-orbit coupling effects on the gate-induced electron gas. By applying gate voltage via organic gate insulator parylene, an electron accumulation layer was successfully formed at the interface of KTaO$_3$. By analyzing magnetoresistance associated with weak ntilocalization at low temperature, we find that the spin precession length is remarkably short in this system, in the 20-70 nm range [1]. The factors possibly leading to this remarkably short spin precession length are 1) heavy electron mass originating from $d$-bands, 2) strongly asymmetric potential well, and 3) strong spin-orbit coupling caused by 5$d$ element, tantalum.\\[4pt] [1] H. Nakamura and T. Kimura, Phys. Rev. B, 80, 121308(R) (2009). [Preview Abstract] |
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