Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P19: Focus Session: Spin Interference and Spin Hall Effect |
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Sponsoring Units: GMAG DMP Chair: Jairo Sinova, Texas A&M University Room: Baltimore Convention Center 316 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P19.00001: Direct observation of the Aharonov-Casher phase M. K\"{o}nig, E. M. Hankiewicz, Jairo Sinova, A. Tschetschetkin, V. Hock, V. Daumer, M. Sch\"{a}fer, C. R. Becker, H. Buhmann, L. W. Molenkamp We report the direct observation of Aharonov-Casher effect, which can occur when electrons propagate in a ring structure in the presence of spin-orbit interactions and external magnetic field perpendicular to the ring plane. The transport measurements have been conducted on the series of ring structures fabricated from HgTe/HgCdTe quantum wells. We study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe non-monotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect and confirm it by numerical calculations of the magneto-conductance for a multichannel ring within the Landauer-B\"uttiker formalism. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P19.00002: Spin Interference Effect in a Square Loop Array including the Rashba and Dresselhaus Terms T. Koga$^2$, H. Okutani, Y. Sekine, J. Nitta$^{2,3}$ The effect of electron wave interference to the electric conductivity ($\sigma$), including the effect of spin degree of freedom, is investigated through nanolithographically defined square (and other) loop array structures fabricated on In$_ {0.52} $Al$_{0.48}$As/In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48} $As quantum wells (QW). In this experiment, we measure $\sigma$'s of QWs as a function of magnetic field $B$ (${\bf B}$$\perp$QW plane). These samples had a gate electrode covering the entire loop array structures, where a gate voltage $V_g$ was applied between the metal gate electrode and the QW. We note that $V_g$ controls both the carrier density and the Rashba and Dresselhaus spin-orbit terms within the QWs. It turned out that the magnetoconductance $\sigma(B)$ oscillates as a function of $B$ with a period corresponding to $h/2e$, which is denoted as the AAS oscillation. We found that the amplitude of the AAS oscillation in this system also oscillated as a function of $V_g$, which is called as a ``spin interference'' effect. We investigated this effect, which is also in close relation to the ``Aharonov-Casher'' effect (electric control of the phase of the electronic wave function), in detail including both the Rashba and Dresselhaus spin-orbit terms quantitatively. [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P19.00003: Numrical simulation of a spin interferometer based on a single square loop with Rashba interaction Zhenyue Zhu, Qing-Feng Sun, Bin Chen, Xin-Cheng Xie We numerically calculate the transverse conductance as a function of magnetic field in two models. One is an exact 1D model and the other is a quasi 1D square loop system which is similar to the experimental setup by Koga et. al. From the conductance curves, we employ FFT (Fast Fourier transform) and IFFT (inverse Fast Fourier transform) to extract separately the oscillatory part of conductance whose period correspond to the magnetic flux quanta (AB oscillation) and half quanta (AAS oscillation). We show that the spin precession angle $\theta$ is modulated by the Rashba interaction strength. From the curves about the amplitude of AB or AAS oscillations at B=0 versus $\theta$, we find that the node positions of $\theta$ in the exact 1D model fits well with previous theoretical calculations, but there are some deviations for the quasi 1D model. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P19.00004: Ahronov-Bohm oscillations in a GaAs 2D hole system Babur Habib, Emanuel Tutuc, Mansour Shayegan We have grown shallow 2D hole samples in GaAs for implementing mesoscopic structures via local anodic oxidation using an AFM. In this talk we present our results of a ring device which shows clear Ahronov-Bohm oscillations. The amplitude of the resistance oscillations are about 10{\%} of the ring resistance, the strongest seen in a 2D hole system to date. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P19.00005: Zero-field spin-splitting in Al$_{x}$Ga$_{1-x}$N/GaN heterostructures Ikai Lo, M.H. Gau, W.T. Wang, J.K. Tsai, S.F. Tsay, J.C. Chiang We have observed the beating Shubnikov-de Haas oscillations with respect to the zero-field spin splitting of 2DEG in Al$_{x}$Ga$_{1-x}$N/GaN heterostructures. The spin-splitting energy was obtained about 9 meV from the beating SdH frequency derived by the non-linear curve fitting. A new mechanism ($\Delta _{C1}-\Delta _{C3}$ coupling) was proposed to describe the large spin splitting in wurtzite GaN, which is originated from the band folding effect and intrinsic wurtzite structure inversion asymmetry. The band-folding effect generates two conduction bands ($\Delta _{C1}$ and $\Delta _{C3})$, in which $p$-wave probability has tremendous change when $k_{z}$ approaches anti-crossing zone. The $\Delta _{C1}-\Delta _{C3}$ coupling can produce a spin-splitting energy much larger than traditional Rashba or Dresselhaus effects. This project is supported in parts by National Science Council, Core Facilities Laboratory in Kaohsiung-Pintung area, Taiwan (ROC). [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P19.00006: Exact Landau Levels in Two-Dimensional Electron Systems with Rashba and Dresselhaus Spin-Orbit Interactions in a Perpendicular Magnetic Field Degang Zhang, C.S. Ting, C.-R. Hu We study a two-dimensional electron system in the presence of both Rashba and Dresselhaus spin-orbit interactions in a perpendicular magnetic field. Defining a suitable boson operator and using the unitary transformations we are able to obtain the exact Landau levels in the range of all the parameters. When the strengths of the Rashba and Dresselhaus spin-orbit interactions are equal, the Zeeman and spin-orbit splittings are independent of the Landau level index $n$. Due to the Zeeman energy, new crossing between the eigenstates $|n, k, s=1, \sigma>$ and $|n+1, k, s^\prime =-1, \sigma^\prime>$ is produced at certain magnetic field for larger Rashba spin-orbit coupling. This degeneracy leads to a resonant spin Hall conductance if it happens at the Fermi level. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 1:03PM |
P19.00007: Spatial imaging of the spin Hall effect and current-induced polarization in two-dimensional electron gases Invited Speaker: Spin-orbit coupling in semiconductors relates the spin of an electron to its momentum, and provides a pathway for electrically initializing and manipulating electron spins in zero magnetic field for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with strain in bulk semiconductors and quantum confinement in semiconductor heterostructures. Using Faraday and Kerr rotation spectroscopies with temporal and spatial resolution, current-induced spin polarization\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{93}, 176601 (2004).} and the spin Hall effect\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, \textit{Science} \textbf{306}, 1910 (2004).} have been observed in bulk semiconductors. More recently, we have investigated the spin Hall effect and current-induced spin polarization in a two-dimensional electron gas confined in (110) AlGaAs quantum wells using Kerr rotation microscopy\footnote{V. Sih, R. C. Myers, Y. K. Kato, W. H. Lau, A. C. Gossard and D. D. Awschalom, \textit{Nature Physics} \textbf{1}, 31 (2005).}. In contrast to previous measurements, the spin Hall profile shows complex structure and the current-induced spin polarization is out-of-plane. The experiments map the strong dependence of the current-induced spin polarization to the crystal axis along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. These results reveal opportunities for tuning a spin source using quantum confinement, strain and device engineering in non-magnetic materials. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P19.00008: Spin Hall effect, spin-accumulation, and spin-currents in mesoscopic structures Mario Borunda, Kentaro Nomura, Jairo Sinova Spin dependent transport effects in strongly spin-orbit coupled paramagnetic systems, such as the spin Hall Effect, have been studied extensively over the last few years. We explore how spin accumulation in a mesoscopic device could be used to observe the effect through electrical and optical means. We report calculations of spin flow in finite size samples with strong spin-orbit coupling using the non-equilibrium Green's function formalism in both the linear and the non-linear regimes. We explore different geometries and spin-orbit coupling mechanism to understand how spin relaxes near the interfaces. We will also report on the progress made in understanding the spin Hall Effect in the bulk regime and how it connects to the closely related effect of the anomalous Hall effect in ferromagnetic materials. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P19.00009: The finite spin Hall effect in semicondutors Naoyuki Sugimoto, Shigeki Onoda, Shuichi Murakami, Naoto Nagaosa We formulate the theory of the spin Hall effect taking into account the impurity scattering effect as general as possible with the focus on the definition of the spin current. The conserved spin current (Zhang {\it et, al}. [cond- mat/0503505]) satisfying the continuity equation of spin in the bulk is considered in addition to the conventional one defined by the anti-symmetric product of the spin and velocity operators. The condition for the non-zero spin Hall current is clarified from a generic viewpoint and the following new results for explicit models are obtained: (i) spin Hall current in Rashba model is non-zero on the non-delta impurity scattering potential, and (ii) spin Hall current vanishes in cubic Rashba model on the delta impurity scattering potential. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P19.00010: What is intrinsic and what is extrinsic in the spin Hall effect? Ewelina Hankiewicz, Giovanni Vignale, Michael Flatt\'{e} Two different forms of the spin Hall effect, intrinsic and extrinsic, have been recently proposed and observed in experiments. The intrinsic effect is caused by spin-orbit coupling in the band structure of the semiconductor and survives in the limit of zero disorder, whereas the extrinsic effect is caused by spin-orbit coupling between Bloch electrons and impurities. We treat both effects on equal footing within the framework of the exact Kubo linear response formalism. We show that the ``side-jump" term, which is usually considered part of the extrinsic spin Hall effect, is really intrinsic, because it is independent of disorder. Furthermore, it is the only non-zero {\it intrinsic} contribution to the spin-Hall effect for the linear Rashba (or Dresselhaus) spin-orbit coupling model. On the other hand, the skew scattering term is the only {\it extrinsic} contribution to the spin-Hall effect within this model. The proof based on gauge invariance holds at all orders in disorder and electron-electron interactions and to first order in spin-orbit coupling, but does not apply to more complex spin-orbit coupled bands (e.g the Luttinger model). We also study many-body effects and predict that the spin Coulomb drag will reduce the spin Hall conductivity. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P19.00011: Spin Hall Effect in Doped Semiconductor Structures Wang-Kong Tse, Sankar Das Sarma We present a microscopic theory of the extrinsic spin Hall effect based on the diagrammatic perturbation theory. Side-jump (SJ) and skew-scattering (SS) contributions are explicitly taken into account to calculate the spin Hall conductivity, and we show their effects scale as $\sigma_{xy}^{SJ}/\sigma_{xy}^{SS} \sim (\hbar/\tau)/\varepsilon_F$, where $\tau$ being the transport relaxation time. Motivated by recent experimental work we apply our theory to n-doped and p-doped 3D and 2D GaAs structures, obtaining analytical formulas for the SJ and SS contributions. Moreover, the ratio of the spin Hall conductivity to longitudinal conductivity is found as $\sigma_s/\sigma_c \sim 10^{-3}-10^{-4}$, in reasonable agreement with the recent experimental results of Kato \textit{et al}. [Science 306, 1910 (2004)] in n-doped 3D GaAs system. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P19.00012: Intrinsic spin Hall conductivity Pavel Krotkov, Sankar Das Sarma In an isotropic 2D gas with general dispersion and linear-in-k spin-orbital interaction of the Rashba or Dresselhaus type in the presence of impurities we find that an intrinsic spin-Hall conductivity is finite and is of the order of the spin-orbit term squared. It vanishes only in the well-studied particular case of a quadratic dispersion. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P19.00013: Imaging Stationary Flow of Spin Hall Effect-Induced Spin Densities in Mesoscopic Nanostructures Liviu Z\^arbo, Satofumi Souma, Branislav Nikoli\'c The spin Hall effect has recently attracted a lot of attention in semiconductor spintronics since it offers a novel way of all-electrical generation and manipulation of pure spin currents by employing spin-orbit (SO) couplings. To describe spin Hall transport on a scale of a few nanometers we introduce the concept of bond spin current and corresponding local flowing spin densities between the sites of the lattice model of a multiterminal SO coupled semiconductor nanostructure, and express them in terms of the nonequilibrium (Keldysh) Green functions. Our predictions for the out-of-plane polarized steady state spin densities flowing into the transverse interaction-free electrodes due to the longitudinal charge current injected into high-mobility two-dimensional electron gas (2DEG) with Rashba SO coupling crucially depend on the size of 2DEG in the units of spin precession length. In the presence of disorder, the flowing spin Hall densities remain non-zero in the bulk of the 2DEG. Moreover, we also find in-plane polarized spin densities flowing into the longitudinal leads due to the magneto-electric effect. These theoretically predicted images of mesoscopic spin Hall flow could be tested via recently advanced Kerr rotation microscopy. [Preview Abstract] |
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