Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X3: Spin Hall and Quantum Spin Hall Effects |
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Sponsoring Units: DCMP Chair: David Awschalom, University of California, Santa Barbara Room: 301/302 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X3.00001: Imaging electrical spin generation and spin Hall dynamics in semiconductors Invited Speaker: The capability to generate and manipulate spin polarization through the spin-orbit interaction drives interest in all-electrical techniques to exploit electron spins for semiconductor spintronics. The spin Hall effect refers to the generation of a pure spin current transverse to a charge current, resulting in a spontaneous spin accumulation near sample boundaries without the need for magnetic fields or materials. Recent experiments toward imaging this electrically generated spin polarization with both spatially and temporally resolved Kerr rotation microscopy in bulk zincblende semiconductors are discussed. Both current-induced in-plane spin polarization and out-of-plane spin accumulation from the spin Hall effect are observed in ZnSe up to room temperature\footnote{N. P. Stern, S. Ghosh, G. Xiang, M. Zhu, N. Samarth, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 126603 (2006)}. In GaAs devices, spatially resolved measurements of steady-state spin Hall accumulation and associated modeling clarify the important role of drift and diffusion in transporting spins generated at sample boundaries to the device interior\footnote{N. P. Stern, D. W. Steuerman, S. Mack, A. C. Gossard, and D. D. Awschalom, \textit{Appl. Phys. Lett.} \textbf{91}, 062109 (2007)}. In these typical optical experiments, electrically-generated spin accumulation is measured using steady-state techniques that do not directly observe dynamics at timescales important for device operation. Here we discuss a time- and spatially-resolved measurement of the spin Hall effect using a pulsed current to drive spin accumulation\footnote{N. P. Stern, D. W. Steuerman, S. Mack, A. C. Gossard, and D. D. Awschalom, \textit{Nat. Physics} \textbf{4}, 843 (2008)}. The dynamical processes of spin accumulation and diffusion reveal spatially-dependent nanosecond timescales comparable to the electric-field dependent spin coherence time. A time-dependent diffusion analysis reconciles the observed spatial and temporal dynamics of spin accumulation from the spin Hall effect in one coherent picture. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X3.00002: The quantum spin Hall effect and the topological magneto-electric effect Invited Speaker: Search for topologically non-trivial states of matter has become a important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction[1] of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation [2]. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators [4]. I shall also present realistic experimental proposals to observe fractional charge [3], spin-charge separation and the deconfinement of the magnetic monopoles in these novel topological states of matter. \\[4pt] [1] A. Bernevig, T. Hughes and S. C. Zhang, Science, 314, 1757, (2006) \\[0pt] [2] M. Koenig et al, Science 318, 766, (2007) \\[0pt] [3] X. Qi, T. Hughes and S. C. Zhang, Nature Physics, 4, 273 (2008) \\[0pt] [4] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, ``Topological Field Theory of Time-Reversal Invariant Insulators", Phys. Rev B. 78, 195424 (2008) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X3.00003: Experimental observation of the quantum spin Hall state in HgTe quantum wells Invited Speaker: Spin-orbit interaction in semiconductors causes many interesting and potentially useful transport effects, such as e.g. the presently very topical spin-Hall effect[1]. So far no direct evidence for a ballistic, intrinsic SHE (i.e. resulting from the band structure) has been obtained by transport experiments. Here, we demonstrate that in specially designed nanostructures[2], which are based on narrow gap HgTe type-III quantum wells, a detection of the spin signal is possible via non-local voltage measurements. Recently, it was pointed out that such HgTe quantum wells, that exhibit an inverted band structure where the ordering of electron- and hole-like states is interchanged, are topologically non-trivial insulators[3], in which the quantum spin Hall insulator state[4] should occur. In this novel quantum state of matter, a pair of spin polarized helical edge channels develops when the bulk of the material is insulating, leading to a quantized conductance. I will present transport data provide very direct evidence for the existence of this third quantum Hall effect: when the bulk of the material is insulating, we observe a quantized electric conductance[5]. Finally, we demonstrate how a combination of the techniques used in the above experiments allows us to verify that the transport in the quantum spin Hall insulator state indeed occurs through spin-polarized helical edge channels. \\[4pt] [1] S. Murakami et al., Science 301 (2003) 1348; J. Sinova et al., Phys. Rev. Lett. 92 (2004) 126603; Y. Kato et al., Science 306 (2004) 1910. \\[0pt] [2] E.M. Hankiewicz, et al., Phys. Rev. B 70 (2004) 241301(R).\\[0pt] [3] B.A. Bernevig et al., Science 314 (2006) 1757. \\[0pt] [4] C.L. Kane and E.J. Mele, Phys. Rev. Lett. 95 (2005) 146802. \\[0pt] [5] M. K\"{o}nig et al., Science 318, 766 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X3.00004: Theory of Topological Insulators Invited Speaker: Topological insulators are materials with a bulk excitation gap generated by the spin orbit interaction, and which are different from conventional insulators. This distinction is characterized by Z$_2$ topological invariants, which characterize the ground state. In two dimensions a single Z$_2$ invariant describes the quantum spin Hall insulator phase. In three dimensions there are four Z$_2$ invariants, distinguishing ``weak'' (WTI) and ``strong'' (STI) topological insulators. The STI phase is characterized by the presence of unique gapless surface states whose Fermi surface encloses an odd number of 2D Dirac points. We will argue theoretically that the semiconducting alloy Bi$_{1-x}$Sb$_x$ is a strong topological insulator -- a prediction that has recently been confirmed experimentally. We will next show that the proximity effect between this unique surface phase and an ordinary superconductor leads to a two dimensional state that resembles a spinless p$_x$+ip$_y$ superconductor, but does not break time reversal symmetry. This state supports zero energy Majorana bound states at vortices, and may provide a new venue to realize proposals for topological quantum computing. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X3.00005: Observation of a New Topological Phase of Quantum Matter : Quantum Hall-like Effect without Magnetic Field Invited Speaker: Most quantum states of condensed-matter are categorized by the spontaneously broken symmetries. The remarkable discovery of charge quantum Hall effects (1980s) revealed that there exists an organizational principle of matter based not on the spontaneously broken symmetry but only on the topological distinctions in the presence of time-reversal symmetry breaking. In the past few years, theoretical developments suggest that new classes of topological states of matter might exist that are purely topological in nature in the sense that they do not break time-reversal symmetry hence can be realized without any applied magnetic field : ``Quantum Hall-like effects without magnetic field.'' In this presentation, I report a series of experimental results documenting and demonstrating the existence of such a topologically ordered time-reversal-invariant state of matter and discuss the exotic electromagnetic and spin properties this novel phase of quantum matter might exhibit and outline their potential use. [Preview Abstract] |
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