Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H35: Topological Insulators: General 
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Sponsoring Units: DCMP Chair: Sukbum Chung, Stanford University Room: C140 

H35.00001: ABSTRACT WITHDRAWN 
Tuesday, March 22, 2011 8:12AM  8:24AM 
H35.00002: Topological Insulators as Substrates for CO Oxidation Catalysis by Ultrathin Au Films Hua Chen, Wenguang Zhu, Di Xiao, Zhenyu Zhang We propose a novel application of three dimensional topological insulators (3DTIs) in heterogeneous catalysis based on first principles calculations within density functional theory. We use a Bi$_2$Se$_3$ substrate as the support of an ultrathin Au film, and show that the Au adatoms are strongly bound to and able to wet the surface of Bi$_2$Se$_3$. More importantly, we find the topological surface states of Bi$_2$Se$_3$ are robust against Au deposition, and it can enhance the interaction between Au and CO, O$_2$ molecules by acting as an $``$electron bath$"$. The present study may broaden the potential technological applications of 3DTIs, and shine some new light on the understanding of the role of surface states in heterogeneous catalysis. [Preview Abstract] 
Tuesday, March 22, 2011 8:24AM  8:36AM 
H35.00003: Topological insulators on the ruby lattice Xiang Hu, Mehdi Kargarian, Gregory Fiete We study a tightbinding model on the twodimensional ruby lattice. This lattice supports two types of second neighbor spindependent hopping parameters in an sband model that preserves timereversal symmetry. We discuss the phase diagram of this model for various values of the hopping parameters, and note an interesting interplay between the two spinorbit terms that individually would drive the system to a Z2 topological insulating phase. The fidelity of each phase is also calculated. [Preview Abstract] 
Tuesday, March 22, 2011 8:36AM  8:48AM 
H35.00004: Extension of the Kitaev model on the square lattice Ryota Nakai, Akira Furusaki, Shinsei Ryu We study an extension of the Kitaev model [1] on the square lattice, where two types of Gamma matrices on neighboring sites have interaction that respects time reversal symmetry. A family of Kitaev models can be classified as the topological insulator/superconductor when described by Majorana fermions [2]. Our model is in class DIII in AltlandZirnbauer classification, and thus a Z$_{2}$ invariant characterizes two distinct phases. There appear helical Majorana edge modes in the topological phase. The same model on the onedimensional ladder is also studied. \\[4pt] [1] A. Kitaev, Annals of Physics 321, 2 (2006).\\[0pt] [2] S. Ryu, Phys. Rev. B 79, 075124 (2009). [Preview Abstract] 
Tuesday, March 22, 2011 8:48AM  9:00AM 
H35.00005: Electrical manipulation and measurement of spin properties of quantum spin Hall edge states Teemu Ojanen, Jukka Vayrynen The existence of the quantum spin Hall state has been confirmed in a series of experiments performed in HgTe quantum wells but a quantitative observation of the helical edge structure is still lacking. We study an electrical manipulation and measurement of helicity properties of the edge states by employing the Rashba spinorbit interaction (SOI). Specifically, we show that a spatially uniform Rashba SOI, controllable by the gate voltage, can be utilized in tuning the spin orientation of the edge modes (J. I. V\"ayrynen and T. Ojanen, arXiv:1010.1353). We introduce a pointcontact geometry where helicity of the edge modes can be accessed by two terminal conductance measurements. [Preview Abstract] 
Tuesday, March 22, 2011 9:00AM  9:12AM 
H35.00006: Electronic Transport in Exfoliated Bismuth Selenide Andrew Bestwick, James Williams, Patrick Gallagher, David GoldhaberGordon, James Analytis, Ian Fisher Recent theoretical and experimental work has identified bismuth selenide as a promising candidate for studies of threedimensional topological insulators due to its large bulk semiconducting gap crossed by topological Dirac surface states. We report on the fabrication and measurement of mesoscale exfoliated bismuth selenide devices, including the effects of electricfieldeffect gating and magnetic field on transport and possible signatures of topological states. We will also discuss fabrication strategies to mitigate surface disorder and doping [Preview Abstract] 
Tuesday, March 22, 2011 9:12AM  9:24AM 
H35.00007: Approaching the insulating state in Cadoped Bi$_{2}$Se$_{3}$ nanodevices Peng Wei, Zhiyong Wang, Jing Shi We report a systematic tuning of the carrier density in Cadoped Bi$_{2}$Se$_{3}$ nanodevices. A clear transition from the metallic to insulating state is observed as both the Caconcentration and the gate voltage are tuned. At high temperatures, the devices behave metallic as indicated by the linear temperature dependence of the electrical resistivity as the devices are initially cooled. As the temperature is lowered, the resistivity shows a minimum then increases. This insulating behavior can be modeled by a thermal activated conductivity, which is taken over by saturation at the lowest temperatures. At 1.5 K, the resistivity undergoes a $\sim $5fold increase as a gate voltage is swept from 60 V to 60 V. The fieldeffect mobility is found to be about $\sim $ 5000 cm2/Vs. We have also observed a systematic evolution of the magnetoresistance as the chemical potential is tuned via the gate voltage. The combination of the chemical and electronic dopings provides an effective way to access the low carrier density gap states in Bi2Se3 topological insulator nanodevices. This work was supported in part by DOE and NSF. [Preview Abstract] 
Tuesday, March 22, 2011 9:24AM  9:36AM 
H35.00008: Topological Response Theory of Doped Topological Insulators Maissam Barkeshli, Xiaoliang Qi We generalize the topological response theory of threedimensional topological insulators (TI) to metallic systems  specifically, doped TI with finite bulk carrier density and a timereversal symmetry breaking field near the surface. We show that there is an inhomogeneityinduced Berry phase contribution to the surface Hall conductivity that is completely determined by the occupied states and is independent of other details such as band dispersion and impurities. In the limit of zero bulk carrier density, this intrinsic surface Hall conductivity reduces to the halfinteger quantized surface Hall conductivity of TI. Our theory is directly related to the TI materials currently under experimental investigation, which have finite residual bulk carrier densities. [Preview Abstract] 
Tuesday, March 22, 2011 9:36AM  9:48AM 
H35.00009: Topological states in one dimensional solids and photonic crystals Timothy Atherton, Harsh Mathur We show that the band structure of a onedimensional solid with particlehole symmetry may be characterized by a topological index that owes its existence to the nontrivial homotopy of the space of nondegenerate real symmetric matrices. Moreover we explicitly demonstrate a theorem linking the topological index to the existence of bound states on the surface of a semiinfinite one dimensional solid. Our analysis is a onedimensional analogue of the analysis of topological insulators in two and three dimensions by Balents and Moore; our results may be relevant to long molecules that are the one dimensional analogue of topological insulators. We propose the realization of this physics in a onedimensional photonic crystal. In this case the topology of the bandstructure reveals itself not as a bound surface state but as a Lorentzian feature in the time delay of light that is otherwise perfectly reflected by the photonic crystal. [Preview Abstract] 
Tuesday, March 22, 2011 9:48AM  10:00AM 
H35.00010: Robustness of topologically protected surface states in layering of Bi$_2$Te$_3$ thin films Kyungwha Park, Jean Heremans, Vito Scarola, Djordje Minic Recently, topological insulators with timereversal symmetry have drawn great attention due to their topologically protected states. Topological insulators differ from band insulators in that a bulk energy gap opens up due to strong spinorbit coupling and that metallic surface states reside in the energy gap. The surface states of topological insulators are topologically protected in that impurities preserving timereversal symmetry neither destroy the surface states nor impact the topological nature of the surface states. Recently, bulk bismuthbased alloys were shown to be topological insulators. Thin films offer valuable probes of topological insulators as well as device applications. Additionally, bismuthbased thin films of a thickness of a few nanometers were fabricated on substrates or suspended across trenches. We investigate surface states of Bi$_2$Te$_3$(111) thin films of one to six quintuple layers using densityfunctional theory including spinorbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi$_2$Te$_3$ thin films, we examine the topological nature of the surface states as a function of the film thickness and compare our results with experimental data and other theoretical reports. [Preview Abstract] 
Tuesday, March 22, 2011 10:00AM  10:12AM 
H35.00011: Spin and Charge Transport on the Surface of a Topological Insulator Anton Burkov, David Hawthorn We derive diffusion equations, which describe spincharge coupled transport on the helical metal surface of a threedimensional topological insulator. The main feature of these equations is a large magnitude of the spincharge coupling, which leads to interesting and observable effects. In particular, we predict a new magnetoresistance effect, which manifests in a nonohmic correction to a voltage drop between a ferromagnetic spinpolarized electrode and a nonmagnetic electrode, placed on top of the helical metal. This correction is proportional to the crossproduct of the spin polarization of the ferromagnetic electrode and the charge current between the two electrodes. We also demonstrate tunability of this effect by applying a gate voltage, which makes it possible to operate the proposed device as a transistor. [Preview Abstract] 
Tuesday, March 22, 2011 10:12AM  10:24AM 
H35.00012: Spin and Charge Transport in Thin Films of Topological Insulators Alexander Zyuzin, Anton Burkov We develop a theory of spincharge coupled transport in thin films of topological insulator materials, when the top and bottom surfaces of the sample hybridize. We find significant differences from the case of transport on unhybridized surfaces. In particular, we find significant reduction of the spin relaxation rates, which enhances all the spinrelated transport effects, compared to the case of a single surface. We also find that the outofplane component of the spin, which is absent from the hydrodynamic transport equations in the single surface case, reappears when the surfaces are hybridized. [Preview Abstract] 
Tuesday, March 22, 2011 10:24AM  10:36AM 
H35.00013: Conductance of a helical edge liquid coupled to a magnetic impurity Yoichi Tanaka, Akira Furusaki, K.A. Matveev In a quantum spin Hall system, which can be realized in HeTe/(Hg,Cd)Te semiconductor quantum wells [1], helical edge states carry a current and the conductance takes the universal value of $2e^2/h$. This is because an impurity without internal degrees of freedom cannot backscatter an electron at the edge in the presence of timereversal symmetry [2]. On the other hand, backscattering by a magnetic impurity is allowed. We study the effect of backscattering from a magnetic impurity on the conductance of a quantum spin Hall system [3], and obtain the correction $\delta G(\omega)$ to the electrical conductance as a function of frequency $\omega$. We find that the correction $\delta G(\omega)$ vanishes in the dc limit ($\omega \to 0$), when our model conserves the total spin $S_z$. Another interesting transport property is the thermal conductance, which is affected by the coupling to the magnetic impurity even at $\omega\to 0$. We find that the temperature dependence of the thermal conductance shows a nonmonotonic behavior with a minimum occurring at the Kondo temperature. \\[4pt] [1] M. Konig {\it et al.}, Science 318, 766 (2007). \\[0pt] [2] C. Wu, B. A. Bernevig and S. C. Zhang, Phys. Rev. Lett. 96, 106401 (2006); C. Xu and J. E. Moore, Phys. Rev. B 73, 045322 (2006). \\[0pt] [3] J. Maciejko {\it et al.}, Phys. Rev. Lett. 102, 256803 (2009). [Preview Abstract] 
Tuesday, March 22, 2011 10:36AM  10:48AM 
H35.00014: Design Principles and Coupling Mechanisms in the 2D QuantumWell Topological Insulator HgTe/CdTe JunWei Luo, Alex Zunger We present atomistic band structure calculations revealing a different mechanism than recently surmised via {$\bf k\cdot p$} calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that {\it 2D interface} (not {\it 1D edge}) TS are possible. We find that the transitions from a topological insulator at critical HgTe thickness of $n= 23$ ML (62.5 {\AA}) to a normal insulator at smaller $n$ is due to the crossing between two interface localized states: one derived from the Slike $\Gamma_{6c}$ and one derived from the Plike $\Gamma_{8v}$ lighthole, not because of the crossing of an interface state and an extended QW state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi$_2$Se$_3$. Ref: J.W. Luo and A. Zunger, Phys. Rev. Lett. {\bf 105}, 176805 (2010). [Preview Abstract] 
Tuesday, March 22, 2011 10:48AM  11:00AM 
H35.00015: Towards Quantum Spin Hall Effect in InAs/GaSb Quantum Wells Ivan Knez, RuiRui Du, Gerrard Sullivan Recently, it has been proposed that inverted InAs/GaSb composite quantum wells (CQWs) should exhibit the Quantum Spin Hall Effect (QSHE), characterized by the energy gap in the bulk and gapless edge modes which are protected from backscattering by time reversal symmetry. We have successfully fabricated a doublegated device on highquality MBEgrown InAs/GaAs CQWs in the inverted regime, in which we were able to vary the band structure via an electrical field, and tune the Fermi level into minigap regime. We observed clear evidence for an energy gap in the inverted regime, with values of the gap consistent with those theoretically predicted; however, the minigap exhibits residual conductivity of nontrivial origin, which complicates transport investigation of proposed edge channels. We note that the InAs surface states around the sample edges may play a role in the observed resistivity features. In ongoing work, we pursue Cooper pair injection experiments by proximity to an swave superconductor, which should provide a novel probe of the proposed helical edge modes. We will discuss our progress towards observing QSHE in this unique material system. The work at Rice was supported by grants from NSF, Keck Foundation, and Hackerman Advanced Research Program. [Preview Abstract] 
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