Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J12: Topological Insulators: Magnetic Transport and Weak Localization |
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Sponsoring Units: DCMP Chair: Hadar Steinberg, Massachusetts Institute of Technology Room: 314 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J12.00001: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J12.00002: Transport Signatures of the Quantum Anomalous Hall Effect in 3D Topological Insulators Brian Dellabetta, Taylor Hughes, Matthew Gilbert The unique physics of spin-orbit coupled topological insulators (TIs) exposed to magnetic moments leads to a quantized conductance known as the quantum anomalous Hall effect (QAHE)\footnote{R. Yu et al., \emph{Science} {\bf 329}, 5987 (2010).}. While magnetic disorder has been experimentally shown to open a gap in the surface states of TIs\footnote{Y. L. Chen et al., \emph{Science} {\bf 329}, 659 (2010).}, no clear transport signatures of the QAHE have been observed in 3-dimensional TIs. We perform 3D real space calculations using the Non-Equilibrium Green's Function Formalism to show that topological insulators in proximity to arrays of ferromagnets offer a unique environment in which to study this phenomenon. We show that ferromagnetic domain walls on patterned surfaces manifest chiral surface modes with quantized currents that can be altered by changing the configuration of the magnetic arrays. We compare topologically trivial and nontrivial models to show a qualitative difference in the induced transport flow based on ferromagnet orientation, and propose a variety of experimental configurations which yield transport signatures of the QAHE. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J12.00003: Singular spin response of topological insulators in ac magnetic fields Haiming Deng, Lukas Zhao, Inna Korzhovska, Zhiyi Chen, Lia Krusin-Elbaum Orbital magnetic susceptibility in weak magnetic fields has several contributions whose physical origin is not simple in contrast to the clear Landau diamagnetism of free electrons. Experimentally, anomalous magnetism has been observed in graphite, and bismuth and Bi-Sb alloys, both of which are narrow gap semimetals. Here we report an observation of a singular response in ac magnetic susceptibility -- a suppression of diamagnetism at low magnetic fields that appears ubiquitous in all topological insulators (Sb$_2$Te$_3$, Bi$_2$Se$_3$, PbBi$_2$Se$_4$) we have studied. We observe two distinct contributions to this effect: a broader one that typically disappears around 40-50~K and is likely related to edges, and a divergent-like one (in the $H \rightarrow 0$ limit) that is robust up to room temperature and is likely related to the bulk. The frequency dependence and the dependence on the Fermi level of these effects will be discussed in the context of separation of orbital and spin effects. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J12.00004: Megnetoresistance in thin Bi2Te3 layers contacted by Indium (In) superconducting electrodes Zhuo Wang, Ramesh Mani Topological Insulators (TIs) are materials that insulate in the bulk but conduct electricity on their surfaces, which topologically protected by time-reversal symmetry. Transport measurements of topological insulators in the proximity of a superconductor are theoretically predicted to be a significant method to detect Majorana Fermions. Our experiment studied the interplay between superconductivity and TI surface states below the critical temperature of a type-I superconductor. Here, we used the four terminal lock-in technique to study the transport properties of Bi2Te3 specimens contacted by Indium superconducting electrodes, while sweeping perpendicular magnetic field, at T \textless\ 4.2 K. The results indicate a sharp suppression of the longitudinal resistance at weak magnetic fields, below the critical temperature of Indium. What's more exciting is that the interaction between superconductivity and TI surface states induces resistance enhancement at T $\le $ 2.8K, well below the critical temperature of Indium, in the absence of a magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J12.00005: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J12.00006: Magneto-optical Studies of Bi$_2$Te$_3$ Flakes Li-Chun Tung, Wenlong Yu, Zhigang Jiang, Dmitry Smirnov Magneto-transmittance spectroscopy is used to probe the magnetic-field induced excitations in topological insulator-Bi$_2$Te$_3$. Bi$_2$Te$_3$ single crystals are repeatedly exfoliated on scotch tape untill the sample flakes are sufficiently thin and become permeable in the infrared frequency range. The sample with the underlying tape is placed in a 4K cryostat and the magneto-optical properties of Bi$_2$Te$_3$ are investigated by a broadband Fourier-Transform infrared spectrometer (Bruker 66) using light-pipe optics. The magneto-transmittance data of the sample on the tape and the bare tape up to 35T are collected and analyzed as a stacked multilayer. The average conductivity of the sample flakes at different magnetic fields is evaluated and several magnetic-field dependent features are revealed. These features coincide with the cyclotron resonance energy of the bulk band electrons and potentially linked to the surface state electrons. Implications of these results will be discussed in the presentation. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J12.00007: Spin-orbit scattering in quantum diffusion of massive Dirac fermions Wenyu Shan We theoretically study the effects of spin-orbit scattering on weak (anti-)localization in two-dimensional massive Dirac systems. We clarify that weak anti-localization and localization of a single massive Dirac cone come from the diffusion of a singlet Cooperon in the massless limit and one of triplet Cooperons in the large-mass limit, respectively. Spin-orbit scattering behaves like random magnetic scattering to the triplet Cooperon, and suppresses the weak localization in the large-mass regime, different from in conventional systems where spin-orbit scattering leads to a crossover from weak localization to antilocalization. This behavior suggests an experiment to detect the weak localization of bulk subbands in topological insulator thin films, in which an enhancement of ``weak anti-localization'' is expected after doping heavy-element impurities. Finally, we compare the conventional electron and Dirac fermion systems in the quantum diffusion transport under ordinary, spin-orbit, and magnetic scattering. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J12.00008: Transport properties of SnPbTe topological crystalline insulator films Badih A. Assaf, Ferhat Katmis, Peng Wei, Jagadeesh S. Moodera, Don Heiman A new phase of topological insulators, the topological crystalline insulator, has been recently predicted to arise in band-inverted Sn-Pb chalcogenides, where the topological surface states are protected by crystal symmetry instead of spin-orbit coupling [1]. We grew epitaxial thin films of SnTe and Sn$_{1-x}$Pb$_{x}$Te by MBE and sputtering on (100) and (111) surfaces of BaF$_{2}$ and Si. We report on magnetotransport measurements on SnTe films having hole densities ranging between 10$^{20}$ and 10$^{21}$ cm$^{-3}$ and mobilities up to 200 cm$^{2}$/Vs. Weak antilocalization is observed in all films, allowing us to study the behavior of the phase coherence length versus temperature in an attempt to shed light on the dimensionality of the transport as a function of the Fermi level. Some evidence of 2D transport is found in low carrier density films. [1]T. H. Hsieh et al. \textit{Nature Communication} \textbf{3,} 982 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J12.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J12.00010: Kondo resistance minimum in topological insulators Jie Wang, Dimitrie Culcer We present a theory of the Kondo resistance minimum applicable to topological insulators (TI) and spin-orbit coupled semiconductors in the high-temperature limit, defined as T \textgreater T\textunderscore K, the Kondo temperature. We derive the T-matrix for a general strongly spin-orbit coupled system, including the many-body Kondo scattering terms. The physics is qualitatively different from the well-known case of metals due to the interplay of impurity degrees of freedom with the spin-orbit induced spin-momentum locking of the conduction electrons. TI have a single Fermi surface, while in spin-orbit coupled semiconductors scattering between the two spin-split Fermi surfaces must be taken into account. We determine the resistance minimum and Kondo temperature, and comment briefly on Kondo screening and Kondo singlet formation in the presence of strong spin-orbit coupling. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J12.00011: Competing weak localization and weak antilocalization in ultrathin topological insulators Murong Lang, Liang He, Xufeng Kou, Pramey Upadhyaya, Yabin Fan, Hao Chu, Nai-Chang Yeh, Kang Wang We demonstrate the evidences of a surface gap opening in (Bi$_{\mathrm{0.57}}$Sb$_{\mathrm{0.43}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ samples for film thickness below 6 quintuple layers, through magnetotransport and scanning tunneling spectroscopy measurements. By tuning Fermi level position relative to the gap, the striking crossover between weak antilocalization and weak localization is observed in nonmagnetic 4 and 5 QL films at low field region, a characteristic feature of quantum interferences competition, possibly owing to the change of net Berry phase. Furthermore, when the Fermi level is swept into the surface gap, the overall unitary behaviors are revealed at higher magnetic field, which are in contrast to the pure WAL signals obtained in thicker films. Besides, the surface bandgap of ultrathin film is also determined by low temperature STS measurements. Our findings show an exotic phenomenon characterizing the gapped TI surface states and point to the future realization of quantum spin Hall effect and dissipationless TI-based applications. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J12.00012: Weak anti-localization in ultrathin Sb(111) films S. Cairns, N. McGlohon, C. Robison, J. Keay, C.K. Gaspe, K.S. Wickramasinghe, T.D. Mishima, M.B. Santos, S.Q. Murphy We report the first studies of localization in ultrathin Sb films. Sb is a topological semi-metal with a negative bandgap of 180meV, however it is anticipated that in ultra-thin films, quantum confinement will open the bulk gap, such that transport is dominated by the topological surface states. We have studied the magneto-transport of ~nominally 4.5nm thick films of Sb(111) grown via molecular beam epitaxy at a temperature of 300C on nearly lattice matched epilayers. The longitudinal resistance shows positive magneto-resistance, well described by the standard weak anti-localization (WAL) theory of Hikami, Larkin and Nagaoka. The WAL response is consistent with that of a single conducting channel with a phase breaking length of $\sim$200nm at 1.8K. Scanning electron microscopy shows that the Sb growth proceeded by a Volmer-Weber (islanding) process resulting in disordered films. More recent growths performed at lower temperature have yielded significantly less resistive, smoother and thinner films for which transport measurements are ongoing. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J12.00013: Weak-Localization-Like Magnetoresistance on a Topological Insulator - Ferromagnetic Insulator Interface Qi Yang, Merav Dolev, Li Zhang, Jinfeng Zhao, Min Liu, Subhash Risbud, Alexander Palevski, Aharon Kapitulnik In this talk, we will present measurements on the interface between a topological insulator (TI) and a ferromagnetic insulator (FI). The results provide a likely indication for gap opening in the TI surface states by its proximity to the FI. While above the Curie temperature ($T_C$) of the FI we observed weak-antilocalization-like positive magnetoresistance as ubiquitously in TIs, below $T_C$ an unusual weak-localization-like negative magnetoresistance was seen at low magnetic fields, which has never been observed in TIs without any magnetic doping. Such proximity introduced gap-opening and resulting massive Dirac fermions will hopefully lead to realization of many intriguing phenomena such as the quantum anomalous Hall effect and the inverse spin-galvanic effect. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J12.00014: Weak Localization and Antilocalization in Topological Insulator Thin Films with Coherent Bulk-Surface Coupling Ion Garate, Leonid Glazman We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional (3D) topological insulator (TI). We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J12.00015: Coherent Topological Transport on the Surface of Bi$_2$Se$_3$ Dohun Kim, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, Michael S. Fuhrer We report weak anti-localization (WAL) measurements in gate-tuned, bulk insulating Bi$_2$Se$_3$ thin crystals with thicknesses varying between 5 and 17 nm. The gate-voltage dependent WAL behavior shows systematic variation as a function of crystal thickness. For the thickest samples, we observe two decoupled surfaces exhibiting perfect WAL as expected for the symplectic class. As the films are made thinner, we observe a gate-voltage tuned crossover from two decoupled surfaces to a single coherently coupled 2D system exhibiting WAL. The observed crossover is governed by competition between the phase coherence time and inter-surface tunneling time associated with the hybridization gap. In contrast to classical transport in which the signature of the hybridization gap appears only in the ultrathin limit ($\le $ 3nm), phase coherent transport is extraordinarily sensitive to sub-meV coupling between top and bottom topological surfaces, and the surfaces of a TI film may be coherently coupled even for thicknesses as large as 12 nm. On further thinning, the WAL behavior is suppressed altogether at small carrier density, which we associate with the opening of a sizable gap on order the Fermi energy and destruction of topological protection. [Preview Abstract] |
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