Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U2: Invited Session: Topological Insulators: Surface State Transport |
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Sponsoring Units: DCMP Chair: Joel Moore, University of California, Berkeley Room: Ballroom II |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U2.00001: Quantum transport in topological insulator nanowires and thin films Invited Speaker: Jens H. Bardarson Topological insulators have an insulating bulk but a metallic surface. In the simplest case, the surface electronic structure of a 3D topological insulator is described by a single 2D Dirac cone. The transport properties of such a surface state are of considerable current interest; they have some similarities with graphene, which also realizes Dirac fermions, but have several unique features in their response to magnetic fields. In this talk, I give an overview of some of the main quantum transport properties of topological insulator surfaces. I focus on the efforts to use quantum interference phenomena, such as weak anti-localization and the Aharonov-Bohm effect, to verify in a transport experiment the Dirac nature of the surface state and its defining properties. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U2.00002: The strong, weak and anomalous sides of weak topological insulators Invited Speaker: Zohar Ringel Disorder and topology can be thought of as two counter-driving forces. While the former pushes electron wave functions to localize in space, the latter requires them to remain coherent over the entire system. We study the interplay between these two on the surface of a ``weakly'' topological phase- the Weak Topological Insulator. Using arguments based on flux-insertions and locality, we show that such surfaces cannot undergo a localization transition even when the surface is strongly disordered. We also present a numerical study which further quantifies this result. We then reformulate the same notions, in field theory language, using a novel $Z_2$-charge-anomaly. This anomaly generalizes the $Z$-charge-anomaly associated with edges of the Integer Quantum Hall Effect. Besides unifying various aspects of Topological Insulators, the anomaly allows us to calculate new topological properties of TIs in the presence of electric fields. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U2.00003: Surface conduction of topological Dirac electrons in bulk insulating Bi$_{2}$Se$_{3}$ Invited Speaker: Michael Fuhrer The three dimensional strong topological insulator (STI) is a new phase of electronic matter which is distinct from ordinary insulators in that it supports on its surface a conducting two-dimensional surface state whose existence is guaranteed by topology. I will discuss experiments on the STI material Bi$_{2}$Se$_{3}$, which has a bulk bandgap of 300 meV, much greater than room temperature, and a single topological surface state with a massless Dirac dispersion. Field effect transistors consisting of thin (3-20 nm) Bi$_{2}$Se$_{3}$ are fabricated from mechanically exfoliated from single crystals, and electrochemical and/or chemical gating methods are used to move the Fermi energy into the bulk bandgap, revealing the ambipolar gapless nature of transport in the Bi$_{2}$Se$_{3}$ surface states. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite-temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be $\sim$60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi$_{2}$Se$_{3}$, which will have implications for topological electronic devices operating at room temperature.~As samples are made thinner, coherent coupling of the top and bottom topological surfaces is observed through the magnitude of the weak anti-localization correction to the conductivity, and, in the thinnest Bi$_{2}$Se$_{3}$ samples ($\sim$ 3 nm), in thermally-activated conductivity reflecting the opening of a bandgap. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U2.00004: Prediction of weak and strong topological insulators in layered semiconductors. Invited Speaker: Claudia Felser We investigate a new class of ternary materials such as LiAuSe and KHgSb with a honeycomb structure in Au-Se and Hg-Sb layers. We demonstrate the band inversion in these materials similar to HgTe, which is a strong precondition for existence of the topological surface states. In contrast with graphene, these materials exhibit strong spin-orbit coupling and a small direct band gap at the point. Since these materials are centrosymmetric, it is straightforward to determine the parity of their wave functions, and hence their topological character. Surprisingly, the compound with strong spin-orbit coupling (KHgSb) is trivial, whereas LiAuSe is found to be a topological insulator. However KHgSb is a weak topological insulators in case of an odd number of layers in the primitive unit cell. Here, the single-layered KHgSb shows a large bulk energy gap of 0.24 eV. Its side surface hosts metallic surface states, forming two anisotropic Dirac cones. Although the stacking of even-layered structures leads to trivial insulators, the structures can host a quantum spin Hall layer with a large bulk gap, if an additional single layer exists as a stacking fault in the crystal. The reported honeycomb compounds can serve as prototypes to aid in the finding of new weak topological insulators in layered small-gap semiconductors. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U2.00005: Manifestation of topological protection in transport properties of epitaxial Bi$_2$Se$_3$ thin films Invited Speaker: Alexey Taskin A topological insulator is a new quantum state of matter which can be realized in some materials with a strong spin-orbit coupling. Due to the spin-momentum locking, massless Dirac fermions residing on the surface of a topological insulator are protected from backscattering and cannot be localized by disorder. However, such protection can be lifted in ultrathin films when the three-dimensionality of the sample is lost due to hybridization between top and bottom surfaces. Recently, using Molecular Beam Epitaxy, we succeeded in growing Bi$_2$Se$_3$ thin films of sufficiently high quality to present quantum oscillations in magnetotransport [1]. By measuring the Shubnikov-de Haas oscillations in a series of high-quality films, we revealed a systematic evolution of the surface conductance as a function of thickness and found a striking manifestation of the topological protection [2]: The metallic surface transport abruptly diminishes below the critical thickness of $\sim$6 nm, at which an energy gap opens in the surface state and the Dirac fermions become massive. At the same time, the weak antilocalization behavior is found to weaken in the gapped phase due to the loss of $\pi$ Berry phase. Our results demonstrate the importance of the spin and momentum coupling in maintaining the topological protection of the surface carriers in topological insulators.\\[4pt] [1] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Adv. Mater. {\bf 24}, 5581 (2012). \newline [2] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. Lett. {\bf 109}, 066803 (2012). [Preview Abstract] |
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