Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y2: Topological Insulators: Transport and Interactions |
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Sponsoring Units: DCMP Chair: Xiaoliang Qi, SLAC National Accelerator Laboratory Room: Ballroom A2 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y2.00001: Quantum oscillations and Hall anomaly of surface electrons on Topological Insulators Invited Speaker: The investigation of Topological Insulators (TI) by transport experiments is a challenge, because the surface currents cannot be well-resolved when the bulk conductance is dominant, as in most crystals. I will review the progress starting from Ca-doped Bi$_2$Se$_3$, and proceeding to Bi$_2$Te$_3$ and to Bi$_2$SeTe$_2$. Using Ca dopants in Bi$_2$Se$_3$, we succeeded in lowering the Fermi energy $E_F$ into the bulk gap. However, in non-metallic crystals, the substantial dopant-induced disorder precluded observation of Shubnikov-de Haas (SdH) oscillations. Fortunately, $E_F$ in undoped Bi$_2$Te$_3$ can be tuned into the gap by heat treatment. The non-metallic samples display a bulk resistivity $\rho$ = 4-12 m$\Omega$cm at 4 K. In these crystals, weak SdH oscillations are observed below 10 K. We confirmed that these oscillations arise from a 2D Fermi Surface by tilting the magnetic field $\bf H$. From the behavior of the SdH amplitude versus temperature $T$ and $H$, we infer a surface Fermi velocity $v_F$ = 3.7-4.2 $\times 10^5$ m/s, and a high surface mobility $\mu$ = 10,000 cm$^2$/Vs. The high mobility of the surface electrons is confirmed by the appearance of an unusual weak-field anomaly in the Hall conductance $G_{xy}$. I will discuss recent progress in further lowering the bulk conductance in the new TI Bi$_2$Se$_3$, in which a Se layer is sandwiched between two Te layers in each quintuplet unit cell. In these crystals, $\rho$ at 4 K is a factor of 1000 larger (6 $\Omega$cm). The interesting pattern of SdH oscillations in this new system will be reported.\\[4pt] Collaborators: D.X. Qu, J. M. Checkelsky, Y. S. Hor, J. Xiong, R. J. Cava [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 9:12AM |
Y2.00002: Dirac Fermions in HgTe Quantum Wells Invited Speaker: Replace this text with your abstract. Narrow gap HgTe quantum wells exhibit a band structure with linear dispersion at low energies and thus are very suitable to study the physics of the Dirac Hamiltonian in a solid state system. In comparison with graphene, they boast higher mobilities and, moreover, by changing the well width one can tune the effective Dirac massfrom positive, through zero, to negative. Negative Dirac mass HgTe quantum wells are 2-dimensional topological insulators and, as a result, exhibit the quantum spin Hall effect. 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 electrical conductance. Apart from the conductance quantization, there are some further aspects of the quantum spin Hall state that warrant experimental investigation. Using non-local transport measurements, we can show that the charge transport occurs through edge channels - similar to the situation in the quantum Hall effect. However, due to the helical character of the quantum spin Hall edge channels, inhomogeneities in the potential profile of the experimental devices have a much stronger effect on the transport properties. Moreover, the quantum spin Hall edge channels are spin polarized. We can prove this fact in split gate devices that are partially in the insulting and partly in the metallic regime, making use of the occurrence of the metallic spin Hall effect to convert the magnetic spin signal into an electrical one. Finally, I will address another aspect of Dirac Fermion physics: HgTe quantum wells at a critical thickness of 6.3 nm are zero gap systems and exhibit transport physics that is very similar to that observed over the past few years in graphene. However, zero gap HgTe wells have a higher mobility than graphene, and also have only a single Dirac valley. This makes them especially suitable to study quantum interference effects under a Dirac Hamiltonian. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:48AM |
Y2.00003: Band Topology, Electron Correlations and 3D Dirac Metal in Pyrochlore Iridates Invited Speaker: We study consequences of strong spin orbit interaction in a class of correlated systems. We discuss the possibility of novel phases such as a $\pi $ axion insulator, protected by inversion, rather than time reversal symmetry and a gapless topological phase, the three dimensional Dirac semimetal. The latter phase has unusual surface states that take the form of `Fermi Arcs', that cannot be realized in any two dimensional band structure. The pyrochlore iridates, (such as Y$_{2}$Ir$_{2}$O$_{7})$ according to LDA+U calculations and existing experimental data, are argued to be promising materials for realizing these states. This work was done in collaboration with Xiangang Wan (Nanjing U.), Sergey Savrasov (UC Davis) and Ari Turner (UC Berkeley). [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:24AM |
Y2.00004: General Theory of interacting Topological insulators Invited Speaker: In this talk, I shall first briefly review the theory of topological insulators and the experimental status. I will then discuss the general theory of an interacting topological insulator, whose topological order parameter is expressed in terms of the full interacting Green function. This topological order parameter is also experimentally measurable in terms of the quantized magneto-electric effect. I shall discuss various applications of this theory to realistic materials which could realize the topological Mott insulator state. \\[4pt] ``Topological Field Theory of Time-Reversal Invariant Insulators,'' Phys. Rev. B. {\bf 78}, 195424, (2008). \\[0pt] ``General theory of interacting topological insulators,'' arXiv:1004.4229. \\[0pt] ``Dynamical Axion Field in Topological Magnetic Insulators,'' Nature Physics {\bf 6}, 284 (2010). \\[0pt] ``Quantum Spin Hall Effect in a Transition Metal Oxide Na2IrO3,'' Phys. Rev. Lett. {\bf 102}, 256403 (2009). \\[0pt] ``Topological Mott Insulators,'' Phys. Rev. Lett. {\bf 100}, 156401, (2008). [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 11:00AM |
Y2.00005: Classification of Topological Insulators and Superconductors: the ``Ten-Fold Way'' Invited Speaker: We review the exhaustive ten-fold classification scheme of topological insulators and superconductors. It is found that the conventional (i.e.: ``$Z_2$'', or `spin-orbit') topological insulator, experimentally observed in 2D (`Quantum Spin Hall') and in 3D materials, is one of a total of five possible classes of topological insulators or superconductors which exist in every dimension of space. Different topological sectors within a given class can be labeled, depending on the case, by an integer winding number, or by a ``binary'' $Z_2$ quantity. The topological nature of the bulk manifests itself through the appearance of ``topologically protected'' surface states. These surface states completely evade the phenomenon of Anderson localization due to disorder. Examples of the additional topological phases in 3D include topological superconductors (i) with spin-singlet pairing, and (ii) with spin-orbit interactions, as well as ${}^{3}{\rm He \ B}$. -- The classification of topological insulators (superconductors) in d dimensions is reduced to the problem of classifying Anderson localization at the (d-1)-dimensional sample boundary which, in turn, is solved. The resulting five symmetry classes of topological insulators (superconductors) found to exist in every dimension of space correspond to a certain subset of five of the ten generic symmetry classes of Hamiltonians introduced 16 years ago by Altland and Zirnbauer in the context of disordered systems (generalizing the three well-known ``Wigner-Dyson'' symmetry classes). For a significant part of the phases of topological insulators (superconductors) of the classification a characterization can be given in terms of the responses of the system. For these, the responses are described by a field theory possessing a [gauge, gravitational (thermal), or mixed] anomaly. This implies that these phases are well defined also in the presence of inter-fermion interactions. [Preview Abstract] |
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