Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H2: New Materials for Spin Quantum Hall Effect and Topological Insulators |
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Sponsoring Units: DCMP Chair: Shoucheng Zhang, Stanford University Room: Ballroom A2 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H2.00001: Bulk Topological Insulators and Superconductors: Discovery and the new Frontiers Invited Speaker: While most known phases of matter are characterized by broken symmetries, the discovery of quantum Hall effects (1980s) revealed that there exists an organizational principle based on topology rather than broken symmetry. In the past few years, theory and experiments have suggested that new types of topological states of matter exist in certain bulk insulators without any applied magnetic field. These topological insulators are characterized by a full band gap in their bulk and gap-less conducting edge or surface states protected by time-reversal symmetry. Unlike the quantum Hall systems, the bulk 3D topological insulators can be doped into superconductors and magnets revealing the interplay between topological-order and broken-symmetry-order [Rev. Mod. Phys 82, 3045 (2010)]. In this talk, I will highlight the experimental observations and focus on recent experimental developments on bulk topological insulators. I will then draw connections between the topological physics and their potential applications in electronics and the emergent new frontiers in fundamental physics. Work in collaboration with D. Hsieh, Y. Xia, L. Wray, D. Qian, C.L. Kane, H. Lin, A. Bansil, D. Grauer, R.J. Cava, Y.S. Hor, J. H. Dil, F. Meier, L. Patthey, J. Osterwalder, A.V. Fedorov. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H2.00002: Tunable multifunctional topological insulators in ternary Heusler and related compounds Invited Speaker: Recently the quantum spin Hall effect was theoretically predicted and experimentally realized in quantum wells based on the binary semiconductor HgTe. The quantum spin Hall state and topological insulators are new states of quantum matter interesting for both fundamental condensed-matter physics and material science. Many Heusler compounds with C1b structure are ternary semiconductors that are structurally and electronically related to the binary semiconductors. The diversity of Heusler materials opens wide possibilities for tuning the bandgap and setting the desired band inversion by choosing compounds with appropriate hybridization strength (by the lattice parameter) and magnitude of spin--orbit coupling (by the atomic charge). Based on first-principle calculations we demonstrate that around 50 Heusler compounds show band inversion similar to that of HgTe. The topological state in these zero-gap semiconductors can be created by applying strain or by designing an appropriate quantumwell structure, similar to the case of HgTe. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity (for example LaPtBi) to magnetism (for example GdPtBi) and heavy fermion behaviour (for example YbPtBi). These properties can open new research directions in realizing the quantized anomalous Hall effect and topological superconductors. Heusler compounds are similar to a stuffed diamond, correspondingly, it should be possible to find the ``high Z'' equivalent of graphene in a graphite-like structure with 18 valence electrons and with inverted bands. Indeed the ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. These materials have a gap at the Fermi energy and are therefore candidates for 3D-topological insulators. Additionally they are centro-symmetric, therefore, it is possible to determine the parity of their wave functions, and hence, their topological character. Surprisingly, the compound KHgSb with the strong SOC is topologically trivial, whereas LiAuSe is found to be a topological non-trivial insulator. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:48AM |
H2.00003: Quantized Anomalous Hall Effect in Magnetic Topological Insulators Invited Speaker: The anomalous Hall effect is a fundamental transport process in solids arising from the spin-orbit coupling. In a quantum anomalous Hall insulator, spontaneous magnetic moments and spin-orbit coupling combine to give rise to a topologically nontrivial electronic structure, leading to the quantized Hall effect without an external magnetic field. Based on first-principles calculations, we predict that the tetradymite semiconductors Bi$_2$Te$_3$, Bi$_2$Se$_3$, and Sb$_2$Te$_3$ form magnetically ordered insulators when doped with transition metal elements (Cr or Fe), in contrast to conventional dilute magnetic semiconductors where free carriers are necessary to mediate the magnetic coupling. In two-dimensional thin films, this magnetic order gives rise to a topological electronic structure characterized by a finite Chern number, with the Hall conductance quantized in units of e$^2$/h. References:\\[4pt] [1] R. Yu, W. Zhang, H.J. Zhang, S. C. Zhang, X. Dai, Z. Fang, ``Anomalous Hall Effect in Magnetic Topological Insulators,'' Science 329, 61 (2010).\\[0pt] [2] Y. Zhang, K. He, C. Z. Chang, et.al., ``Crossover of the 3D topological insulator Bi$_2$Se$_3$ to the 2D limit,'' Nature Physics 6, 584 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:24AM |
H2.00004: Search for Topological Insulators in Ternary Chalcogenides Invited Speaker: A topological insulator (TI) is a novel quantum state, which is a bulk insulator but has gapless surface states. Recently, binary chalcogenides, Bi2Te3, Bi2Se3 and Sb2Te3 have been theoretically predicted and experimentally observed to be a family of TIs [1]. In this talk, we extend our search of TIs to ternary chalcogenides by replacing some of Bi or Sb atoms by other atoms, such as thallium and rare earth atoms. It is found that for thallium-based materials [2], only TlSbS2 is trivial and all the others are TIs, while for rare earth-based materials[3], LaBiTe3 is a TI and the others are trivial. The search in ternary chalcogenides not only bring new members of TIs in the family of chalcogenides but also may provide candidates for other new topological states such as topological superconductor, quantum anomalous Hall insulator, axionic insulator and topological Kondo insulator. \\[4pt] Reference:\\[0pt] [1] Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface, Haijun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang and Shou-ChengZhang, Nature Physics 5, 438 - 442 (2009).\\[0pt] [2] Theoretical prediction of topological insulators in thallium-based III-V-VI2 ternary chalcogenides, Binghai Yan, Chao-Xing Liu, Hai-Jun Zhang, ChiYung Yam, Xiao-Liang Qi, Thomas Frauenheim and Shou-Cheng Zhang, Europhysics Letters, 90, 37002 (2010).\\[0pt] [3] Theoretical prediction of topological insulator in ternary rare earth chalcogenides, Binghai Yan, Hai-Jun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Thomas Frauenheim, Shou-Cheng Zhang, Phys. Rev. B 82, 161108(R) (2010). [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 11:00AM |
H2.00005: Magnetotransport studies of new topological insulators: Bi$_2$Te$_2$Se and others Invited Speaker: A topological insulator (TI) is a material that has a gapped insulating bulk and a gapless metallic surface. However, presently available TI materials are not truly insulating, making surface transport measurements to be a challenge. The second generation of TIs, Bi$_2$Se$_3$ and related compounds, turned out to be more suitable for the experimental studies of the topological 2D states than the first discovered Bi-Sb alloys, due to a much larger bulk gap and a simpler surface state consisting of a single Dirac cone. Unfortunately, near-stoichiometric Bi$_2$Se$_3$ is always a metallic n-type material owing to a finite amount of Se vacancies. We searched for new TI materials that are better suited for achieving a bulk insulating state and found that Bi$_2$Te$_2$Se, which has an ordered tetradymite structure with the Te-Bi-Se-Bi-Te layer sequence, is a very promising material. It was found that high-quality single crystals of Bi$_2$Te$_2$Se show a high resistivity exceeding 1 $\Omega$cm, together with a variable-range hopping behavior which is a hallmark of an insulator; yet, they present Shubnikov-de Haas oscillations which signify the 2D surface state consistent with the topological one observed by photoemission spectroscopy. Moreover, we are able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that Bi$_2$Te$_2$Se is the best material to date for studying the surface quantum transport in a topological insulator. Transport properties of other new TI materials are also presented. [Preview Abstract] |
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