Physics Colloquium, April 7, 2009
Topological insulators and topological superconductors Shoucheng Zhang

Stanford University

Search for topologically non-trivial states of matter has become a important goal for condensed matter physics. Recently, a new class of topological states has been proposed and experimentally realized. These topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction[1] of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation[2]. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators[3]. Bi2Te3, Bi2Se3 and Sb2Te3 are theoretically predicted to be topological insulators with a single Dirac cone on the surface[4]. I shall present a realistic experimental proposals to observe the magnetic monopoles on the surface of topological insulators[5]. Finally, I shall discuss topological superconductors and superfluids in two and three dimensions, and discuss various experimental proposals[6].
[1] A. Bernevig, T. Hughes and S. C. Zhang, Science, 314, 1757, (2006)
[2] M. Koenig et al, Science 318, 766, (2007)
[3] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, Phys. Rev B. 78, 195424 (2008)
[4] Haijun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang, and Shou-Cheng Zhang, arXiv:0812.1622.
[5] Xiao-Liang Qi, Run-Dong Li, Jiadong Zang and Shou-Cheng Zhang, www.sciencexpress.org/10.1126/science.1167747.
[6] Xiao-Liang Qi, Taylor L. Hughes, Srinivas Raghu and Shou-Cheng Zhang, arXiv:0803.3614.

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