Liang Wu
Institute of Quantum Matter & Department of Physics and Astronomy
The Johns Hopkins University, Baltimore, MD 21218 USA
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时间：8月1日（周五）下午2:00
地点：唐仲英楼A213会议室
Abstract:
Topological insulators (TIs) are newly discovered states of matter with robust metallic surface states protected by the topological properties of the bulk wavefunctions. A quantum phase transition (QPT) from a TI to a conventional insulator and a change in topological class can only occur when the bulk band gap closes. In this work, we have utilized time-domain terahertz spectroscopy (TDTS) to investigate the low frequency conductance in (Bi1-xInx)2Se3 as we tune through this transition by indium substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit x ~ 0.06 where a maximum in the mid-infrared absorption is exhibited. This absorption can be identified with the bulk band gap closing and a change in topological class. The correlation length associated with the QPT appears as the evanescent length of the surface states. The observation of the thickness-dependent collapse of the transport lifetime shows the unusual role that finite size effects play in this topological QPT. Possibility of realization of Weyl Semi-metal near the Quantum critical point (x ~ 0.06) by breaking symmetries will be discussed. THz measurements on a new generation of Bi2Se3 samples with insulating bulk are also investigated. We observed giant Faraday rotation and cyclotron resonance from surface states in TIs.