Low frequency electrodynamics of topological insulator surface states
I will report on our studies of high quality MBE grown Bi2Se3 and strained HgTe
topological insulator thin films using time domain terahertz spectroscopy (TDTS). In the
Bi2Se3 case, we explicitly demonstrate the 2D character of the response by studying
films of different thicknesses. In our measurements we take advantage of a unique
feature of TDTS that allows to use the time structure of the THz pulses to measure the
Faraday and Kerr rotation angles in a single experiment. We find an unprecedentedly
large value of the Kerr rotation that is due to the cyclotron resonance of the 2D Dirac
fermions. I will also talk about our extension of these measurements to the strained
HgTe topological insulator system. Our most recent work is on doping Bi2Se3 with the
indium toward the topologically trivial end member In2Se3 . Here, the thickness
independent Drude peak shows only minor broadening at low In substitutions.
However, above x ∼ 0.05 we observe a sudden collapse in the transport lifetime. This
substitution level closely coincides with a maximum in the mid-infrared (MIR)
absorption coefficient that can be identified with the substitution level where the band
gap closes, the band structure inverts, and hence the topological class changes. We
therefore associate the collapse in the transport lifetime with the loss of topological
protection of surface states as the system enters the topologically trivial phase.
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