Aging and reduced bulk conductance in thin films of the topological insulator Bi₂Se₃

We report on the effect of exposure to atmospheric conditions on the THz conductivity of thin films of the topological insulator Bi₂Se₃. We find (1) two contributions of mobile charge carriers to the THz conductivity immediately after growth and (2) the spectral weight of the smaller of these decays significantly over a period of several days as the film is exposed to ambient conditions, while the other remains relatively constant. We associate the former with a bulk response and the latter with the surface. The surface response exhibits the expected robustness of the carriers from 2D topological surface states. We find no evidence for a third spectral feature derived from topologically trivial surface states.