Aging and reduced bulk conductance in thin films of the topological   insulator Bi$_2$Se$_3$
  We report on the effect of exposure to atmospheric conditions on the THz conductivity of thin films of the topological insulator Bi$_2$Se$_3$. 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. 
R. Vald\'es Aguilar, L. Wu, A. V. Stier, L. S. Bilbro, M. Brahlek, N.   Bansal, S. Oh and N. P. Armitage
cond-mat.str-el cond-mat.mes-hall