Possible flat band bending of the Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) crystal cleaved in an ambient air probed by terahertz emission spectroscopy

We investigate an evolution of the surface electronic state of the Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) single crystal, which is one of the most bulk insulating topological insulators, by examining terahertz light emitted from the sample surface upon the illumination of the near-infrared femtosecond laser pulses. We find that the surface state with a flat band bending can appear in the course of the natural maturation process of the surface state in an ambient air. Furthermore, we demonstrate that the evolution of the surface electronic state can be accelerated, decelerated, or even stopped by controlling environmental conditions to contain different amount of H(2)O, in particular.