Mass Acquisition of Dirac Fermions Near the Topological Quantum Phase Transition(Current Topics)

Abstract

We report an unexpected discovery that the surface Dirac fermions in the topological insulator TlBi(S_<1-x>Se_x)_2 solid-solution system acquire a mass without explicitly breaking the time-reversal symmetry. We found by high-resolution ARPES that the massless Dirac state in TlBiSe_2 switches to a massive state with a small Dirac gap by slightly replacing Se with S. Further replacement of Se results in an enhancement of the Dirac gap, and the gap eventually disappears around the quantum phase transition point between the topological and non-topological phases. This result provides a new route to achieving the massive Dirac state required for some of the exotic topological phenomena.