PbSnSe: a model system to study topological phases*

Pb_1-xSn_xSe hosts three-dimensional (3D) massive Dirac fermions across the entire composition range for which the crystal structure is cubic. In this talk, we will present a comprehensive experimental mapping of the 3D band structure of Pb_1-xSn_xSe as a function of chemical composition (0<x<0.3), temperature (1.6<T<200 K), and magnetic field (0<B<34 T). We use magneto-infrared spectroscopy to determine all the Dirac parameters in the trivial and topologically non-trivial regimes.
In 3D Pb_1-xSn_xSe topological insulators, an effective closure of the bulk energy gap with increasing temperature, magnetic field, or lead content is expected at the critical point of the phase transition. However, in this talk we experimentally demonstrate that such a gap closure consistently does not occur. Avoided-crossing of either bulk bands or N=0 Landau levels are evidenced. These results show that Pb_1-xSn_xSe is a model system to study topological phases and the nature of the phase transition.