Landau level spectroscopy of the PbSnSe topological crystalline insulator

We report on an infrared magnetospectroscopy study of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_x\mathrm{Se}$, a topological crystalline insulator. We have examined a set of samples, all in the inverted regime of electronic bands, with the tin composition varying from $x=0.2$ to 0.33. Our analysis shows that the observed response, composed of a series of interband inter-Landau-level excitations, can be interpreted and modeled using the relativistic-like Hamiltonian for three-dimensional massive Dirac electrons, expanded to include diagonal quadratic terms that impose band inversion. In our data, we have not found any clear signature of massless electron states that are present on the surface of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_x\mathrm{Se}$ crystals in the inverted regime. Reasons for this unexpected result are discussed.