Role of Chalcogen-p States in the Optical Response of Bulk PtSe2 and PtTe2

The type-II Dirac semimetals PtSe${}_2$ and PtTe${}_2$ are isostructural and isovalent layered compounds but exhibit distinct electronic properties. We measure the in-plane infrared reflectivity of bulk crystals between 10 and 300 K and compare the interband contribution to the optical conductivity with band-resolved density-functional calculations. At low temperature, the Drude spectral weight of PtTe${}_2$ is an order of magnitude larger than that of PtSe${}_2$. PtTe${}_2$ also shows a broad mid-infrared absorption continuum, whereas the absorption of PtSe${}_2$ remains weak. The calculations attribute this continuum to transitions from a predominantly chalcogen-$p$ band into a Pt-$d$ band. In PtTe${}_2$, the chalcogen-$p$ band lies higher in energy, which lowers the onset of these transitions and increases the semimetallic band overlap, in line with the larger Drude spectral weight. The type-II Dirac points lie far below the Fermi level, so that transitions between the bands forming the Dirac cones do not contribute appreciably to the interband conductivity.