Optical spectroscopy study on pressure-induced phase transitions in the three-dimensional Dirac semimetal Cd3As2

We report a room-temperature optical reflectivity study performed on [112]-oriented ${\mathrm{Cd}}₃{\mathrm{As}}₂$ single crystals over a broad energy range under external pressure up to 10 GPa. The abrupt drop in the band dispersion parameter ($z$ parameter) and the interruption of the gradual redshift of the band gap at $\ensuremath{\sim}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ confirm the structural phase transition from a tetragonal to a monoclinic phase in this material. The pressure-induced increase in the overall optical conductivity at low energies and the continuous redshift of the high-energy bands indicate that the system evolves towards a topologically trivial metallic state, although a complete closing of the band gap could not be observed in the studied pressure range. Furthermore, a detailed investigation of the low-pressure regime suggests the possible existence of an intermediate state between 2 and 4 GPa, that might be a precursor of the structural phase transition or due to the lifted degeneracy of the Dirac nodes. Several optical parameters show yet another anomaly at 8 GPa where low-temperature superconductivity was found in an earlier study.