Optical spectroscopy study on pressure-induced phase transitions in the three-dimensional Dirac semimetal Cd_3As_2

摘要

We report a room-temperature optical reflectivity study performed on [112]-oriented Cd_3As_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 ~4 GPa conlirm 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.