In-plane and out-of-plane optical response of the nodal-line semimetals ZrGeS and ZrGeSe

Polarization-dependent reflectivity measurements were carried out over a broad frequency range on single-crystalline ZrGeSe and ZrGeS compounds, which are closely related to the prototype nodal-line semimetal ZrSiS. These measurements revealed the strongly anisotropic character of both ZrGeSe and ZrGeS, with a reduced plasma frequency for the out-of-plane direction E∥c as compared with the in-plane direction E∥ab. For E∥ab, the optical conductivity spectrum consists of two Drude terms followed by a shoulder or plateaulike behavior and a distinct U shape at higher energies, while for E∥c, one Drude term is followed by a peaklike behavior, and the U shape of the profile is less developed. Under external pressure, two prominent excitations appear in the out-of-plane optical conductivity spectrum of ZrGeSe, whose frequency position and oscillator strength show a weak anomaly at ∼3 GPa. Overall, the pressure-induced changes in the profile of the E∥c conductivity spectrum are much enhanced >∼3 GPa. We compare our results to those recently reported for ZrSiS in a quantitative manner.