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

TL;DR

This study investigates the anisotropic optical responses of ZrGeS and ZrGeSe, revealing their polarization-dependent spectra, pressure effects, and similarities to ZrSiS, advancing understanding of their electronic properties.

Contribution

It provides the first detailed polarization-dependent optical measurements of ZrGeS and ZrGeSe, highlighting their anisotropic behavior and pressure-induced spectral changes.

Findings

Strong optical anisotropy observed in both compounds

Pressure causes notable changes in out-of-plane optical spectra

Comparison with ZrSiS shows similar optical features

Abstract

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 {\bf E}$\| c$ as compared to the in-plane direction {\bf E}$\| ab$. For {\bf E}$\| ab$ the optical conductivity spectrum consists of two Drude terms followed by a shoulder or plateau-like behavior and a distinct U shape at higher energies, while for {\bf E}$\| c$ one Drude term is followed by a peak-like 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 $\sim$3~GPa. Overall, the pressure-induced changes in the profile of the {\bf E}$\| c$ conductivity spectrum are much enhanced above $\sim$3~GPa. We compare our results to those recently reported for ZrSiS in a quantitative manner.