Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS

TL;DR

This study explores how high pressure affects the out-of-plane optical response of ZrSiS, revealing pressure-induced excitations that are not due to structural changes or electronic correlations, and discussing potential excitonic insulator behavior.

Contribution

It provides the first detailed analysis of pressure effects on out-of-plane optical excitations in ZrSiS, combining experimental and theoretical approaches.

Findings

Strong pressure effects on out-of-plane optical conductivity

Appearance of two prominent excitations under pressure

No structural phase transition observed

Abstract

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.