Effect of mechanical strain on the optical properties of nodal-line semimetal ZrSiS.

Optical properties of nodal-line semimetal ZrSiS are studied using first-principles calculations. Frequency-independent optical conductivity is a fingerprint of the infrared optical response in ZrSiS. We find that this characteristic feature is robust with respect to external pressure of up to 10 GPa, yet with the flat region being narrowed with increasing pressure. Upon tensile stress of 2 GPa, the Fermi surface undergoes a topological transition accompanied by a weakening of the interband screening, which reduces the spectral weight of infrared excitations. We also show that the high-energy region is characterized by low-loss plasma excitations at $\sim$20 eV with essentially anisotropic dispersion. Strongly anisotropic dielectric properties suggest the existence of a hyperbolic regime for plasmons in the deep ultraviolet range. Although the frequencies of high-energy plasmons are virtually unaffected by external pressure, their dispersion can be effectively tuned by strain.