Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS
The anisotropic optical response of the layered, nodal-line semimetal ZrSiS was investigated for the first time by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. Under external pressure, two prominent excitations appear in the out-of-plane optical conductivity, which are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.