Nodal-line plasmons in ZrSiX (X = S, Se, Te) and their temperature dependence

Topological nodal-line semimetals gain extensive attention for their peculiar linear band crossings along closed lines in the Brillouin zone leading to various interesting properties. Here, we systematically study the nodal-line plasmons of prototypical nodal-line semimetals ZrSiX (X = S, Se, Te) by high-resolution electron energy loss spectroscopy, and first-principles calculations. In energy range from near- to mid-infrared frequencies, plasmons induced by the intraband correlations (similar to 0.8 eV) and the interband correlations (similar to 0.3 eV) of surface states related to the nodal-line electrons are universally observed in all the ZrSiX family. The bulk plasmon (similar to 0.6 eV) is observed in ZrSiS but is indiscernible in both ZrSiSe and ZrSiTe owing to their stronger screening effect from the surface states and large interlayer distance. Although the plasmons in ZrSiS seem to be temperature independent, the frequencies of the intraband plasmons in ZrSiSe and ZrSiTe show large redshifts (similar to 10%) with temperature increasing from 30 K to room temperature, which can be ascribed to the prominent thermal occupation effect of nodal-line electrons when the Fermi level is close to the nodal line.