Transient Drude Response Dominates Near-Infrared Pump-Probe Reflectivity in Nodal-Line Semimetals ZrSiS and ZrSiSe.

The ultrafast optical response of nodal-line semimetals ZrSiS and ZrSiSe was studied in the near-infrared using transient reflectivity. The materials exhibit similar responses, characterized by two features, well-resolved in time and energy; the first decays after hundreds of femtoseconds, and the second lasts for nanoseconds. Using Drude-Lorentz fits of the materials' equilibrium reflectance, we show that these are well-represented by a sudden change of the electronic properties (increase of screening or reduction of the plasma frequency) followed by an increase of the Drude scattering rate. This directly connects the transient data to a physical picture in which carriers, after excitation into the conduction band, return to the valence band by sharing excess energy with the phonon bath, resulting in a hot lattice that relaxes through slow diffusive processes. The emerging picture reveals that the sudden electronic reorganization instantaneously modifies the materials' electronic properties on a time scale not compatible with electron-phonon thermalization.