Evolution of AC conductivity in non-equilibrium warm dense gold

AC conductivity has long been of interest in the study of electronic structure and transport properties of Warm Dense Matter. Using a chirped pulse probe technique, we have obtained single-shot measurements of the temporal evolution of AC conductivity during electron energy relaxation in non-equilibrium warm dense gold produced by femtosecond-laser heating with energy density up to 4.1MJ/kg (8×10¹⁰J/m³). The results uncover important changes that have been masked in earlier studies. Equally significant, they provide the first benchmark for testing an ab-initio model that is used to calculate electron heat capacity, electron-ion coupling and AC conductivity in a single, first principles framework. While measurements of the real part of AC conductivity corroborate our theoretical temperature-dependent electron heat capacity, they point to an electron-ion coupling factor of ~2.2×10¹⁶W/m³ K, substantially below that predicted by theory. In addition, measurements of the imaginary part of AC conductivity reveal the need to improve theoretical treatment of intraband contribution at very low photon energy.