Unveiling structural effects on the DC conductivity of warm dense matter via terahertz spectroscopy and ultrafast electron diffraction

The electrical conductivity of materials at extreme conditions is a highly sought parameter due to its importance in simulations of many different processes. Ultrafast laser pulses can drive materials to the relevant conditions, but experimental determination of the conductivity remains challenging due to a lack of direct probes. Interpreting any results also requires consideration of the atomic structure, as this is expected to influence the electrical conductivity. We present a study using single-shot terahertz time-domain spectroscopy and mega-electron-volt ultrafast electron diffraction (MeV-UED) to study the electrical conductivity and structure of Warm Dense aluminum. The relatively long temporal period (~1 ps) compared to the relevant interaction timescales (~0.03 ps) makes THz fields essentially static. By measuring changes in the THz field, we determine the conductivity of the WD-Al at different electron temperatures. We combine these results with studies using MeV-UED, which is ideally suited to probe of the structure of thin films because of the high scattering cross section, short wavelength, and sub-picosecond pulse duration of relativistic electron bunches. These measurements provide crucial characterization of the state and density of the WD-Al probed by the THz pulse, as well as the timescale of melting of Al films at high excitation densities. We separate the effects of changes in lattice structure with electron and ion temperatures, and compare our data with state of the art simulations.