Probing electron transport and thermodynamic properties in expanded warm dense matter

The expanded warm dense matter regime, an intermediate state between condensed matter and high-energy-density plasma at low density, remains poorly constrained experimentally, particularly regarding transport properties across the metal-to-nonmetal transition. Here, we explore expanded warm dense aluminum using a newly developed pulsed Joule heating platform combined with first-principles simulations. Using a largely model-independent approach with time-resolved diagnostics, we measure thermodynamic and transport properties from ambient density down to 0.6 g ⋅ cm^-3 and temperatures exceeding 37 kK, providing benchmark data for electrical conductivity and equation-of-state models. Our results reveal electronic state localization associated with the onset of the metal-to-nonmetal transition and show distinct conductivity behaviors depending on the thermodynamic path followed during expansion. Crucially, electronic relaxation time - rather than ionization - dominates conduction in this supercritical regime. These observations advance our understanding of expanded warm dense matter and open perspectives for future studies on alloys and high-Z elements relevant to giant impact modeling and fusion energy.