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. Despite the ubiquity of warm dense matter states, there is lack of well-established models describing the expanded regime. To address this gap, the authors provide theoretical and experimental analysis of warm dense aluminium by means of pulse Joule heating. This provides constraints to the equation of state and insight into the electrical conductivity behaviour.