Increased Electron-Phonon Coupling Strengths in Nonequilibrium Warm Dense Copper*

This study investigates the strength of electron-phonon couplings in nonequilibrium laser-heated warm dense copper (WDCu). Using XFEL absorption spectroscopy, which offers a unique combination of ultrafast temporal and high-energy resolution, the equilibration process involving both non-thermal and thermal electrons alongside the lattice is examined. Significant enhancements in electron-phonon coupling strengths are observed. Theoretical analysis, considering the degree of electron nonequilibrium, sheds light on the mechanisms driving these enhancements.