Progress in the study of electron–ion coupling in nonequilibrium Warm Dense Au and Cu

Abstract

Electron–ion coupling is a fundamental process in nonequilibrium Warm Dense Matter. It also plays a central role in governing the states resulting from the interaction of high-intensity ultrafast lasers and free-electron-lasers with matter, an area that is of growing interest. Our inadequate understanding of the process was revealed in 1992 by the discovery of much weaker than expected electron–ion coupling in the nonequilibrium state at a shock front in Si where energy was transferred from hot ions to cold electrons. This necessarily raised questions about the behavior of electron–ion coupling in states with hot electrons and cold ions. It became a new focus of the field with the discovery of apparently constant electron–ion coupling in $fs$-laser heated Au, prompting a wide range of experimental and theoretical investigations of non-equilibrium warm dense Au as well as Cu for almost two decades. Fueling the pursuit were the findings of both constant and temperature-dependent coupling from subsequent experiments while the results of theoretical models were revealing reduced electron temperature dependence of electron–ion coupling. The goal of this review is to provide a concise historical account of the reported investigations, focussing on the salient characteristics of the experiments and models and how the diverse findings may be reconciled. Surprisingly, with alternative interpretations of some of the experimental results, a consistent behavior of weak electron–ion coupling has now emerged for $fs$-laser heated Au and Cu for electron temperature up to 20,000 K, corroborated by the evolution of theoretical predictions towards weak coupling. This is a significant progress. It will incentivize new research to gain further understanding of electron–ion coupling in nonequilibrium Warm Dense Matter.