Flux-limited non-equilibrium electron energy transport in warm dense gold

Abstract

Summary form only given. Between solid and plasma lies the so-called Warm Dense Matter regime where temperature of the state is comparable to Fermi energy and the ions are strongly coupled. Accordingly the physics of Warm Dense Matter is dominated by the complex interplay of electronic excitation, electron degeneracy, and strong ion-ion correlation. Understanding the behavior of Warm Dense Matter thus offers an opportunity to uncover the convergence of condensed matter and plasma physics. Earlier experiments have led to development of theoretical models for electron energy relaxation, dielectric function and lattice stability [1–5]. In this paper, we will present results of our new study on non-equilibrium electron energy transport, a process that is pertinent to not only ultrafast excitation of solids but also carrier dynamics in ultrafast electronics. In our experiment, an abrupt change in energy transport has been observed in femtosecond-laser heated gold when the absorbed laser flux exceeds ∼7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Below this value, the absorbed flux is carried by ballistic transport of non-thermal electrons produced in interband excitation. Above this value, energy transport appears to include ballistic transport by non-thermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ∼7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This yields a unique benchmark for comparison with theory on non-equilibrium electron transport.