Picosecond-scale Heterogeneous Melting of Metals at Extreme Non-equilibrium States

TL;DR

This paper reveals that ultrafast laser excitation causes nonthermal melting in metals through electronic pressure relaxation, leading to rapid, surface-initiated phase changes much faster than traditional thermal melting.

Contribution

It introduces neural network-enhanced multiscale simulations to identify electronic pressure relaxation as key to nonthermal melting, a novel mechanism in laser-induced phase transformations.

Findings

Electronic pressure relaxation drives rapid surface melting.

Nonthermal expansion enables melting below equilibrium temperatures.

Melting interface propagates at 2500 m/s, much faster than thermal mechanisms.

Abstract

Extreme electron-ion non-equilibrium states, generated by ultrafast laser excitation, lead to melting processes that are fundamentally different from those under conventional thermal equilibrium and remain not fully understood. Through neural network-enhanced multiscale simulations of tungsten and gold nanofilms, we identify electronic pressure relaxation as critical to heterogeneous phase transformations. This nonthermal expansion generates a density decrease that enable surface-initiated melting far below equilibrium melting temperatures, creating electronic pressure-driven solid-liquid interface propagation at a high speed of 2500 m/s -- tenfold faster than that of thermal heterogeneous melting mechanisms. Simulated time-resolved X-ray diffraction signatures distinguish this nonthermal expansion from thermal expansion dynamics driven by thermoelastic stress. These results establish hot-electron-mediated lattice destabilization as a universal pathway for laser-induced structural transformations, providing new insights for interpreting time-resolved experiments and controlling laser-matter interactions.