Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

TL;DR

This study combines ultrafast X-ray diffraction experiments and molecular dynamics simulations to reveal the microscopic pathways of non-equilibrium melting in thin polycrystalline palladium films, emphasizing defect roles and model validity.

Contribution

It provides a detailed microscopic understanding of ultrafast melting mechanisms in Pd, validating existing models under extreme conditions and highlighting defect influences.

Findings

Validation of non-equilibrium melting models at ultrafast timescales

Identification of defect roles in the melting process

Real-time atomic structure snapshots during melting

Abstract

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.