Femtosecond phase-transition in hard x-ray excited bismuth

TL;DR

This paper reports the first observation of an ultrafast, nonthermal phase transition in bismuth induced by hard x-ray pulses, revealing rapid lattice disordering within 300 femtoseconds.

Contribution

It demonstrates the capability of hard x-ray pulses to induce ultrafast phase transitions in bismuth, expanding understanding of nonthermal lattice dynamics.

Findings

Peak intensity drops to zero within 300 fs at high fluence

The process is nonthermal and occurs faster than A1g phonon oscillation

Excludes electron-ion and thermally driven plasma formation explanations

Abstract

The evolution of the bismuth crystal structure upon excitation of its A$_{1g}$ phonon has been intensely studied with short pulse optical lasers. Here we present the first-time observation of a hard x-ray induced ultrafast phase transition in a bismuth single crystal, at high intensities (~$10^{14}$ W/cm$^2$). The lattice evolution was followed using a recently demonstrated x-ray single-shot probing setup. The time evolution of the (111) Bragg peak intensity showed strong dependence on the excitation fluence. After exposure to a sufficiently intense x-ray pulse, the peak intensity dropped to zero within 300fs, i.e. faster than one oscillation period of the A1g mode at room temperature. Our analysis indicates a nonthermal origin of a lattice disordering process, and excludes interpretations based on electron-ion equilibration process, or on thermodynamic heating process leading to a plasma formation.