Direct observation of ultrafast amorphous-amorphous transitions indicated by bond stretching and angle bending in phase-change material GeTe

TL;DR

This study uses femtosecond electron diffraction and simulations to directly observe ultrafast amorphous-amorphous transitions in GeTe, revealing rapid bond stretching and angle bending that elucidate atomic-scale glass transition mechanisms.

Contribution

It provides the first direct observation of ultrafast amorphous-amorphous transitions in GeTe, linking atomic motions to structural features and dynamics at femtosecond timescales.

Findings

Ultrafast Ge-Te bond stretching within 0.2 ps

Angle bending of Ge-Te-Ge motif on 0.5-2 ps timescale

Identification of 3.10 THz localized oscillation modes

Abstract

The intrinsic nature of glass states and glass transitions at the atomic scale remain a fundamental open question in condensed-matter physics and materials science. By combining femtosecond electron diffraction with time-dependent density-functional theory molecular dynamics simulations, we directly observe ultrafast amorphous-amorphous transitions in amorphous GeTe, manifested as rapid Ge-Te (Ge) bond stretching within 0.2 ps and subsequent angle bending of the Ge-Te (Ge)-Ge motif on a 0.5-2 ps timescale. Critically, the ultrafast bond stretching is accompanied by localized oscillation modes with the frequency of 3.10 THz, unambiguously signaling the local Peierls-like bonding structure and the flexibility of these polarized bonds. These ultrafast collective atomic motions provide a direct structural origin for the boson peak and pay the way for systematic optimization of relaxation and crystallization kinetics.