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

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.