First-order rhombohedral to cubic phase transition in photoexcited GeTe

TL;DR

This study reveals that photoexcited GeTe undergoes a strongly first-order, non-thermal phase transition from rhombohedral to cubic, driven by electronic gap closure, without phonon softening, and confirmed by ultrafast X-ray diffraction data.

Contribution

The paper demonstrates that the photoinduced phase transition in GeTe is first-order and driven by electronic effects, contrasting with thermal transitions, using advanced theoretical modeling.

Findings

Transition is strongly first order with phase coexistence.

No phonon softening occurs during the transition.

Ultrafast X-ray data supports phase coexistence evidence.

Abstract

Photoexcited GeTe undergoes a non-thermal phase transition from a rhombohedral to a rocksalt crystalline phase. The microscopic mechanism and the nature of the transition are unclear. By using constrained density functional perturbation theory and by accounting for quantum anharmonicity within the stochastic self-consistent harmonic approximation, we show that the non-thermal phase transition is strongly first order and does not involve phonon softening, at odd with the thermal one. The transition is driven by the closure of the single particle gap in the photoexcited rhombohedral phase. Finally, we show that ultrafast X-ray diffraction data are consistent with a coexistence of the two phases, as expected in a first order transition. Our results are relevant for the understanding of phase transitions and bonding in phase change materials.