Light induced phase transitions in GeTe and SnSe monochalcogenides.*

In this talk I will demonstrate the capability of constrained density functional perturbation theory (1) coupled with a non-perturbative treatment of quantum anharmonicity (2) to describe light-induced phase transition in GeTe and SnSe monochalcogenides.

In SnSe I will show that ultrafast lasers can permanently transform the topologically-trivial orthorhombic structure of SnSe into the topological crystalline insulating rocksalt phase via a first-order non-thermal phase transition (3). I will describe the reaction path and evaluate the critical fluence and the possible decay channels after photoexcitation.

Our simulations of the photoexcited structural and vibrational properties are in excellent agreement with recent pump-probe data in the intermediate fluence regime below the transition with an error on the curvature of the quantum free energy of the photoexcited state that is smaller than $2\%$.

In GeTe I will investigate the non-thermal phase transition from a rhombohedral to a rocksalt crystalline phase. The microscopic mechanism and nature of the transition are unclear. I will show that the non-thermal phase transition is strongly first order and does not involve phonon softening, in contrast to the thermal one. The transition is driven by the closure of the single-particle gap in the photoexcited rhombohedral phase. Finally, I will show that ultrafast XRD data are consistent with the coexistence of the two phases, as expected in a first order transition.

The high accuracy of our results demonstrates that an approach based on a complete self-consistent constrained density functional perturbation theory in the presence of an electron-hole plasma captures the light-induced structural deformations and transitions extremely accurately.