Photo-induced gap closure in an excitonic insulator

TL;DR

This paper investigates how photo-excitation induces a phase transition in an excitonic insulator, revealing two critical points and the mechanisms behind gap closure through advanced time-dependent simulations.

Contribution

It introduces a time-dependent GW approach to study dynamical phase transitions and clarifies the role of screening and energy transfer in gap melting.

Findings

Identification of two dynamical phase transition points.

Main mechanism for gap melting is energy transfer from carriers to excitons.

Screening effects are minor on low energy dynamics.

Abstract

We study the dynamical phase transition out of an excitonic insulator phase after photo-excitation using a time-dependent extension of the selfconsistent GW method. We connect the evolution of the photoemission spectra to the dynamics of the excitonic order parameter and identify two dynamical phase transition points marked by a slowdown in the relaxation: one critical point is connected with the trapping in a nonthermal state with reduced exciton density and the second corresponds to the thermal phase transition. The transfer of kinetic energy from the photoexcited carriers to the exciton condensate is shown to be the main mechanism for the gap melting. We analyze the low energy dynamics of screening, which strongly depends on the presence of the excitonic gap, and argue that it is difficult to interpret the static component of the screened interaction as the effective interaction of some low energy model. Instead we propose a phenomenological measure for the effective interaction which indicates that screening has minor effects on the low energy dynamics.