Comparing the generalized Kadanoff-Baym ansatz with the full Kadanoff-Baym equations for an excitonic insulator out of equilibrium

TL;DR

This study compares the generalized Kadanoff-Baym ansatz (GKBA) with full nonequilibrium Green's function methods in modeling excitonic insulator dynamics after laser excitation, revealing regimes of agreement and limitations of GKBA.

Contribution

It demonstrates the validity and limitations of GKBA in simulating out-of-equilibrium excitonic insulator dynamics, especially under strong excitation conditions.

Findings

Good agreement between GKBA and full Green's functions in weak and intermediate regimes.

GKBA shows persistent oscillations without complete order melting under strong excitation.

Coupling to an external bath enables GKBA to model transition to the normal state.

Abstract

We investigate out-of-equilibrium dynamics in an excitonic insulator (EI) with a finite momentum pairing perturbed by a laser-pulse excitation and a sudden coupling to fermionic baths. The transient dynamics of the excitonic order parameter is resolved using the full nonequilibrium Green's function approach and the generalized Kadanoff-Baym ansatz (GKBA) within the second-Born approximation. The comparison between the two approaches after a laser pulse excitation shows a good agreement in the weak and the intermediate photo-doping regime. In contrast, the laser-pulse dynamics resolved by the GKBA does not show a complete melting of the excitonic order after a strong excitation. Instead we observe persistent oscillations of the excitonic order parameter with a predominant frequency given by the renormalized equilibrium bandgap. This anomalous behavior can be overcome within the GKBA formalism by coupling to an external bath, which leads to a transition of the EI system towards the normal state. We analyze the long-time evolution of the system and distinguish decay timescales related to dephasing and thermalization.