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.
Highlights
Quantum dynamics out of equilibrium can be used to disentangle interesting mechanisms of materials’ properties, such as origin of ordered states and their subsequent control
We have considered the out-of-equilibrium dynamics in a prototypical ordered-phase material, namely the excitonic insulator
We have studied out-of-equilibrium conditions due to a laser-pulse excitation and coupling the excitonic insulator (EI) system to a fermionic bath
Summary
Quantum dynamics out of equilibrium can be used to disentangle interesting mechanisms of materials’ properties, such as origin of ordered states and their subsequent control. Recent experimental progress in pump-probe-spectroscopical approaches to excitonic insulator [1], charge-density wave [2], and superconducting phases [3,4] has prompted extensive research interest in both simulating [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] and measuring [4,24,25,26,27,28,29] ultrafast quantum correlation effects far from equilibrium Simulating these processes can be challenging since an accurate but computationally feasible theoretical description is required for simultaneously dealing with strong external fields, many-particle interactions, and transient effects.
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