# Pump driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES

## TL;DR

This paper models a pump-induced transition from a band insulator to a nonequilibrium excitonic insulator, revealing oscillations, spectral shape changes, and BEC-BCS crossover signatures observable via time-resolved ARPES and photocurrent oscillations.

## Contribution

It provides the first ab initio band structure of LiF in NEQ-EI states and links spectral and photocurrent oscillations to Josephson-like phenomena in a pump-driven system.

## Key findings

- Spectral convex-to-concave transition during pump driving.
- Observation of BEC-BCS crossover signatures in ARPES spectra.
- Photocurrent oscillations with pump-tunable frequency post-pump.

## Abstract

We consider a ground-state wide-gap band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a Nonequilibrium Green's Function treatment on the Konstantinov-Perel' contour. We present the first {\em ab initio} band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states and show that these states can be generated by currently available pump pulses. We highlight two general features of time-resolved ARPES spectra: (1) during the pump-driving the excitonic spectral structure undergoes a convex-to-concave shape transition and {\em concomitantly} the state of the system goes through a BEC-BCS crossover; (2) attosecond pulses shone after the pump-driving at different times $t_{\rm delay}$ generate a photocurrent which {\em oscillates} in $t_{\rm delay}$ with a pump-tunable frequency -- we show that this phenomenon is similar to the AC response of an exotic Josephson junction.