# Excitonic Gap Formation in Pumped Dirac Materials

## TL;DR

This paper theoretically explores how optical pumping can induce transient excitonic states in Dirac materials, reducing the critical Coulomb interaction needed for exciton formation and suggesting experimental conditions for observing these states.

## Contribution

It demonstrates the feasibility of transient excitonic instabilities in pumped Dirac materials and provides guidelines for experimental observation and material parameters for large excitonic gaps.

## Key findings

- Reduction of critical coupling for exciton formation
- Identification of signatures of transient excitonic states
- Estimation of excitonic gaps and critical temperatures in Dirac materials

## Abstract

Recent pump-probe experiments demonstrate the possibility that Dirac materials may be driven into transient excited states describable by two chemical potentials, one for the electrons and one for the holes. Given the Dirac nature of the spectrum, such an inverted population allows the optical tunability of the density of states of the electrons and holes, effectively offering control of the strength of the Coulomb interaction. Here we discuss the feasibility of realizing transient excitonic instabilities in optically-pumped Dirac materials. We demonstrate, theoretically, the reduction of the critical coupling leading to the formation of a transient condensate of electron-hole pairs and identify signatures of this state. Furthermore, we provide guidelines for experiments by both identifying the regimes in which such exotic many-body states are more likely to be observed and estimating the magnitude of the excitonic gap for a few important examples of existing Dirac materials. We find a set of material parameters for which our theory predicts large gaps and high critical temperatures and which could be realized in future Dirac materials. We also comment on transient excitonic instabilities in three-dimensional Dirac and Weyl semimetals. This study provides the first example of a transient collective instability in driven Dirac materials.