Self-consistent screening enhances stability of the nonequilibrium excitonic insulator phase

The nonequilibrium excitonic insulator (NEQ-EI) is an excited state of matter characterized by a finite density of coherent excitons and a time-dependent macroscopic polarization. The stability of this exciton superfluid as the density grows is jeopardized by the increased screening efficiency of the looser excitons. In this work we put forward a Hartree plus Screened Exchange HSEX scheme to predict the critical density at which the transition toward a free electron-hole plasma occurs. The dielectric function is calculated self-consistently using the NEQ-EI polarization and found to vanish in the long-wavelength limit. This property makes the exciton superfluid stable up to relatively high densities. Numerical results for the MoS$_{2}$ monolayers indicate that the NEQ-EI phase survives up to densities of the order of $10^{12}mathrm{cm}^{-2}$.