Ultrafast creation and melting of nonequilibrium excitonic condensates in bulk WSe$_{2}$

TL;DR

This paper investigates how ultrafast laser excitation causes rapid formation and melting of nonequilibrium excitonic condensates in bulk WSe$_{2}$, revealing the dynamics of screening and coexistence of excitons and free carriers.

Contribution

It provides a detailed theoretical analysis of the ultrafast melting process of excitonic condensates, matching recent experimental observations.

Findings

Ultrafast melting of the excitonic condensate due to plasma screening.

Coexistence of coherent excitons and free electron-hole pairs during melting.

Time-resolved spectral features match experimental data.

Abstract

We study the screened dynamics of the nonequilibrium excitonic consensate forming in a bulk WSe$_{2}$ when illuminated by coherent light resonant with the lowest-energy exciton. Intervalley scattering causes electron migration from the optically populated K valley to the conduction band minimum at $\Sigma$. Due to the electron-hole unbalance at the K point a plasma of quasi-free holes develops, which efficiently screens the interaction of the remaining excitons. We show that this plasma screening causes an ultrafast melting of the nonequilibrium consensate and that during melting coherent excitons and quasi-free electron-hole pairs coexist. The time-resolved spectral function does exhibit a conduction and excitonic sidebands of opposite convexity and relative spectral weight that changes in time. Both the dependence of the time-dependent conduction density on the laser intensity and the time-resolved spectral function agree with recent experiments.