Ultrafast creation and melting of nonequilibrium excitonic condensates in bulk WSe2

We show the formation of a nonequilibrium excitonic condensate in a bulk WSe2 coherently pumped in resonance with the lowest-energy exciton. The lifetime of the superfluid is addressed by studying the screened dynamics during and after the pump pulse. Intervalley scattering causes electron migration from the optically populated K valley to the conduction band minimum at ς. Due to the electron-hole imbalance at the K point a plasma of quasifree holes develops, which efficiently screens the interaction of the remaining excitons. We show that this plasma screening causes an ultrafast melting of the nonequilibrium condensate and that during melting coherent excitons and quasifree 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 ARPES experiments.