Speaker
Description
The time--resolved angle--resolved photoemitton spectra of WSe$_2$, a
paradigmatic metal dichalcogenide, is dominated by a peak that,
initially observed withing the gap at the $K$ valley, scatters, on an
ultra--fast time scale of $\sim 30$ fs to the S valley.
In this work we question the commonly used interpreation of the
experimental observations in terms of a massive bound electron--hole
exciton
scattering with phonon and behaving as a quasi--particle. By using a
combined theoretical and experimental investigation we demonstrate that
the observed dynamics can be interpreted as the photo--induced
transition from a direct to and indirect excitonic insulating phase. The
peaks that appear in the experimental spectrum correspond to
single--particle levels renormalized by the excitonic spontaneous
polarization.