Dynamics of valley pseudospin in single-layer WSe2. Inter-valley scattering mediated by electron-phonon interaction

In a time-dependent Kerr experiment a circularly polarized laser field is used to selectively populate the $K^{\pm}$ electronic valleys of single-layer WSe$_2$. This carrier population corresponds to a finite pseudospin polarization that dictates the valleytronic properties of WSe$_2$, but whose decay mechanism still remains largely debated. Time-dependent Kerr experiments provide an accurate way to visualize the pseudospin dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized and short pump pulse. We present here a clear, accurate and parameter-free description of the valley pseudospin dynamics in single-layer WSe$_2$. By using an ab-initio approach we solve unambiguously the long—standing debate about the dominant mechanism that drives the valley depolarization. Our results are in excellent agreement with recent time-dependent Kerr experiments. The decay dynamics and peculiar temperature dependence is explained in terms of electron—phonon mediated processes that induce spin-flip inter-valley transitions.