Ab initio calculations of ultra-short carrier dynamics in 2D materials: valley depolarization in single-layer WSe$_2$

摘要

In single-layer WSe$_2$, a paradigmatic semiconducting transition metaldichalcogenide, a circularly polarized laser field can selectively exciteelectronic transitions in one of the inequivalent $K^{\pm}$ valleys. Suchselective valley population corresponds to a pseudospin polarization. This canbe used as a degree of freedom in a valleytronic device provided that the timescale for its depolarization is sufficiently large. Yet, the mechanism behindthe valley depolarization still remains heavily debated. Recent time-dependentKerr experiments have provided an accurate way to visualize the valley dynamicsby measuring the rotation of a linearly polarized probe pulse applied after acircularly polarized pump pulse. We present here a clear, accurate andparameter-free description of the valley dynamics. By using an atomistic, abinitio, approach we fully disclose the elemental mechanisms that dictate thedepolarization effects. Our results are in excellent agreement with recenttime-dependent Kerr experiments. We explain the Kerr dynamics and itstemperature dependence in terms of electron-phonon mediated processes thatinduce spin-flip inter-valley transitions.