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

TL;DR

This paper uses ab initio calculations to accurately model valley depolarization in single-layer WSe$_2$, revealing electron-phonon interactions as the key mechanism behind valley pseudospin relaxation, consistent with recent Kerr experiments.

Contribution

It provides a parameter-free, atomistic understanding of valley depolarization mechanisms in WSe$_2$, clarifying the role of electron-phonon interactions in spin-flip inter-valley transitions.

Findings

Electron-phonon interactions induce valley depolarization.

Results agree with recent Kerr experiment data.

Temperature influences valley depolarization dynamics.

Abstract

In single-layer WSe$_2$, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent $K^{\pm}$ valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a valleytronic device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio, approach we fully disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time-dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon mediated processes that induce spin-flip inter-valley transitions.