Spin States Protected from Intrinsic Electron-Phonon-Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe$_2$

TL;DR

This study demonstrates that monolayer MoSe$_2$ exhibits spin lifetimes exceeding 100 ns at room temperature, with a temperature-dependent Kerr amplitude influenced by electron-phonon interactions and valley dynamics, revealing protected spin states.

Contribution

The paper uncovers long-lived spin states in MoSe$_2$ that are protected from electron-phonon coupling, advancing understanding of spin preservation in 2D materials.

Findings

Spin lifetimes over 100 ns at room temperature.

Temperature-dependent Kerr amplitude with sign change.

Electron-phonon coupling does not limit long spin lifetimes.

Abstract

We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe$_2$. These long lifetimes are accompanied by an intriguing temperature dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations we explain the latter behavior in terms of the intrinsic electron-phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photo-excited electron-hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated inter-valley scattering rates. However, the electron-phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states which are protected from the intrinsic electron-phonon coupling, and are most likely resident charge carriers which are not part of the itinerant valence or conduction band states.