# Inter-valley dark trion states with spin lifetimes of 150 ns in WSe$_2$

## TL;DR

This study reports long-lived dark trion states with spin lifetimes up to 150 ns in WSe$_2$ monolayers, revealing complex spin dynamics influenced by extrinsic factors and demonstrating the formation of dark trion states affecting spin properties.

## Contribution

The paper uncovers long-lived dark trion states in WSe$_2$ monolayers and provides a detailed analysis of their spin dynamics and the influence of extrinsic effects.

## Key findings

- Long trion spin lifetimes of up to 150 ns at 5 K.
- Detection of a Kerr rotation signal from the triplet state.
- Observation of inhomogeneous spin dephasing and extrinsic effects on spin dynamics.

## Abstract

We demonstrate long trion spin lifetimes in a WSe$_2$ monolayer of up to 150 ns at 5 K. Applying a transverse magnetic field in time-resolved Kerr-rotation measurements reveals a complex composition of the spin signal of up to four distinct components. The Kerr rotation signal can be well described by a model which includes inhomogeneous spin dephasing and by setting the trion spin lifetimes to the measured excitonic recombination times extracted from time-resolved reflectivity measurements. We observe a continuous shift of the Kerr resonance with the probe energy, which can be explained by an adsorbate-induced, inhomogeneous potential landscape of the WSe$_2$ flake. A further indication of extrinsic effects on the spin dynamics is given by a change of both the trion spin lifetime and the distribution of g-factors over time. Finally, we detect a Kerr rotation signal from the trion's higher-energy triplet state when the lower-energy singlet state is optically pumped by circularly polarized light. We explain this by the formation of dark trion states, which are also responsible for the observed long trion spin lifetimes.