Magnetic field control of light-induced spin accumulation in monolayer MoSe₂

Abstract

Semiconductor transition metal dichalcogenides (TMDs) have equivalent dynamics for their two spin/valley species. This arises from their energy-degenerated spin states, connected via time-reversal symmetry. When an out-of-plane magnetic field is applied, time-reversal symmetry is broken and the energies of the spin-polarized bands shift, resulting in different bandgaps and dynamics in the K+ and K- valleys. Here, we use time-resolved Kerr rotation to study the magnetic field dependence of the spin dynamics in monolayer MoSe2. We show that the magnetic field can control the light-induced spin accumulation of the two valley states, with a small effect on the recombination lifetimes. We unveil that the magnetic field-dependent spin accumulation is in agreement with hole spin dynamics at the longer timescales, indicating that the electron spins have faster relaxation rates. We propose a rate equation model that suggests that lifting the energy-degeneracy of the valleys induces an ultrafast spin-flip toward the stabilization of the valley with the higher valence band energy. Our results provide an experimental insight into the ultrafast charge and spin dynamics in TMDs and a way to control it, which will be useful for the development of new spintronic and valleytronic applications.

Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor

“…We use the time-resolved magneto-optic Faraday effect to monitor the magnetization dynamics in our system using a single-color pump-probe setup similar to the one described in refs. 54 , 55 (more information in Methods). The process of optical excitation of magnetization dynamics in van der Waals magnets has been previously reported as purely thermal 44 – 48 , 53 , similar to many studies on conventional metallic thin films 56 – 58 .…”

“…The pump beam goes through a mechanical delay stage, allowing us to modify the time delay between pump and probe by a change in the optical path length. To allow for a double-modulation detection 54 , 55 , the pump beam goes through an optical chopper working at 2173 Hz. The polarization of the pump is set to be horizontal (p-polarized with respect to the sample), to allow us to block the pump beam through a polarization filter at the detection stage.…”