INTRODUCTION AND MOTIVATION
A group at The Ohio State University studied monolayer tungsten disulfide (WS2), which is a transition metal dichalcogenide (TMD). TMDs have recently been studied for their characteristics which are useful in spintronic and nanoscale device applications. Properties of TMDs that are desirable for these applications include the large valley dependent spin orbit splitting and the spin/valley-selective optical selection rules.
Chemical vapor deposition was used to form monolayer WS2 on SiO2/Si substrates. Photoluminescence (PL) and transport measurements confirmed that the samples were monolayer and n-type respectively.
The Cryostation was used to cool samples to 6K and then performed time-resolved Kerr rotation (TRKR) microscopy using ultra-fast pulses focused to 1 micron on the sample. The pump beam was circularly polarized which led to the creation of valley-polarized excitons and the linear polarized probe beam measured the spin/valley polarization (Figure 1). Scans of the pump/probe spot overlapped to yield spatial images of spin density. Mapping with different time delays revealed the evolution of the spin dynamics on the monolayer WS2.
Figure 1: Optical setup for TRKR microscopy. Source
The researchers imaged spin dynamics of the monolayer WS2 using time-resolved Kerr rotation (TRKR) microscopy and micro-photoluminescence. They found an anti-correlation between A exciton luminescence and high spin density implying that there is a transfer of spin angular momentum of excitons to long lived spin states. The initial decay is due to the loss of valley polarization of excitons, but the Kerr rotation persists due to spin and/or valley polarization of n-type conduction electrons.
The researchers then examined the spin stabilization. Conventionally, with small-spin orbit coupling, electron spins precess in an external field, but with large-spin orbit field spins are stabilized such that even with changing external magnetic field or temperatures, the lifetime stays constant. The researchers measured a section of their sample where the spin density was high and found that the spin lifetime of WS2 stayed constant up to 130K and external magnetic fields up to 700mT (Figure 2).
Figure 2: Kerr Rotation vs Time Delay for varying external magnetic field (left); Plot of spin lifetime vs external magnetic field (top right); Spin Lifetime vs. Temperature, the inset showing different time delays at various temperatures (bottom right). Source
The experiments showed that spin lifetime stabilization is due to large spin-orbit coupling of the monolayer WS2. Other examples of TMD monolayers are MoS2, which has a smaller spin-orbit coupling and therefore has the attribute of short lifetimes above 40K. The TMD monolayer WSe2 has short spin-lifetimes of roughly 1ns, but maintains these lifetimes up to room temperature. Further work on WS2 will be conducted to investigate the nature of the spin transfer mechanism.
Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy arXiv:1602.03568v1