Optical detection of long electron spin transport lengths in a monolayer semiconductor

TL;DR

This study demonstrates long-distance spin/valley electron transport in a monolayer WSe2 using optical methods, revealing diffusion lengths of about 18 micrometers and emphasizing the influence of temperature and excitation conditions.

Contribution

It provides the first direct optical measurement of long-range spin/valley transport in a monolayer semiconductor, highlighting the roles of spin relaxation and pumping efficiency.

Findings

Spin/valley polarization of up to 75% achieved

Transport distances of up to 20 micrometers observed

Diffusion length of approximately 18 micrometers measured

Abstract

Using a spatially-resolved optical pump-probe experiment, we measure the lateral transport of spin/valley polarized electrons over very long distances (tens of micrometers) in a single WSe2 monolayer. By locally pumping the Fermi sea of 2D electrons to a high degree of spin/valley polarization (up to 75%) using circularly-polarized light, the lateral diffusion of the electron polarization can be mapped out via the photoluminescence induced by a spatially-separated and linearly-polarized probe laser. Up to 25% spin-valley polarization is observed at pump-probe separations up to 20 microns. Characteristic spin/valley diffusion lengths of 18 +/- 3 um are revealed at low temperatures. The dependence on temperature, pump helicity, pump intensity, and electron density highlight the key roles played by spin relaxation time and pumping efficiency on polarized electron transport in monolayer semiconductors possessing spin-valley locking.