Controlling spin and valley hall effect in monolayer WSe2 at elevated temperatures

Abstract

The spin and valley physics in 2-dimensional van der Waals materials provides a unique platform for novel applications in spintronics and valleytronics. 2H phase transition metal dichalcogenides (TMD) monolayers possesses broken inversion symmetry and strong spin-orbit coupling, leading to a coupled spin and valley physics that makes them better candidates for these applications. For practical device applications, spin and valley Hall effect (SVHE) is a good way of charge to spin and charge to valley conversion, making the electrical generation of spin and valley polarization possible. While SVHE has been observed via optical measurements at cryotemperatures below 30 K, the behavior at elevated temperatures and thorough understanding of the data are still lacking. In this work we conduct spatial Kerr rotation (KR) measurements on monolayer tungsten diselenide (WSe₂) field effect transistors and study the electrical control and temperature dependence of SVHE. We image the distribution of the spin and valley polarization directly and find clear evidence of the spin and valley accumulation at the edges. We show that the SVHE can be electrically modulated by the gate and drain bias, and the polarization persists at elevated temperatures. We then conduct four-port electrical test reflection spectra measurement and use a drift-diffusion model to interpret the data and extract key parameters. A lower-bound spin/valley lifetime is predicted of 40 ns and a mean free path of 240 nm below 90 K. The spin/valley polarization on the edge is calculated to be ~4% at 45 K. WSe₂-on-hBN samples are prepared as well, and the KR measurements on these samples are discussed.