Observation of intervalley quantum interference in epitaxial monolayer WSe2

TL;DR

This study uses low-temperature STM/S to observe quantum interference patterns in monolayer WSe2, revealing spin-valley coupling and long valley/spin lifetimes, crucial for future spintronic and valleytronic devices.

Contribution

First experimental observation of quasi-particle interference patterns in monolayer WSe2 confirming spin-valley coupling and large spin-splitting at Q valleys.

Findings

Intervalley quantum interference involving Q-valleys observed

Spin-conserved scattering processes dominate, spin-flip scattering absent

Long valley and spin lifetimes inferred from inefficient spin-flip scattering

Abstract

Monolayer (ML) transition metal dichalcogenides (TMDs) have been attracting great research attentions lately for their extraordinary properties, in particular the exotic spin-valley coupled electronic structures that promise future spintronic and valleytronic applications1-3. The energy bands of ML TMDs have well separated valleys that constitute effectively an extra internal degree of freedom for low energy carriers3-12. The large spin-orbit coupling in the TMDs makes the spin index locked to the valley index, which has some interesting consequences such as the magnetoelectric effects in 2H bilayers13. A direct experimental characterization of the spin-valley coupled electronic structure can be of great interests for both fundamental physics and device applications. In this work, we report the first experimental observation of the quasi-particle interference (QPI) patterns in ML WSe2 using low-temperature (LT) scanning tunneling microscopy/spectroscopy (STM/S). We observe intervalley quantum interference involving the Q-valleys in the conduction band due to spin-conserved scattering processes, while spin-flip intervalley scattering is absent. This experiment establishes unequivocally the presence of spin-valley coupling and affirms the large spin-splitting at the Q valleys. Importantly, the inefficient spin-flip intervalley scattering implies long valley and spin lifetime in ML WSe2, which represents a key figure of merit for valley-spintronic applications.