Ultrafast Strongly Anisotropic Valleytronics in SnSe

TL;DR

This study investigates ultrafast valley polarization dynamics in SnSe, revealing nearly perfect and stable polarization when selectively excited, but rapid decay and reversal due to intervalley scattering when excited differently, highlighting unique anisotropic valley physics.

Contribution

It provides the first detailed exploration of ultrafast nonequilibrium valley dynamics in SnSe using combined experimental and theoretical approaches.

Findings

Nearly unity, stable valley polarization with selective excitation.

Ultrafast decay and reversal of valley polarization due to intervalley scattering.

Distinct anisotropic valley physics compared to other 2D materials.

Abstract

Valleytronics aims to control electrons in a valley-specific manner for quantum information manipulation. Due to their strong in-plane anisotropy, which enables polarization-controlled optical transitions to distinct nondegenerate valleys, group-IV monochalcogenides have been recently proposed as promising candidates for next-generation valleytronic materials. However, ultrafast nonequilibrium dynamics following optical preparation of valley-polarized states remain completely unexplored in these systems. Combining time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with time-dependent Boltzmann equation simulations, we investigate ultrafast valley polarization dynamics following polarization-controlled photoexcitation in SnSe. We show that selective excitation to valleys at global conduction minima yields nearly unity and time-independent valley polarization. In contrast, photoexcitation to the other valley channel leads to ultrafast decay and reversal of valley polarization on sub-picosecond timescales due to intervalley scattering mediated by strong electron-phonon coupling with an optical phonon mode. Our findings reveal strongly anisotropic and radically different nonequilibrium valley physics than in most common two-dimensional valleytronics materials.