Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2

TL;DR

This study demonstrates that circularly polarized light can generate and control spin-, valley-, and layer-polarized excited carriers in centrosymmetric WSe2, revealing ultrafast dynamics and potential for quantum device applications.

Contribution

It shows for the first time that centrosymmetric WSe2 can host polarized excited states manipulated by light, combining experimental and theoretical insights into their dynamics.

Findings

Circularly polarized light induces spin-, valley-, and layer-polarized states in WSe2.

Carrier scattering occurs within 100 femtoseconds to states enabling inter-layer charge transfer.

The excited states exhibit strong circular dichroism and two-dimensional character in the K valleys.

Abstract

Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies, for which atomically thin transition metal dichalcogenides (TMDCs) have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry gives rise to a spin-valley correlation of the band structure allowing for valley-selective electronic excitation with circularly polarized light. Here we show that, even in centrosymmetric samples of 2H-WSe2, circularly polarized light can generate spin-, valley- and layer-polarized excited states in the conduction band. Employing time- and angle-resolved photoemission spectroscopy (trARPES) with spin-selective excitation, the dynamics of valley and layer pseudospins of the excited carriers are investigated. Complementary time-dependent density functional theory (TDDFT) calculations of the excited state populations reveal a strong circular dichroism of the spin-, valley- and layer-polarizations and a pronounced 2D character of the excited states in the K valleys. We observe scattering of carriers towards the global minimum of the conduction band on a sub-100 femtosecond timescale to states with three-dimensional character facilitating inter-layer charge transfer. Our results establish the optical control of coupled spin-, valley- and layer-polarized states in centrosymmetric materials and suggest the suitability of TMDC multilayer materials for valleytronic and spintronic device concepts.