Proximity-enhanced valley Zeeman splitting at the WS₂/graphene interface

Abstract

The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS$_2$, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS$_2$/graphene stacks employing first-principles calculations and find that the lower conduction band of WS$_2$ at the $K/K'$ valleys (the $CB^-$ band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at the $Q$ point of WS$_2$ mediates the coupling between $CB^-$ and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr

“…These distinct g -factors provide valuable insights into the intricate interplay between electronic and magnetic degrees of freedom, underscoring the importance of considering the magnetic state of CrSBr in understanding the behavior of excitonic systems in this heterostructure. The changes in the magnitude of the g -factors are consistent with proximity effects due to the hybridization between the layers, as previously demonstrated in MoSe 2 /WSe 2 , , WSe 2 /CrI 3 , and WS 2 /graphene systems . A systematic analysis of the microscopic features behind the asymmetric g -factors is beyond the scope of the current manuscript; however, we point out that asymmetric signatures in valley Zeeman splitting have recently been observed in MoSe 2 /CrBr 3 heterostructures at zero magnetic field.…”

“…The changes in the magnitude of the g-factors are consistent with proximity effects due to the hybridization between the layers, as previously demonstrated in MoSe 2 /WSe 2 , 53,72 WSe 2 /CrI 3 , 73 and WS 2 / graphene systems. 74 A systematic analysis of the microscopic features behind the asymmetric g-factors is beyond the scope of the current manuscript; however, we point out that asymmetric signatures in valley Zeeman splitting have recently been observed in MoSe 2 /CrBr 3 16 heterostructures at zero magnetic field. In these systems, the magnetic moments in CrBr 3 point in the out-of-plane direction and act already as an external magnetic field.…”