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The valley Zeeman physics of excitons in monolayer transition metaldichalcogenides provides valuable insight into the spin and orbital degrees offreedom inherent to these materials. Being atomically-thin materials, thesedegrees of freedom can be influenced by the presence of adjacent layers, due toproximity interactions that arise from wave function overlap across the 2Dinterface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B-excitons in monolayer WS$_2$, systematically encapsulated in monolayergraphene. While the observed variations of the valley Zeeman effect for the A-exciton are qualitatively in accord with expectations from the bandgapreduction and modification of the exciton binding energy due to thegraphene-induced dielectric screening, the valley Zeeman effect for the B-exciton behaves markedly different. We investigate prototypical WS$_2$/graphenestacks employing first-principles calculations and find that the lowerconduction band of WS$_2$ at the $K/K'$ valleys (the $CB^-$ band) is stronglyinfluenced by the graphene layer on the orbital level. This leads to variationsin the valley Zeeman physics of the B- exciton, consistent with theexperimental observations. Our detailed microscopic analysis reveals that theconduction 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 Diraccone. Our results therefore expand the consequences of proximity effects inmultilayer semiconductor stacks, showing that wave function hybridization canbe a multi-step process with different bands mediating the interlayerinteractions. Such effects can be exploited to resonantly engineer thespin-valley degrees of freedom in van der Waals and moir\'e heterostructures.

Proximity-enhanced valley Zeeman splitting at the WS$_2$/graphene interface