Investigation of the correlations in hybrid graphene-WTe₂ structures

Monolayer WTe₂ exhibits an assortment of intriguing electronic phenomena, including a quantum spin Hall insulating state with a thermodynamic gap at the neutral point and gate-tunable superconductivity when moderately electron-doped. The nature of the insulating state, the gap and the superconductivity remain unclear, but it seems likely that spontaneous excitons, and possibly exciton condensation, are involved. To gain insights into this possibility, we fabricated dual-gated heterobilayers of monolayer WTe₂ and monolayer graphene motivated by the hypothesis that electron-hole correlations in the WTe₂ would be modified by screening by the graphene. Our results show that the hybrid material exhibits modified graphene-derived features, including a resistance peak at the Dirac point and quantum oscillations in a magnetic field which can be used to probe the distribution of charge between the two layers as a function of both gate voltages. We observe a spontaneous charge transfer due to the work function mismatch between the two materials. We can identify the insulating and gapped states of the WTe₂ and determine the size of the gap as the doping in the graphene is varied, finding that the variation is small. Additionally, negative compressibility is observed in the WTe₂ at low doping levels. The implications of these findings are discussed for the correlated nature of the insulating state.