Band Gap Opening in Bilayer Graphene-CrCl$_3$/CrBr$_3$/CrI$_3$ van der Waals Interfaces

We report experimental investigations of transport through bilayer graphene (BLG)/chromium trihalide (CrX$_3$; X=Cl, Br, I) van der Waals interfaces. In all cases, a large charge transfer from BLG to CrX$_3$ takes place (reaching densities in excess of $10^{13}$ cm$^{-2}$), and generates an electric field perpendicular to the interface that opens a band gap in BLG. We determine the gap from the activation energy of the conductivity and find excellent agreement with the latest theory accounting for the contribution of the $\sigma$ bands to the BLG dielectric susceptibility. We further show that for BLG/CrCl$_3$ and BLG/CrBr$_3$ the band gap can be extracted from the gate voltage dependence of the low-temperature conductivity, and use this finding to refine the gap dependence on the magnetic field. Our results allow a quantitative comparison of the electronic properties of BLG with theoretical predictions and indicate that electrons occupying the CrX$_3$ conduction band are correlated.