Researchers measured electrical transport in a twisted double bilayer graphene (TDBG) device placed on the antiferromagnetic insulator CrOCl and found that a charge-transfer process at the interface strengthens a correlated insulating state when the moiré band is half full. The enhancement shows up most clearly under high magnetic field and at very low temperature, and it appears to come from moving charge across the interface rather than from a magnetic exchange effect on graphene.

The team made a TDBG device with a twist angle near 1.1 degrees, cooled it to about 10 millikelvin, and mapped longitudinal and Hall resistance while changing gate voltages, displacement field, temperature and magnetic field up to the tens of tesla range. The dual-gate maps split into a region affected by charge transfer and a region that is not. In the charge-transfer region the data show that electrons are injected into the CrOCl substrate, which alters the carrier type in TDBG and bends the usual gate-response lines. They also observed Brown–Zak oscillations, Landau-fan features from the moiré bands, and a Chern insulating state originating from the half-filled band with an extracted Chern number C = 2 around 5 T.

To explain the stronger insulating behavior at half-filling, the authors point to a charge-transfer driven long-wavelength electronic pattern on the CrOCl surface. This pattern can act like a superlattice Coulomb potential on the graphene layers and open or enlarge an energy gap. Experimentally, the half-filling resistance becomes much larger in the charge-transfer region and shows a re-entrant, strongly enhanced insulating state near about 20 T. Fits of the temperature dependence give larger thermal activation gaps and a larger effective g-factor when charge transfer is present (g ≈ 18.1 with charge transfer versus g ≈ 11.8 without), which the authors interpret as stronger valley polarization under those conditions.