Charge sensing of an unconventional ferroelectric metal in bilayer graphene

Moiré superlattices in twisted and lattice-mismatched van der Waals systems have recently led to a dazzling display of correlated phenomena including superconductivity, magnetism, and correlated insulating states. Here, we show that Bernal-stacked bilayer graphene in moiré superlattices with insulating boron nitride can result in a surprising electronically-driven ferroelectric state, exhibiting dramatic and unconventional hysteresis in transport and capacitive charge sensing. This ferroelectricity manifests as a robust and switchable electric polarization that coexists with metallicity, unexpected for this well-studied system. By measuring capacitances between the bilayer graphene and the top and bottom gates separately, we find large layer-asymmetric charge compressibility arising from an electronic phase transition. Moiré structures of Bernal-stacked bilayer graphene thus offer a new paradigm for exploring emergent electronic correlations and spontaneous symmetry breaking in an ultraclean two-dimensional system.