Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

Understand

Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries.

• By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extrinsic doping.
• This insulating state has an associated transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly suggesting a gapped broken symmetry state that is destroyed by very weak disorder.
• The transition to the intrinsic broken symmetry state can be tuned by disorder, out-of-plane electric field, or carrier density.