Competing ordered states with filling factor two in bilayer graphene

The quantum Hall effect, in which a two-dimensional sample’s Hall conductivities become quantized, is a remarkable transport anomaly commonly observed at strong magnetic fields. However, it may also appear at zero magnetic field if time-reversal symmetry is broken. Charge-neutral bilayer graphene is unstable to a variety of competing and closely related broken symmetry states, some of which have non-zero quantized Hall conductivities. Here we explore those states by stabilizing them with external fields. Transport spectroscopy measurements reveal two distinct states that have two quantum units of Hall conductivity, stabilized by large magnetic and electric fields, respectively. The majority spins of both phases form a quantum anomalous Hall state, and the minority spins constitute a Kekulé state with spontaneous valley coherence for phase I and a quantum valley Hall state for phase II. Our results shed light on the rich set of competing ordered states in bilayer graphene. Single layers of carbon atoms are now well known for their useful properties, but a combination of two sheets can also exhibit some unusual electronic characteristics. Here, the authors identify two distinct electron states in bilayer graphene that result from spontaneous symmetry breaking.