▎ 摘　　要

While single-layer graphene shows extraordinary phenomena which are stable against electronic interactions, the noninteracting state of bilayer graphene is unstable to infinitesimal interactions leading to one of many possible exotic states. Indeed a gapped state is found in experiments but none of the states proposed so far can provide full accounts of its properties. Here we show that a magnetoelectric (ME) state is consistent with the experimental observations. This state breaks time-reversal symmetry through a pair of spontaneously generated current loops in each layer, and has odd parity with respect to the two layers. We also suggest further experiments to check whether the ME state is indeed the gapped state found in experiments. DOI: 10.1103/PhysRevB.87.035427