Posters

Bilayer graphene exhibits a rich phase diagram in the quantum Hall regime, arising from a multitude of internal degrees of freedom, including spin, valley, and orbital indices. The variety of fractional quantum Hall states between filling factors 1 and 2 suggests, among other things, a quantum phase transition between valley-unpolarized and polarized states induced by a perpendicular electric field. We find the behavior of the critical electric field with filling factor changes markedly as the magnetic field is reduced. The critical field may even vanish at filling factor 2 for a sufficiently small magnetic field. We present a theoretical model for lattice-scale interactions which explains these observations; surprisingly, both repulsive and attractive components in the interactions are required. Within this model we analyze the nature of the state at filling factor 2 as a function of the magnetic and electric fields, and predict that valley-coherence may emerge in the high field regime. This suggests the system supports Kekule bond-ordering, which could in principle be verified via STM measurements.