Spin-Polarized to Valley-Polarized Transition in Graphene Bilayers at ν=0 in High Magnetic Fields

We investigate the transverse electric field (E) dependence of the nu = 0 quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic fields. The longitudinal resistivity rho(xx) measured at nu = 0 shows an insulating behavior which is strongest in the vicinity of E = 0, as well as at large E fields. At a fixed perpendicular magnetic field (B), the nu = 0 QHS undergoes a transition as a function of the applied E, marked by a minimum, temperature-independent rho(xx). This observation is explained by a transition from a spin-polarized nu = 0 QHS at small E fields to a valley-(layer-) polarized nu = 0 QHS at large E fields. The E field value at which the transition occurs follows a linear dependence on B.