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

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