Magnetoresistance of Bilayer Graphene in Parallel Magnetic Fields

We have measured the magnetoresistance of bilayer graphene in parallel magnetic fields to compare it with the results of monolayer graphene reported previously. It was revealed that the magnetoresistance consists of three components. The positive magnetoresistance due to charge inhomogeneity (electron--hole puddles) contributes significantly around the charge neutrality point (CNP) and was found to disappear at a high carrier density away from the CNP. The negative magnetoresistance observed at all carrier densities is considered to be due to the weak localization and the perpendicular component [$\delta B_{\bot}(\mbi{r})$] arising from ripples and the parallel magnetic field. The positive magnetoresistance due to the random distribution of $\delta B_{\bot}(\mbi{r})$ shows weak magnetic field dependence in contrast to the parabolic dependence observed in monolayer graphene.