Magnetoresistance induced by inhomogeneity in graphene

We study the magnetoresistance of graphene samples with varying disorder as a function of carrier density. We observe a quadratic low-field classical magnetoresistance which is largest at low carrier density reflecting the inhomogeneous nature of transport in the electron-hole puddle dominated minimum conductivity region, as observed previously[\textit{Phys. Rev. B} \textbf{77}, 084102(R) (2008)]. However we observe the magnetoresistance persists to carrier densities well outside the electron-hole puddle region, where single band transport is expected. We find that the magnetoresistance for all samples follows a universal form which depends only on the ratio of the carrier density $n$ to the characteristic electron-hole puddle density $n*$. The results are in excellent quantitative agreement with a recent theory based on an effective medium approximation for disordered graphene.