Characteristic Sign Change of the Magnetoresistance of Strongly Correlated GaAs Two-dimensional Holes

High quality strongly correlated two-dimensional (2D) electron systems at low temperatures $T\rightarrow 0$ exhibits an apparent metal-to-insulator transition (MIT) at a large $r_s$ value around 40. We have measured the magnetoresistance of 2D holes in weak perpendicular magnetic field in the vicinity of the transition for a series of carrier densities ranging from $0.2-1.5\times10^{10}$ $cm^{-2}$. The sign of the magnetoresistance is found to be charge density dependent: in the direction of decreasing density, the sign changes from being positive to negative across a characteristic value that coincides with the critical density of MIT.