High quality strongly correlated two-dimensional (2D) electron systems at low temperatures $Trightarrow 0$ exhibits an apparent metal-to-insulator transition (MIT) at a large $r_s$ value around 40. We have measured the magnetoresistance of 2D holes i
n weak perpendicular magnetic field in the vicinity of the transition for a series of carrier densities ranging from $0.2-1.5times10^{10}$ $cm^{-2}$. 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.
Comparing the results of transport measurements of strongly correlated two-dimensional holes in a GaAs heterojunction-insulated-gate field-effect-transistor obtained before and after a brief photo-illumination, the light-induced disorder is found to
cause qualitative changes suggesting altered carrier states. For charge concentrations ranging from $3times10^{10}$ $cm^{-2}$ down to $7times10^{8}$ cm$^{-2}$, the post-illumination hole mobility exhibits a severe suppression for charge densities below $2times10^{10}$ cm$^{-2}$, while almost no change for densities above. The long-ranged nature of the disorder is identified. The temperature dependence of the conductivity is also drastically modified by the disorder reconfiguration from being nonactivated to activated.
