Thermodynamic Density of States of Two-Dimensional GaAs Systems near the Apparent Metal-Insulator Transition

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition—a crossover in the sign of ∂R/∂T with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r_s, the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.