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 dR/dT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a ten-fold 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. 
G.D.Allison, E.A.Galaktionov, A.K.Savchenko, S.S.Safonov, M.M.Fogler,   M.Y.Simmons and D.A.Ritchie