Negative compressibility of interacting two-dimensional electron and quasiparticle gases

Direct measurements of the compressibility K of the 2D electron gas at both zero magnetic field B and in the high-field extreme quantum limit, 1, are reported. A new method is used to determine both the magnitude and IsignR of K. At B=0 we find K^-1 changes sign and becomes negative as the 2D density is reduced. At high B, K^-1 exhibits negative anomalies near both =0 and 1. Compressibility features near the fractional fillings =1/3 and 2/3 give thermodynamic evidence for dilute interacting quasiparticle gases.