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, ν<1, are reported. A new method is used to determine both the magnitude and IsignR of K. At B=0 we find ${\mathit{K}}^{\mathrm{-}1}$ changes sign and becomes negative as the 2D density is reduced. At high B, ${\mathit{K}}^{\mathrm{-}1}$ exhibits negative anomalies near both ν=0 and 1. Compressibility features near the fractional fillings ν=1/3 and 2/3 give thermodynamic evidence for dilute interacting quasiparticle gases.