  A two-dimensional electron liquid is formed at the n-type interface between
SrTiO3 and LaAlO3. Here we report on Kelvin probe microscopy measurements of
the electronic compressibility of this electron system. The electronic
compressibility is found to be negative for carrier densities of
\approx10^13/cm^2. At even smaller densities, a metal-to-insulator transition
occurs. These local measurements corroborate earlier measurements of the
electronic compressibility of LaAlO3-SrTiO3 interfaces obtained by measuring
the capacitance of macroscopic metal-LaAlO3-SrTiO3 capacitors.
