Thermodynamic properties of a two-dimensional electron gas with attractive interactions

We present a calculation of the thermodynamic properties of a two-dimensional negative-U Hubbard model in the low-density limit. We show that the standard expansion of the thermodynamic potential in terms of particle-particle ladder diagrams evaluated in the undressed fermion propagator, leads to unphysical results like negative entropy and/or specific heat. We propose a phenomenological pair susceptibility that avoids these problems and calculate the entropy and specific heat for different values of the parameters.