Partition functional and thermodynamic properties of the infinite-dimensional Hubbard model

An approximate partition functional is derived for the infinite-dimensional Hubbard model. This functional naturally includes the exact solution of the Falicov-Kimball model as a special case, and is exact in the uncorrelated and atomic limits. It explicitly keeps spin symmetry. For the case of the Lorentzian density of states, we find that the Luttinger theorem is satisfied at zero temperature. The susceptibility crosses over smoothly from that expected for an uncorrelated state with antiferromagnetic fluctuations at high temperature to a correlated state at low temperature via a Kondo-type anomaly at a characteristic temperature T*. We attribute this anomaly to the appearance of the Hubbard pseudogap. The specific heat also shows a peak near T*. The resistivity goes to zero at zero temperature, in contrast to other approximations, rises sharply around T*, and has a rough linear temperature dependence above T*.