Thermodynamics of the neutral Hubbard model
By use of Bogoljubov's variational principle the free energy of the neutral Hubbard model is calculated as a function of the total spinSz. A transition from an antiferro-magnetic to a paramagnetic phase is found for all values of the coupling constantV0. The calculations are performed for the simple cubic and the body-centred cubic lattice; some numerical results are given for the transition temperature, the free energy, the sublattice magnetization, the transversal magnetic susceptibility and the response to a finite magnetic field.