MEAN-FIELD STATES IN AN ITINERANT HEISENBERG-MODEL

We use symmetry to find the mean-field states that can arise at a continuous transition from the normal phase in a two-dimensional itinerant Heisenberg model on a square lattice with unit cell aXa. All states consistent with a square-root 2a X square-root 2a unit cell for both ferromagnetic and antiferromagnetic couplings of the exchange constant are considered. A sublattice SU(2) symmetry is used to map between particle-hole and particle-particle states, charge and singlet-pairing states, and spin and triplet-pairing states. We calculate the critical temperatures for these states and find, at weak coupling, a spin-current state that has a higher critical temperature than the ferromagnetic state.