0 a New Approach to Doped Mott Insulators

We describe a new microscopic approach for analyzing interacting electron systems with local moments or, in principle, any local order parameter. We specialize attention to the doped Mott insulator phase of the Hubbard model, where standard weak–coupling perturbation methods fail. A rotationally invariant Stratonovich–Hubbard field is introduced to decouple the static spin components of the interaction at each site. Charge degrees of freedom are then treated by " slave " Hartree–Fock in the presence of a spatially varying random spin field. The effective action reduces to a classical Heisenberg model at half filling, insuring that the system has (i) finite–range order at T > 0 with an exponentially diverging correlation length and (ii) a one–electron Mott–Hubbard gap in the presence of disorder. Away from half–filling properties are determined by strongly non–Gaussian fluctuations in the amplitude and orientation of the local spin fields. The saddle point equations of the theory at zero temperature reduce to inhomogeneous Hartree–Fock, so that disordering of domain walls at finite temperature is in principle included. We present preliminary small system results for the intermediate and large interaction regimes obtained by Monte Carlo simulation of random spin field configurations.