A new approach to strongly correlated fermion systems: the spin-particle-hole coherent-state path integral

We describe a new path integral approach to strongly correlated fermion systems, considering the Hubbard model as a specific example. Our approach is based on the introduction of spin-particle-hole coherent states which generalize the spin-1/2 coherent states by allowing the creation of a hole or an additional particle. The action of the fermion system $S[\gamma^*,\gamma;{\bf\Omega}]$ can be expressed as a function of two Grassmann variables ($\gamma_\uparrow$,$\gamma_\downarrow$) describing particles propagating in the lower and upper Hubbard bands, and a unit vector field ${\bf\Omega}$ whose dynamics arises from spin fluctuations. In the strong correlation limit, $S[\gamma^*,\gamma;{\bf\Omega}]$ can be truncated to quartic order in the fermionic fields and used as the starting point of a strong-coupling diagrammatic expansion in $t/U$ ($t$ being the intersite hopping amplitude and $U$ the on-site Coulomb repulsion). We discuss possible applications of this formalism and its connection to the t-J model and the spin-fermion model.