BCS-type mean-field theory for the t-J model in the SU(2|1) superalgebra representation

A simple version of the Bardeen-Cooper-Schrieffer (BCS)-type mean-field theory for the t-J model is developed. The present approach rigorously treats the constraint of no doubly occupied states and invokes two local order parameters to implement spontaneous breaking of the global U(1) x U(1) symmetry. This is achieved by identifying the Hubbard operators with generators of the SU(2\1) superalgebra in the fundamental representation and employing the ensuing CP1\1 parametrization for dynamical variables. As a result, an improved phase diagram T-c(delta) in the paramagnetic phase is obtained.