BCS-type mean-field theory for t-J model in 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)\times 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 CP^{1|1} parameterization for dynamical variables. As a result, an improved phase diagram T_c(\delta) in the paramagnetic phase is obtained.