  We study the phase diagram of a new model that exhibits a first order
transition between s-wave superconducting and antiferromagnetic phases. The
model, a generalized Hubbard model augmented with competing spin-spin and
pair-pair interactions, was investigated using the projector Quantum Monte
Carlo method. Upon varying the Hubbard $U$ from attractive to repulsive we find
a first order phase transition between superconducting and antiferromagnetic
states.
