PHASE-SEPARATION AT ALL INTERACTION STRENGTHS IN THE T-J MODEL

We investigate the phase diagram of the two-dimensional t-J model usin g a recently developed Green's function Monte Carlo method for lattice fermions. We use the technique to calculate exact ground-state energi es of the model on large lattices. In contrast to many previous studie s, we find the model phase separates for all values of J/t. In particu lar, it is unstable at the hole dopings and interaction strengths at w hich the model was thought to describe the cuprate superconductors.