Phase Separation at all Interaction Strengths in thet-JModel

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