EXACT-DIAGONALIZATION STUDY OF THE T-J MODEL IN THE LOW-DENSITY LIMIT- IMPLICATIONS FOR PHASE-SEPARATION

The ground state of the two-dimensional (2D) t-J model is studied by e xact diagonalization for seven different unit cells with N = 10, 16, 1 8, 20, 26, 32, and 36 sites on a square lattice. In the limit of low e lectron densities (up to six electrons) we calculate the lowest eigenv alues and classify the ground state according to wave vector K, total spin S, and point-group symmetry both in the limit J = 0 (U = infinity Hubbard model) and for finite J > 0. The transition to a completely p hase-separated state for J > J(PS)(N) is examined by calculating the f ree energy as a function of the electron concentration. We find that J (PS)(N) increases with the lattice size N. An extrapolation of our fin ite-size results to the thermodynamic limit gives J(PS)(infinity)/t = 4.1 (+/- 0.1).