0 Numerical Studies of the two - leg Hubbard ladder

The Hubbard model on a two-leg ladder structure has been studied by a combination of series expansions at T = 0 and the density-matrix renormal-ization group. We report results for the ground state energy E 0 and spin-gap ∆ s at half-filling, as well as dispersion curves for one and two-hole excita-tions. For small U both E 0 and ∆ s show a dramatic drop near t/t ⊥ ∼ 0.5, which becomes more gradual for larger U. This represents a crossover from a " band insulator " phase to a strongly correlated spin liquid. The lowest-lying two-hole state rapidly becomes strongly bound as t/t ⊥ increases, indicating the possibility that phase separation may occur. The various features are collected in a " phase diagram " for the model.