Numerical studies of the two-leg Hubbard ladder

The Hubbard model on a two-leg ladder structure has been studied by a combi nation of series expansions at T = 0 and the density-matrix renormalization group. We report results for the ground-state energy E-0 and spin gap Delt a (s) at half-filling, as well as dispersion curves for one- and two-hole e xcitations. For small U both E-0 and Delta (s) show a dramatic drop near t/ t(perpendicular to) similar to 0.5, which becomes more gradual for larger U . This represents a crossover from a 'band insulator' phase to a strongly c orrelated spin liquid. The various features are collected in a 'phase diagr am' for the model.