Numericalstudies of the two-leg Hubbard ladder

Abstract

The Hubbard model on a two-leg ladder structure has been studied bya combination of series expansions at T = 0 and the density-matrixrenormalization group. We report results for the ground-stateenergy E0 and spin gap Δs at half-filling, as well asdispersion curves for one- and two-hole excitations. For small Uboth E0 and Δs show a dramatic drop near t/t⊥~0.5, which becomes more gradual for larger U. This represents acrossover from a `band insulator' phase to a strongly correlatedspin liquid. The various features are collected in a `phasediagram' for the model.