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    Numerical Studies of the two-leg Hubbard ladder

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    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 renormalization group. We report results for the ground state energy E0E_0 and spin-gap Ξ”s\Delta_s at half-filling, as well as dispersion curves for one and two-hole excitations. For small UU both E0E_0 and Ξ”s\Delta_s show a dramatic drop near t/tβŠ₯∼0.5t/t_{\perp}\sim 0.5, which becomes more gradual for larger UU. 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/tβŠ₯t/t_{\perp} increases, indicating the possibility that phase separation may occur. The various features are collected in a "phase diagram" for the model.Comment: 10 figures, revte
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