Correlations in a Two-Chain Hubbard Model

Equal-time spin-spin and pair-field correlation functions are calculated for a two-chain Hubbard model using a density-matrix numerical renormalization group approach. At half filling, the antiferromagnetic and pair-field correlations both decay exponentially with the pair field having a much shorter correlation length. This is consistent with a gapped spin-liquid ground state. Below half filling, the antiferromagnetic correlations become incommensurate and the spin gap persists. The pair-field correlations appear to follow a power law decay which is similar to their noninteracting $U=0$ behavior.