REDUCED-DENSITY-MATRIX ANALYSIS OF SUPERCONDUCTING CORRELATION IN 2-DIMENSIONAL AND 2-CHAIN HUBBARD MODELS

We have calculated the largest eigenvalues of the reduced density matrix of the singlet superconducting correlation function and pair-field susceptibility in two-dimensional and two-chain Hubbard models with quantum Monte Carlo methods. A singlet superconducting channel opens upon doping but is otherwise closed at half-filling when U/t = 4. The interaction vertex contribution to the largest eigenvalue of the singlet pair-field susceptibility grows with a power law as temperature is decreased. With these two observations, we revise our previous understanding of the interaction vertex in the two-dimensional Hubbard model. We observe that some phases reside in the two-chain Hubbard model. An analysis of the 2k(F) charge correlation up to 32 X 2 sites suggests that the 2k(F) correlation is dominant over the superconducting correlation (0.25 less-than-or-equal-to K(rho) less-than-or-equal-to 1.00) in the two-component Luttinger-liquid phase (U/t = 2 and t(upsilon)/t = 1.4). We conclude the reduced-density-matrix analysis discussed here is needed to unveil the nature of Hubbard and related models.