We show that there are qualitative differences between the temperature dependence of the spin and charge correlations in the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. The one-particle density of states shows a pseudogap above
Tc with a depleted
N(0) with decreasing
T. The susceptibility ${\ensuremath{\chi}}_{s}$ and the low frequency spin spectral weight track
N(0), which explains the spin-gap scaling: ${1/T}_{1}T\ensuremath{\sim}{\ensuremath{\chi}}_{s}\left(T\right)$. However, collective excitations contribute to the charge channel, and the compressibility $dn/d\ensuremath{\mu}$ is
T independent. This anomalous ``spin-charge separation'' is shown to exist even at intermediate
∣U∣ where the momentum distribution
n(k) gives evidence for a degenerate Fermi system.
We study the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. We show that singlet pairing correlations develop above
Tc, and the normal state of a short coherence length superconductor deviates from a canonical Fermi liquid. In the intermediate U regime, the spin susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$ is strongly temperature dependent, and the low-frequency spectral weight, as measured by the NMR relaxation rate 1/
T1T, is shown to track ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$. This provides a simple, qualitative explanation for the spin-gap behavior observed in several high-
Tc systems.
We study the evolution of the superconducting phases in a model with competing short-range attractive (W) and on-site repulsive (U) interactions. The influence of the correlations on the phase diagram of $s\ensuremath{-}$ and d-wave superconductors is studied by means of various approximations. The knowledge of the normal state correlation effects allows the tracking of the influence of U on the superconductivity in correlated systems. We have found the qualitatively different response of the superconducting phases in systems with
W<Ucr, when the system ceases to superconduct for large enough U, and in the limit of strong
W>Ucr, when the system remains superconducting for arbitrarily large U.