Deviations from Fermi-Liquid Behavior aboveTcin 2D Short Coherence Length Superconductors

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 $T_c$ with a depleted $N\left(0\right)$ with decreasing $T$. The susceptibility ${\chi }_s$ and the low frequency spin spectral weight track $N\left(0\right)$, which explains the spin-gap scaling: ${1/T}_{1}T\sim {\chi }_s\left(T\right)$. However, collective excitations contribute to the charge channel, and the compressibility $dn/d\mu$ is $T$ independent. This anomalous “spin-charge separation” is shown to exist even at intermediate $|U|$ where the momentum distribution $n\left(\mathbf{k}\right)$ gives evidence for a degenerate Fermi system.