Mixeds-wave andd-wave superconductivity in high-Tcsystems

Abstract

We investigate the nature of the superconducting state of high-${\mathit{T}}_{\mathit{c}}$ materials from a microscopic theory and from a phenomenological analysis of experimental data. The simplest model for a microscopic electronic mechanism is the one-band Hubbard model, for which the parameters can be derived from experiment and cluster calculations. For these parameter values and moderate doping levels, we use the variational Monte Carlo method to show that a mixed s-d Cooper-pair state is favored. These results are compared to quantum Monte Carlo results on the Hubbard model. This mixed state is compared with experimental results on nuclear-magnetic-resonance relaxation times and Knight shifts, penetration depth, and tunneling measurements. The comparison with experiment indicates that a mixed-symmetry state is a strong candidate for the ground state of the system, comparing favorably to pure d-wave and pure s-wave states.