TEMPERATURE DERIVATIVE OF THE SUPERFLUID DENSITY AND FLUX-QUANTIZATION AS CRITERIA FOR SUPERCONDUCTIVITY IN 2-DIMENSIONAL HUBBARD MODELS

Based on extensions of quantum Monte Carlo algorithms to incorporate m agnetic fields, two criteria to detect superconductivity in two-dimens ional Hubbard models are investigated. In order to provide such criter ia, we calculate both the internal energy E(Phi,T) as well as the grou nd-state energy, E(0)(Phi), for Hubbard models on a cylinder geometry threaded by a flux Phi. The temperature derivative of the superfluid d ensity, partial derivative beta D-s(beta)/partial derivative beta, is obtained from the difference in internal energy of systems which diffe r by a phase twist pi/2 in the boundary condition along one lattice di rection. In the framework of a Kosterlitz-Thouless transition, partial derivative beta D-s(beta)/partial derivative beta scales to a Dirac d elta function at the transition temperature. On finite-sized lattices, partial derivative beta D-s(beta)/partial derivative beta shows a res ponse which increases with lattice size. Flux quantization is a T = 0 method. From the functional form of E(0)(Phi), superconducting or nons uperconducting ground states may be identified. In both approaches, su perconductivity may be detected without prior knowledge of the symmetr y and nature of the pairing correlations. For single-band Hubbard mode ls, our main quantum Monte Carlo results include numerical data which (a) confirm the existence and pin down the transition temperature of a Kosterlitz-Thouless-type transition in the attractive Hubbard model a way from half-band filling and (b) show that the quarter-filled repuls ive Hubbard model is not superconducting. For the three-band Hubbard m odel we consider two parameter sets which take into account the differ ences in static magnetic structure and Fermi surfaces between La-Sr-Cu -O and Y-Ba-Cu-O materials. For both parameter sets, the finite-temper ature approach showed no sign of a Kosterlitz-Thouless-type transition up to inverse temperatures beta = 17.5, in units of the Cu-O hopping, and hole doping delta = 0.25. Flux quantization results for the Y-Ba- Cu-O parameters on clusters up to 8 x 8 unit cells equally showed no s ign of superconductivity at a hole doping delta = 0.25.