Mechanisms were considered which could account for an observed rapid decrease in the critical current density as a function of the misorientation angle of grain boundaries in high-Tc superconductors. It was shown that the angular dependence was governed mainly by a decrease in the current-carrying cross-section, due to insulating dislocation cores and to a progressive local suppression of the superconducting order parameter near to the grain boundaries as the angle increased. The insulating regions near to the dislocation cores resulted from a strain-induced local transition to the insulating antiferromagnetic phase of the high-temperature superconductor. The structure of the non-superconducting core regions and current channels in the grain boundaries was markedly affected by an anisotropy of the strain dependence of the critical temperature. A mechanism was proposed for the progressive suppression of superconductivity, at grain boundaries, as a function of the misorientation angle. This supposed that there was an excess ionic charge, on the grain boundaries, which shifted the chemical potential in the layer by an amount which was of the order of the screening length near to the grain boundaries. The local suppression of the order parameter was magnified by the proximity, of any high-temperature superconductor, to a metal-insulator transition. This behavior was due, in turn, to their low carrier densities and extended saddle-point singularities in the electron density of states near to the Fermi surface. By taking account of these mechanisms, the angular variation of the critical current was calculated analytically by solving the Ginzburg-Landau equation. The model described well the observed quasi-exponential decrease in critical current, with misorientation, for many high-temperature superconductors. The d-wave symmetry of the order parameter weakly affected the critical current density, at small angles, and could not account (by several orders of magnitude) for the fall in the critical density as the angle increased from 20 to 40º.

A.Gurevich, E.A.Pashitskii: Physical Review B, 1998, 57[21], 13878-93