It was shown that long-range strain fields around structural defects in high-temperature superconductors could give rise to localized superconducting domains at temperatures that were appreciably higher than the bulk critical temperature; regardless of the microscopic mechanism of superconductivity and the nanostructure of the defects. This effect was attributed to the strongly non-monotonic dependence of the bulk critical temperature upon pressure and hole concentration. The increase in critical temperature was calculated for edge dislocations, low-angle grain boundaries, and metastable linear dislocation arrays; while taking account of the anisotropic strain dependence of the critical temperature in the ab-plane. It was suggested that the superconducting state on the grain boundaries resulted from the proximity coupling of superconducting domains that were localized on a periodic chain of edge dislocations. In this case, the change in the critical current decreased with the misorientation angle and vanished at a critical value of the latter. This angle was governed by competition between a strain-field enhancement of the critical temperature and the suppression of superconductivity in the dislocation cores. In the case of metastable dislocation arrays which were caused by plastic deformation, the strain-induced increase in critical temperature was much more pronounced than that for grain boundaries and occurred in macroscopic domains which were much larger than the coherence length. The localized remanent strains in these domains could be high enough to reveal the absolutely maximum critical temperature. It was expected that this might not be attainable by using hydrostatic pressures.

A.Gurevich, E.A.Pashitskii: Physical Review B, 1997, 56[10], 6213-25