Enhancement of superconductivity at structural defects in high-temperature superconductors

It is shown that long-range strain fields around structural defects in high-temperature superconductors can give rise to localized supercoducting domains at temperatures noticeably higher than the bulk critical temperature T-c0, regardless of the microscopic mechanism of superconductivity and the nanostructure of the defects. The effect is due to the strong nonmonotonic dependence of T-c0 on pressure and hole concentration characteristic of high-T-c superconductors. We calculated the T-c increase Delta T-c = T-c - T-c0 for edge dislocations, low-angle grain boundaries, and metastable linear dislocation arrays, taking into account the anisotropic strain dependence of T-c in the ab plane. The superconducting state on the grain boundaries results from the proximity coupling of superconducting domains localized on the periodic chain of edge dislocations. In this case Delta T-c(theta) decreases with the misorientation angle theta, vanishing at the critical angle theta(0) determined by the competition between the strain fields enhancement of T-c and the suppression of superconductivity in the dislocation cores. We calculated the magnetic susceptibility and the critical current along the grain boundary network at T-c0 < T < T-c. For metastable dislocation arrays caused by plastic deformation, the strain-induced T-c enhancement is much more pronounced than for grain boundaries and occurs in macroscopic domains much larger than the coherence length. The localized remanent strains in these domains can be strong enough to reveal the absolute maximum of T-c which may not be seen in hydrostatic pressure experiments. The compositional change in the strain fields of defects and the implications of the T-c variations on flux pinning and magnetic granularity are discussed.