A study was made of the effect of defects upon the structure of the order parameter for a superconductor with x2-y2 symmetry. By using a Ginzburg-Landau theory which coupled the d-wave and s-wave components of the order parameter, it was shown that - in the vicinity of defects - there was an induced s-wave pairing that extended up to a length-scale that was governed by the coherence length. Results were presented for columnar defects along various directions, for twin boundaries, and for point defects. In the case of columnar defects, the rich structure of the induced s-wave component led to the conclusion that the inner structure of a vortex core could change significantly when the vortex was pinned at one of these defects. Ideal twin boundaries were simulated by inducing a depression of the d-wave component of the order parameter on a thin slab. This model did not account for a small anisotropy in the xy-plane that was due to the orthorhombic distortion of the CuO planes. In the present model, an ideal twin boundary did not induce an s-wave component since the mixed-gradient term in the free energy cancelled by symmetry. However, when a twin boundary met a surface or when 2 twin boundaries met one another, a small s-wave component was induced.

J.J.V.Alvarez, G.C.Buscaglia, C.A.Balseiro: Physical Review B, 1996, 54[22], 16168-71