However recently, data probing the property in the bulk gave mounting evidence that inside the cuperate superconductor a substantial s-component is present, and therefore I proposed a changing symmetry from pure d at the surface to more s inside, at least (2).
This suggestion was made to reconcile the observations stemming from the surface and bulk. But such a behavior would be at variance with the accepted classical symmetry properties in condesed matter. (1,3).
In this respect, Iachello, applying the interacting boson-model, successful in nuclear theory, to the C4v symmetry of the cuperates, showed that indeed a crossover from a d-phase at the surface, over a d + s, to a pure s-phase could be present(4).
Attempts to estimate this crossover from known experiments will be presented. It makes also plausible why the face stiffness of the d-component is preserved over a whole sample, i.e. in a SQUID. Furthermore most recent experiments indicating a full gap in the bulk at low temperatures will be commented on.
1) C.C. Tsuei and R.J. Kirtley, Rev. Mod. Phys. 72, 969 (2000)
2) K.A. Müller, Phil. Mag. Lett. 82, 270 (2002)
3) J.F. Annett, N.D. Goldenfeld, and J.J. Legett, 1996 in „Physical Properties of High-Temperature Superconductors V”, edited by D.M. Ginsberg (World Scientific, Singapore), p. 571
4)F. Iachello, Phil. Mag. Lett. 82, 289 (2002)