In any superconductor with a non-spherical Fermi surface, one expects anisotropy in the superconducting gap function . In general, the gap function will vary according to the angle with respect to the crystalline axes . That is, the energy required to break a Cooper pair will depend on the direction of .
In , one expects the size of the energy gap to show some variation in the ab-plane. However this does not necessarily imply nodes in the gap. If the Fermi surface in the ab-plane is not a perfect circle, then the gap will certainly be anisotropic; but as is the case for an anisotropic s-wave pairing state, the gap may remain finite over the entire Fermi surface. Furthermore, if we are to think of the energy gap in the superconducting state of as having dx2-y2 symmetry, - anisotropy will produce nodes in the gap which are not precisely along .
As mentioned above, conventional superconductors also have some anisotropy in the gap function. However, in most of these materials the mean free path is such that and also , so that the anisotropy is negligible when interpreting the experimental results . In the high-Tc superconductors where , the anisotropy in the gap might play a significant role.