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 [70]. 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 d_{x2-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 [71]. In the high-T_{c} superconductors where , the anisotropy in the gap might play a significant role.