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
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 [71].
In the high-Tc superconductors where
,
the
anisotropy in the gap might play a significant role.