People have recently been pondering over the symmetry of the order parameter in the newly discovered (iron-based) pnictide superconductors. Being layered materials, like the cuprates, it was thought that they might be d-wave. However, there is some theoretical basis to them being s-wave, see here and here (Parental Advisory: contains explicit band structure calculations).
Well, here’s the usual experimental way of settling this matter:
(Picture © Van Harlingen 1995)
The basis of the method involves making a tunnel junction using a ‘corner’ of a piece of the superconductor of interest. The corner encompasses tunnel directions along both the a and b crystal axes. The other side of the junction is a superconductor which is known to have a direction-independent order parameter symmetry, such as Pb. If the order parameter changes phase with respect to crystal direction, there is a built in phase shift across the junction, which manifests as a shift in the Fraunhofer-type diffraction pattern of the critical current with respect to applied field.
They conclude that the superconductor is probably s-wave (or at least MOSTLY s-wave). No more yummy pi-junction candidates yet then.
I love the last line of the paper:
“This indicates that the superconducting wavefunction of
the iron based superconductor is definitely not like that of a
[EDIT 19/12/08] Here is another related paper