  The possibility of p-wave pairing in superconductors has been proposed more
than five decades ago, but has not yet been convincingly demonstrated. One
difficulty is that some p-wave states are thermodynamically indistinguishable
from s-wave, while others are very similar to d-wave states. Here we studied
the self-field critical current of NdFeAs(O,F) thin films in order to extract
absolute values of the London penetration depth, the superconducting energy
gap, and the relative jump in specific heat at the superconducting transition
temperature, and find that all the deduced physical parameters strongly
indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further
investigation revealed that single atomic layer FeSe also shows p-wave pairing.
In an attempt to generalize these findings, we re-examined the whole inventory
of superfluid density measurements in iron-based superconductors show quite
generally that most of the iron-based superconductors are p-wave
superconductors.
