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Understanding the structure of the order parameter of the
iron-based superconductors is the key to unveil their pairing mechanism.
Although there has been much theoretical and experimental indications
that the order parameter changes its sign in momentum space, direct
evidence is still lacking. The difficulty stems from the fact that the
order parameter is likely to be of s-wave symmetry, and therefore
designing a phase sensitive experiment that would clearly reveal the
sign change is non-trivial. Here, we examine a contact between a
sign-changing superconductor and an ordinary, uniform-sign
superconductor. If the the barrier between the two superconductors is
not too high, the frustration of the Josephson coupling between
different portions of the Fermi surface across the contact can lead to
surprising consequences, such as time-reversal symmetry breaking at
the interface and unusual energy-phase relations with multiple local
minima. We propose this mechanism as a possible explanation for the
half-integer flux quantum transitions in niobium–iron pnictide loops,
which were discovered in a recent experiment (C-T. Chen et. al.,
2010).