Phase-Sensitive Evidence for the Sign-Reversal s_{±} Symmetry of the Order Parameter in an Iron-Pnictide...

Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional s-wave superconductor Nb and a multiband iron-pnictide Ba_{1-x}Na_{x}Fe_{2}As_{2}. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the I_{c}R_{n} product is very small ≃3μV, which is not consistent with the sign-preserving s_{++} symmetry either. We argue that the small I_{c}R_{n} value, along with its unusual temperature dependence, provides evidence for the sign-reversal s_{±} symmetry of the order parameter in Ba_{1-x}Na_{x}Fe_{2}As_{2}. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.