Phase-Sensitive Evidence for the Sign-Reversal s ± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb / Ba 1 − x Na x Fe 2 As 2 Josephson Junctions

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.