No abstract

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 multi-band iron-pnictide Ba1−xNaxFe2As2. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the IcRn product is very small 3 µV, which is not consistent with the sign-preserving s++ symmetry either. We argue that the small IcRn value along with its unusual temperature dependence provide evidence for the sign-reversal s± symmetry of the order parameter in Ba1−xNaxFe2As2. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (bellow a sub-percent) cancellation of supercurrents from sign-reversal bands in the pnictide.Symmetry of the order parameter provides one of the main clues about the mechanism of superconductivity. Attractive electron-phonon interaction leads to a simple s-wave symmetry in conventional low-T c superconductors. Unconventional superconductivity in cuprates and iron-pnictides is characterized by a proximity to an antiferromagnetic state, suggesting importance of spin interactions. The corresponding direct electron-electron interaction is non-retarded and, therefore, repulsive. It was predicted that this could favor superconductivity with a sign-reversal symmetry [1][2][3]. In single band cuprates the sign-reversal can be only achieved with a d-wave symmetry [1,4]. But in a multi-band pnictides the sign change may also take place between different bands, resulting in the s ± symmetry [2,3,[5][6][7]. On the other hand, presence of the nematic order [8][9][10][11][12][13][14] suggests importance of charge/orbital interactions [15,16], which could lead to a sign-preserving s ++ symmetry [17]. Thus, establishing of the gap symmetry provides a key evidence towards the mechanisms of unconventional superconductivity.At present determination of the gap symmetry in ironpnictides remains ambiguous. For example, a resonant peak observed in inelastic neutron scattering [18][19][20] can be due to either a zero in the denominator of the dynamic spin susceptibility, caused by the sign-reversal order parameter, or to the nominator (Lindhard function) [21], if one takes into account quasiparticle damping [22]. Gap nodes, deduced from angular resolved photoemission spectroscopy [23] and heat conductance [24] may indicate either s ± or d-wave symmetry. Alternatively, the strong reduction of the gap can be related with the change of the orbital character from d xz/yz to d z 2 −1 [25].Josephson effect facilitates phase-sensitive probe of the order parameters [1,[4][5][6][7]. So far few reliable phasesensitive experiments were reported for pnictides [26][27][28][29]. Both integer and half-integer flux-quantum transitions were observed [26] and large variations of the I c R n product, where I c is the critical current and R n is the junction resistance, were reported [27]. Interpretat...