[hal-01094773] Disorder-induced topological change of the superconducting gap structure in iron pnictides

In superconductors with unconventional pairing mechanisms, the energy gap in the excitationspectrum often has nodes, which allow quasiparticle excitations at low energies. In manycases, such as in d-wave cuprate superconductors, the position and topology of nodes areimposed by the symmetry, and thus the presence of gapless excitations is protected againstdisorder. Here we report on the observation of distinct changes in the gap structure of iron–pnictide superconductors with increasing impurity scattering. By the successive introductionof nonmagnetic point defects into BaFe2(As1-xPx)2 crystals via electron irradiation, we findfrom the low-temperature penetration depth measurements that the nodal state changes to anodeless state with fully gapped excitations. Moreover, under further irradiation the gappedstate evolves into another gapless state, providing bulk evidence of unconventionalsign-changing s-wave superconductivity. This demonstrates that the topology of the superconductinggap can be controlled by disorder, which is a strikingly unique feature of ironpnictides.



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