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An essential step toward elucidating the mechanism of superconductivity is todetermine the sign/phase of superconducting order parameter, as it is closelyrelated to the pairing interaction. In conventional superconductors, theelectron-phonon interaction induces attraction between electrons near the Fermienergy and results in a sign-preserved s-wave pairing. For high-temperaturesuperconductors, including cuprates and iron-based superconductors, prevalentweak coupling theories suggest that the electron pairing is mediated by spinfluctuations which lead to repulsive interactions, and therefore that asign-reversed pairing with an s+-or d-wave symmetry is favored. Here, by usingmagnetic neutron scattering, a phase sensitive probe of superconducting gap, wereport the observation of a transition from the sign-reversed to sign-preservedCooper-pairing symmetry with insignificant changes in Tc in the S-doped ironselenide superconductors KxFe2-y(Se1-zSz)2. We show that a rather sharpmagnetic resonant mode well below the superconducting gap (2delta) in theundoped sample (z = 0) is replaced by a broad hump structure above 2delta under50% S doping. These results cannot be readily explained by simple spinfluctuation-exchange pairing theories and, therefore, multiple pairing channelsare required to describe superconductivity in this system. Our findings mayalso yield a simple explanation for the sometimes contradictory data on thesign of the superconducting order parameter in iron-based materials.

Transition from sign-reversed to sign-preserved Cooper-pairing symmetry in sulfur-doped iron selenide superconductors