  An essential step toward elucidating the mechanism of superconductivity is to
determine the sign/phase of superconducting order parameter, as it is closely
related to the pairing interaction. In conventional superconductors, the
electron-phonon interaction induces attraction between electrons near the Fermi
energy and results in a sign-preserved s-wave pairing. For high-temperature
superconductors, including cuprates and iron-based superconductors, prevalent
weak coupling theories suggest that the electron pairing is mediated by spin
fluctuations which lead to repulsive interactions, and therefore that a
sign-reversed pairing with an s+-or d-wave symmetry is favored. Here, by using
magnetic neutron scattering, a phase sensitive probe of superconducting gap, we
report the observation of a transition from the sign-reversed to sign-preserved
Cooper-pairing symmetry with insignificant changes in Tc in the S-doped iron
selenide superconductors KxFe2-y(Se1-zSz)2. We show that a rather sharp
magnetic resonant mode well below the superconducting gap (2delta) in the
undoped sample (z = 0) is replaced by a broad hump structure above 2delta under
50% S doping. These results cannot be readily explained by simple spin
fluctuation-exchange pairing theories and, therefore, multiple pairing channels
are required to describe superconductivity in this system. Our findings may
also yield a simple explanation for the sometimes contradictory data on the
sign of the superconducting order parameter in iron-based materials.
