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Transition from Sign-Reversed to Sign-Preserved Cooper-Paring Symmetry in Sulfur-Doped Iron Selenide Superconductors



Qisi Wang, J. T. Park, Yu Feng, Yao Shen, Yiqing Hao, Bingying Pan, Jeffrey W. Lynn, A. Ivanov, Songxue Chi, M. Matsuda, Huibo Cao, R. J. Birgeneau, D.V. Efremov, Jun Zhao


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 (high-Tc) 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 u1-3^. Here, by using magnetic neutron scattering, a phase sensitive probe of superconducting gap4, we report the observation of a crossover 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 (2{Δ}) in the undoped sample (Z=0) is replaced by a broad hump structure above 2{Δ} under 50% S doping. The 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 materials5-7.
Physical Review Letters


Iron Superconductor, Superconducting Pairing Symmetry, Neutron inelastic scattering
Created May 13, 2016, Updated February 19, 2017