Spin excitations and pairing symmetry of sulfur-doped iron selenide superconductors

In conventional BCS superconductors, electron-phonon coupling gives rise to a sign-preserved s-wave pairing. In high-temperature superconductors, weak coupling theories suggest that superconductivity is mediated by spin fluctuations which lead to a sign reversal between the superconducting order parameters on different parts of the Fermi surfaces. A strong evidence for this comes from the observation of a magnetic resonance mode in the spin excitation spectrum. Here, we report the observation of a transition from sign-reversed to sign-preserved Cooper-pairing symmetry in S-doped iron selenide superconductors K$_x$Fe$_{2-y}$(Se$_{1-z}$S${_z}$)$_2$ [1]. Our neutron scattering data show that a sharp magnetic resonance mode well below the superconducting gap (2Δ) in the undoped sample is replaced by a broad hump structure above 2Δ under 50\% S doping. These findings suggest that multiple channels are required to understand the superconductivity in this system. [1] Q. Wang et al., Phys. Rev. Letts. 116, 197004 (2016).