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

TL;DR

This study reveals a transition from sign-reversed to sign-preserved Cooper-pairing symmetry in sulfur-doped iron selenide superconductors, challenging simple spin fluctuation theories and suggesting multiple pairing mechanisms.

Contribution

It provides direct experimental evidence of a pairing symmetry transition in sulfur-doped iron selenide, highlighting the complexity of superconducting pairing mechanisms.

Findings

Magnetic neutron scattering shows a transition from a sharp resonance to a broad hump.

The transition occurs with minimal change in critical temperature (Tc).

Results suggest multiple pairing channels are involved in superconductivity.

Abstract

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.