Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn₅, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn₅ likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.

自旋涨落介导的非常规超导性可以出现在磁性边界，其特征是超导序参数可以改变动量空间中的符号。检测这种符号变化在实验上具有挑战性，因为大多数探针都不是相敏的。通过假设 SRM 是自旋激子束缚态，从非弹性中子散射中观察到的自旋共振模式 (SRM) 通常被视为超导有序参数符号变化的有力相敏证据。在这里，我们表明，对于重费米子超导体CeCoIn ₅ ，其SRM违背了对自旋激子束缚态的预期，并且不是符号改变超导性的表现。相反，CeCoIn ₅中的 SRM 可能是由于超导状态下的阻尼减少到类磁振子模式而产生的，因为它接近磁量子临界点。我们的研究结果强调，当使用 SRM 的存在来证明变号超导性时，需要更严格地测试 SRM 是否是自旋激子。