The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out ⁷⁷Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature T_c increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature T_nem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on T_c.

轨道和自旋自由度的相互作用是许多凝聚态现象的基本特征，其中包括高温超导，量子自旋液体和拓扑半金属。在铁基超导体（FeSC）中，这会导致超导电性出现在另外两个不稳定性附近：向列和磁性。揭示向列序，自旋涨落和超导性之间的相互关系一直是FeSCs研究的主要挑战，但仍存在争议。在此，通过对掺有钴和硫作为控制参数的FeSe单晶进行⁷⁷ Se核磁共振（NMR）测量，我们证明了超导转变温度T _c与自旋涨落强度成比例地增加，而与向列转变温度T _nem无关。因此，我们的观察结果直接表明，FeSe的超导性基本上是由中间耦合机制中的自旋波动驱动的，而向列波动对T _c的影响很小。