  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 Tc increases in proportion to the
strength of spin fluctuations, while it is independent of the nematic
transition temperature Tnem. 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 Tc.
