  The sulfur substituted FeSe system, FeSe$_{1-x}$S$_{x}$, provides a versatile
platform for studying the relationship between nematicity, antiferromagnetism,
and superconductivity. Here, by nuclear magnetic resonance (NMR) and
resistivity measurements up to 4.73 GPa on FeSe$_{0.91}$S$_{0.09}$, we
established the pressure($p$)-temperature($T$) phase diagram in which the
nematic state is suppressed with pressure showing a nematic quantum phase
transition (QPT) around $p$ = 0.5 GPa, two SC regions, separated by the QPT,
appear and antiferromagnetic (AFM) phase emerges above $\sim$3.3 GPa. From the
NMR results up to 2.1 GPa, AFM fluctuations are revealed to be characterized by
the stripe-type wavevector which remains the same for the two SC regions.
Furthermore, the electronic state is found to change in character from
non-Fermi liquid to Fermi liquid around the nematic QPT and persists up to
$\sim$ 2.1 GPa. In addition, although the AFM fluctuations correlate with
$T_{\rm c}$ in both SC states, demonstrating the importance of the AFM
fluctuations for the appearance of SC in the system, we found that, when
nematic order is absent, $T_{\rm c}$ is strongly correlated with the AFM
fluctuations, whereas $T_{\rm c}$ weakly depends on the AFM fluctuations when
nematic order is present. Our findings on FeSe$_{0.91}$S$_{0.09}$ were shown to
be applied to the whole FeSe$_{1-x}$S$_{x}$ system and also provide a new
insight into the relationship between AFM fluctuations and SC in Fe-based
superconductors.
