Interrelationships between nematicity, antiferromagnetic spin fluctuations, and superconductivity: Role of hotspots in FeSe1−xSx revealed by high pressure Se77 NMR study

The sulfur-substituted FeSe, ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_x$, is one of the unique systems that provides an independent tunability of nematicity, antiferromagnetism, and superconductivity under pressure ($p$). Recently, Rana et al. [K. Rana et al., Phys. Rev. B 101, 180503(R) (2020)] reported, from $⁷⁷\mathrm{Se}$ nuclear magnetic resonance (NMR) measurements on ${\mathrm{FeSe}}_0.91{\mathrm{S}}_0.09$ under pressure, that there exists a clear role of nematicity in the relationship between antiferromagnetic (AFM) spin fluctuations and the superconducting transition temperature (${T}_{\mathrm{c}}$), where the AFM spin fluctuations are more effective in enhancing ${T}_{\mathrm{c}}$ in the absence of nematicity than with nematicity. Motivated by the work, we carried out $⁷⁷\mathrm{Se}$ NMR measurements on ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_x$ with $x$ = 0.15 and 0.29 under pressure up to 2.10 GPa to investigate the relationship in a wide range of $x$ in the ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_x$ system. Based on the new results together with the previously reported data for $x=0$ [P. Wiecki et al., Phys. Rev. B 96, 180502(R) (2017)] and 0.09 [K. Rana et al. Phys. Rev. B 101, 180503(R) (2020)], we established a $p\ensuremath{-}x$--temperature ($T$) phase diagram exhibiting the evolution of AFM spin fluctuations. From the systematic analysis of the NMR data, we found that the superconducting (SC) state in a nematic state arises from a non-Fermi-liquid state with strong stripe-type AFM spin fluctuations, while the SC state without nematicity comes from a Fermi-liquid state with mild stripe-type AFM spin fluctuations. Furthermore, we show that the previously reported impact of nematicity on the relationship between AFM fluctuations and superconductivity holds throughout the wide range of $x$ from $x=0$ to 0.29 in ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_x$ under pressure. We discuss the origin of the role of nematicity in terms of the different numbers of hotspots on Fermi surfaces with and without nematicity.