Order-parameter evolution in the Fulde-Ferrell-Larkin-Ovchinnikov phase

We report on the temperature dependence of the spatially modulated spin-polarization amplitude ΔK_spin, which is a hallmark of the superconducting Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. For that, we use ¹³C nuclear magnetic resonance (NMR) spectroscopy performed on the organic conductor β''-(ET)₂SF₅CH₂CF₂SO₃. From a comparison of our experimental results to a comprehensive modeling of the ¹³C NMR spectra, we determine the evolution of ΔK_spin upon condensation of the FFLO state. Further, the modeling of the spectra in the superconducting phase allows to quantify the decrease of the average spin susceptibility, stemming from the spin-singlet coupling of the superconducting electron pairs in the FFLO state of β''-(ET)SF₅CH₂CF₂SO₃.