Quasiparticle dynamics and phonon softening in FeSe_0.9S_0.1 superconductors

The quasiparticle and longitudinal acoustic (LA) coherent phonon dynamics of the FeSe_0.9S_0.1 single crystal were investigated by femtosecond transient optical spectroscopy. The FeSe_0.9S_0.1 single crystal undergoes metal-superconductor transition at T_c = 9.5 K and structural phase transition at T_s = 72 K. To investigate the correlation between the quasiparticle/LA coherent phonon dynamics and the antiferromagnetic spin/nematic fluctuation, we measured the transient differential reflectivity ΔR(t)/R₀ in time series (R is the reflectivity) between 4-130 K. The ΔR(t)/R₀ time series showed a negative sign below T_s = 72 K, while its sign reversed from negative to positive above T_s. The ΔR(t)/R₀ time series was contributed by a combination of two exponential decays and an oscillation term. The slow decay attributed to electron-phonon and phonon-phonon coupling showed a negative amplitude below T_s and a positive amplitude above T_s. A fast decay with positive amplitude appeared at T ≥ 55 K was induced by the electron-phonon coupling of the high-temperature orthorhombic phase FeSe_0.9S_0.1. The oscillation contributed by the LA coherent phonon emerged below T_s but was not observed above T_s, indicating that it was impacted by the nematic fluctuation of FeSe_0.9S_0.1.