Quasiparticle dynamics and phonon softening in FeSe 0.9 S 0.1 superconductors.

The quasiparticle and longitudinal acoustic (LA) coherent phonon dynamics of the FeSe 0.9 S 0.1 single crystal were investigated by femtosecond transient optical spectroscopy. The FeSe 0.9 S 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 Δ R ( t )/ R 0 in time series ( R is the reflectivity) between 4-130 K. The Δ R ( t )/ R 0 time series showed a negative sign below T s = 72 K, while its sign reversed from negative to positive above T s . The Δ R ( t )/ R 0 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 0.9 S 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 0.9 S 0.1 .