Quasiparticle dynamics and phonon softening in FeSe superconductors
  Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements ({\Delta}R/R) provides unprecedented information on Fe-based superconductors. The amplitude of fast component in {\Delta}R/R clearly tells a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 K and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant {\lambda} is found to be 0.16, identical to the value of theoretical calculations. Such a small {\lambda} demonstrates an unconventional origin of superconductivity in FeSe. 
C. W. Luo, I. H. Wu, P. C. Cheng, J-Y Lin, K. H. Wu, T. M. Uen, J. Y.   Juang, T. Kobayashi, D. A. Chareev, O. S. Volkova, a...
cond-mat.supr-con cond-mat.str-el