Ultrafast dynamics and phonon softening in Fe1+ySe1−xTexsingle crystals

The ultrafast quasiparticle dynamics of Fe1+ySe1?xTex single crystals were investigated by dual-color transient reflectivity measurements (?R/R) from 4.3 to 290?K. The electron?phonon coupling strength ??(=0.16?0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in ?R/R. Such a small ? demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent ?R/R exhibits anomalous changes at Tc, 90?K and 230?K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.