Ultrafast dynamics and phonon softening in Fe1+ySe1-xTex single crystals

The ultrafast quasiparticle dynamics of Fe1+ySe1-xTex single crystals were investigated by dual-color transient reflectivity measurements (Delta R/R) from 4.3 to 290 K. The electron-phonon coupling strength lambda (= 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 Delta R/R. Such a small lambda demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent Delta R/R exhibits anomalous changes at T-c, 90 K and 230 K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.