title: Quasiparticle dynamics and phonon softening in FeSe superconductors.
abstract: Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 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 λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.
authors.affiliation: Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan. cwluo@mail.nctu.edu.tw
authors.forename: C W
authors.initials: CW
authors.lastname: Luo
authors.forename: I H
authors.initials: IH
authors.lastname: Wu
authors.forename: P C
authors.initials: PC
authors.lastname: Cheng
authors.forename: J-Y
authors.initials: JY
authors.lastname: Lin
authors.forename: K H
authors.initials: KH
authors.lastname: Wu
authors.forename: T M
authors.initials: TM
authors.lastname: Uen
authors.forename: J Y
authors.initials: JY
authors.lastname: Juang
authors.forename: T
authors.initials: T
authors.lastname: Kobayashi
authors.forename: D A
authors.initials: DA
authors.lastname: Chareev
authors.forename: O S
authors.initials: OS
authors.lastname: Volkova
authors.forename: A N
authors.initials: AN
authors.lastname: Vasiliev