Distinct quasiparticle relaxation dynamics in an electron-doped superconductor, BaFe1.9Ni0.1As2

We report an investigation of the distinct quasiparticle (QP) relaxation dynamics in an electron-doped superconductor, BaFe1.9Ni0.1As2, employing femtosecond pump-probe measurements. Two distinct relaxation components and one sub-nanosecond long-lived component are clearly observed in our transient reflectivity spectra. The slow relaxation component, which is on a picosecond time scale, strongly correlates with the superconducting (SC) transition, and we attribute it to the recombination dynamics of Cooper pairs. We calculated the SC gap, Delta(0) approximate to 5.7 meV. The fast relaxation component, on a sub-picosecond scale, is ascribed to the QP relaxation from a range of large gap states to the state above the SC gap. We obtained the low end of these bandgaps to be Delta(G) approximate to 9 meV. Furthermore, we estimated the electron-phonon (e-ph) coupling constant and the Coulomb pseudopotential from the fast relaxation lifetime. The small values of the e-ph constant and the negative Coulomb pseudopotential suggest that the e-ph interaction is not the dominant contribution of the SC transition.