Quasiparticle relaxation across the multiple superconducting gaps in the electron-doped BaFe1.85Co0.15As2

We study the quasiparticle relaxation dynamics in the electron-doped superconductor, BaFe1.85 Co0.15As2 (BFCA) by using optical spectroscopy in both time and frequency domains. The estimate of the electron-phonon (el-ph) coupling constant from quasiparticle relaxation rate in time domain gives a value of similar to 0.16 that is much too small to give a T-C of 25 K. We obtain a negative value of the Coulomb pseudopotential that implies that the el-ph interaction alone is not strong enough to induce a superconducting transition in BFCA. More importantly, the electron-boson spectral function determined from optical scattering rate in frequency domain does not resemble the measured phonon density of states and has a dominant peak (at 53 meV) beyond the phonon frequencies. Our results support that the BFCA is a multi-gap s-wave superconductor and that its electron pairing is based on a spin-mediated pairing mechanism. (C) 2012 American Institute of Physics. [doi:10.1063/1.3677659]