Conventional and Time-Resolved Infrared Spectroscopy of La-1111 Thin Films

We have performed both conventional as well as time-resolved far-infrared spectroscopy on LaFeAsO$_{1-x}$F$_x$ pnictide thin films. The conventional spectroscopy results can be fit using a simple gapped superconductor + normal conductor two-component model. Absorption by quasiparticles in a gap system with nodes is a plausible explanation for the normal component [Lobo \textit{et al}. Phys. Rev. B 82, 100506(R) (2010)]. The time-resolved study is performed by laser-pump, far-IR probe spectroscopy using synchrotron radiation at NSLS beamline U4IR. A laser pulse breaks superconducting pairs and the synchrotron probe is used to sense the recombination process. In contrast to the picosecond response observed for cuprate superconductors, we observe a nanosecond response typical of a fully gapped superconductor where phonon-bottleneck effects slow the effective recombination rate. This result suggests the presence of a full isotropic gap, as might exist at lower energies due to electronic scattering [Carbotte \textit{et al}. Phys. Rev. B 81, 104510 (2010)].