Doping Dependent Quasiparticle Relaxation Dynamics in SmFeAsO1−x F x Single Crystals: Comparison of Spin-Density Wave and Superconducting States

Abstract

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO1−x F x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T SDW, the second moment of the Eliashberg function is determined to be λ〈ω 2〉≈135 meV2. Below T SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2ΔSDW/k B T SDW=7±3, at 4.2 K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.