Electron-phonon coupling and the charge gap of spin-density wave iron-pnictide materials from quasiparticle relaxation dynamics

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SrFe(2)As(2) exhibiting spin-density wave (SDW) ordering using optical pump-probe femtosecond spectroscopy. A remarkable critical slowing down of the quasiparticle relaxation dynamics at the SDW transition temperature T(SDW) = 200 K is observed. From temperature dependence of the transient reflectivity amplitude we determine the SDW-state charge gap magnitude, 2 Delta(SDW)/k(B)T(SDW)= 7.2 +/- 1. The second moment of the Eliashberg function, lambda <((h) over bar omega)(2)>=110 +/- 10 meV(2), determined from the relaxation time above T(SDW), is similar to SmFeAsO and BaFe(2)As(2) indicating a moderate electron phonon coupling.