Electron-phonon coupling and charge gap in spin-density-wave iron-pnictides from quasiparticle relaxation dynamics

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SrFe_2As_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 = 200K is observed. From temperature dependence of the transient reflectivity amplitude we determine the SDW-state charge gap magnitude, 2Delta_SDW/k_BT_SDW=7.2+-1. The second moment of the Eliashberg function, lambda<(hbar omega)^2>=110+-10meV^2, determined from the relaxation time above T_SDW, is similar to SmFeAsO and BaFe_2As_2 indicating a rather small electron phonon coupling constant unless the electron-phonon spectral function (alpha^2F(omega) is strongly enhanced in the low-energy phonon region.