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

Abstract

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped ${\text{SrFe}}_{2}{\text{As}}_{2}$ 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 ${T}_{\text{SDW}}=200\text{ }\text{K}$ is observed. From temperature dependence of the transient reflectivity amplitude we determine the SDW-state charge gap magnitude, $2{\ensuremath{\Delta}}_{\text{SDW}}/{k}_{B}{T}_{\text{SDW}}=7.2\ifmmode\pm\else\textpm\fi{}1$. The second moment of the Eliashberg function, $\ensuremath{\lambda}⟨{(\ensuremath{\hbar}\ensuremath{\omega})}^{2}⟩=110\ifmmode\pm\else\textpm\fi{}10\text{ }{\text{meV}}^{2}$, determined from the relaxation time above ${T}_{\text{SDW}}$, is similar to SmFeAsO and ${\text{BaFe}}_{2}{\text{As}}_{2}$ indicating a moderate electron phonon coupling.