Doping dependence and electron–boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1–x Co x )2As2

Abstract

Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in $\mathrm{Ba}{({\mathrm{Fe}}{1-x}{\mathrm{Co}}{x})}{2}{\mathrm{As}}{2}$ in a range of $0\leqslant x\lt 0.15$ at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at $0\lt E-{E}{{\rm{F}}}\lt 0.2,\mathrm{eV}$ are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at $E-{E}{{\rm{F}}}=200,\mathrm{meV}$ which we explain by coupling of the excited electronic system to a boson of this energy. We compare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping-dependence, respectively. We discuss the effect of the electron–boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.