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

Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in Ba(Fe1-xCox)2As2 in a range of $0\leq \ $x< 0.15 at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at 0 < E - E$_{F}$ < 0.2 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$_{F}$ = 200 meV whichwe explain by coupling of the excited electronic system to a boson of this energy.Wecompare our results with static ARPES and transport measurements and find disagreement and agreement concerning the dopingdependence, respectively.Wediscuss the effect of the electron–boson coupling on the energydependent relaxation and assign the origin of the boson to a magnetic excitation