Doping dependence and electron-boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1-xCox)(2)As-2

Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in Ba(Fe1-xCox)(2)As-2 in a range of 0 <= x < 0.15 at temperatures slightly above the Neel 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 which we 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 doping-dependence, respectively. W ediscuss the effect of the electron-boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.