[en] Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in $\mathrm{B}\mathrm{a}{({\mathrm{F}\mathrm{e}}_{1-<!--\; ???\; -->x}{\mathrm{C}\mathrm{o}}_x)}_2{\mathrm{A}\mathrm{s}}_2$ in a range of $0⩽<!--\; ???\; -->x<<!--\; <\; -->0.15$ 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 $0<<!--\; <\; -->E-<!--\; ???\; -->E_{\mathrm{F}}<<!--\; <\; -->0.2\mathrm{e}\mathrm{V}$ 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 $E-<!--\; ???\; -->E_{\mathrm{F}}=200\mathrm{m}\mathrm{e}\mathrm{V}$ 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. (paper)

[en] Using ARPES we have studied the scattering rates and effective masses of the ferropnictides (Ba/Eu)Fe_2(As_1_-_xP_x)_2 and NaFe_1_-_x(Co/Rh)_xAs as a function of the control parameter (chemical pressure/electron doping). The detected scattering rates of all electron and hole pockets are nearly independent of the control parameter, strongly differ for pockets having different orbital character, and are linear in energy indicating marginal Fermi liquid behavior near optimal substitution/doping. The measurements also indicate a crossing of the top of that hole pocket, having the largest scattering rate, through the Fermi level. A calculation as well as the experiments show that a coaction of marginal Fermi liquid behavior and the weakly dispersive band crossing the Fermi level leads to an extended singularity. The later can explain, possibly also in other unconventional superconductors, the strong mass enhancement near optimal doping/substitution and a superconducting phase with a small effective Fermi energy favoring a BCS-BE crossover state.

[en] We employed femtosecond time- and angle-resolved photoelectron spectroscopy to analyze the response of the electronic structure of the 122 Fe-pnictide parent compounds Ba/EuFe₂As₂ and optimally doped BaFe_1.85Co_0.15As₂ near the Γ point to optical excitation by an infrared femtosecond laser pulse. We identify pronounced changes of the electron population within several 100 meV above and below the Fermi level, which we explain as a combination of (i) coherent lattice vibrations, (ii) a hot electron and hole distribution, and (iii) transient modifications of the chemical potential. The responses of the three different materials are very similar. In the coherent response we identify three modes at 5.6, 3.3, and 2.6 THz. While the highest frequency mode is safely assigned to the A_1g mode, the other two modes require a discussion in comparison to the literature. Employing a transient three temperature model we deduce from the transient evolution of the electron distribution a rather weak, momentum-averaged electron–phonon coupling quantified by values for λ〈ω²〉 between 30 and 70 meV². The chemical potential is found to present pronounced transient changes reaching a maximum of 15 meV about 0.6 ps after optical excitation and is modulated by the coherent phonons. This change in the chemical potential is particularly strong in a multiband system like the 122 Fe-pnictide compounds investigated here due to the pronounced variation of the electron density of states close to the equilibrium chemical potential. (paper)

[en] In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally ''doped'' compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally ''doping.'' Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of λ(ω²) and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of k_F, hot electrons dissipate their energy by electron-phonon coupling with a characteristic time constant of 200 fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin-dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fe-pnictides because it establishes experimental access to spin-dependent dynamics in materials with spin density waves. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)