Coherent excitations and electron–phonon coupling in Ba/EuFe₂As₂ compounds investigated by femtosecond time- and angle-resolved photoemission spectroscopy

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 BaFe1.85Co0.15As2 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 A1g 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