[en] The advent of LaO_1-xF_xFeAs with T_c=26 K has brought enormous interest to the high-Tc superconductor community. It is most important to study the electronic structure of these new superconductors, i.e., Fermi surfaces and band dispersions near the Fermi level at high symmetry points in order to obtain microscopic understanding of superconductivity. We have studied the electronic structure of Fe based 122 type parent compounds and their superconducting derivatives to reveal the important information on the Fermi surface nesting conditions (between hole pockets at the Brillouin zone center and electron pockets at zone corner) as a function of electron, hole doping, and isovalent substitution of P at As site in Ba(Eu)Fe₂As₂. Our findings show that electron doping into parent BaFe₂As₂ destroy the nesting conditions and superconductivity emerges, accompanied by increase in the dimensionality of electronic structure. We observe that charge carrier doping into parent compounds show a rigid-band-like behavior of electronic structure where as isovalent substitution show a non rigid-band-type nature.

[en] We report high-resolution ARPES studies of the electronic structure of BaFe_2-xCo_xAs₂, Ba_1-xK_xFe₂As₂, FeTe(Se), and EuFe₂As₂. The results are compared with DFT band structure calculations. From photon energy dependent measurements, information on the band dispersion perpendicular to the Fe layers could be derived. with increasing Co doping in BaFe_2-xCo_xAs₂, the dimensionality increases and, due to the filling of the hole pockets, the nesting condition decreases. In the AFM phase of EuFe₂As₂ back-folded bands strongly hybridize with the non-folded bands leading to the opening of gaps around both high-symmetry points. This transforms the large Fermi surface of the PM phase into droplet Fermi surfaces in the AFM low-T phase.

[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] The recently discovered class of Fe-based high-T_c superconductors and their parent compounds presents an interesting correlated electronic system to study the effects of intra- and interband scattering. The influence of the electronic bandstructure on the fundamental relaxation processes of excited carriers leading to intra- and interband scattering is of fundamental interest in solid state physics. Here, we report on fs time- and angle-resolved photoemission spectroscopy (trARPES) on the parent compound EuFe₂As₂ of the new class of FeAs based high-T_c superconductors. Using intense fs laser pulses (hν=1.5 eV), part of the electronic population is excited to states above the Fermi level. The transient evolution of both occupied and unoccupied states is probed by energy- and angle-resolved photoelectron spectroscopy using a time-delayed ultraviolet pulse (hν=6.0 eV). Upon excitation, occupied states around the hole-pocket at the Γ-point of the Brillouin zone become partially depopulated by excited holes, whereas electrons are filling the empty states within the hole-pocket. The timescales of electron and hole dynamics within this band differ drastically and cannot be explained solely by intraband e-h pair generation, but an additional interband excitation channel has to be considered.

[en] Thermal conductivity, point contact spectroscopy, angle-resolved photoemission and Raman spectroscopy measurements were performed on BaFe_1_._9Pt_0_._1As_2 single crystals obtained from the same synthesis batch in order to investigate the superconducting energy gap structure using multiple techniques. Low temperature thermal conductivity was measured in the superconducting state as a function of temperature and magnetic field, revealing an absence of quasiparticle excitations in the T→0 limit up to 15 T applied magnetic fields. Point-contact Andreev reflection spectroscopy measurements were performed as a function of temperature using the needle-anvil technique, yielding features in the conductance spectra at both 2.5 meV and 7.0 meV scales consistent with a multi-gap scenario. Angle-resolved photoemission spectroscopy probed the electronic band structure above and below the superconducting transition temperature of T_c = 23 K, revealing an isotropic gap of magnitude ∼3 meV on both electron and hole pockets. Finally, Raman spectroscopy was used to probe quasiparticle excitations in multiple channels, showing a threshold energy scale of 3 meV below T_c. Overall, we find strong evidence for an isotropic gap structure with no nodes or deep minima in this system, with a 3 meV magnitude gap consistently observed and a second, larger gap suggested by point-contact spectroscopy measurements. We discuss the implications that the combination of these results reveal about the superconducting order parameter in the BaFe_2_−_xPt_xAs_2 doping system and how this relates to similar substituted iron pnictides. (paper)

[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] The phase diagram of Sr₃Ru₂O₇ shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr₃Ru₂O₇. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets. (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)