[en] It has been found that the self-energy of high-T_C cuprates indeed exhibits a well pronounced structure, which is currently attributed to coupling of the electrons either to lattice vibrations or to collective magnetic excitations in the system. To clarify this issue, the renormalization effects and the electronic structure of two cuprate families Bi₂Sr₂CaCu₂O_8+δ and YBa₂Cu₃O_7-δ were chosen as the main subject for this thesis. With a simple example of an electronic system coupled to a collective mode unusual renormalization features observed in the photoemission spectra are introduced. It is shown that impurity substitution in general leads to suppression of the unusual renormalization. Finally an alternative possibility to obtain a purely superconducting surface of Y-123 via partial substitution of Y atoms with Ca is introduced. It is shown that renormalization in the superconducting Y-123 has similar strong momentum dependence as in the Bi-2212 family. It is also shown that in analogy to Bi-2212 the renormalization appears to have strong dependence on the doping level (no kinks for the overdoped component) and practically vanishes above T_C suggesting that coupling to magnetic excitations fits much better than competing scenarios, according to which the unusual renormalization in ARPES spectra is caused by the coupling to single or multiple phononic modes. (orig.)

[en] The Hall coefficient of the hole-doped iron arsenide Ba_1-xK_xFe₂As₂ (BKFA) is calculated purely on the basis of the electronic structure, revealed in the angle-resolved photoemission spectroscopy (ARPES) experiments, and compared to the one measured directly. The observed agreement allows us to state that upon cooling the Fermi surface (FS) in the optimally doped BKFA gradually evolves to the propeller-like topology, on which the superconductivity develops. Persistence of the notable temperature dependence in both photoemission and magnetotransport experiments well above the spin-density-wave (SDW) transition suggests that the FS reconstruction in BKFA is partially decoupled from the emergence of static magnetism. (author)

[en] To analyse the charge dynamics of La,Sr-bilayer manganites we performed angle resolved photoemission measurements on (La_1-xPr_x)_1.2Sr_1.8Mn₂O₇ single crystals. The spectra are compared to a tight-binding Wannier model. The measured bilayer splitting is much smaller than the calculated one, indicating a strongly reduced hybridization between the bilayer-planes in the real material. Further the spectral function exhibits fingerprints of a periodic perturbation with an interaction strength of around 400 meV. A possible origin of such a perturbation are CE-type charge-order fluctuations.

[en] Several angle resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within hole-like bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the hole-like bands. The resolution follows from an excellent agreement between experimental quasiparticle scattering data and T-matrix QPI calculations (based on experimental band structure data), which allows disentangling interband and intraband scattering processes.

No abstract available

[en] We report on a temperature dependent scanning tunneling microscopy (STM) and angle resolved photoemission (ARPES) study of the Cu intercalated dichalcogenide 2H-TaSe₂. The Cu intercalation leads not only to a lowering of the transition temperature into the commensurate charge-density wave state (CDW) but also to the formation of a √(13) x √(13) superstructure, previously observed for the 1T polytype only. The origin, spectroscopic appearance, and influence of these superstructures on the electronic properties of 2H-TaSe₂ are discussed.

[en] Strontium ruthenates are famous for p-type superconductivity, metamagnetism, and notable spin-orbit coupling. Understanding all these phenomena requires a detailed knowledge of the electronic structure. Unfortunately recent ARPES on Sr₂RuO₄ was confronted with a problem of an acute 'surface state' (SS). Here we argue that, instead of the earlier proposed remedy of high temperature cleaving, one may rely on excitation energy dependence of matrix elements enhancing surface or bulk features, while circularly polarized light can be used to establish their origin. Owing to the minimized surface degradation we observe bulk α, β, γ bands and their surface counterparts along with additional δ feature. According to the dichroic pattern the new feature must be yet another surface counterpart of the β band. Also the narrower momentum width together with negligible k_z dispersion are distinct properties specific to a surface state. Since there are numerous examples where the surface state undergoes splitting due to spin-orbit interaction we suggest that full relativistic calculation might be needed to understand the origin of the new feature.

[en] We present first-time measurements of the Fermi surface and low-energy electronic structure of intermetallic compounds Gd₂PdSi₃ and Tb₂PdSi₃ by means of angle-resolved photoelectron spectroscopy (ARPES). We show that the Fermi surface in both compounds consists of an electron barrel at the Γ point surrounded by spindle-shaped electron pockets originating from the same band, with the band bottom of both features lying at 0.5 eV below the Fermi level. From the experimentally measured band structure, we estimate the momentum-dependent RKKY coupling strength and demonstrate that it is peaked at the 1/2Γ K wave vector. Comparison with neutron diffraction data from the same crystals shows perfect agreement of this vector with the propagation vector of the low-temperature in-plane magnetic order, thereby demonstrating the decisive role of the Fermi surface geometry in explaining the complex magnetically ordered ground state of ternary rare earth silicides.

[en] The electronic structure of the iron chalcogenide superconductors FeSe_1_-_x and Rb_0_._7_7Fe_1_._6_1Se_2 was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations and μSR measurements. Both compounds share ''cigar-shaped'' Fermi surface sheets in their electronic structure, that can be found in almost all iron-pnictide superconductors. These features originate from a strong interplay of two hole- and electron-like bands in the Brillouin zone center, leading to a pronounced singularity in the density of states just below the Fermi level. This facilitates the coupling to a bosonic mode responsible for superconductivity.

[en] The lack of nesting of Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a T_c of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from ARPES. The gaps obtained are qualitatively different from previous 2D theoretical works and in good agreement with ARPES on the main Fermi-surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets still dominate the pairing as in other Fe-based superconductors despite the lack of nesting, leading to gaps with anisotropic s_± structure.