[en] The main motivation behind this thesis has been to determine both the advantages and the disadvantages of the soft X-ray application of angle-resolved photoemission spectroscopy. The investigation of a well known system, Ag(001), enables us to discuss several questions such as the role of the photon momentum, the applicability of the free-electron-like final state approximation and the role of phonons. The polarisation of the incoming light has also been exploited. The choice of such a system also comes from our desire to compare the results with calculations of angle-resolved photoemission spectra in this energy range. The anomalous low temperature properties of Ce-based compounds are generally related to the Kondo effect. Original results have been obtained by investigating the valence band of three iso-structural single crystalline Ce compounds, CeCu₂Ge₂, CeNi₂Ge₂ and CeCo₂Ge₂. The location of the Ce M5 absorption edge within the soft X-ray energy range is exploited in order to isolate the 4f contribution to the spectra. Added to this, the use of relatively high incoming photon energies allows minimising the surface contribution. Temperature dependent, resonant, angle-resolved and angle-integrated photoemission measurements have been performed. The former are in qualitative agreement with the single impurity Anderson model, whereas the latter suggest the importance of taking the lattice into account. (author)

[en] The (001) surface of Silver (Ag) has been chosen as a simple and well known system for investigating the band mapping potentialities of angle-resolved photoemission spectroscopy in the soft X-ray energy range. In order to determine the intrinsic limits of this technique, the valence band of Ag has been measured both as a function of the measuring temperature and of the incoming photon energy and the issue of phonon-assisted non-direct transitions is addressed. (authors)

[en] The authors present results of electronic Raman-scattering experiments in differently doped YBa₂Cu₃O_6+x. In B_2g symmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates T(ω,T) and mass-enhancement factors 1 + λ(ω,T) for the carriers are obtained. Starting from temperatures T > 180K, T(ω,T) and 1 + λ(ω,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with the data, they find a loss of spectral weight between Tc lt T lt T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below Tc, the superconducting gap is observed. It depends on x and scales with Tc whereas the energy scale of the pseudogap remains the same

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[en] We present results of Raman-scattering experiments in differently doped YBa₂Cu₃O_6+x (Y-123) and Bi₂Sr₂(Ca_xY_1-x)Cu₂O_8+δ (Bi-2212) single crystals. When reducing the doping level x the scattering intensity in B_1g symmetry increases while it decreases in the B_2g channel. This anisotropic behavior can be attributed to a qualitative change of the Fermi surface topology. At B_2g symmetry we find evidence of a pseudogap Δ^* in the normal state below a characteristic temperature T^* ∼ 180 K. In the superconducting state, the spectra obtained from overdoped samples show the well-known pair-breaking (2Δ) features in both B_1g and B_2g symmetries while in the underdoped regime pair-breaking maxima can only be observed in the B_2g channel. The magnitude of the gap can be estimated to be approximately 8kT_c and is, from our experiments, different from Δ^*. (orig.)