Inelastic neutron scattering has been employed to investigate the low-lying crystal-field excitations of Pr2−xCuO4−δ as a function of Ce content x. The observed energy spectra are found to results from a superposition of three different components which we interpret in terms of local regions of antiferromagnetic, intermediately doped and highly doped character. The volume fraction of the doped clusters is shown to reach the percolation limit for x≈0.14 where the onset of superconductivity is actually observed.

O K alpha X-ray fluorescence of YBa2Cu3O7- delta and Sr0.85Nd0.15CuO2 samples containing CO32- impurities, as well as pure BaCO3, have been studied using tunable monochromatic synchrotron radiation excitation. When the excitation energy is set to the most intense features in the O 1s absorption edge of cuprates and to bound states above this edge the O K alpha spectra recorded are very similar to those of pure BaCO3 even if the admixture of CO32- is small. The observed changes in the O K alpha profiles of multi-phase YBa2Cu3O7- delta and Sr0.85Nd0.15CuO2 samples are discussed in terms of different X-ray absorption cross sections for O in the main phase and in the CO32$fraction.

We present investigations on the electronic structure of electron doped Nd2−xCexCuO4−δ and Sr1−xNdxCuO2−δ (infinite layer compound) as well as hole doped Nd1.4Ce0.2Sr0.4CuO4−δ by means of photoelectron spectroscopy and auxiliary measurements. The CuO-layers of electron type HTSC exhibit a comparably high Cu3d-Coulomb-correlation. The CuO hybridization of the new infinite layer compound is found to be remarkably low. Rare earth layers of T′- and T∗-systems are very similar with respect to electronic parameters and antiferromagnetic correlations between Nd spins. For Sr1−xNdxCuO2−δ, the Nd dopants are found to be trivalent.

The authors present investigations on the electronic structure of the recently discovered, electron-doped infinite-layer compound Sr[sub 1-x]Nd[sub x]CuO[sub 2-[delta]] (T[sub c][approx]40 K) by means of photoelectron spectroscopy with synchrotron radiation. This system is compared with the two closely related cuprates Nd[sub 2-x]Ce[sub x]CuO[sub 4-[delta]] (electron doped) and Nd[sub 1.4]Ce[sub 0.2]Sr[sub 0.4]CuO[sub 4-[delta]] (hole doped). Valence band spectra in the Cu 3p[yields]3d resonance are interpreted in terms of CuO[sub 4] (n-doped compounds) and CuO[sub 5] (p-doped system) cluster models. From the author's calculations they find a considerably higher Cu 3d Coulomb correlation for the electron-doped compounds as well as a very low Cu-O hybridization for the infinite-layer system. Partial Nd spectra of Sr[sub 0.85]Nd[sub 0.15]CuO[sub 2-[delta]] show the Nd dopants to be trivalent, thus confirming the infinite-layer compound to be electron doped. Analysis of partial Nd valence bands and magnetic susceptibilities of Nd[sub 2-x]Ce[sub x]CuO[sub 4-[delta]] and Nd[sub 1.4]Ce[sub 0.2]Sr[sub 0.4]CuO[sub 4-[delta]] gives very similar electronic parameters and antiferromagnetic correlations between Nd spins for both cuprates.

We present investigations on the electronic structure of the recently discovered, electron-doped infinite-layer compound Sr1−xNdxCuO2−δ (Tc≈40K) by means of photoelectron spectroscopy with synchrotron radiation. This system is compared with the two closely related cuprates Nd2−xCexCuO4−δ (electron doped) and Nd1.4Ce0.2Sr0.4CuO4−δ (hole doped). Valence band spectra in the Cu 3p→3d resonance are interpreted in terms of CuO4 (n-doped compounds) and CuO5 (p-doped system) cluster models. From our calculations we find a considerably higher Cu3d Coulomb correlation for the electron-doped compounds as well as a very low Cu-O hybridization for the infinite-layer system. Partial Nd spectra of Sr0.85Nd0.15CuO2−δ show the Nd dopants to be trivalent, thus confirming the infinite-layer compound to be electron doped. Analysis of partial Nd valence bands and magnetic susceptibilities of Nd2−xCexCuO4−δ and Nd1.4Ce0.2Sr0.4CuO4−δ gives very similar electronic parameters and antiferromagnetic correlations between Nd spins for both cuprates.

Polycrystalline samples of Ln2−xCexCuO4−δ (Ln=Nd, Sm) (the T' type high-Tc superconductors) with x=0 and 0.15 as a “simple” model compound for other HTSC's have been irradiated with 30 MeV 32S ions at room temperature in order to study the differences in electronic structure between ideal and disordered materials. Irradiation has been performed in an ultra-high vacuum chamber with subsequent in-situ photoelectron spectroscopy (PES). 32S fluences have been varied between 2×1012 cm-2 and 1×1015 cm-2. After irradiation the valence band and several core-level spectra of Cu, O and Ln=Nd, Sm have been recorded. For each compound and fluence we observe preferential depletion of oxygen and deposition of these fragments on the surface, indicated by characteristic changes in the valence band and O 1s core-level spectra. For low fluences (Φt<10-13cm-2) oxygen removement takes place preferentially in the Ln-O planes (i.e. in the dopant layers). At a certain threshold fluence (Φt)Cu+, which depends essentially on the Ln-O hybridization, we observe strong formation of monovalent copper in the valence band and Cu 2p spectra indicating considerable oxygen depletion also of the Cu-O layers.

We compare the electronic structure of n-type and p-type dopable high-T(c) cuprates, namely the hole-doped Nd1.4Ce0.2Sr0.4CuO4-delta (T*) system as well as the electron-doped Nd2-CexCuO4-delta (T') and Sr0.85Nd0.15CuO2-delta (''infinite-layer'') systems. Investigations were done mainly by means of core-level and valence-band photoemission spectroscopy. Also we performed auxiliary measurements of Hall effect and magnetic susceptibility. From the investigations on the Cu-O layers we propose that one criterion for electron dopability in high-T(c) cuprates is a comparatively high value of the Cu 3d Coulomb interaction U(dd). This is concluded from model calculations on core-level and valence-band spectra. Also the superconducting infinite-layer compound Sr0.85Nd0.15CuO2-delta exhibits (besides a remarkably low Cu-O hybridization) this enhanced value of U(dd). For the rare-earth layers of T' and T* we find small but characteristic differences in the electronic properties (Nd-0-hybridization and charge-transfer energy), which can be attributed to structural differences. Crystal-field splitting of Nd 4f levels and antiferromagnetic coupling of Nd3+ moments in Nd1.85Ce0.15CuO4-delta and Nd1.4Ce0.2Sr0.4CuO4-delta have been investigated by evaluation of the magnetic susceptibility. The dopant ions cerium for T' and T* and neodymium for Sr0.85Nd0.15CuO2-delta are found to be tetra- and trivalent, respectively, which confirms again that the infinite-layer compound is an electron-doped cuprate.

Oxygen and chlorine ions at energies of 40 keV and fluences between 2 x 10(16) and 2 x 10(17) cm-2 have been implanted into epitaxial YBa2Cu3O7-delta films. By X-ray photoemission spectroscopy (XPS or ESCA) depth profiles of the implanted O/Cl ions as well as of Y and Cu have been determined. By analyzing the peak positions of characteristic core level lines of Ba and Cl it was possible to obtain a depth profile of the chemical surrounding of these elements. For all fluences and projectiles we observe a nearly amorphized layer of a thickness of approximately 500 angstrom with disturbed stoichiometry. The degree of perturbation correlates with ion fluence and projectile mass, whereas the thickness of the off-stoichiometric layer depends only on the mass of the implanted ions. Experimental results are compared with theoretical concentration and DPA profiles obtained by TRIM-DYN calculations which give hints on diffusion of implanted ions and implantation defects during and after implantation.

Investigations on the electronic structure of T*-type Nd1.4Ce0.2Sr0.4Cu04−δ by means of photoelectron spectroscopy are presented and compared with results for electron doped compounds of the T′-structure. Concerning the Cu-O-layers we find slightly altered Cu-O-hybridization and -charge transfer energy, which can be explained by the different surrounding of copper in both compounds. Also the electronic properties of the rare earth layers exhibit small but distinct differences in both systems which also can be related to different structural properties.

We present our method of fabricating superconducting compounds of the infinite-layer structure Sr1-xNdxCuO2-delta with various doping concentrations. The samples undergo a high temperature high pressure process and are then bulk superconductors without any further treatment. A slightly reducing atmosphere was necessary to get doped layer-phases. Refinement of the crystallographic data was done by a least squares method. The T(c)onset values are in the range of 36-41 K. The lower critical field H(c1)ab(T=0) is extrapolated to 15 G, while magnetization cycles show extremely weak pinning behaviour. From photoemission data a considerably diminished Cu-O-hybridisation is found. The particular shape of partial Nd-valence band spectra for these compounds shows Nd to work as a trivalent dopant for divalent Sr.

Oxygen and chlorine ions at energies of 40 keV and doses up to 2 × 1017 cm-2 have been implanted into epitaxial Y1Ba2Cu3O7−δ films with different Tc≈90 K and ≈60 K, so that ion ranges are smaller than the sample thickness. As has been established by both resistivity and Hall-effect measurements the layer underlying the implanted near-surface layer still retains its superconducting properties, thus indicating a possibility to realize a two-layered structure by means of ion implantation. Depth profiles of Y, Ba, Cu and Cl concentrations obtained by step-by-step sputtering of the samples and recording the core-level electron spectra of these elements by means of XPS could be accounted for by T-DYN Monte-Carlo code simulation, besides the differences caused by diffusion processes. A striking inadequacy of chlorine and oxygen ion implantation has been observed. XPS-spectra of the Cu2p-core level reveal Cu to remain in a divalent oxidation state even after Ar-sputtering for O-implanted films whereas in Cl-implanted specimens Cu is totally converted to Cu+ by sputtering.

Polarization-dependent x-ray-absorption spectroscopy on the O 1s and Cu 2p edges using the nonsurface-sensitive fluorescence method has been performed on single crystals of La2-xSrxCuO4+delta, R2-xCexCuO4-delta (R = Nd and Sm), and the insulating compound Ca0.86Sr0.14CuO2. The experimental results support the picture of a doped charge-transfer insulator. The symmetry of hole states on O and Cu sites has been determined. These have predominantly in-plane, i.e., O 2p(x,y) and Cu 3d(x2-y2) character. In p-type doped cuprates for 0 < x < 0.15 about 8% of the total number of hole states on O sites have O 2p(z) character, which probably originates from apical O sites. At higher dopant concentration, this number increases. In the n-type doped system 4-11% of the O 2p states in the energy range of the upper Hubbard band have O 2p(z) character. In nearly all the systems investigated in this work the fraction of unoccupied Cu 3d3z2-r2 states with respect to the total number of Cu holes in the upper Hubbard band is 3+/-3%. At higher energies a high spectral weight with Cu 3d3z2-r2 character (about 40% of the total amount of unoccupied Cu 3d states in the upper Hubbard band) is observed, which is probably caused by a hybridization with Cu 4s and 4p states. The implication of holes in nonplanar orbitals on high-T(c) superconductivity in cuprates is discussed.

The electronic structure of high-temperature superconductors near the Fermi level which is of primary interest for the understanding of the mechanism of superconductivity, is not yet well understood. Most theoretical investigations deal with Cu3dx−y and O2px,y orbitals at EF, but whether or not O2pz and Cu3d3z−r orbitals contribute to states at EF, is still under discussion. The contribution of states with out-of-plane orbital character has been investigated by polarized x-ray absorption spectroscopy (XAS) on HTSC single crystals. For La2−xSrxCuO4 a small contribution from O2pz orbitals (8% of holes in O2p valence band states) was observed which increased to about 13% on 30% Sr doping. In n-type doped HTSC there is no clear evidence for O2pz orbitals at EF, but states are observed slightly above EF with O2pz character depending on the doping and annealing conditions. An observed admixture of Cu 3d3z−r states to the predominant 3dx hole states at EF of a few percent did not significantly exceed the expermental error limits.

We present investigations of the electronic structure of T∗-type Nd1.4Ce0.2Sr0.4CuO4 by means of photoelectron spectroscopy. The CuO-hybridization for Nd1.4Ce0.2Sr0.4CuO4 seems to have a smaller value than in n-doped T'-HTSCs, but is slightly larger than e.g. for Bi2Sr2CaCu2O8−δ. We attribute this to the structural properties of these compounds. On the other hand, the effective Coulomb repulsion on the copper site for T∗ is found to be comparable to the electron doped systems. Concerning the mean NdO- hybridization within the rare-earth layers only small differences between Nd1.4Ce0.2Sr0.4CuO4 and T'-systems are observed. Doped Ce was confirmed to be tetravalent in both T'-and T∗-compounds. The CuO-valence band region shows no significant changes compared with other HTSCs.

We present investigations of the processing and doping dependence of the Cu 2p- and Nd 3d-core level spectra of Nd2−xCexCuO4−δ. The sintering process seems to enhance the overlap between Cu 3d- and O 2p-orbitals in the CuO- layers, whereas the reducing procedure as well as Ce-doping introduce electrons with Cu 3d-character into the T'-structure. Nd 3d-spectra show a decreasing mean overlap of Nd-orbitals with the neighbouring oxygen atoms as a result of both the sintering and reducing processes, whereas hybridizations is enhanced with increasing Ce-content of the samples. This behaviour can be explained by the removal of oxygen from the NdO-layers of Nd2−xCexCuO4−δ during the sintering and reducing procedures.

The influence of substitutions in the characteristic structure elements of the n-doped HTSC Sm2-xCexCuO4-delta on magnetic, superconducting and electronic properties has been investigated by means of magnetic measurements and photoelectron spectroscopy. Significant changes in the magnetic behaviour of the rare earth layers (T(Neel), chi(T)) and the superconducting properties (T(c), H(c1) (T)) have been observed.