Measurements of the linear thermal expansion coefficients αi at the superconducting transition temperature Tc and measurements of the hydrostatic pressure dependence dTc/dP of high-quality single crystals of Sm2−xCexCuO4−y are presented. The uniaxial stress dependencies dTc/dσi are deduced thermodynamically from the observed discontinuous changes in αi at Tc. The uniaxial stress effects along the a and c crystallographic directions are dTc/dσa=−5.78±1.6 K/GPa and dTc/dσc=+11.66±3.5 K/GPa, respectively. Their sum agrees well with the measured dTc/dP near zero. Thus, a decrease in the interlayer spacing is found to have a strong positive affect on Tc, and a decrease in the intralayer atomic spacing has a strong negative influence on Tc.

In a previous paper [1] we described the influence of Ce-doping on the distribution of the electron density in the HTSC-compound Nd2-xCexCuO4-delta. The main feature of the doping which appeared on the electron density maps is the rearrangement of the charge density at the Cu position. Also the formation of a strong positive charge density maximum at the Nd/Ce position could be observed. By cooling down the sample from 300 K to 25 K this maximum splits up. Further calculations to explain this behaviour have been carried out.

Langmuir-Blodgett (LB) films of fatty acid salts (Y-stearate, Y-arachidate, Cu-arachidate, Ba-stearate) were thermally oxidized. As a result one obtains ultrathin (a few Å thick) metal-oxide films at the substrate. The surface of the metal-oxide films was found to be rather inhomogeneous. Y and Cu ions remain quantitatively at the substrate despite the heating procedure. A linear dependence between area density of the metal ions in the oxidized films and the number of monolayers of the LB films was observed. The preparation of a mixed metal-oxide film containing Y, Ba, Cu with a given stoichiometry was found to be difficult due to the effect of counter ion exchange. The samples were investigated by means of plasma-desorption and spontaneous-desorption mass spectrometry, by Rutherford back-scattering and electron microscopy.

Ultrathin ferric oxide films (10(15) Fe ions cm(-2)) were prepared from Langmuir-Blodgett (LB) films of Fe arachidate. The LB films were heated for this purpose to a temperature of 300 degrees C. It turns out that the Fe ions remain quantitatively at the substrate during the heating procedure and that a linear dependence exists between the Fe area density in the ferric oxide film and the number of Fe arachidate monolayers. The homogeneity of the ferric oxide films was found not to be satisfactory if LB films with a small number of monolayers are used. The homogeneity increases with increasing number of monolayers. The analysis of the films was performed with mass spectrometric methods, Rutherford backscattering spectroscopy and electron microscopy.

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.

A fullyc-axis oriented thin film of YBa2(Cu0.9857Fe0.02)3O6.8 prepared by planar dcsputtering has been investigated by means of Mössbauer spectroscopy. Room temperature57Fe conversion electron Mössbauer spectra taken at different angles between the γ-ray direction and the normal (=c-axis) of the film show four subspectra: A (quadrupole splitting ΔEQ≈1.9 mm/s), B (ΔEQ≈1 mm/s), C (ΔEQ≈0.5 m/s) and D (ΔEQ≈1.6 mm/s). For subspectra A, B and C, we found the same hyperfine parameters as already published on other samples. The hyperfine parameters for subspectrum D are determined for the first time using a fully oriented sample. For D, we found the asymmetry parameterη ≥0.6 andVzz (the main component of the electric field gradient) lying in the a-b-plane.

Changes in the distribution of electron density in single crystals of Nd2−xCexCuO4−δ caused by Ce-doping have been studied using X-ray diffraction data. The substitution of Nd by Ce changes as the electronic state of Cu as the whole picture of the electron density in crystals.

A fully c-axis oriented thin film of YBa2((Cu0.98Fe0.02)-Fe-57)(3)O-6.8 prepared by planar dc-sputtering has been investigated by means of Mossbauer spectroscopy. Room temperature Fe-57 conversion electron Mossbauer spectra taken at different angles between the gamma-ray direction and the normal (= c-axis) of the film show four subspectra: A (quadrupole splitting Delta E(Q) approximate to 1.9 mm/s), B (Delta E(Q) approximate to 1 mm/s), C (Delta E(Q) approximate to 0.5 mm/s) and D (Delta E(Q) approximate to 1.6 mm/s). For subspectra A, B and C, we found the same hyperfine parameters as already published on other samples. The hyperfine parameters for subspectrum D are determined for the first time using a fully oriented sample. For D, we found the asymmetry parameter eta greater than or equal to 0.6 and V-zz (the main component of the electric field gradient) lying in the a-b-plane.

Stoichiometrically optimized, epitaxial SmBa2Cu3O7-delta thin films with high T-c,T-R=0 and high critical current densities j(c) have been prepared for the first time in a tightly controlled molecular beam epitaxy process in non-reactive molecular oxygen, followed by an in situ loading process with molecular oxygen. The surface roughness (on a submicrometre scale) of single-crystal films with their c axes perpendicular to the surface depends markedly on the surface temperature of the substrate during the deposition of the epitaxial films, within a range of only a few degrees centigrade. The calibrated optimal temperature for the preparation of epitaxial films 200 nm thick of this single orientation is found to be 680 +/- 5 degrees C. In scanning tunnelling microscopy investigations, they show a surface roughness of less than 6 nm (five SmBa2Cu3O7-delta unit cells) on a 2 mu m x 2 mu m scale, At deposition temperatures below this optimal deposition temperature, the well-known a-axis growth increases rapidly, whereas higher temperatures give a significantly higher surface roughness, which can be observed by scanning electron microscopy.

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.

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.

The perturbed angular correlation (PAC) method was applied to study the electron-doped HTSC Nd2−xCexCuO4 (NCxCO) forx=0, 0.06, 0.1 and 0.15.111In was implanted at 400 keV into sintered bulk samples. After annealing the radiation damage at 1223 K in air, PAC spectra were taken at measuring temperatures between 15 and 823 K. Point charge model (pcm) calculations support the assumption that111In replaces a Nd-ion. The width of the main frequency depends on the amountx of Ce-doping. The magnitude of the observed electric field gradient (efg) can be explained if the111In probe catches simultaneously an oxygen vacancy at the O(2)-site.

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.

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