H. Riesemeier

INFLUENCE OF HYDROGEN ON THE 89Y NMR IN HXYBA2CU3O7

The ambient-temperature {sup 89}Y magnetic resonance line in H{sub {ital x}}YBa{sub 2}Cu{sub 3}O{sub 7} was measured at 14.707 MHz as a function of hydrogen concentration {ital x}. The Knight shift relative to an aqueous solution of YCl{sub 3} increases from {minus}100 ppm for {ital x}=0 to +115 ppm for {ital x}=1.82. The linewidth at half-amplitude is 1.2 kHz and increases slowly with {ital x} in the superconducting samples (0{lt}{ital x}{lt}0.74), then increases sharply in the normal samples. It was seen that, for a number of parameters, increasing the hydrogen content gives similar behavior to decreasing the oxygen concentration. Our {sup 89}Y NMR data and those of other investigators are analyzed and are consistent with previous evidence for local antiferromagnetic correlations in the superconducting compound.

NMR investigations on oriented grains of superconducting YBa2Cu3O7

Oriented powdered samples of YBa2Cu3O7 were prepared by exposing the mechanically vibrated powder at 4.5 K to a magnetic field of 1.2 T. NMR experiments on 63,65Cu were performed and the results are compared with those of non-oriented samples. The data confirm the assignment of νQ1 ≅ 22 MHz to the Cu(1) and νQ2 ≅ 31.5 MHz to the Cu(2) sites. By means of the less common low-field NMR technique (νLarmor < νQuadrupole) the asymmetry parameter at the Cu(2) sites was determined to be η < 0.01, and at 100 K the Knight shift in the (a,b) plane was found to be K⊥ = (0.35 ± 0.1)% which is about a factor 2 smaller than the values reported in literature so far. Using published NMR data for 63Cu in Cu2O and in antiferromagnetic YBa2Cu3O6, our results are indicative for fluctuations of the Cu(1) ions between the ionic states Cu+ and Cu3+, and of the Cu(2) ions between Cu+ and Cu2+.

Evidence of local magnetic order in the superconducting phase of hydrogen doped YBa2Cu3O7 using nuclear magnetic resonance

Abstract Nuclear magnetic resonance absorption traces were obtained for the 1H line in H0.11YBa2Cu3O7 as a function of temperature. The superconducting temperature Tc is 93K. A spectral splitting was observed which we attribute to local antiferromagnetism. The Neel temperature is approximately 320K.

Comparison of the magnetization behaviour in conventional and high-Tc YBaCuO superconductors

Abstract For various magnetic field strengths and different cooling and heating cycles we have investigated the temperature dependence of the magnetic susceptibility of YBa2Cu3O7−δ and of the conventional superconductors V3Si and non-annealed Nb. It is shown that both types of superconductors exhibit a “hard type II” magnetic behaviour.

Possible relationship between Cu-NQR frequencies and Cu valence states in CuO2-planes for high-T c superconductors

We report on63Cu-NQR frequency νNQR measurements on various high-Tc superconductors. An empirical relationship betweenTc and νNQR is presented. We attribute this finding to variations in the valence state of the in-plane Cu ions which, due to the unusual electric polarizability of the O2− ions, depends sensitively on the Cu−O distance.

Susceptibility and NQR investigations on NaCuO2

Abstract Crystalline samples of NaCuO 2 have been synthesized and characterized by magnetic susceptibility and 23 Na-NMR investigations. Furthermore, a 63 Cu-NQR line at 37.48 MHz (4.2 K) could be detected. This frequency is higher than all 63 Cu-NQR frequencies found so far in high- T c superconductors, which is in coincidence with the high formal valence of 3+ in NaCuO 2 .

Influence of 5f electrons on transport properties in uranium-based metallic glasses

Abstract In order to study the influence of 5f electrons on the superconducting and the magnetic properties of uranium glasses, X-ray, upper critical magnetic field and susceptibility measurements were performed on three binary metallic glass systems: U-Fe, U-Co and U-Ni. For each alloy system a continuous range of uranium-rich compositions prepared by splat cooling has been examined. It is found that these samples have ferromagnetic inclusions which may be due either to ferromagnetic impurities or to the formation of clusters with high transition metal densities. The superconducting transition temperature is reduced by strong spin fluctuations. Both the superconductivity and the spin fluctuation are due to the presence of the 5f electrons.

Influence of magnetic fields on NQR line shape parameters in oriented YBa2Cu3O7−δ powder samples

63Cu nuclear quadrupole resonance (NQR) experiments on the Cu(1) site of oriented YBa2Cu3O7−δ powder samples are performed at 4.2 K. The application of magnetic fields during the cooling procedure or after zero-field cooling influences the NQR lineshape parameters, i.e. linewidth, intensity, and amplitude. The NQR measurements for different magnetic states of the samples are presented together with corresponding measurements of the remanent magnetic field.

Temperature dependence of Cu-NQR frequencies and linewidths of high-Tc superconductors

Abstract The temperature dependence of Cu-NQR spectra was investigated at both Cu sites in several YBa2Cu3O7 samples. All lines exhibit a strong broadening. Furthermore, the NQR frequency at the Cu(1) site shows an anomaly around Tc which is most likely of dielectric origin.

Resistivity and superconductivity of uranium based metallic glasses

Amorphous U-Fe, U-Co and U-Ni alloys were prepared by splat quenching over the uranium concentration range of 50 – 83 at%. The temperature dependence of the electrical resistivity, ρ (T), was investigated in the range of 4.2 – 300 K. The results below the Debye temperature ( θ ≃ 165 K) may be explained within the frame of the extended Ziman theory and the paramagnon model. The concentration dependence of ρ and of the superconducting transition temperature T c might be explained by the variation of the d-electron contribution.