E.-W. Scheidt

Observation of a lambda-shaped ultrasonic attenuation peak in superconducting UPt3

Abstract We report measurements of the temperature dependence of the ultrasonic attenuation in superconducting UPt3. In the vicinity of the superconducting transition temperature a pronounced longitudinal attenuation peak is observed. Such a peak does not appear for transverse ultrasound. Our experimental results can be explained by a “correlation hump” of the density-density correlation function.

NQR investigations of the high-Tc superconductor Ba2YCU3O7

Abstract NQR measurements of 63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba2YCu3O7. An anomalous behaviour in the vicinity and below Tc is observed: after a steep decrease at Tc the quadrupole frequency increases with decreasing temperature.

Magnetic penetration depth in oriented YBaCuO powder samples

Abstract For oriented YBa2Cu3O7-δ powder samples we report for what we believe to be the first time on measurements of the temperature dependence of the magnetic penetration depth and of the lower critical magnetic field Bc1(T). The results are found to agree with theoretical expressions derived for weak-coupled BCS-type superconductors in the clean local limit. From the lower critical field values Bc1∣(0) = (340 ± 50)G and Bc1⊥(0) = (113 ± 30)G the magnitude of the anisotropic penetration depth has been determined yielding λ ∣ (0) = (1400 ± 200) A and λ ⊥ (0) = (6100 ± 1000) A .

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.

Specific heat of amorphous U−Fe, U−Co and U−Ni-alloys

Alloys of U-X with X=Fe, Co, Ni were prepared by rapid quenching in the concentration range between 50 and 83 at% U. The samples were obtained in the amorphous state as determined by X-ray diffraction. Parts of the samples were used to measure the specific heat by the relaxation method. The calculated density of states is twice as large as that of transition metals like amorphous Zr-alloys. Therefore these results are discussed within the frame of the paramagnon model by taking into account values of the magnetic susceptibility and the supercon-ducting transition temperature.

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.

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.

Critical field, susceptibility and NQR measurements on the high-Tc superconductor Y-Ba-Cu-O

Abstract We have prepared high temperature superconducting compounds Y2−xBaxCuO4−δ and YBa2Cu3O7. Superconductivity was detected resistivity as well as inductively with onset temperatures up to 100 K. The critical magnetic field was determined up to 8 T by means of a superconducting magnet. For YBa2Cu3O7 a slope -(dBc2/dT)Tc of about 2.3 T/K was found. Furthermore, NQR measurements were performed on YBa2Cu3O7. A steep decrease of the nuclear quadrupole frequency at Tc is observed followed by an increase with decreasing temperature.

Superconducting phases in UPt3

Abstract A brief discussion is given concerning the superconducting properties of UPt3 based on extensive ultrasonic investigations. Unsuccessful attempts in deriving a consistent picture for the complex behaviour observed, lead to the suggestion that not only superconductivity but also other types of ordering processes should be involved.