Siegfried Ewert

Ultrasonic investigations on superconducting YBa2Cu3O7-δ samples of different microstructure

Abstract Ultrasonic velocity and attenuation measurements down to 4.2 K were performed on well characterized ceramic superconducting YBa 2 Cu 3 O 7-δ samples of different microstructure. In fine grained samples of high volume density and excellent superconducting properties no softening in the sound velocity and no thermal hysteresis were found. A broad maximum at about 180 K and an anomaly near T c were observed in the ultrasonic attenuation.

Evidence for Three-Dimensional Magnetic Ordering in the High-Tc Superconductor ErBa2Cu3O7

Neutron diffraction experiments on polycrystalline samples of ErBa2Cu3O7 show that the Er moments are ordered three-dimensionally at T = 140 mK. Two independent propagation vectors k1 = (1/2, 0, 0) and k2 = (1/2, 0, 1/2) are required to describe the magnetic reflections. The most likely model is one in which all the moments are inclined at an angle of 32° to the crystallographic c-axis and the magnitude of the ordered moment at 140 mK is 4.9(2)μB. The two propagation vectors indicate the existence of both ferromagnetic and antiferromagnetic coupling between moments along the c-axis.

Ultrasonic attenuation by the vortex lattice of Bi1.6Pb0.4Sr2Ca2Cu3Oy

Abstract The ultrasonic attenuation and velocity shift, induced by the vortex lattice of high- T c superconductors, are measured as a function of field, frequency and temperature. Two different ceramic samples, both of the Bi-2223 composition, are investigated: One is an isotropic and the other one a strongly textured sample. For the latter, eight different modes are analysed, differing from each other by the relative orientation of the wave vector, the displacement vector of sound and the applied magnetic field. The field induced attenuation shows a distinct peak and the sound velocity a step at the same temperature. These effects are explained by a transition of the flux line lattice from a pinned into a depinned state. The results can be described quantitively by a simple model based on flux line diffusion.

Acoustic measurements on high-tc superconductors in the systems YBaCuO and Bi,PbSrCaCuO☆

Abstract Longitudinal and transverse sound velocity and attenuation measurements as a function of temperature (4.2–280 K) and frequency (3–30 MHz) have been carried out in compounds of the type YBaCuO and Bi,PbSrCaCuO. The elastic properties of the ceramic samples, characterized by bulk modulus and Poisson ratios, are strongly affected by the microstructure (grain size, density) and the oxygen stoichiometry; YBa2Cu3O7 − δ samples of different δ values have different Poisson ratios (νortho Anelastic processes in the absorption were found in orthorhombic and tetragonal YBaCuO samples. A single thermally activated Debye process was observed with an activation energy H = 90 meV and an attempt frequency ν0≈ 1011 Hz. This was derived from the line width of the absorption peaks and the shift of the absorption maximum with frequency.

Ultrasound study of YBa2Cu3O7−δ samples of different microstructure

Abstract Investigations of elastic properties were performed on ceramic single-phase YBa 2 Cu 3 O 7−δ samples. The temperature dependence of the sound velocity and attenuation between 260 K and 4.2 K depends strongly on the microstructure of the samples. In constrast to fine grained samples both softening in the sound velocity and thermal hysteresis were found in coarse grained samples.

Messung der spezifischen Wärme und der Fehlordnungsenergie von amorphen Blei-Wismut-Schichten

Amorphous Pb-Films with a 30 atomic-% Bi-content are produced by evaporation onto a cooled Be-foil at 20 °K in high vacuum. The specific heat of these films in the temperature range from 5–8 °K is found to be 1.6 times larger than after crystallization, which takes place during warming up between 30 and 40 °K. The stored energy released during this crystallization amounts to 1.5 kcal/mol.

Superconductivity of amorphous and disordered crystalline indium films

Abstract Amorphous In(+Sb) films are prepared by evaporation onto a liquid helium cooled substrate. The Eliashberg function α 2 F (ω) and the related superconducting parameters are determined by electron tunneling for the amorphous phase and after annealing for subsequent crystalline states.

Ultrasonic attenuation in high-Tc superconductors: a new approach to the problem of flux pinning

Measurements of the transverse sound attenuation and velocity in ceramic YBa2Cu3Ox superconductors were performed with and without a magnetic field. We observe a pronounced peak in the field-induced attenuation and an increase of the velocity at low temperatures. We attribute these effects to the transition of a rigidly pinned flux line lattice into a depinned state at higher temperatures and give a quantitative explanation in terms of thermally assisted flux flow. Depinning temperatures show a drastic dependence on the oxygen stoichiometry.

Übergangstemperatur zur Supraleitung, Energielücke und Widerstandsverlauf amorpher und kristallisierter Thallium-Tellur-Schichten

Thin Tl/Te-films are condensed onto a cooled substrate at 4 K. An amorphous phase is obtained. The maximum superconducting transition temperature of the amorphous Tl/Te-films is 4.2 K. Measurements of the energy gap by superconducting tunneling yield a ratio 2Δ/k Tc=4.6 for amorphous Tl/Te-films and 2Δ/k Tc=3.8 for disordered, pure Tl-films.

Messung der spezifischen Wärme von abschreckend kondensierten Indium-, Blei- und Wismut-Schichten bei tiefer Temperatur

Thin metal films are evaporated onto a cooled Be-foil at 25–32 °K in high vacuum. After condensation the films have a large degree of disorder. The specific heat is measured in the temperature range from 5–8 °K after condensation and after annealing. No differences in the specific heat are found for disordered and annealed In- and Pb-films. The specific heat of amorphous Bi-films, however, is 1.7 times larger than that of crystalline films. The results are discussed together with investigations of disordered films by superconducting tunneling.