Albert Comberg

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.

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.

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.

Magnetoacoustic determination of pinning energies in high-Tc superconductors

Abstract By means of the recently established ultrasonic technique, activation energies for the vortex lattice of various high- T c superconductors are determined. These are deduced from the characteristic maximum in the field induced attenuation which is associated with the depinning of the flux line lattice from the superconductor. It is demonstrated that the resulting energies are basically the same as those determined by resistive measurements on single crystals, even though the ultrasound experiments are performed on ceramic bulk samples.

Anomalous poisson ratio and bulk modulus in high-Tc superconductors

Abstract Longitudinal and transverse sound velocity and attenuation measurements as function of temperature and frequency have been carried out in well characterised ceramic YBa2Cu3Ox samples of different oxygen content but the same controlled microstructure. At the occurrence of superconductivity we found a pronounced steplike behaviour in both bulk modulus and Poisson ratio. Together with a general decrease towards higher oxygen content an effect of 15 % is observed. Both effects are discussed in connection with the mixed valence of the Cu-3d electrons. Thermally activated relaxation processes in the sound attenuation are systematically studied in dependence of oxygen content. The derived activation energy shows a dependence on oxygen content which is attributed to oxygen vacancy ordering in the basal plane.

Lattice specific heat of superconducting amorphous indium films

The specific heat of amorphous and crystallized In(+ Sb) films has been measured in the low-temperature range from 0.6 K to 5 K. In the amorphous phase the lattice specific heat is enhanced by a factor of 3.5, but no indication of a term linear in temperature is found. The results are compared with calculations of the lattice specific heat fromα2F(ω) spectra obtained from electron tunneling measurements.