I. Abrego Castillo

Coherence transition in granular high temperature superconductors

Abstract We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa 2 Cu 3 O 7− δ , YBa 2 (Cu 2.98 Zn 0.02 )O 7− δ and GdBa 2 Cu 3 O 7− δ . The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T co , which is close to the point where resistivity vanishes. The specific heat results, when analyzed as d C /d T , show a weak but reproducible cusp-like anomaly at T co . This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.

Magnetic and magnetoresistive properties of the series Ce(Ni1-xCox)2B2C

Abstract Structural, magnetic, calorimetric and magnetoresistive studies on the solid solution Ce(Ni 1- x Co x ) 2 B 2 C demonstrate that the Co-substitution decreases the unit-cell volume, the Sommerfeld constant γ, and the Pauli susceptibility χ 0 . It increases the Debye temperature and the strength of the crystalline electric field. The structural modification can be related to the variation of the tetrahedral angle subtended by the B-(Ni, Co)-B bonds: a decrease in the tetrahedral angle contracts the parameter a and elongates the length c and vice versa. The variation in the magnetic and electronic properties can be traced down to the shift of E F (predominantly due to the less filling of the 3d-band) away from the peak maximum of the density of state and the consequent reduction of N ( E F ). The influence of the variation of the band filling and the f-d hybridization on the magnetic behavior of CeT 2 B 2 C will be considered.

Dimensional crossover in the irreversibility line in (Bi, Pb)2Sr2Ca2Cu3O10

We report on magnetic measurements in the vicinity of the irreversibility line showing a field-induced dimensional crossover in the vortex ensemble in (Bi, Pb)2Sr2Ca2Cu3O10. The temperature dependence of the irreversibility line changes its behaviour at a crossover field, Hcross = 3.0±0.5 kOe, observed in both DC magnetization and AC susceptibility data, independently of the alternating field and frequency. This yields an estimate for the anisotropy factor in (Bi, Pb)2223, = 45±5. Magnetization fluctuation analysis shows that the applicability limit of the 3D-XY scaling model coincides with the change of behaviour in the IL, which provides further evidence for a dimensionality change in the vortex fluctuations. The pinning potential of the system also shows a change in its field dependence at the same crossover field.

Linearized description of the AC susceptibility of ceramic high- superconductors

This paper presents a linearized solution for the AC magnetic susceptibility equations of ceramic superconductors, here interpreted as an approximation to the exact equations based on the actual nonlinear E - J behaviour. The method not only provides solutions for the AC susceptibility equations in analytical form but also yields an approximation to the true E - J resistive behaviour of the sample. A detailed comparison between AC susceptibility and magnetoresistivity data is made, which leads to a clearer understanding about the meaning of the parameters in the Bean model description of the electromagnetic properties of ceramic superconductors.

Thermal expansion and specific heat of non-random YBCO/Ag composites

We report on low-temperature thermal expansion and specific heat measurements in a series of non-random composites YBCO/Ag x wt% (x = 0, 6, 11 and 23). A reference YBCO polycrystalline sample is also investigated. At the lowest temperatures, the expansivities and specific heat are analysed as sums of a extrinsic T-linear term and a -Debye contribution. The Gruneisen parameters are determined and shown to be almost constant for . The values of the Gruneisen parameters increase with the silver content in the composites.

Irreversibility line in deoxygenated YBa2Cu3O7−δ single crystal

Abstract We have measured the AC susceptibility of a YBa2Cu3O7−δ single crystal, and determined the irreversibility line through the dissipation peak in the imaginary component χ″ The sample investigated has a transition temperature Tc ≈ 62.8 K, and transition width less than 1.0 K. At low fields we may fit the results with a typical power law of the type (1-T/Tc)″, with n = 1.35, whereas at higher fields the data is better adjusted with 1/T. We observed a clear change in behavior of the IL at Hcr = 9.0 kOe, which characterizes two different regimes related to the dimensionally of the vortex fluctuations. This could be interpreted as a transition from three dimensional-vortex lines to two-dimensional pancake-like vortices. The higher anisotropy of this system, as compared with a fully oxygenated crystal, favors the decoupling of the vortices in adjacent CO2 planes.

AC susceptibility of YBaCuO-Ag composites

Abstract We have performed measurements of χ′ and χ

Specific heat of La1−xCaxMnO3−δ

in YBaCuO-Ag nonrandom composites. Results of χ′ vs. temperature show a strong reduction of intra-granular shielding in the superconducting composites. Cooling the samples in large superimposed DC fields, measurements of χ′ and χ″ at fixed temperatures as a function of AC field can be fitted with a modified critical state model. This description gives an interpretation for our results in terms of a reduction of the superconducting volume fraction in the composite samples.