S.K. Agarwal

Mo¨ssbauer studies of Sn in the high T c superconductor YBa 1.8Sn 0.2Cu 3O 9−y

Abstract Mo¨ssbauer measurements have revealed that the Sn in YBa 1.8 Sn 0.2 Cu 3 O 9−y occupies two sites, possibly those of Ba and Y. Marked variations in the intensity of one of the resonance lines, most probably corresponding to Sn in Ba sites, are observed in the superconducting transition region 115 to 82 K, indicating changes in the lattice rigidity. This implies the important role of phonons in the superconductivity phenomenon, which seems to occur in the Ba—Cu—O cuboctahedron.

High temperature superconductivity in Y-Ba-Cu-O and Y-Ba-Sr-Cu-O systems

Abstract Mixed metal oxides Y-Ba-Cu-O and Y-Ba-Sr-Cu-O have been prepared in varying compositions and by changing process parameters. Superconductivity with critical temperatures in the range of 70 to 100 K is reported.

Superconductivity of Y Ba2 Cu3 O9−y containing zirconium

Abstract The effect of Zr incorporation on the superconductivity of YBa 2 Cu 3 O 9−y has been investigated. A systematic study made to replace the three cation sites reveals that the partial replacement of Y and both Y and Ba by Zr results in the formation of superconducting multiphases, whereas Cu replacement leads only to the single superconducting phase of usual 123 composition. The data obtained indicate that variation in the Cu 2+ Cu 3+ ratio does not affect superconductivity of these compounds significantly.

Electroluminescence in high Tc Y-Ba-Cu-Zr-O superconductors

Abstract The phenomenon of electroluminescence in high Tc ceramic superconductors has been investigated for the first time. The electroluminescent brightness has been studied as a function of excitation voltage and frequency for various samples of Y-Ba-Cu-Zr-O of different compositions. Interestingly, only those samples which showed superconductivity exhibited luminescence, suggesting the possible importance of both copper and oxygen stoichiometry in the two phenomena.

Observation of inverse a. c. Josephson effect in bulk Y-Ba-Sr-Cu-O up to room temperature

Microwave induced d.c. voltage due to inverse a.c. Josephson effect has been observed across bulk samples of multiphase Y2BaSrCu3O8. Results indicate that weakly coupled superconducting grains probably exist up to 26°C.

Observation of inverse a.c. Josephson effect in bulk Y-Ba-Cu-O possessing highT c

Microwave-induced d.c. voltage due to inverse a.c. Josephson effect has been observed across bulk samples of Y-Ba-Cu-O. Variation of the d.c. voltage with small external magnetic field and temperature has been investigated. Our results indicate that weakly coupled superconducting grains exist up to 230 K.

Superconductivity in Pr-doped

Substitution of Pr at the Ca site in , with , has been reported. Pr is shown to substitute in the (BiPb)-2223 system, albeit with a simultaneous formation of Pr-doped (BiPb)-2212 phase in the samples. The x-ray analysis indicates that the latter phase grows continuously with increasing Pr concentration up to x = 0.4. For only Pr-doped (BiPb)-2212 phase is observed in the samples. The superconductor transition temperatures measured resistively shift to lower temperatures, with increasing Pr, at a rate of 5 K/(% Pr). The normal-state resistivity value and its temperature dependence are found to depend on the amount of Pr doping. We have further analysed the resistivity data of the pure and 5% (x = 0.1) doped samples for the fluctuation conductivity. Both samples exhibit 2D behaviour for the superconducting order parameter in the critical regime, within the Aslamazov-Larkin formulation.

Study of inverse a.c. Josephson effect in Y–Ba–Cu–O and Y–8a–Sr–Cu–O

Abstract Microwave induced d.c. voltage due to inverse a.c. Josephson effect has been observed across bulk samples of Y-Ba-Cu-O and Y-Ba-Sr-Cu-O. The d.c. voltage is found to vary with microwave power, frequency and also with small external magnetic fields. Although the resistivity curve of Y-Ba-Cu-O does not show any appreciable resistance drop around 230 K, the microwave induced d.c. voltage due to the inverse a.c. Josephson effect has been found to exist upto 230 K. The resistivity behaviour of Y-Ba-Sr-Cu-O shows a sharp resistivity drop above 230 K. In this sample the inverse Josephson effect is found to exist upto +26 °C, indicating the presence of a phase having a superconducting onset around this temperature.

Thermoelectric power studies on Nd1.82−xSrxCe0.18CuOy:x≤0.18 superconductors

Thermoelectric power (S) studies on a Nd1.82−xSrxCe0.18CuOy:x≤0.18 superconducting system in the temperature range 35–250 K are reported here. In the x=0.09 sample, synthesized in the reduced environment, the small magnitude of S is highly metalliclike and its sign is negative, a characteristic of electron conduction. The sign of S for the x=0.18 sample shows a crossover below 75 K from negative to positive, in apparent conflict with electronic conduction. Interestingly, after oxygenation this sample exhibits a broadened but positive phonon draglike peak. This oxygenated sample shows overcompensation of the carrier (electron) concentration. Critical analysis of the data suggests that Sr doping seemingly causes a competition between electron‐ and holelike conduction. The slope dS/dT is, in general, negative suggesting that the main contribution is coming from the diffusive part. The observed thermopower features seem to fall in line with the theoretical curves of Durczewski and Ausloos [Z. Phys. B 85, 59 (...

Temperature dependence of microwave loss signal in hafnium doped YBaCuO

Abstract The microwave loss signal in Hf doped YBaCuO as a function of temperature is investigated. The parameters of the signal like peak-to-peak height, peak position and area under the hysteresis curve are observed to decrease from LNT to T c and to vanish at T c . The results are interpreted in terms of a Josephson junction network picture. However, the decreasing amount of trapped flux with increasing T is difficult to understand in terms of the available model for type II superconductors.