C. Politis

SUPERCONDUCTIVITY AND THERMOELECTRIC POWER IN Tl2Ca2Ba2Cu3O10 AND TlCa3BaCu3O9

We report the synthesis and superconducting temperature of the high-Tc compounds Tl2Ca2Ba2Cu3O10 and TlCa3BaCu3O9. The transition to the superconducting state was measured by the magnetic susceptibility changes and by the temperature dependence of the electrical a.c. and d.c. resistivity and by thermoelectric power measurements. For Tl2Ca2Ba2Cu3O10 a superconducting transition with zero resistance at 114.5K have been observed. Higher Tc with zero resistance at 122K have been found in TlCa3BaCu3O9.

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa2Cu3O7−x with resistiveTc∼96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu3+ ions are uncorrelated, and do not affect the superconducting state.

FLUCTUATION CONDUCTIVITY EFFECTS ON THERMOELECTRIC POWER OF GRANULAR Bi1.75Pb0.25Ca2Sr2Cu3O10 SUPERCONDUCTOR

We report precise measurements of the thermoelectric power (TEP) of granular superconducting Bi1.75Pb0.25Ca2Sr2Cu3O10. The TEP is strictly linear at high temperature. Superconductivity fluctuations set in at about 140 K. From the temperature derivative of the excess TEP (with respect to a straight line at “high temperature”), the critical behavior is obtained in the mean field regime, and is found identical to that of the temperature derivative of the excess electrical resistivity.

Magnetic properties of GdBa2Cu3O7−x in the normal and superconducting states

The magnetic properties of superconducting GdBa2Cu3O7−x withTc∼95 K are measured. In the normal state, its susceptibility closely follows the Curie law with an effective moment of 7.77μB.Hc1 is determined to be 650 Oe. No superconducting fluctuation can be delineated. Flux pinning indicated behavior of type II superconductors. A 25–405 diamagnetic state indicating bulk superconductivity in GdBa2Cu3O7−x.

Paraconductivity Measurements and Comparative Studies of Fluctuations in BiSrCaCuO, YBaCuo and YBaCuO with Gd Substitution

Information concerning the dimensionality of the superconductive fluctuations in the new high temperature superconductors can be derived from the excess conductivity (or paraconductivity) near the transition temperature. Here the “lowering” of the sample resistance in the same temperature region is used for extraction of dimensionality and generally for comparative studies of the critical behaviour of three different samples such as YBa2Cu3O7, Y0.8Gd0.2Ba2Cu3O7 and Bi0.85Pb0.15SrCa1.2CU2O6. The experimental data of the three samples show a similar behaviour with some distinct differences depending rather on the “metallurgical” state of the material than the composition itself. A logarithmic behaviour is present for YBaCuO and Y(Gd)BaCuO samples not close to the critical temperature and, the most important, closer to Tc YBaCuO shows a fractal behaviour, observed for the first time, similar to the one of Bi-compound observed many times before.