S.K. Patapis

Thermoelectric power of granular ceramic oxide superconductors

Abstract We discuss the Seebeck effect or thermoelectric power of granular ceramic oxide superconductors in an effective medium approximation. We compare experimental results to the theory. We find no evidence for attributing the behavior above the critical temperature to any phonon drag. The bump above T c is simply due to electron-phonon (inelastic) scattering. The sign of the coefficient does not indicate the type of carrier. We show the necessity of theoretical work for compensated semi-metals as well.

Superconductivity fluctuations in Bi(Pb) based granular ceramics superconductors: evidence for fractal behavior

We report precise measurements of the electrical resistivity in three different Bi(Pb) based granular ceramics superconductors. We show that a single critical exponent (5/6) describes the superconductivity fluctuations. Such a critical exponent indicates a fractal behavior of the superconducting path. Our results thus indicate a strict two dimensional fluctuation percolation set (below the superconductivity onset temperature 195 K down toTc), and provide some proof for Tarascon et al. “shell conductivity path” hypothesis. We estimate the shell thickness to be of the order of 10 Å.

Evidence for anomalous fluctuations in superconducting Bi/sub 1. 75/Pb/sub 0. 25/Ca/sub 2/Sr/sub 2/Cu/sub 3/O/sub 10/

We report very precise measurements of the electrical resistivity of granular Bi1.75Pb0.25-Ca2Sr2Cu3O10 superconducting ceramics from room temperature down to the superconducting percolating temperature. We analyze the temperature behavior of the excess conductivity and of the temperature derivative variation in electrical resistivity in order to extract the superconductivity fluctuation dimensionality regimes. We obtain unusual power law behaviors which do not correspond to predictions based on Aslamazov and Larkin theory (nor to its generalizations) nor to the regimes found in YBa2Cu3O7, e.g., here, dR/dT≃E−11/6 for ln E in [−4,0] with E=|T−Tc|Tc, where Tc is the critical temperature.

Superconductivity Fluctuation Effects on Electrical and Thermal Transport Phenomena. H=0,T>TcI

The shape of the electrical resistance versus temperature curve in the vicinity of a superconductivity transition depends on the amount of superconducting materials (among other things) in the sample. The same is true for the thermoelectric power. In both cases, this amount fluctuates according to various theoretical laws. These effects are highly visible in high critical temperature superconductors. The data analysis also differs from authors to authors. We briefly review old and present theories of superconductivity fluctuation effects on transport phenomena with emphasis on the approach of the critical temperature from above. We discuss the various ways of analyzing the data on the electrical resistivity and on the thermoelectrical power. We distinguish between data on quasi single crystals, on polycrystalline materials and on films for various high critical temperature superconductors. We show the range of validity of theoretical laws, and see that homogeneous or fractal behaviors can be distinguished.

Electrical resistivity of a-Fe2O3 and comparison to TEP at the first order Morin transition

Abstract The electrical resistivity of hematite is measured in the region of the first order Morin transition and the above trasport property is compared to the thermoelectric power (TEP) in the same region. Some similarities can be found in the detailed behaviour, of the electrical resistivity which can be distinguished generally as a step-like change, at the temperature of the transition, and the behaviour of the thermoelectric power at the same temperature. The above behaviour of the two transport effects is compared to that observed in other materials at a first order transition.

Magnetic investigation of the uranium-molybdenum silicide

Abstract An investigation is made of the magnetic behaviour of the ternay intermetallic compound Uranium-Molybdenum Silicide (U 2 Mo 3 Si 4 ) by monitoring the temperature spectrum of the magnetisation and hence the magnetic susceptibility under different magnetic fields. Novel results are presented and discussed.

Thermodynamic Fluctuations and their Dimensionality in Ceramic Superconductors Out of Transport Properties Measurements

Information concerning the dimensionality of the thermal flustuations of the new high temperature ceramic superconductors, can be derived from the excess conductivity (paraeonductivity) near the transition temperature according to Aslamasov-Larkin (AL) theory. Here the above dimensionality is derived from the excess resistivity and thermoelectric power measured in different samples of HTS materials such as YBaCuO and different Bi (PB) SrCaCuO compounds.

THE EPR STUDIES OF HIGH-Tc SUPERCONDUCTORS (Bi, Pb)-Ca-Sr-Cu-O

The high-Tc superconductors Bi1.55Pb0.25Sr2Ca2.2Cu3O10, Bi1.75Pb0.25Sr2Ca2Cu3O10 and Bi1.75Pb0.25Sr2Ca3Cu4O12 (with about Tc=106, 103 and 101 K, respectively) have been investigated using EPR method. Differences in intensities of the EPR spectra of Cu2+ ions have been observed. These differences could be caused by the skin effect.

Two thermomagnetic effects and the electronic thermal conductivity in p-GaSb

Abstract Two thermomagnetic effects are studied in a single crystal ϱ-GaSb. The influence of a weak magnetic field, up to 20 kG, upon the thermoelectric effect and the thermal conductivity of the semiconductor for different temperatures is investigated. From the measurement of the change of the above transport properties in a magnetic field conclusions concerning their field and temperature dependence, the r (exponent in scattering law) and the electron component of the thermal conductivity and its dependence on temperature have been derived.

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.