P. Vašek

Influence of combined mechanical and heat treatment on Bi2Sr2Ca2Cu3O10+γ phase formation

Abstract A comparative study of the influence of mechanical and heat treatments on the mechanism of Bi 2 Sr 2 Ca 2 Cu 3 O 10+γ phase formation was carried out on samples of the nominal composition Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+γ . Uniaxial cold pressing apparently creates lattice defects which play a substantial role as centers for phase formation and exaggerated grain growth which in turn promotes c -axis alignment.

Weak links in Pb-doped BiSrCaCuO ceramic superconductors

Abstract The dependence of the resistive transition curve R ( T ) on measuring current and magnetic field along with AC and DC magnetic susceptibility χ( T ) has been measured on ceramic superconductors of the nominal composition Bi 0.7 Pb 0.3 SrCaCu 1.8 O x to elucidate the nature of weak intergranular links in the samples. These wear links are shown to be responsible for pronounced current-dependent resistive tails analogous to those reported for YBaCuO [1:2:3] ceramics. The R ( T ) curves can be interpreted in terms of percolative conduction in a multiphase system with superconducting regions coupled via nonsuperconducting barriers. External magnetic fields below 0.1 mT are shown to be sufficient for partial decoupling of the superconducting grains. A possible role of the Pb atoms in the formation of weak links resulting in a suppression of the transport critical current density J c is discussed.

Intrinsic pinning and guided motion of vortices in high-Tc superconductors☆

Abstract The anisotropy of the vortex motion has been studied on textured samples of Bi based superconductors. For some orientation of the magnetic field and transport current with respect to the preferential orientation of grains a Hall (transverse) voltage even in a magnetic field has been observed in the mixed state in the low-temperature range. The observed dependences have been discussed as a result of the guided motion of vortices due to intrinsic pinning due to the layered structure of the material under study. The increase of the temperature suppresses the anisotropy of the pinning and even the voltage disappears.

Reciprocity theorem in high-temperature superconductors

Abstract This article is devoted to the problem of the validity of the reciprocity theorem in high-temperature superconductors (HTSC). The violation of the reciprocity theorem in zero external magnetic fields has been studied. Experimental data obtained for two different superconducting materials BiSrCaCuO and YBaCuO are presented. Results show that the basic form of the reciprocity theorem (without consideration of any additional anisotropy) is not valid near the critical temperature. We assume that the breakdown of the reciprocity theorem is connected with the existence of an extraordinary transverse electric field originating from additional anisotropy and that a more general form of the reciprocity relations should be valid. However, the origin of this anisotropy is not clear yet. We suggest that the vortex–antivortex dynamics model taking into account vortex guiding can be responsible for the observed effect. Also the explanation based on weak P and T symmetry breaking in HTSC, which is supported by the observation of the spontaneous magnetisation, cannot be excluded.This article is devoted to the problem of the validity of the reciprocity theorem in high-temperature superconductors (HTSC). The violation of the reciprocity theorem in zero external magnetic fields has been studied. Experimental data obtained for two different superconducting materials: BiSrCaCuO and YBaCuO are presented. Results show that the basic form of the reciprocity theorem (without consideration of any additional anisotropy) is not valid near the critical temperature. We assume that the reciprocity theorem breaking is connected with the existence of an extraordinary transverse electric field originated from additional anisotropy and more general form of the reciprocity relations should be valid. However, the origin of this anisotropy is not clear yet. We suggest that the vortex-antivortex dynamics model taking into account vortex guiding can be responsible for the observed effect. Also the explanation based on weak P and T symmetry breaking in HTSC which is supported by the observation of the spontaneous magnetisation can not be excluded.

Structural and transport properties of YBa2Cu3−yCoyO7−x solid solutions

Abstract The X-ray study of YBa 2 Cu 3- y Co y O 7 solid solutions showed that in addition to the orthorhombic-tetragonal transition existing in the compositional range of 0.0 y y >0.4. A model based on an arrangement of microwatwins, formed owing to a tendency of cobalt ions to be six-fold coordinated, was proposed to explain the observed structural changes. A comparison of the oxygen stoichiometry and Hall charge carriers density data showed that this tendency is decisive for the charge distribution in the (Me(1)O) sublattice. Besides the geometrical arrangement, the quality of the bonds and consequent character of the transport properties is influenced by a more ionic character of cobalt ions in comparison with copper ones.

Aging effects in ceramic high-TC superconductors

Abstract Changes of the resistance R(T) and the magnetic susceptibility χ(T) due to prolonged room temperature storing in various atmospheres have been studied on YBaCuO, BiSrCaCuO and (Bi, Pb)SrCaCuO ceramic superconductors. In all the samples studied the aging resulted in a systematic increase of the resistivity above TC and in an enhancement of the current-dependent resistive tails without any appreciable change of TC itself. The degradation of samples can be interpreted in terms of a deterioration of the weak intergrain links within samples due to chemical reactions with environment. The aging effects can be substantially suppressed simply by improving the quality of samples.

Bi-2223 PHASE DEVELOPMENT UNDER INFLUENCE OF ADDITIONS OF 5B METAL OXIDES

Formation of the high temperature Bi-2223 phase has been studied for samples with nominal composition Bi3,2-xPb0.8MxSr4Ca5Cu7Oy. M represented V, Nb, Ta, their contents was altered from 0 to 0.6 mol %. The effect of the additives on the growth of the phase 2223, lattice parameters and the superconducting properties of the samples was detected. It was found that the additives enhanced the volume of the 2223 phase, improved the intergrain connection resp. the critical current density. The additives did not affect the lattice parameters and Tc significantly.

Superconductivity and electric transport of Bi2+xSr2−yCuO6+δ single crystals

A series of Bi2+xSr2−yCuO6+δ single crystals in the compositional range 0.11 < x < 0.4 and x = y was investigated by means of resistivity, thermopower and Hall effect measurements. In order to elucidate the influence of the oxygen stoichiometry the studies were carried out successively in the as-grown, oxygen and argon post-annealed crystals. The superconductivity appears in an extremely narrow composition range, 0.11 < x < 0.15. This fact is discussed in the context of the doping level and charge carrier localization ascribed to bismuth ions which occupy the Sr sites partially.

The hall effect anomaly in single phase Bi based ceramics

Abstract The temperature dependence of the Hall effect and resistivity have been measured in magnetic field up to 4T. At the temperatures near Tc anomalous behaviour of Hall effect has been observed. The temperature dependence below Tc is more complicated than that in the Y based system. The measured magnetic field dependence of this anomaly for the field in the range from 1T to 4T shows the opposite tendency in comparison with the behaviour below 1T, i.e. increase with decreasing magnetic field. Results are discussed in the framework of recent theories.

INFLUENCE OF TECHNOLOGICAL PARAMETERS ON THE QUALITY OF THE Bi-BASED SUPERCONDUCTORS

The effect of the sintering temperature, the cooling rate and the additive on the critical parameters was studied on samples with nominal composition Bi1.8(Pb0.26)Sr2Ca2Cu3O8+x both undoped and with addition of 5, 9, 12.7 mol % of boron (as B2O3), respectively. The sample with 5 mol % of boron cooled slowly had the critical current density five times higher than undoped sample. The rapid cooling of the samples caused enhancement of the volume of the 2223 phase.