F. Miguélez

Probing thermodynamic fluctuations in high temperature superconductors

Abstract We probe thermodynamic fluctuations in HTSC by measuring the excess electrical conductivity, Δσ, abovr T c in single-phase (within 4%) Ba 2 LnCu 3 O 7−δ compounds, with LnY, Ho and Sm. As expected, the measured relative effect, Δσ / σ (300 K), is much more important in HTSC than for low-temperature superconductors (at least one order of magnitude). In the reduced temperature region −5 ϵ ≡ ln [ (T-T c )/T c ] Δσ / σ ( 300 K ) = Aϵ x , where A is a temperature-independent amplitude. x is found to be similar for all compounds, with average value =−0.47 ± 0.06. This result confirms an universal critical behaviour of Δσ in HTSC, and the value of agrees with that predicted by the Aslamazov-Larkin (AL) theory for three-dimensional BCS superconductivity. However, A shows a normal conductivity dependence which is not accounted for by the AL theory.

AC and DC electrical resistivity measurements in Ba2HoCu3O7−δ compounds

Detailed AC and DC electrical resistivity measurements in Ba2HoCu3O7- delta samples are reported. Although all samples have a very similar transition temperature to the superconductor state (about 92 K), their normal resistivity shows in some cases important differences. In the whole frequency range examined (up to 2 kHz), their AC resistivity is frequency independent and qualitatively similar to the DC resistivity. The behaviour of resistivity around Tc seems not be affected by the presence of the magnetic holmium ions. The excess conductivity above Tc strongly depends on the properties of the samples in the normal state.

Influence of preparative conditions on Tc of superconducting Bi2SrCa2Cu2O8

Abstract Four polycrystalline samples of composition Bi2Ca2SrCu2O8 have been prepared from nominal mixtures of analytical grade Bi2O3, CuO and either Ca(OH)2 and Sr(OH)2 · 8H2O (samples A and B) or CaCO3 and SrCO3 (samples C and D). The mixtures were ground, heated in air at the same temperatures from 800 to 850°C, sintered (samples A and C were quenched; samples B and D were slowly cooled) and pelletized. Electrical resistivity measurements by a four-probe method showed that all the samples were superconducting with zero resistivity at 20, 55, 35 and 65 K for A, B, C and D, respectively. The higher Tc corresponds to the sample that was prepared from carbonates and slowly cooled. The resistivity rounding above Tc is consistent with thermodynamic fluctuations in two dimensions of an order parameter of two components. X-ray diffraction data for sample D are given. It is orthorhombic, S.G. Amaa (No. 66, Cccm), Z = 4, a = 5.389 (3), b = 5.408 (3), c = 30.55 (2) A , and shows a clear tendency to preferential orientation in the c direction.

Influence of the a.c. and d.c. electrical current density on the normal-superconductor transition in high-temperature superconductors☆

Abstract Detailed a.c. and d.c. measurements of the electrical resistivity, ρ, of Ba2Ln1Cu3O7 − δ(Ln ≡ Y, Ho) polycrystalline samples, using a current density, J, spanning the range 4 × 10−3A cm−2⩽ J ⩽ 2 A cm−2 are reported. In particular, the resistivity behaviour near and above the superconducting critical temperature Tc is shown to have no dependence on the current density. This finding gives further support to the theory that fluctuations are responsible for the resistivity rounding above Tc and suggests that no percolative process is involved.

Measurements of pool boiling heat transfer from ceramic Y1Ba2Cu3O7−δ with different coating to liquid nitrogen.

Abstract We present measurements of pool boiling heat transfer from ceramic Y 1 Ba 2 Cu 3 O 7−δ (YBCO) samples to liquid nitrogen for low heat flux densities, q e , up to 9 W/cm 2 . To analyze the influence of the surface treatment on the heat transfer characteristics, some of the samples were coated with a Ag film that closes most of the microchannels at the surface. We observed that the heat transfer process is less effective for the coated samples, so circulating fluid within the superficial microchannels may have a non negligible role in the refrigeration mechanism of these materials.

A phenomenological model for the rounding of the electrical conductivity above Tgco in polycrystalline copper oxide superconductors

Abstract The amplitude and temperature behavior of the electrical resistivity of ceramic superconductors above the mean-field critical temperature of the individual grains is explained by combining the current phenomenological ideas on the electrical conductivity in granular superconductors with the effects of the fluctuations of the superconducting order parameter amplitude. As an example of application, we analyze the phase-breaking time in ceramic YBCO samples.

Critical current density on Bi(2212) samples with a needle-like grain microstructure

Abstract In an early work we have reported that a simplified version of the melt-textured growth (MTG) process on Bi(2212) leads to a microstructure consistent in superconducting needle-like grains. The size of the resulting needles is about 1.5 mm in length and 100 μ in diameter. The internal structure of these needles seems to be the result of the melting of the typical Bi(2212) plate-like grains along a longitudinal axe, so the superconducting ab plane must run continuously along the needle length. The critical temperature, deduced from the magnetisation curve, is about 91K. Now, we have performed critical current density and resistivity measurements on single needle-like grains to ensure its practical application possibilities for making superconducting Ag-sheated wires and tapes. The critical current density at 77.4 K in absence of magnetic field of a single needle is about 2500 A/cm 2 .

Needle-like microstructure in Bi2Sr2CaCu2O8+x

We have obtained superconducting Bi2Sr2CaCu2O8+x ceramic compounds with a grain structure that consists of needle-like particles of about 1.5 mm in length and 100 μm in diameter, which seem to be formed by flake-like small grains. The Bi(2212) compound has been prepared following the polymer solution synthesis method. The thermal treatment used is a simplified version of the melt-textured growth process (MTG), with the sample placed in a zone of a simple tube furnace where the temperature gradient was about 15°C/cm. We have performed scanning electron microscopy (SEM) observations on the samples, before and after thermal treatment. In addition, we have measured the ac susceptibility of both samples to confirm their superconducting nature and critical temperatures of about 91 K.The authors have obtained superconducting Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x} ceramic compounds with a grain structure that consists of needle-like particles of about 1.5 mm in length and 100 {mu}m in diameter, which seem to be formed by flake-like small grains. The Bi(2212) compound has been prepared following the polymer solution synthesis method. The thermal treatment used is a simplified version of the melt-textured growth process (MTG), with the sample placed in a zone of a simple tube furnace where the temperature gradient was about 15{degrees}C/cm. The authors have performed scanning electron microscopy (SEM) observations on the samples, before and after thermal treatment. In addition, they have measured the ac susceptibility of both samples to confirm their superconducting nature and critical temperatures of about 91 K.

Microstructural evolution through a thermal treatment to obtain needle-shaped grains of Bi2Sr2CaCu2O8 + x

Abstract In a previous paper, we presented a novel grain structure for Bi(2212), which consists of needle-shaped grains of around 1 mm length and 100 μm diameter (Cabeza O.,

Measurements of the fluctuation-induced conductivity, magnetoconductivity and diamagnetism of an Untwinned Y1Ba2Cu3O7−δ single crystal: absence of appreciable pair-breaking effects

Abstract Measurements of the fluctuation-induced conductivity, Δσ a , magnetoconductivity, Δ σ a , and diamagnetism, Δ X ab , of an untwinned YBa 2 Cu 3 O 7−δ single crystal, with the applied magnetic field perpendicular to the ab-planes, are presented. These results suggest the absence of appreciable Maki-Thompson effects in copper-oxide superconductors.