J. Maza

Excess electrical conductivity in polycrystalline Bi-Ca-Sr-Cu-O compounds and thermodynamic fluctuations of the amplitude of the superconducting order parameter

Abstract Measurements of the rounding effects on the electrical resistivity above the superconducting transition in Bi-Ca-Sr-Cu-O polycrystalline compounds are reported, to our knowledge for the first time in this HTSC system. These effects are analyzed in terms of thermodynamic fluctuations of the amplitude of the superconducting order parameter (SCOPF). In the mean-field-like region, the experimental critical exponent seems to be compatible with an order parameter of two components (2d) fluctuating in two dimensions (2D). This contrasts with previous results for A-Ba-Cu-O ( A = Y , Ln ) and Ln-M-Cu-O ( M = Ba , Sr ) superconductors, where SCOPF seem to be 2d-3D in all the different dynamic critical regions.

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.

Excess electrical conductivity above Tc in high-temperature superconductors, and thermal fluctuations

The excess electrical conductivity, Delta sigma , above Tc in single-phase (within 4%) Ba2LnCu3O7- delta compounds, with Ln=Y, Ho and Sm, has been measured in the reduced temperature range 5*10-4<or approximately= epsilon <or approximately=1; this extends previous results almost one order of magnitude closer to Tc. This allows the authors to observe, for the first time, four different Delta sigma ( epsilon ) regions, with different critical exponents. These regions are analysed in terms of the Aslamazov-Larkin (AL) theory for thermal fluctuations in BCS superconductors and also using some dynamical scaling ideas. The results suggest that the Ginzburg-Landau mean-field region is located in the high-temperature range. In that case, the corresponding critical exponent is of the order of -1/2, in agreement with 3D AL superconductivity, although the strong intrinsic anisotropy, the polycrystallinity and the local defects of the samples may cause the disagreement with the AL model observed for the amplitude of the fluctuation effects.

Superconducting order parameter fluctuations above Tc in polycrystalline Ho1Ba2Cu3O7−δ compounds

Abstract We report measurements of the excess electrical conductivity, Δσ, above T c in polycrystalline HoBa 2 Cu 3 O 7−δ single-phase 0 (within 4%) compounds. The relative temperature resolution is of the order of 10 −2 K which, in spite of the broadening of the transition by nonintrinsic effects, should probably make accessible the whole mean-field regime for Δσ and also to penetrate inside the full critical dynamic region. The general behavior of Δσ(ϵ) in these Ho-based samples is very similar to that previously observed in our laboratory for Y-based high-temperature superconductors. In particular, when analyzed in terms of the Aslamazov-Larkin theory and by using some dynamic scaling ideas, the Δσ(ϵ) data are compatible with a superconducting order parameter of two components fluctuating in three dimensions. No influence of the magnetic Ho ions on Δσ is observed in the whole reduced-temperature range studied.

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.

Electrical resistivity and magnetic susceptibility roundings above the superconducting transition in Y1Ba2Cu3O7- delta

The authors report high-resolution data of the electrical resistivity and of the magnetic susceptibility rounding effects above the superconducting transition in single-phase polycrystalline YBa2Cu3O7- delta compounds. These data may be explained quantitatively and consistently, over two orders of magnitude in reduced temperature, by direct three-dimensional thermodynamic fluctuations of the amplitude of a two-component order parameter in unconventional (non-s-wave pairing) superconductors. The resulting order parameter correlation length amplitudes are xi ab(0)=(12+or-3) AA, and xi perpendicular to (0)=(1.4+or-0.4) AA, the Lawrence-Doniach effective interlayer distance is dc=(8+or-2) AA, and the Ginzburg reduced temperature is epsilon G=(0.7+or-0.3)*10-2.

Critical current versus normal‐state resistivity in granular Y1Ba2Cu3O7−δ

Precision measurements of critical current densities at zero applied magnetic field Jc for six granular Y1Ba2Cu3O7−δ samples have been performed by using a low frequency induction method. Also, the intergrain resistivity ρct for each sample has been extracted from the normal‐state resistivity. It is found experimentally that at any temperature distance from the transition, Jc is inversely proportional to ρct. A way to estimate Jc from the resistivity of the granular samples in the normal state is thus open.

Critical currents versus normal‐state resistivity in granular BiPb‐based copper‐oxide superconductors

Both the critical current intensity at zero applied magnetic field Ic(T), from nitrogen temperature up to the critical temperature, and the normal‐state resistivity ρn(T) have been measured in granular Bi1.5Pb0.5Sr2Ca2Cu3Oy (BiPbSCCO) superconductors, with different granular characteristics. It is found that the combination [Ic(T)/P][ρn(0)/ρ’n], where ρ(0) is the residual normal‐state resistivity, ρ’n is the temperature derivative and P is the sample’s perimeter, is essentially constant from sample to sample at any temperature. It thus constitutes a universal temperature‐dependent critical current for the polycrystalline BiPbSCCO family.

Scaling of critical currents versus temperature in granular Y1Ba2Cu3O7-δ superconductors at zero applied magnetic field

Abstract Simultaneous measurement of critical currents at zero applied magnetic field, Jc, as a function of the temperature, and of the average intergrain resistivity, ϱct, have been performed for six single phase (to better than 5%) polycrystalline Y1Ba2Cu3O7-δ (YBCO) samples, with distinct granularity characteristics, i.e., having 20 μΩcm ⪅ϱct⪅110 μΩcm. Jc has been measured by using a low-frequency induction method in order to increase the precision and eliminate possible spurious heating. Also, ϱct for each sample has been extracted from the normal-state resistivity. It is found experimentally that for all single phase granular samples of the same composition, but having different granulosity, the corresponding critical current scaled as ϱctJc fit into the same curve as a function of the reduced temperature. This result opens a way to estimate Jc in granular samples simply from normal state resistivity measurements.

Transition to the normal state of superconducting Y1Ba2Cu3O7-δ thin films induced by high current densities

We report measurements at zero applied magnetic field of the characteristic curves (E-J) in Y1Ba2Cu3O7-δ thin films in the high current density regime, up to current densities which induce the transition to the normal state. To minimize spurious heating effects due to Joule dissipation, the E-J curves were measured with a pulse technique and with the samples submerged in a liquid nitrogen bath, changing the temperature by varying the nitrogen vapour pressure. This experimental set-up allows us to observe not only the abrupt voltage jump appearing at current densities J* much larger (typically one order of magnitude) than the critical current density, Jc, but also the entire transition up to the normal state. Our results show that the product J*ρn is roughly sample independent, ρn being the slope of the E-J curves for J>J*. This finding is in qualitative agreement with the existing models that explain the jump at J* in terms of a vortex instability at high flux-flow velocities.