Félix Vidal

Paraconductivity of ceramic YBa2Cu3O7−δ in the mean-field-like region

A simple and unified phenomenological picture of the rounding of the electrical resistivity, ρ( T ), of ceramic HTSC above the critical temperature of individual grains is proposed. This approach is centered here on the mean-field-like region of Y 1 Ba 2 Cu 3 O 7−δ samples in zero-applied magnetic field. The approach combines the current phenomenological ideas on the electrical conductivity in granular superconductors, together with the empirical features of the Anderson-Zou proposal for the background conductivity, and with the Lawrence-Doniach treatment of the superconducting order parameter fluctuations in layered superconductors. To check this approach we present new experimental data on ρ( T ) in zero-applied magnetic field in different YBCO ceramic compounds, all with the same nominal composition but with important differences in their granular structure. These new experimental data strongly confirm the adequacy of that approach. In particular, the amplitude of the excess conductivity in the mean-field-like region (MFR) is explained quantitatively, to our knowledge for the first time, in ceramic superconductors. Our results also confirm that the paraconductivity in ceramic samples is essentially associated with the in-plane conductivity of the individual grains. Also, our results suggest that in the MFR the critical exponents of the excess conductivity are almost independent of the granular nature of the samples, provided that the correlation length amplitude is smaller than the characteristic dimensions of the grains. In the case of YBCO samples, our results confirm the 3D nature of SCOPF in most of the MFR in zero-applied magnetic field, and for the average amplitude of the Ginzburg-Landau correlation length in the c direction we find, ξ 1 (0) = 2.2 A.

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.

Measurements of the paraconductivity in the a-direction of untwinned Y1Ba2Cu3O7−δ single crystals

Abstract We report detailed experimental results of the paraconductivity in the a -direction, Δσ a , of two high-quality Y 1 ba 2 Cu 3 O 7- δ single crystals having a large (∼ 0.3 mm 2 ) untwinned region in their centers, and with transition temperatures of 90.0 K and 90.8 K. The exceptional sharpness of their resistive transitions (their upper half widths are less than 0.15 K) allows us to obtain quantitative information of Δσ a (ϵ), which is not affected by the CuO chains, up to reduced temperatures, ϵ, of the order of 10 -3 . The general scenario compatible with our experiments consists of the assumption of (1): A mean field-like region (MFR) up to ϵ≈2×10 -2 , where Δσ a (ϵ) may be explained in terms of the Lawrence-Doniach-like (LD) approach by taking into account the existence of two Josephson-coupled CuO 2 superconducting planes per unit cell. The characteristic superconducting length amplitudes in this MFR are ς ab (0) = (1.1±0.2) nm, and ς c (0) = (0.12±0.02) nm. (2) In agreement with our earlier qualitative resul ts, below ϵ≈2×10 -2 the measured ϱ a ( T ) separates from the mean-field behavior. Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these Δσ a (ϵ) data for ϵ⪅2×10 -2 may be explained, also on a quantitative level, on the grounds of the 3D-XY model, with a critical exponent of x = - 1 3 for Δσ a (ϵ), and x = - 2 3 for fluctuation-induced diamagnetism. This scenario correponds to one complex component order parameter, i.e., conventional 1 s 0 -wave pairing or one complex component unconventional pairing.

On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, Tc, is dominated by the uncertainty principle which, in particular, leads to a well-defined temperature, Tc, above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced temperature, ec ≡ ln (TcTc, is estimated to be around 0.55, i.e., above Tc 1.7 Tc coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different low- and high-Tc superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ec.

Resistivity anomalies above the superconducting transition in Y1Ba2Cu3O7−δ crystals and non-uniformly distributed critical-temperature inhomogeneities

Abstract We report the observation of a sharp peak (20% of the normal resistivity), just above the superconducting transition, in the curve of the ab plane electrical resistivity versus temperature,ϱab(T), of some non-fully oxygenated Y1Ba2Cu3O7−δ crystals, with transition temperatures, Tc, below 90 K. The peak amplitude decreases in the presence of an applied magnetic field perpendicular to the ab plane and it is also affected by the electrical-contact configuration or by further re-oxygenation of the crystals. This resistivity anomaly may easily be explained by assuming the presence of PTc inhomogeneities associated with small stoichiometric (oxygen-content) inhomogeneities, at long length scales, non-uniformly distributed in the crystals. Also, we show that depending on their spatial distribution, the Tc inhomogeneities may just deform (but without a peak) the resistivity versus temperature curve above Tc, very close to the transition, without appreciably broadening the measured resistive transition. These results may help to the understanding of the anomalies observed near Tc in different transport properties and materials by other authors.

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.

Fluctuation-induced diamagnetism in biperiodic layered superconductors in the weak magnetic field regime

Abstract By using a generalization of the Lawrence-Doniach free-energy functional, we calculate in this work the fluctuation-induced diamagnetism above the superconducting transition, Δ χ , in a layered superconductor having two different interlayer distances, d 1 and d 2 , in the superconducting layer periodicity length, s = d 1 + d 2 , and two different Josephson-coupling strengths, γ 1 and γ 2 , between adjacent superconducting layers. The calculations are performed in the weak magnetic field limit. We include in our treatment the possibility of an unconventional superconducting pairing by considering in each plane a superconducting wave function of g complex components. For the first time, Δ χ in a multiperiodic layered superconductor is obtained explicitly, as a function of the coherence length amplitudes, of g and of γ 1 and γ 2 . Our results show that by introducing an effective number, N e , of independent fluctuating layers in the layer periodicity length, it is possible to express Δ χ in terms of the fluctuation-induced diamagnetism for one-single periodicity layered superconductor, with the same periodicity length s = d 1 + d 2 . As an example of their interest, these theoretical results for the superconducting-order parameter fluctuations above T c are used to quantitatively analyze the existing experimental Δ χ ( T ) data in the mean-field0like region, measured in the weak magnetic field limit in single crystals of two different copper oxide systems having two Josephson coupled superconducting CuO 2 planes in s : The Y 1 Ba 2 Cu 3 O 7− δ and the Bi 2 Sr 2 Ca 1 Cu 2 O 8 compounds.

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.

Measurements of pool boiling heat transfer from ceramic Y1Ba2Cu3O7- delta superconductors to liquid nitrogen

We present measurements of pool boiling heat transfer from ceramic Y1Ba2Cu3O7- delta to liquid nitrogen in the range of heat flux density up to qe=35 W cm-2. Along the convective pool boiling regime, a linear relationship between the interface overtemperature, Tw, and qe, has been observed, with a value of the heat transfer coefficient h=1.3 W cm-2 K-1. In the nucleate regime the qe dependence on Tw follows quite well a power law, with an exponent close to 0.7. In the low heat flux regime qe<or approximately= 5 W cm-2, the heat transfer curve shows a double hysteresis, which seems to be closely related to the porosity of the samples.