C. Torrón

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.

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.

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.

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.

Measurements of the magnetic susceptibility above the superconducting transition of YBa2Cu3O7−δ single crystals in the weak magnetic field limit

Abstract We report detailed experimental results of the magnetic susceptibility of high quality YBa 2 Cu 3 O 7−δ single crystals for the applied magnetic field, H , parallel (χ ab ) and perpendicular (χ c ) to the c direction. The data were obtained from 5 K to 300 K, and with μ 0 H ≤ 0.4 T, for which χ ab and χ c around the superconducting transition correspond to the so-called weak magnetic field limit, even for reduced temperatures of the order of 10 −2 . The excess (or para-) diamagnetism above but near the superconducting transition, extracted from these data following consistent procedures, was analyzed in terms of the existing approaches for independent gaussian fluctuations of the superconducting order parameter amplitude in layered materials. A scenario compatible with our experimental results is characterized by values of the correlation length amplitudes in the ab plane and in the c direction of, respectively, (1.0 ± 0.2) nm and (0.12 ± 0.03) nm, an effective number of superconducting planes in the unit cell length varying in the mean field-like region between 1 and 1.6, and by conventional 1 s 0 -wave pairing or one complex component unconventional pairing.

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.

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.

On the extraction of the paraconductivity from the resistivity data in cuprate oxide superconductors

We propose an unified and simple procedure to extract the paraconductivity, Δσ, from the electrical resistivity in cuprate oxide superconductors. As an example, this procedure is applied to Y 1 Ba 2 Cu 3 O 7−δ single crystal and granular samples. The resulting Au is compared with two of the mean field approaches actually available.

Influence of the granularity on the thermoelectric power of polycrystalline Bi1.5Pb0.5Sr2Ca2Cu3Oy superconductors

Abstract We first present an empirical picture of the influence of the sample structural inhomogeneities, at length scales much larger than the superconducting correlation length amplitude, on the thermoelectric power, S(T) in copper oxide superconductors. Then, we present experimental results on S(T) near the superconducting transition obtained in two polycrystalline Bi 1.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O y samples with different granularity. In full agreement with our empirical model, our results show that the reduced temperature (e) behaviour of the thermopower excess near but above the transition, ΔS(e), is the same for the two different samples. For completness we also analize the excess of the thermopower coefficient, LSσ.

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.