A. Pomar

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.

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.

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.

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.

Measurements of the fluctuation-induced magnetoconductivity in the a-direction of an untwinned Y1Ba2Cu3O7−δ single crystal in the weak magnetic field limit

Abstract Measurements of the fluctuation-induced magnetoconductivity in the a -direction of an untwinned Y 1 Ba 2 Cu 3 O 7−δ single crystal, not affected by the presence of CuO chains, are presented. The measurements were performed with the applied magnetic field, H , perpendicular to the ab -planes, and with H ≤1 T, which corresponds to the weak magnetic field limit even for reduced temperatures, ϵ, of the order of 10 −2 . In the mean-field region, i.e., for 10 −2 ⪅ϵ⪅10 −1 , these data may be explained in terms of direct thermal order-parameter fluctuations, in agreement with our previous paraconductivity and fluctuation-induced dia-magnetism measurements in the same sample. These results suggest that the pair-breaking parameter, δ, is less than 5×10 −2 at T =100 K. This upper limit not only leads to the absence of appreciable Maki-Thompson effects in cooper oxide superconductors (HTSCs), but also has implications in other central open HTSC problems. By using the BCS-like expressions to relate δ and the Cooper-pair relaxation time, τ φ , we obtain, at T =100 K, τ φ ⪅10 -−15 s in the clean limit and τφ⪅10 −14 s in the dirty limit. This last value is comparable with the expected electronic-scattering time.

Resistivity‐peak anomaly in Y1Ba2Cu3O7−δ crystals and nonuniformly distributed critical‐temperature inhomogeneities

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 of some nonfully oxygenated Y1Ba2Cu3O7−δ crystals, with transition temperatures below 90 K. This resistivity anomaly may easily be explained by assuming the presence of critical temperature inhomogeneities associated with small stoichiometric (oxygen content) inhomogeneities, at long length scales, nonuniformly distributed in the crystals. Our findings directly apply to the resistivity, the thermopower, and the Hall effect behavior in other superconducting materials having nonuniformly distributed Tc inhomogeneities.

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 the polycrystallinity on the paraconductivity in copper-oxide superconductors

Abstract The influence of long length scale structural inhomogeneities on the electrical resistivity of polycrystalline YBa2Cu3O7−δ samples is analyzed on the grounds of an empirical picture proposed by our group some time ago. For that, the electrical resistivity, ρ(T), has been measured in different polycrystalline YBa2Cu3O7−δ samples, all single phase to 4% and with δ ≲ 0.15, and the results compared with ρab(T), the intrinsic resistivity in the ab plane of single crystals with the same nominal composition. Our analysis fully confirms our earlier proposal that the structural inhomogeneities at scales much larger than the superconducting correlation length strongly affects the paraconductivity amplitude, but its reduced temperature behavior is not affected.