Дифференциальная термоЭДС углеродных нанотрубок типа "zigzag" во внешнем электрическом поле

The influence of (0.00, 0.20, 0.40, 0.50, and 0.60wt%) nano-sized tin oxide (SnO2) particles on electrical conductivity fluctuation in normal and superconducting state of the Y3Ba5Cu8O18±δ (denoted as Y-358) polycrystalline samples is studied. Phase formation and microstructures have been systematically examined. By increasing the content of SnO2 in YBCO matrix, X-ray diffraction technique showed slightly variation in lattice parameters and overall reduction in the orthorhombicity. Scanning electron microscopy observations and the crystallite size calculation also revealed that the grain size and the average crystallite size decreased compared to the SnO2 free sample. Aslamazov–Larkin and Lawrence–Doniach prototypes were performed to analyse conductivity fluctuations based on the electrical resistivity ρ(T) measurements. Superconducting transition temperatures Tc and TLD have been reported. The influence of SnO2 addition on the superconducting properties indicates that with the addition of SnO2 nanoparticles into Y-358 compound, some parameters values such as zero-resistance critical temperature Tc zero, coherence distance alongside the c axis at 0K ξc(0), and super-layer length d decrease in total, while anisotropy γ, critical magnetic fields Bc1(0), Bc2(0), and critical current density Jc(0) increase in SnO2-added Y-358 specimens compared to the pure one. The reasons corresponding to these scenarios are discussed in details.

Crystallographic analysis and conductivity fluctuations of the Y2Ba5Cu8O17 superconductor

Synthesis of the Y 2 Ba 5 Cu 8 O 17 superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk Tc = 104.95 K which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.8712(0) Å, b = 3.8481(4) Å and c = 27.1601(4) Å. In order to study the pairing mechanism close to Tc, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above Tc, the conductivity fluctuation analysis reveal the occurrence of two fluctuation regimes characterized by the critical exponents λ 3D = 0.52 and λ 1D = 1.51, corresponding to 3D and 1D Gaussian regimes, respectively. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory.

INHOMOGENEITY INDUCED CONDUCTIVITY FLUCTUATION IN YBa2Cu3O7-δ/BaTiO3–CoFe2O4 COMPOSITE

Polycrystalline (1-x) YBa 2 Cu 3 O 7-y + x BaTiO 3– CoFe 2 O 4(x = 0.0, 0.2, 0.4, 0.6 wt. %) superconductors were prepared by solid state route. XRD analysis reveals no significant change in "b" parameter and increase in "a" and "c" parameters. SEM micrographs show no change in grain size of the samples. With the increase of BaTiO 3– CoFe 2 O 4 (BTO–CFO) addition it has been analyzed that the superconducting transition temperatures (Tc) determined from standard four-probe method was decreased and dropped sharply with higher wt.% addition. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above Tc. The decrease in 2D–3D crossover temperature T LD (Lawerence–Doniach temperature) in the mean field region has been observed as a consequent dominance of 3D region to increase in wt.% in the composite. The increasing value of ρwl and ρ0 and the decreasing trend in the value of zero-resistance critical temperature (Tc0) indicates that the connectivity between grains decreases gradually with the addition of magneto–electric composite BTO–CFO.

STRUCTURE AND CONDUCTIVITY FLUCTUATIONS OF THE Y3Ba5Cu8O18 SUPERCONDUCTOR

Synthesis of the Y 3 Ba 5 Cu 8 O 18 superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk Tc = 97.86 K, which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.9211(3) Å, b = 3.8514(1) Å and c = 31.0170(0) Å. In order to study the pairing mechanism close to Tc, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above Tc, the conductivity fluctuation analysis revealed the occurrence of two fluctuation regimes characterized by the critical exponents λ 3D = 0.48 and λ 2D = 0.97. These regions were interpreted as corresponding to 3D and 2D Gaussian regimes, respectively. By the utilization of the Ginzburg–Landau theory, we experimentally determined the Ginzburg number. Our results are in agreement with that reported on YBa 2 Cu 3 O 7-δ, but an enhancement of the Gaussian fluctuation regimes was experimentally detected. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory. Closer to Tc, a genuinely critical regime was identified. Scaling of our results permits to establish that the dynamical of fluctuation system has the universality class described by the 3D-XY model. Below Tc, it is observed that for several values of transport current density, this material evidences a paracoherent regime followed by a coherent transition, which occurs in the regime of the zero resistance state.