J. Feng

Thermodynamic fluctuation under high pressure in Hg-1223 superconductors

Excess conductivities of polycrystalline Hg-1223 superconductor have been studied under high-pressure conditions. The critical temperature was chosen as the mean field transition temperature . Above this temperature, the normal state resistivity decreases with increasing pressure. The relative change in resistivity w.r.t. pressure, namely , is about . It is much smaller than the typical value of for most high- superconductors. As a consequence of the change in the normal state resistivity due to the applied pressure, the fluctuation-induced conductivity is also changed. The experimental results were analysed within the framework of Aslamazov-Larkin theory. The logarithmic plots of the excess conductivity versus reduced temperature reveal that there is a cross-over temperature which determines the transition from two- to three-dimensional conductivity of charge carrier transportation. With the help of the cross-over temperature , the values of the coherence length for the Hg-1223 superconductor could be estimated. The change in the fluctuation conductivity is discussed in terms of the coherence length and the thermodynamical critical field .

Powder XRD investigation of crystal structure modification effects on superconductivity in the Sn-doped YBa2Cu3-xSnxO7-delta systems

The relationships between the crystal structure modification, due to the doping element of Sn, and the critical temperature Tc of Sn-doped YBa2Cu3-x SnxO7- , with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, have been investigated by means of the peak shift and the peak broadening of the powder x-ray diffraction pattern. The profiles of x-ray diffraction peaks, due to crystal planes (006), (020), (021) and (200), were studied using the multi-peak fitting technique which is based on the Gaussian distribution function. It is interesting to note that the Sn-doped atoms effectively occupy the Cu2 sites in the CuO2 layer. The critical temperature of the Y-123 phase depends more sensitively on the lattice parameters a and b, rather than on the c-axis parameter c. The samples used for this investigation underwent the same treatment to ensure the requirement for maintaining the same oxygen content, particle size and residual stress. In this way, we can make sure that the Sn doping basically causes the peak shift and the peak broadening. Based on our results, we conclude that Sn doping in YBCO leads to an increase of the lattice constants a and b. The greater the expansion in a and b, the lower the critical temperature that results.

Thermal stabilization of impurity and superconducting phase in superconductor

Tin-doped samples were synthesized in sealed silica tubes by a solid-state reaction method. The dominant phase involved in the sample is a superconducting phase of (Hg(Sn)-1223), which is a solid solution of Sn in the compound . An impurity phase also exists together with the dominant phase. The results of thermogravity analysis and XRD analysis show that the impurity phase and superconducting phase Hg(Sn)-1223 have different decomposition temperatures in flowing He gas and in flowing gas atmosphere. The onset decomposition temperatures of these two phases measured in flowing He gas are 560 and respectively. They increase to 576 and respectively in flowing gas. Studies of the effect of annealing temperature on by resistance measurement for this sample show that the superconductivity is reversible in flowing gas below the decomposition temperature of the superconducting phase Hg(Sn)-1223, namely . The quality of the superconductor, including the critical temperature and the sharpness of the transition, can be improved by removing the impurity phase from the sample using a high-temperature annealing process under flowing gas.

Effects of Sn doping in Hg1−xSnxBa2Ca2Cu2O8+δ superconductors

Abstract Tin-doped samples of Hg1−xSnxBa2Ca2Cu3O8+δ (hereafter symbolized as (Hg1−xSnx)-1223) with x = 0, 0.05, 0.1, 0.2, 0.4 and 0.5 have been synthesized in sealed silica tubes by the solid state reaction method. The precursor can be prepared in air. The effects of Sn doping on the formation and stability, microstructure of grain growth and critical temperature of the (Hg1−xSnx)-1223 superconductor have been investigated by X-ray diffraction, scanning electron microscopy (SEM) and electron probe microanalysis (EDS), thermogravity analysis, resistance measurements and ac susceptibility measurement techniques. Experimental results show that a small amount of Sn doped in Hg-1223 promotes the formation of the triple CuO2-layer superconducting compound. The stability of the product compounds is also enhanced. Sn doping may play a role as a nucleating centre. The Tc for Sn-doped samples (Hg1−xSnx)-1223 with x ≤ 0.1 does not notably change as compared with the pure Hg-1223 phase (about 134 K). Sn doping is favourable for synthesizing the Hg-1223 superconductor. However, further increasing of Sn doping into the sample leads to a deterioration on both Tc and the fraction of superconducting phase.

Intergranular pinning properties in high-Tc superconductor by ac susceptibility response

Abstract From the ac susceptibility measurements, both the intergranular and intragranular peaks in the χ″ imaginary part for the granular YBa 2 (Cu 0.9 Cd 0.1 ) 3 O 7−δ superconductor can be observed clearly. Our finding of the exponent n ∼ 1 in the scaling law of temperature dependence of critical current density indicates that the grain boundary junction in YBa 2 (Cu 0.9 Cd 0.1 ) 3 O 7−δ should be SIS-type. The effective pinning potential is found to be inversely proportional to the power of magnetic field. The current density dependence of the U(T,H,J) is found to obey the inverse power law of the collective pinning theory. This indicates that the intergrain Josephson tunneling, which forms as a weak link network, exhibits a collective pinning behaviour.

Effects of In doping in

Indium-doped samples of (hereafter symbolized as with x = 0, 0.1, 0.2, 0.4 and 0.5 have been synthesized in sealed silica tubes by the solid state reaction method. The precursor can be prepared in air. The effects of In doping in Hg(In)-1212 superconductor have been investigated by x-ray diffraction, resistance and susceptibility measurements, scanning electron microscopy (SEM), electron probe microanalysis (EDX) and thermogravity analysis techniques. Experimental results show that a small amount of In doped in Hg-1212 promotes the formation of compound Hg(In)-1212 which has two layers. The stability of the product compounds is also enhanced. The volume fraction of the impurity phase is reduced obviously in the In-doped samples. However, the critical temperature of the In-doped Hg(In)-1212 is slightly decreased. A maximum of about 5% (x = 0.05) of In can be doped into the crystal structure of Hg-1212. The solubility of the In makes the lattice parameter c decrease as compared to the pure Hg-1212 phase.

The surface characterization and stability of Pb-doped Hg-based HTSC

Abstract The stability of the homologous series of mercury copper oxide superconductors is still a serious problem that receives attention from scientists all over the world. In this paper, we show that Pb-doping in Hg-1223 can improve the thermal stability of the system. The transition temperature, as well as the flux pinning effect of the system can be enhanced. The surface characterization was performed by means of Auger electron spectroscopic (AES), X-ray photoelectron spectroscopic (XPS) and sputtered neutral-particle mass spectroscopic analyses (SNMS). The results of our experiment indicate that the Pb-enriched regions, together with the Hg deposition on the surface of the ceramic sample, play the role of flux pinning centres to resist the flux creep.

Vortex dynamics in Pb-doped Hg-1234 superconductor by AC susceptibility response

Abstract The general equation that describes the AC magnetic susceptibility response of the superconducting system due to the change of varying AC field H = d H d t has been reviewed. By using the AC susceptibility measurements, the temperature, magnetic field and current density dependence of the effective pinning potential U(T, H, J) for our Pb-doped Hg-1234 superconductor has been determined. It is found that the fast drop of the effective pinning potential with current density is due to the large value of the characteristic exponent μ which depends on the existing types of non-superconducting phases that form the intergrowth structures with the dominant matrix. The characteristic curve of electric field E(J) against the current density J has been obtained from the AC susceptibility technique. For consideration of current relaxation due to the presence of giant flux creep, we have studied and determined the temperature dependence of the critical current density J c for our specimen.

Effect of hydrogenation on positron annihilation in the superconductor of YBa2Cu3O7−δ

Abstract The effect of hydrogenation on the positron annihilation in different oxygenated YBa 2 Cu 3 O 7−δ superconductors have been investigated. Experimental results showed that the effect of the hydrogenation process is to affect the concentrations of monovacancy and microvoid in both of intragrain and intergrain of the sample. The hydrogenation effect in the YBCO superconductor can be classified into four stages. In the first stage, i.e. for the initial hydrogen charging, the hydrogen atoms fill into the monovacancies and microvoids. The long lifetime τ 2 for positron annihilation is increased, and reaches the maximum value when monovacancies and microvoids have been fully filled with hydrogen atoms. In the second stage, the further hydrogen charging leads to the formation of new monovacancies. The long lifetime τ 2 decreases and reaches a minimum value at the end of this stage. In the third stage, the further hydrogen charging promotes the formation of microvoids by coalescence of clustered monovacancies and leads to increase τ 2 again. Finally in the fourth stage, τ 2 reaches a steady state which corresponds to the equilibrium concentrations for both monovacancy and microvoid maintained at the temperature of the material.

The thermal-assisted-flux-flow effect in infinite length rectangle-section sample of Pb-doped HgBa2Ca2Cu3O8+δ

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The theory of thermal-assisted-flux flow in an infinite length specimen of superconductor, with rectangle cross-section, is discussed by using somewhat different in the mathematical method rather than the conventional one. The formalism is also extended to the one-dimensional case. The results of our formalism are consistent with that obtained by Kes et al. The pinning potential and the crossover from TAFF to Flux Creep regime for the Pb-doped mercury copper oxide superconductor with nominal composition of Hg 0.69 Pb 0.31 Ba 2 Ca 2 Cu 3 O 8+δ were studied experimentally by means of ac magnetic susceptibility responses with a bias of dc field parallels to the longitudinal axis of the sample.