Study of Thermally Activated Phase-Slip and Scaling of Magneto-Resistance Near TC in YBa2Cu3O7−δ/Ag bulk composites

Journal of Superconductivity and Novel Magnetism - Dissipation in YBCO polycrystalline bulk samples added with nominally different amounts of silver (0, 5, 15, 25 wt%) has been studied through...     

Relationship between Growth, Structure and Superconductivity of Single Crystal YBa2Cu3O7-δ

We study the crystal growth of YBa ₂ Cu ₃ O _7- with additives, using ac susceptibility, X-ray diffraction and STM. One specific additive, BaF ₂ , dramatically improves both the crystal growth and the superconducting properties. A cluster growth model is proposed to describe the function of the additive. F _– reduces the Al contamination of Cu-O chains, and the orthorhombicity increases. On a nanometer scale, the roughness of the chains decreases markedly. Superconductivity is further enhanced using non-contaminating crucibles. Large single crystals with T _C = 93.7 K and T _C = 0.1K were grown in Y ₂ O ₃ or BaZrO ₃ crucibles using the BaF ₂ additive.     

Thermally Assisted Flux Creep of a Driven Vortex Lattice in Untwinned YBa2Cu3O7−δ Single Crystals

We have studied the driven vortex lattice in untwinned, clean YBa ₂ Cu ₃ O _7– single crystals, showing the first order (melting) transition T _m . At high enough driving currents (j 10 ³ A/cm ² , j j _c ) and temperatures T < T _m , a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T _m , associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Superconductivity Enhancement in Fe3O4 Doped YBa2Cu3O7−δ

Flux pinning can lead to the enhancement of J _c in superconductor. In order to study the effect of magnetic particles on superconductivity, the composites of (YBa₂Cu₃O_7?δ )_0.98(Fe₃O₄)_0.02 and (YBa₂Cu₃O_7?δ )_0.98(α-Fe₂O₃)_0.02 are synthesized with former sintered at different temperatures. The field-cooling measurements show that the T _c of the samples has not changed. The superconducting properties are enhanced when the samples are sintered at high temperature (750 ^°C), but suppressed for the samples without sintering or sintered at low temperature (350 ^°C). Scanning electron microscopy images show that the doped Fe₃O₄ are diffused into intergrain sites. With the increase of sintering temperatures, the grain size of the sample grows up and the flux pinning, which leads to the enhancement of J _c is formed by the addition of Fe₃O₄ nanoparticles. The extended X-ray absorption fine structure and X-ray absorption near edge structure experiments prove that after sintered at high temperatures the addition of Fe₃O₄ can induce disorder of local structures of YBa₂Cu₃O_7?δ . Also, the transition from the tetragonal phase to the orthorhombic phase is observed by the X-ray diffraction spectra. By the above analysis of experimental data, the enhancement of diamagnetic signal, which represents the superconducting properties can be attributed to the disorder enhancement of local structures and structural phase transition of the cuprates induced by the addition of Fe₃O₄.     

Normal State Resistivity of YBa2Cu3O7-δ /PrBa2Cu2.8Ga0.2O7-δ c-axis Superlattices : Evidence for an Underdoping Effect of YBa2Cu3O7-δ

We measured the ab-plane resistivity of c-axis YBa ₂ Cu ₃ O _7- /PrBa ₂ Cu _2.8 Ga _0.2 O ₇ superlattices grown by pulsed laser deposition, from T _C to 325 K. The normal-state resistivity and the fluctuation regime above T _C were studied for different thickness of YBa ₂ Cu ₃ O _7- and PrBa ₂ Cu _2.8 Ga _0.2 O _7- in the superlattice period. As the concentration of PrBa ₂ Cu _2.8 Ga _0.2 O _7- is raised, the T _C decreases and the resistivity increases. Evidence is given for an underdoping effect in the YBa ₂ Cu ₃ O _7- layers and our measurements are compared to similar measurements on oxygen-deficient YBa ₂ Cu ₃ O _7- thin films, grown on the same equipment. They are also compared to measurements on YBa ₂ Cu _3-x Co _x O _7- single crystals made by other groups. We present here the different measurements and investigate the mechanisms that could be held responsible for this depletion in charge carriers.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

Mechanisms of interaction between the reactive species of a (CF₄+O₂) plasma and YBa₂Cu₃O_7– ceramics have been proposed through detailed angle-resolved X-ray photoelectron spectroscopic analyses. At the surface of the outer grains, the plasma treatment removes (OH)^– and (CO₃)^2– species contained in the degradation layer and gives rise to a fluoride-rich layer, whereas in the bulk of the material the occurrence of metal-fluorine bonds in the superconducting phase has been assumed. An increase of the oxidation state of copper has been also detected, confirming the oxidizing effect of the plasma treatment. A comparison with the oxidation mechanisms obtained by fluorine gas treatment is proposed.     

Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

The radiofrequency plasma technique involving mixtures of CF₄+O₂ gases has been applied to the surface treatment of high T_c superconducting oxides (YBa₂Cu₃O_7–). Investigation of the various experimental parameters of the process has shown that the improvement of the critical current density, J _c, mainly depends on the inlet precursor composition CF₄+%O₂, on the total pressure and on the reaction time. The presence of fluorine in the bulk of the ceramics has been observed from electron microprobe analysis, together with an increase of the Cu³⁺ content. The plasma-enhanced fluorination (PEF) treatment improves the superconducting properties of the materials: both values of the resistivity in the normal state and of the superconducting transition width are reduced and the critical transition temperature is improved by about 1 K.     

Oxygen mobility and charge transport in YBa2Cu3O7-δ

The out-diffusion of oxygen during heat treatment of fully oxygenated c-axis oriented epitaxial films of YBa₂Cu₃O_7- leads to large electric fields in the films. During heat treatment under oxygen pressure a transient electric resistance peak has been observed also.     

Cation interdiffusion between thick film of YBa2Cu3O7-δ and ceramic substrate

Thick films of superconducting oxides, YBa₂Cu₃O_7-, were successfully made by conventional screen-printing technology on Al₂O₃, MgO, and ZrO₂ substrates. Interdiffusion between the superconductive film and substrate was investigated using analytical electron microscopy. The results indicate that MgO and ZrO₂ are superior to Al₂O₃ for substrate materials.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

The superconducting state of YBa2Cu3O7−δ

Many low temperature properties of high T_c superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa₂Cu₃O₇- and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.     

Growth and processing of single crystal superconducting YBa2Cu3O7−δ

A method for growing millimeter-sized crystals of YBa₂Cu₃O_7– is described. Processing conditions were developed to improve the superconducting transition temperature and sharpness, as observed by measurements of the dc magnetic susceptibility. Some of our crystals have been used in observations of the Raman Effect, specific heat, nuclear magnetic resonance, and nuclear quadrupole resonance, with results indicating that the samples are excellent.     

Study on properties and microstructure of YBa2Cu3O7-δ-Agx composite superconductors prepared by sol-gel method

The relation between the microstructure and properties of YBa₂Cu₃O_7--Ag_x composite superconductors prepared via sol-gel processing has been studied. Optical microscopy, SEM and TEM analyses indicated that Ag-doped samples were dense, with small and regular grains and narrow spaces between grains. Ag particles were distributed homogeneously in samples and these could increase their J ₀ values. The mechanisms of the effect of Ag doping on the superconducting properties and microstructure of samples were hindrance of the movements of grain boundaries and metallization of grain boundaries. However, too much Ag doping would weaken the coupling among superconducting grains and induce J ₀ to decrease. The experimental results confirmed these ideas.     

Correlation Between Ferromagnetism and Superconductivity at Interfaces of La2/3Ca1/3MnO3/YBa2Cu3O7−δ /La2/3Ca1/3MnO3 Trilayers Grown by dc Sputtering

In this work, we study the magnetoelectrical properties of La_2/3Ca_1/3MnO₃/YBa₂Cu₃O_7?δ /La_2/3Ca_1/3MnO₃ heterostructures grown by means of high oxygen-pressure dc sputtering onto (001)-oriented SrTiO₃ substrates. The test heterostructures are composed of ferromagnetic layers of constant thicknesses (～280 unit cells, ～110 nm) and superconducting interlayers with thicknesses ranging between 5 (～6 nm) and 15 unit cells (～17 nm). Transport measurements show a strong suppression of the superconducting properties when the thickness of the YBa₂Cu₃O_7?δ interlayer is reduced to a value below 10 unit cells. Magnetic measurements show superconducting transition in junctions with YBa₂Cu₃O_7?δ interlayers with thicknesses larger than ～15 unit cells. The observed difference in the values of the superconductivity onset when determined by electrical and magnetic measurements might be related with the presence of a spontaneous vortex phase in the temperature range around the superconducting transition.     

Low-Field Phenomenology and Characteristic Lengths of Hydrogen- and Halogen-Doped YBa2Cu3O7−δ Compounds

We measure the diamagnetic response of YBa ₂ Cu ₃ O _y A _x (A=Cl, F, H) ceramics. We obtain low-field effective penetration depths of a few 10 m, first flux-penetration field H₁ distinguishable from the volume penetration field H_C1 and two characteristic temperatures T_s and T_c, T_cs, defined by the onset of the real and imaginary parts of the susceptibility, respectively. H ₁ (T) goes to zero as a power law H ₁ (T, x)=H ₁ (0, x) (T–T/T_c) ^2.7 near T_C. From the values of and H ₁ we infer the correlation length at T=4.2 K. A large range of values for (4.2) suggests that chemical disorder plays an essential role in the phenomenology of cuprates. We find (4.2)H ₁ (4.2) ^–1/3 . This, and the power law (4.2)H ₁ (4.2) ^–2/3 result from the scaling properties of a phase transition governed by disorder.     

Preparation,crystallization and properties of rapidly solidified YBa2Cu3O7−δ

YBa₂Cu₃O_7−δ was prepared in the glassy state by rapid solidification from the melt. The quenched material was paramagnetic down to liquid-He temperature. Diamagnetic behavior developed after crystallization of the glass at high temperatures (> 800°C). A well-defined superconducting transition occurred at 90 K for material heat treated and crystallized above 900 °C.     

超导PbO/YBa2Cu3O7-δ复合氧化物摩擦磨损性能

为适应从低温到高温宽温范围的使用条件,用溶胶-凝胶法制备了YBa2Cu3O7-δ超导材料,用摩擦磨损试验机测试了YBa2Cu3O7-δ从室温至液氮温度的摩擦学性能.结果表明:室温20℃下,YBa2Cu3O7-δ与对偶件不锈钢盘对摩时,摩擦因数在0.5左右,当温度降到超导转变温度以下时(液氮温度-196℃)摩擦因数大幅度降低,YBa2Cu3O7-δ超导态摩擦因数是正常态值的一半,实验直接证明了电子激励对摩擦能量耗散的作用.为改善室温下YBa2Cu3O7-δ摩擦学性能,掺杂不同质量分数PbO作为润滑组元,制备了PbO/YBa2Cu3O7-δ超导固体润滑复合材料,取得良好效果.PbO掺杂不影响PbO/YBa2Cu3O7-δ复合材料的超导电性,在正常的载荷和滑行速度下15%PbO/YBa2Cu3O7-δ复合材料摩擦因数为0.2至0.3,磨损率为4.35×10-4mm3·(N·m)-1,分析了PbO/YBa2Cu3O7-δ复合材料减摩耐磨机制.