Relationship between chemical bond parameters and superconducting temperature in HgBa2Can−1CunO2n+2+δ (n=1, 2, 3, 4)

Bond covalency and valence of elements in HgBa₂Ca_n−1Cu_nO_2n+2+δ (n=1, 2, 3, 4) were calculated and their relationship with T_c was discussed. For both oxygen and argon annealed samples, the results indicated that with the increase of n, the trend of bond covalency of Hg-O and Cu-O was the same or opposite compared with that of superconducting temperature. This may suggest that the magnitudes of Cu-O and Hg-O bond covalency are important in governing the superconducting temperature. For the highest T_c sample, Hg had the lowest valence, implying that lower valence of Hg was preferred in order to produce higher T_c. For fixed n, the valence of Cu in oxygen annealed samples was larger than that in argon annealed samples, indicating that oxygen annealed samples produced more carriers than argon annealed samples.     

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB₂ junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB₂ samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Δ/k_BT_c=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit).     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

n型T'-214相超导体的结构和性质

T＇－214相化合物R2-xCexCuO4（R为稀土离子）成为超导体决定于R3+的离子半径大小、Ce4+的取代量和化合物的热处理时的稳定性．从Raman光谱实验结果提出热处理过程中发生电荷转移．Ce4+的取代引起TN（Cu）下降，当TN（Cu）降到0时，超导性出现。不同的稀土离子TN（Cu）不同，稀土离子的反铁磁性（AFM）与超导性（SC）共存。热电势的测量结果可以用双通道模型解释，n型超导体中电子与空穴共存．     

Determination of oxygen content in YBa2Cu3O6.5+x

A method for determining the oxygen content of the high-temperature superconductor YBa₂Cu₃O_6.5+x is described. The superconductor is dissolved in 4.4 M hydrobromic acid, forming bromine. The mixture is then diluted with hydrochloric acid to obtain a solution of bromine and Cu(II) in 0.44 M hydrobromic acid and 1.1 M hydrochloric acid. As(III) is added in slight excess of that required to react with the bromine and the unreacted As(III) is determined by titration with potassium bromate. Oxygen does not interfere. The results of this method are in agreement with those of other iodimetric procedures. The dissolution of the superconductor in 4.4 M hydrobromic acid is much faster than in hydrochloric acid, the medium used in one iodimetric technique. YBa₂Cu₃O_6.5+x sintered fibers and powder samples weighing from ca. 0.5 to 200 mg were analyzed for oxygen content.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

Crystallographic and microstructural aspects of the superconductor YBa2Cu3O7-y —an overview

Crystal structures and structural inhomogeneities observed in YBa₂Cu₃O_7-y are reviewed. It is brought out that a proper understanding of the nature of the structural inhomogeneities is essential for exploiting the technological potential of this material. The need for an adequate characterization of specimens used for experiments is emphasized. Practical implications of the ferroelastic nature of the material are discussed.     

Effects of Cd2+ substitution on elastic properties and step-like anomalies in Tl0.9Bi0.1Sr1.8Yb0.2Ca1−xCdxCu1.99Fe0.01o7−δ superconductors

Cd-doped Tl_0.9Bi_0.1Sr_1.8Yb_0.2Ca_1−xCd_xCu_1.99Fe_0.01O_7−δ (x=0–0.4) bulk superconductor samples were prepared by solid-state reaction method, to examine the effect of Cd on ultrasonic velocity and elastic behavior of the samples. The samples were characterized by X-ray diffraction, DC electrical resistivity and temperature dependent ultrasonic velocity measurements. DC electrical resistivity measurement showed all the samples exhibit metallic normal-state behavior with the highest T_{c zero} observed at around 76.4 K (x=0.3). Ultrasonic velocity measurements at 80 K showed a non-linear increase in both absolute longitudinal and shear velocities as well as elastic moduli with Cd substitution with the largest increase observed for the x=0.3 sample. Temperature dependant longitudinal modulus showed elastic anomaly characterized by a step-like slope change at around 230 K for x=0 & x=0.3 and at around 250 K for x=0.4 with the x=0.3 sample showing the sharpest slope change. A comparison between experimental data and calculated lattice anharmonicity curve based on the model proposed by Lakkad, showed large deviation of the experimental longitudinal modulus curves for (x=0.3) from the calculated anharmonicity curves indicating that the elastic behavior was strongly influenced by the existence of the step-like longitudinal anomaly. On the other hand, our analysis using the Landau free energy model found that the anomalous step-like elastic behavior fitted well with the equation derived from the model for regions below and above the elastic anomaly temperature, T_A. The fitting indicated that the anomaly is related to a phase transition that is suggested to involve ordering of oxygen which introduces strain in the system.     

Probing spatial correlations in the inhomogeneous glassy state of the cuprates by Cu NMR

We discuss the crossover of the form of the Cu Nuclear magnetic resonance (NMR) spin echo decay at the onset of Cu wipeout in lanthanum cuprates. Experimentally, the echo decay undergoes a crossover from Gaussian to exponential form below the temperature where the Cu NMR intensity drops. The wipeout and the change in behavior both arise because the nuclei experience spatially inhomogeneous spin fluctuations at low temperatures. We argue that regions where the spin fluctuations remain fast are localized on length scales of order 1-2 lattice spacings. The inhomogeneity is characterized by the local activation energy E_a(r); we estimate the functional form of E_a(r) for points where E_a>(r)∼0.