Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

Superconductivity in the BaPb1−x Bi x O3 system

Superconductivity properties of the BaPb_1−x Bi _x O₃ system have been investigated by precise measurements of electrical resistivity and Hall coefficient in specimens prepared by the hot press technique. Sharp superconducting transitions have been observed in those specimens, indicating the homogeneities of the specimens thus prepared. superconductivity has been observed for 0.05<x≦0.35 and a maximum critical temperature of 11.7 K has been observed atx=0.25. The carrier concentration deduced from the Hall coefficient has been found to show nearly the samex dependence as does the critical temperature. The upper critical field has been determined from the magnetic field dependence of the lowcurrent-density resistivity. Discussions are given about the superconductivity in the BaPb_1−x Bi _x O₃ system on the basis of the experiment results.     

Microstructural characterization of the 85 K superconducting phase in the Bi−Ca−Sr−Cu−O system

Summary Transmission electron microscopy has been applied to investigate the microscopic features of a superconducting phase present in the system Bi−Sr−Ca−Cu−O. Sintered samples showing a single transition temperature of 85K, which points out the formation of aB-phase, were mechanically and ion-beam thinned. Electron diffraction shows that the Bi₂Sr₂CaCu₂O₈ superconductor possesses a primitive orthorhombic unit cell withPnnn space group. High-resolution electron microscopy reveals that the most common defect corresponds to a (001) lamellar structure caused from frequent twins with (001) composition plane and [001] fourfold axis twin operator. Possible derivative structures, essentially based on the occurrence ofm b/5+n a shifts everyc period, are discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

A new compound, NdBa₂Ca₃Sr₄Cu₅O_x, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed.     

Superconductivity in La1.85Sr0.15Cu1-xZnxO4

Based on the multiple exchange-mediated interaction mechanism of high-T_c superconductivity and the Abrikosov-Gorfiov theory of magnetic impurity superconductors, this letter quantitatively explains the Zn-substitution experiment of La_1.85Sr_0.15Cu_1-xZn_xO₄     

Comparative photoemission study of Bi2(Sr,Ca)3Cu2O8+y and Bi1.7Pb0.3Sr2Ca2Cu3O10+y

The electronic structure of Bi-based 2223 single phase superconductor has been studied by ultraviolet and X-ray photoemission. By comparison with that of 2212 phase superconductor, we find a higher density of states nearE _F for 2223 phase. From analysis of the Cu 2p core-level spectroscopy, we obtain a relatively smaller charge transfer energy between copper and oxygen as well as the Coulomb repulsion energy on Cu site for 2223 phase. We relate these changes to the increase of hole concentration from 2212 to 2223 phase. The experimental results support the viewpoint that the transition temperature should be correlated with the density of states atE _F .     

Energy gaps and phonon structures in tunneling spectra of Bi2Sr2Ca1Cu2O8+x and Bi2Sr2Ca2Cu3O10+y superconductors

Polycrystalline Bi-2212 or Bi-2223 systems and single crystals of the Bi-2212 system were investigated at several temperatures by means of the break-junction and point-contact tunneling techniques, respectively. In the case of Bi-2223 we obtained an averaged gap value 2=65±4 meV. The Bi-2212 system yields 2= 45±5 meV in the case of polycrystalline samples and 2=47±5 meV in the case of single crystals. Hence there result BCS-ratios 2/k _B T _c of 7.3, 6.8 and 6.5, respectively. A dc-Josephson-type supercurrent could be observed in polycrystalline Bi-2212 samples in addition to the usual gap structure. The point-contact tunneling spectra of Bi-2212 single crystals exhibit structures in the second derivative d² I/d V ² which we interpret as inelastic tunneling processes due to electron-phonon scattering most likely in the barrier region. The phonon energies deduced from these structures are in accordance with data obtained from inelastic neutron scattering and Raman spectroscopy. Good agreement is obtained with a lattice dynamical calculation of the partial phonon densities of states of individual atoms in the unit cell.     

Thermoelectromotive force in Bi2Sr2Ca2Cu4O11 bismuth-based high-temperature superconductor

The temperature dependence of the thermoemf in Bi₂Sr₂Ca₂Cu₄O₁₁ bismuth-based high-temperature superconductors is studied. The thermoemf is positive at high temperatures (T > 300 K) and negative at low temperatures. As the temperature decreases, the thermoemf reaches a maximum slightly above T _c and then sharply drops to zero. The results can be explained in the framework of the two-band theory with a linear temperature term.     

X-ray characterization of La−Sr−Cu−O system

The high-temperature (38 K) superconducting La–Sr–Cu–O sample was investigated by X-ray diffractometry with diffracted beam monochromator. Four phases were identified. It is possible that the interaction between two of them can influence the superconducting properties of the sample. The EPMA analysis revealed the precipitates enriched by Sr up to 97%. The compositions of the matrix and the area with excess of Cu were estimated. The correlations between X-ray diffraction and EPMA results are discussed.