Modulated structure of the superconducting compounds Bi2Sr2Ca n-1Cu n O y withn=1 and 2

The incommensurate modulated structures of high-T _c superconducting phases Bi₂Sr₂Ca _n–1Cu _n O _y (n=1 and 2) have been studied using symmetry properties of four-dimensional super-spacegroups and electron diffraction as well as high resolution electron microscopy are used to describe the properties of the incommensurate modulated structure in the compounds. The main results are planar monoclinic symmetryP ^B _{2/b 11} orP B _{b 1} for Bi₂Sr₂CuO _y (n=1) and orthorhombic symmetryN ₁₁₁ ^Bbmb orN ₁₁₁ ^Bb2b for Bi₂Sr₂CaCu₂O _y (n=2). The temperature dependence of the modulated structure of Bi₂Sr₂CaCu₂O _y has been investigated from –190°C to 800°C. Some structure properties of Bi₂Sr₂Ca₂Cu₃O _y (n=3) are presented for comparison.     

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.     

Effect of chemical substitution and oxide additives on the Tc behaviour of the oxide superconductor,Bi4Sr3Ca3Cu4O16

Studies on the superconductivity behaviour of pure and doped (Pb,TI) Bi₄Sr₃Ca₃Cu₄O₁₆ phases are reported. Effect of oxide additives PbO, BaPbO₃ and BaBiO₃ has also been examined. Results show that an oxygen treatment at 600°C for 12 h during the final stages of the synthesis has the beneficial effect of sharpening the T_c ^zero of the 4334 Bi-composition. Pb and TI substitution has only a marginally beneficial effect on the T_c , though it is not definitely known whether substitution is actually occurring in the system. PbO as an additive raises the T_c ^zero to 89 K, whereas other additives do not basically affect the properties of the 4334 system.     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

Specifics of third-harmonic generation in Bi–Sr–Ca–Cu–O superconductors in the region of superconducting transition temperatures

The specifics of third-harmonic generation in Bi–Sr–Ca–Cu–O two-phase superconductors are studied. It is shown that the emergence of a maximum in the temperature dependence of the third-harmonic voltage in the region of superconducting transition temperatures is associated with the redistribution of the eddy current in the bulk of the semiconductor between its regions with different superconducting parameters (critical current and critical temperature).     

Influence of heat treatment on the superconducting properties of Bi1.6Pb0.3Sb0.1Sr2Ca2Cu3O y

Differently heat-treated samples with the nominal composition Bi_1.6Pb_0.3Sb_0.1Sr₂Ca₂Cu₃O _y were studied by resistance and ac susceptibility measurements in the temperature range 77 K<T<200 K. The X-ray diffraction patterns at room temperature were also investigated. The experimental results show that repeated quenchings can depress the formation of the 80 K phase, and enhance the formation of the 110 K phase.     

Anisotropic fluctuation magnetoconductivity in Bi2Sr2Ca2Cu3Ox thin films

Measurements of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin films of Bi₂Sr₂Ca₂Cu₃O_x are presented. The variation of the magnetoconductivity with the magnetic field strength is quadratic at temperatures distantly above the critical temperatureT _c and changes to an entirely negative curvature nearT _c . The latter behavior is attributed to an inhomogeneous critical temperature in the films. The results are analyzed in terms of recent theoretical models for fluctuation-enhanced magnetoconductivity in layered superconductors. The magnetoconductivity in a magnetic field oriented normal to the copper-oxide layers is dominated by the orbital contribution of the Aslamazov-Larkin effect. The magnetoconductivity in the parallel orientation is distinctly smaller and provides evidence for the corresponding Zeeman contribution. The latter measurement also indicates that the Maki-Thompson process may be insignificant in Bi₂Sr₂Ca₂Cu₃O_x.     

Evidence of critical fluctuations from the magnetoconductivity data in Bi2Sr2Ca2Cu3O10+x phase

The fluctuation-induced magnetoconductivity of the Bi₂Sr₂Ca₂Cu₃O_10+x phase is studied above zero-field critical temperature Tc(0) and for moderate magnetic fields. It is found that the Gaussian approximation for superconducting fluctuations underestimates the negative fluctuation magnetoconductance drastically in the Tc(0) < T < Tc(0) + 20 K temperature range. Taking into account the critical fluctuation contribution on the base of self-consistent Hartree approximation makes it possible to explain the data quantitatively in terms of the only Aslamazov-Larkin contribution for different magnetic fields and temperatures, consistently with the zero field data. Received 14 April 2000 and Received in final form 13 July 2000     

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.     

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.     

Symmetry of the charge-ordered phases in Pr0.5Ca0.5MnO3

The crystallographic symmetry of the charge-ordered Pr_0.5Ca_0.5MnO₃ manganite is studied at ～98?K using convergent beam electron diffraction technique. These studies have revealed appearance of two different type of charge-ordered phases in different domains of the same grain. These correspond to be monoclinic and triclinic phases. These studies reveal that the symmetry of the charge-ordered phase is highly sensitive to the local strain conditions.     

Electronic structure of the Sr0.4Ca13.6Cu24O41 incommensurate compound

We extracted, from strongly-correlated ab-initio calculations, a complete model for the chain subsystem of the Sr0.4Ca13.6Cu24O41 incommensurate compound. A second neighbor t-J+V model has been determined as a function of the fourth crystallographic parameter , for both low and room temperature crystallographic structures. The analysis of the obtained model shows the crucial importance of the structural modulations on the electronic structure through the on-site energies and the magnetic interactions. The structural distortions are characterized by their long range effect on the cited parameters that hinder the reliability of analyses such as BVS. One of the most striking results is the existence of antiferromagnetic nearest-neighbor interactions for metal-ligand-metal angles of 90. A detailed analysis of the electron localization and spin arrangement is presented as a function of the chain to ladder hole transfer and of the temperature. The obtained spin arrangement is in agreement with antiferromagnetic correlations in the chain direction at low temperature.