Effect of pressure on superconductive properties of BaPb1-xBixO3 monocrystals

Abstract

Extraordinary properties of superconductive BaPb, -xBixO, (BPB) led to the assumption of out-of-ordinary mechanism of the superconductivity of localized pairs^1,2     

Superconductivities of high-pressure phases in the metal-hydrogen systems

Abstract

The superconducting properties of the d-metal hydrides rank among the most interesting and less studied of their characteristics. Correct and complete data have only been obtained for one superconducting hydride, the palladium hydride'. The long studies on phases forming in the d-metal-hydrogen system in the well- mastered pressure range up to tens of atmospheres have not exhibited any new superconductors, and the scope of systems which could be of interest from this viewpoint has been mainly exhausted'.     

Superconductivity and spin density waves

In the electron-electron interaction the r-space structure caused by magnetic fluctuations at the phase transition from a nonmagnetic metal to an antiferromagnetic metal gives rise to a d-wave attractive interaction for Cooper pairing. This is a contribution to some total electron-electron interaction which in total may or may not give rise to Cooper pairing and superconductivity.     

Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes

In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-[O₂-(Ce,Y)]_s−1, between two identical CuO₂ planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO₄-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO₂ plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent Ca^II-for-Y^III substitution in CoSr₂YCu₂O_7+δ) is found not to be completely accommodated in the CuO₂ planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr₂YCu₂O_7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr₂(Ce_0.25Y_0.75)₂Cu₂O_9+δ) and Co-1232 (CoSr₂(Ce_0.67Y_0.33)₃Cu₂O_11+δ) samples Cu valence does not increase high enough to induce superconductivity.     

添加Cu2S对Bi-Pb-Sr-Ca-Cu-O体系超导性的影响

本文对Cu_2S添加的Bi-Pb-Sr-Ca-Cu-O体系进行了低温电阻、X射线衍射和XPS研究．实验结果表明，加入少量的Cu_2S能够降低高T_c相的形成温度，改善超导性能，加入Cu_2S引起了Cu 2p_(3/2)谱的主峰向低结合能方向移动，并减少其卫星峰的面积．这表明Cu_2S的加入减少了Cu~(2+)的量，即导致空穴载流子浓度的减小．     

Investigation of the Rb-W-O system in connexion with the superconducting properties of the hexagonal tungsten bronzes

The previous studies of the superconducting properties of the hexagonal tungsten bronzes have repeatedly come up against the lack of reproducibility of the data and, at the first place, of the data relating to the stability of superconducting state. We revisited this problem and identified the main causes of these contradictory data in Rb_xWO₃, among which the major one is the ordering of the alkali atoms. Our study has emerged onto a determination of the x dependence of the order-disorder transition, of the lattice parameters and of the superconducting transition temperature T_c. We have also clarified the crystal structure and examined the mechanisms involved in the oxidation and reduction of this compound. Finally, we have assembled our data to draw a plausible Rb-W-O phase diagram.     

Supraleitung in Seltenerdmetall-Carbidhalogeniden des Typs SE2X2C2

Superconductivity in Rare Earth Metal Carbide Halides of the Type SE₂X₂C₂ The metallic nature of the carbide halides Y₂X₂C₂ is due to Y? C covalency. The superconductivity of the compounds is attributed to a pairwise attraction of conduction electrons by C₂-π* states at the Fermi level. The hypothesis is followed by experiments and band structure calculations. – Neutron powder diffraction reveals d(C? C) = 128(1) pm for Y₂Br₂C₂. X-ray single crystal investigations on Y₂Br₂C₂ and Y₂I_1.5Br_0.5C₂ show a characteristic variation of the coordination of the C₂ unit. Systematic changes of the average halide radius in Y₂(X,X′)₂C₂ (X,X′ = Br, Cl I, Cl and I, Br) lead to a monotonic increase of T_c = 2.3 K (X = Cl) via T_c = 5.05 K (X = Br) to a maximum T_c = 11.2 K for Y₂I_1.6Br_0.4C₂. No isotope effect for ¹²C/¹³C could be detected. Photoelectron spectra of Y₂Br₂C₂ (excitation energies between 40 and 140 eV) are compared with the results of band structure calculations (LMTO, E.H.). The electronic structure reveals two bands crossing the Fermi level. One of them has C₂-π*-Y-d_xz,yz character and exhibits a saddle-point at E_F. The other intersects the Fermi level with large dispersion and has exclusively Y-d character at the crossing point. The results are discussed with respect to theoretical models (van Hove singularity, local pairs and itinerant electrons).     

Vortices in weakly coupled layered superconductors

The vortex system in high-temperature layered superconductors exhibits a rich phase diagram with many proposals of phase transitions modifying the correlations both within and between the layers. We focus on the limit where the magnetic coupling between “pancake” vortices dominates over the interlayer Josephson coupling. The weak, but long-ranged nature of this magnetic interaction allows for an accurate “mean-field” treatment where the pancakes in each layer move independently in a self-consistent substrate potential. We calculate the form of the two relevant phase transitions in this system. First, we determine when the substrate potential is too weak to stabilize the two-dimensional (2D) fluctuations and the lattice evaporates to a pancake gas. Second, within the lattice we find a Kosterlitz–Thouless unbinding transition of vacancies and interstitials. For a small but finite Josephson term, this is identified with the phase-decoupling transition.     

Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.