Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

Recently a new Y–Ba–Cu–O superconducting oxide has been prepared as a distinct phase in thin films1–3. The new phase was first seen as a defect structure in bulk samples of YBa2Cu3O7 (refs 4–7) and found to contain an additional copper layer7. The proposed defect structure has a c-axis spacing of ∼27 A and a cation ratio of YBa2Cu4. Y–Ba–Cu–O films containing a majority phase with a 27 A unit cell have been proposed as evidence of a distinct phase with the same structure as the defects (ref. 1 and K. Char et al., unpublished results). Films of the new phase superconduct with a transition temperature of 80 K (K. Char et al., unpublished results and refs 2 and 3). We show by X-ray crystallography of a film containing 85% of the new phase that the stoichiometry can be written as YBa2Cu4O8. The new structure differs from YBa2Cu3O7 in that the single, linear Cu–O chain parallel to the b-axis has been replaced by a double Cu–O chain with edge-sharing, square-planar oxygen coordination.

Crystal structure of the 80 K superconductor YBa2Cu4O8

A new superconducting phase has been identified during measurements of the pressure–temperature–composition phase diagram of the pseudobinary system YBa2Cu3O6+x–O2 (ref. 1). The new phase has formula unit Y2Ba4Cu7O14+x. Preliminary crystallographic results from electron diffraction showed a doubling of the c-axis1. Here we report the results of a single-crystal X-ray structural study and measurements of superconductivity in the new compound. The superconducting onset, from a.c. susceptibility measurements, is ˜40 K. The structure (space group Ammm) consists of single YBa2Cu3O6+x -type blocks containing single corner-sharing CuO4 square-planar chains, alternating with single YBa2Cu4O8-type blocks containing double edge-sharing CuO4 square-planar chains. This suggests the possibility of similar compounds with other ordered sequences of single and double chains.

Structure determination of the new high-temperature superconductor Y2Ba4Cu7O14+x

The crystal structures of the known copper oxide superconductors are described, with particular emphasis on the manner in which they fall into structural families. The local charge picture, a framework for understanding the influence of chemical composition, stoichiometry, and doping on the electrical properties of complex structures, is also described.

https://science.sciencemag.org/content/sci/247/4943/656.full.pdf

Structural chemistry and the local charge picture of copper oxide superconductors.

Recently Siegrist et al. proposed a structure for a high-Tc superconductor Ba2YCu3O(9-∂) based on an orthorhombic (a = ap, b = ap, c = 3ap) perovskitelike model containing Ba and Y cations ordered over the A-sites of the ABO3 structure. This ordering is responsible for the tripling of the c-axis. Half of the oxygen vacancies (at z = 1/2) are ordered, while the other half (at z = 0) are disordered over two sites. Using profile refinement of high-resolution neutron powder data at six different temperatures between 5 K and 300 K, we have refined the structure of a pure and well-characterised powder sample with onset of superconductivity at 100 K. At all temperatures we confirmed the previous model except that in our structure, all oxygen vacancies are ordered. Two-thirds of the copper cations have a pyramidal coordination and one-third has a square coordination. In our structure all squares are parallel to the (b, c)-plane, while in the one reported by Siegrist et al. the squares are disorderedly parallel to either the (a, c)- or (b, c)-plane. The difference between the two models is probably due to the fact that the single crystal used in the earlier work was highly twinned. Empirical calculations of the copper valences show that the Cu+++ cations are almost equally distributed over the two sites. No structural change has been detected at the transition.

https://iopscience.iop.org/article/10.1209/0295-5075/3/12/009/pdf

Structure of the 100 K Superconductor Ba2YCu3O7 between (5 ÷ 300) K by Neutron Powder Diffraction

Properties of a new perovskite oxyde Sr2VO4

The magnetic phase diagram of the system ZnCr2xGa2-2xO4 has been experimentally determined by using low-field DC and AC susceptibility measurements, neutron diffraction experiments and Mossbauer spectroscopy A spin glass behaviour has been found in the concentration range 040<or=x<085; this behaviour is a result of the frustration inherent in the octahedral spinel sublattice with antiferromagnetic interactions only

Magnetic phase diagram of ZnCr2xGa2-2xO4 spinels: an antiferromagnetic frustrated lattice

A randomly dilute antiferromagnetic system with spinel structure ZnCr2xGa2-2xO4 (octahedral magnetic sublattice) was studied by means of low-field susceptibility measurements, neutron diffraction experiments and Mossbauer spectra With decreasing x the antiferromagnetic structure existing for x=1 (TN=13K) is progressively perturbed and a spin-glass phase is established for 040<or=x<085 as a consequence of the frustration inherent in the octahedral spinel sublattice with only antiferromagnetic interactions

J L Soubeyroux

Papers

Structure of the 100 K Superconductor Ba2YCu3O7 between (5 ÷ 300) K by Neutron Powder Diffraction

Properties of a new perovskite oxyde Sr2VO4

Magnetic phase diagram of ZnCr2xGa2-2xO4 spinels: an antiferromagnetic frustrated lattice

From the antiferromagnetic regime to the spin-glass state in the frustrated spinel system ZnCr2xGa2-2xO4