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

Spectacular advances in superconductors have taken place in the past two years. The upper temperature for superconductivity has risen from 23 K to 122 K, and there is reason to believe that the ascent is still ongoing. The materials causing this excitement are oxides. Those oxides that superconduct at the highest temperatures contain copper-oxygen sheets; however, other elements such as bismuth and thallium play a key role in this new class of superconductors. These superconductors are attracting attention because of the possibility of a wide range of applications and because the science is fascinating. A material that passes an electrical current with virtually no loss is more remarkable when this occurs at 120 K instead of 20 K.

https://science.sciencemag.org/content/sci/242/4885/1519.full.pdf

Chemistry of High-Temperature Superconductors

The superconducting phase YBa2Cu4O8 ('124') differs from YBa2Cu3O7 ('123') in having a double, instead of single, Cu–O chain running parallel to the b axis. It was first observed as a lattice defect in partly decomposed 123 powders1, and then as an ordered defect structure in inhomogeneous 123 thin films2,3, but has not up to now been synthesized in bulk. As part of a systematic study of the pressure–temperature–composition phase diagram of pseudo-binary Y–Ba–Cu–O systems at high oxygen pressure4,5, we have determined the approximate field of thermodynamic stability of the 124 phase. Here we report synthesis of the phase in bulk at 400 bar O2 and 1,040 °C. In agreement with previous measurements in thin films, the transition temperature is 81 K. Unlike the 123 compound, the oxygen content in 124 is thermally stable up to 850 °C, which may prove important for applications.

Bulk synthesis of the 81-K superconductor YBa2Cu4O8 at high oxygen pressure

Rapid depletions of atmospheric mercury (Hg0) and ozone coinciding with high halogen concentrations have recently been observed in several high-Arctic and sub-Arctic regions. The lack of kinetic data on the halogen-initiated reactions of elemental mercury precluded the drawing of any conclusions on the chemistry of these mercury depletion events. We carried out extensive kinetic and product studies on the reactions of gaseous Hg0 with molecular and atomic halogens (X/X2 where X = Cl, Br) at atmospheric pressure (750 ± 1 Torr) and room temperature (298 ± 1 K) in air and N2. Kinetics of the reactions with X/X2 was studied using relative and absolute techniques by cold vapor atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS). The measured rate constants for reactions of Hg0 with Cl2, Cl, Br2, and Br were (2.6 ± 0.2) × 10-18, (1.0 ± 0.2) × 10-11, <(0.9 ± 0.2) × 10-16, and (3.2 ± 0.3) × 10-12 cm3 molecule-1 s-1, respectively. The reaction products were analy...

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Reactions of Gaseous Mercury with Atomic and Molecular Halogens: Kinetics, Product Studies, and Atmospheric Implications

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

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.

L. J. Martinez-Miranda

Papers

Crystal structure of the 80 K superconductor YBa2Cu4O8