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

Results are reported on a room temperature static compression study of iron and the Fe(0.8)Ni(0.2) alloy to above 260 GPa, which provide direct pressure-volume measurements on geophysically important materials at the conditions close to those at the inner core boundary. The diffraction patterns obtained by XRD indicate that pure iron remains in the hcp structure to 304 GPa and that Fe(0.8)Ni(0.2) is stable to at least 255 GPa. The results of this study, in conjunction with work at higher temperatures, will make it possible to address directly the question of the composition of the inner core with a level of certainty that were previously applicable only to the mantle.

Static compression of iron to 300 GPa and Fe0.8Ni0.2 alloy to 260 GPa: Implications for composition of the core

Bulk superconductivity is reported in T12Ca2 Ba2 Cu3 Ox with a transition to zero resistance at ≃ 125 K, the highest transition temperature (Tc) yet found. Transmission electron spectroscopy shows that the unit cell is body-centered tetragonal and contains trilayer Cu perovskitelike units separated by bilayer TI-O units. (T c in this material varies from 118 to 125 K depending on preparation conditions. A high density of perovskitelike bilayer intergrowths are observed in the 118-K material, which we speculate result in the decreased (T c. A second structure containing only bilayer perovskitelike units—TI2 Ca1 Ba2Cu2Ox—is a bulk superconductor with Tv ranging from 95 to 108 K.

Bulk superconductivity at 125 K in Tl2Ca2Ba2Cu3Ox.

There is now a new series of high-temperature superconductors that may be represented as (A(III)O)(2)A(2)(II)Can-1CunO2+2n where A(III) is Bi or Tl, A(II) is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when A(III) is Tl. This compound, Tl(2)Ba(2)Ca(2)Cu(3)O(10), has the highest transition temperature( approximately 125 K) of any presently known bulk superconductor. The structure of Tl(2)Ba(2)Ca(2)Cu(3)O(10) has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 A and c = 35.9 A. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor

Since the report that multiphase mixture with the nominal composition La2−xBaxCCuO4−J, exhibit possible superconductivity1, research on this class of quaternary oxides has increased enormously. The superconducting compound in the original mixture was rapidly identified as La1.85Ba0.15CuO42, and subsequent research concentrated on the most suitable cation substitutions in this compound. A new oxide with a considerably higher critical temperature, Tc, was very soon found in the phase system Y2O3-BaO–CuO3, later identified by several groups as YBa2Cu3O7–δ; and now commonly known as the 1–2–3 compound. Intensive activity in structural characterization followed including reports of various planar defects. Here we report an analysis, using high-resolution electron microscopy of planar defects in superconducting YBa2Cu3O7, revealing the identity of a CuO double layer intercalation. Within this double layer, the Cu atoms remain in planar fourfold coordination with O, identical to their arrangement in the single CuO layers of the YBa2Cu3O7 matrix.

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Structure of (CuO) 2 double layers in superconducting YBa2Cu3O7

Two superconducting phases, ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$ (2223) and ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{2}$${)}_{8+\mathrm{\ensuremath{\delta}}}$ (2122), both with onset ${\mathrm{T}}_{\mathrm{c}}$ near 120 K and zero resistivity at 100 K, have been isolated from samples in the Tl-Ca-Ba-Cu-O system. The new 2223 superconductor has a 5.40\ifmmode\times\else\texttimes\fi{}5.40\ifmmode\times\else\texttimes\fi{}36.25-A${\mathrm{\r{}}}^{3}$ pseudotetragonal unit cell. The 2122 superconductor, which appears to be structurally related to ${\mathrm{Bi}}_{2}$${\mathrm{CaSr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$, has a 5.44\ifmmode\times\else\texttimes\fi{}5.44\ifmmode\times\else\texttimes\fi{}29.55-A${\mathrm{\r{}}}^{3}$ pseudotetragonal subcell. The 2223 phase is probably related to 2122 by the addition of extra calcium and copper layers.

100-K superconducting phases in the Tl-Ca-Ba-Cu-O system.

Single-crystal x-ray techniques were used to determine two structures that comprise the Y-Ba-Cu-O superconductor. A green phase has the orthorhombic ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ structure. An opaque phase (${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$, 0\ensuremath{\le}x\ensuremath{\le}1.0) is tetragonal (a=3.859 A\r{}, c=11.71 A\r{}; space group P4\ifmmode\bar\else\textasciimacron\fi{}m2;Z=1). The structure is related to a 1:1:3 AB${\mathrm{O}}_{3}$ perovskite: Ba and Y order in A sites and Cu is in B sites. Systematic oxygen vacancies between adjacent copper atoms lead to a fivefold-coordinate Cu in proximity to Y and a twofold-coordinate Cu in the vicinity of the Ba site.

Crystallographic description of phases in the Y-Ba-Cu-O superconductor

Superconductivity up to about 110 K in the CrTlSrCaCuO system was observed by resistance and ac susceptibility measurements. Powder X-ray diffraction analyses showed that a 1212-type phase, Cr-substituted TlSr2CaCu2O7, is responsible for the observed superconductivity. This is the first time that single-element substitution for TlSr2CaCu2O7 increases Tc of the 1212 phase to above 100 K. The Cr-substituted TlSr2CaCu2O7 is easily prepared and has very good quality suitable for practical applications.

A NEW 1212-TYPE PHASE: Cr-SUBSTITUTED TlSr2CaCu2O7 WITH Tc UP TO ABOUT 110 K

Experimental phase relations of iron to 360 kbar, 1400°C, determined in an internally heated diamond-anvil apparatus

The formation of the high temperature superconducting phases in the quinary Bi-Ca-Sr-Cu-O compound system has been investigated. The 120K-superconducting transition appears in samples prepared between 850 and 900°C. A certain homogeneity range is found to exist in this superconducting system, suggesting that higher Tc may be achievable in the same system.

C. G. Hadidiacos

Papers

100-K superconducting phases in the Tl-Ca-Ba-Cu-O system.

Crystallographic description of phases in the Y-Ba-Cu-O superconductor

A NEW 1212-TYPE PHASE: Cr-SUBSTITUTED TlSr2CaCu2O7 WITH Tc UP TO ABOUT 110 K

Experimental phase relations of iron to 360 kbar, 1400°C, determined in an internally heated diamond-anvil apparatus

THE 120K-SUPERCONDUCTING PHASE IN Bi-Ca-Sr-Cu-O