The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

Crystal chemistry and superconductivity of the copper oxides

A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

Crystal structures of high- T c oxides

Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure 100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400012498

Reactive coevaporation of YBaCuO superconducting films

Hole concentration and T c in Bi 2 Sr 2 CaCu 2 O 8+δ

The electron deficiency (hole concentration) in the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ system is determined by a chemical method. Our results show a direct correlation between ${T}_{c}$ and hole concentration. The hole concentration equals the Sr concentration to about $x=0.15$. For $xg0.15$, the hole concentration decreases and oxygen vacancies are formed. These results strongly support an all-electronic mechanism for superconductivity in this system.

Correlation of Tc with hole concentration in La2-xSrx

One of the fundamental questions involving the high-temperature copper oxide containing superconductors is to understand the relationship between hole concentration and the superconducting transition temperature, ${T}_{c}$. We help resolve this question by a study of Nd-, La-, Ga-, Ca-, and Zn-substituted $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ with controlled oxygen stoichiometry. The total hole concentration is determined by a chemical method and trends in the mobile hole concentration by Hall measurements. By considering ${T}_{c}$ to be a function of the mobile sheet holes only, there is a maximum in ${T}_{c}$ (92 K) at a sheet-hole concentration of about 0.20-0.25 per Cu site.

http://www.w2agz.com/Library/HTSC%20History/Shafer,%20Penney,%20Olson%20(1989)%20Hole%20concentrations,%20Hall%20number,%20and%20Tc%20relationships%20in%20substituted%20YBCO.pdf

Hole concentrations, Hall number, and Tc relationships in substituted YBa2Cu3Oy.

The compensation doped systems Nd (1+x)−c Ca c Ba 2−x Cu 3 O y and Nd 1+x Ba 2−x Cu 3−z Zn z O y are studied to elucidate both the dependence of T c on the Cu O plane hole concentration and the effect of disorder on the planes. Nd substitution for Ba reduces T c and the plane holes. Ca substitution compensates for Nd, raising T c and the plane holes. Zn has a small effect on the hole concentration. More importantly, Zn causes disorder on the Cu O planes, suppressing T c . The choice of these Nd systems is very important, because, contrary to the YBa 2 Cu 3−x Zn x O y based systems, the Hall number can be unambiguously interpreted and is in good agreement with the plane hole concentration derived from chemical analysis and the method of Tokura et al. for dividing the total holes into mobile plane holes and local chain holes.

Hole concentration, Hall number, and disorder in compensation doped NdBa2Cu3Oy

We report a study of the magnetic susceptibility, Hall effect, resistivity and specific heat of pure and Sr-doped La 2 CuO 4 . Undoped La 2 CuO 4 exhibits antiferromagnetic order and traces of superconductivity, sensitivity dependent on stoichiometry. The data provide evidence for strong electron-electron correlations in the Sr-doped superconductors, and a scaling of T c with the electron deficiency, supporting an all-electronic model for the oxide superconductivity.

B. L. Olson

Papers

Correlation of Tc with hole concentration in La2-xSrx

Hole concentrations, Hall number, and Tc relationships in substituted YBa2Cu3Oy.

Hole concentration, Hall number, and disorder in compensation doped NdBa2Cu3Oy

ANTIFERROMAGNETIC CORRELATIONS AND SUPERCONDUCTIVITY IN THE La2CuO4 CLASS OF OXIDES

Comparison of Hall Number, Hole Concentration and Tc in Nd and Zn Substituted YBa2Cu3Oy Superconductors