In narrow-band systems electrons can interact with each other via a short-range nonretarded attractive potential. The origin of such an effective local attraction can be polaronic or it can be due to a coupling between electrons and excitons or plasmons. It can also result from purely chemical (electronic) mechanisms, especially in compounds with elements favoring disproportionation of valent states. These mechanisms are discussed and an exhaustive list of materials in which such local electron pairing occurs is given. The authors review the thermodynamic and electromagnetic properties of such systems in several limiting scenarios: (i) Systems with on-site pairing which can be described by the extended negative-$U$ Hubbard model. The strong-attraction limit of this model, at which it reduces to a system of tightly bound electron pairs (bipolarons) on a lattice, is extensively discussed. These electron pairs behaving as hard-core charged bosons can exhibit a superconducting state analogous to that of superfluid $^{4}\mathrm{He}$ II. The change-over from weak-attraction BCS-like superconductivity to the superfluidity of charged hard-core bosons is examined. (ii) Systems with intersite pairing described by an extended Hubbard model with $Ug0$ and nearest-neighbor attraction and/or nearest-neighbor spin exchange as well as correlated hopping. (iii) A mixture of local pairs and itinerant electrons interacting via a charge-exchange mechanism giving rise to a mutually induced superconductivity in both subsystems. The authors discuss to what extent the picture of local pairing, and in particular superfluidity of hard-core charged bosons on a lattice, can be an explanation for the superconducting and normal-state properties of the high-${T}_{c}$ oxides: doped BaBi${\mathrm{O}}_{3}$ and the cuprates.

Superconductivity in narrow-band systems with local nonretarded attractive interactions

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

The local oxygen density of states of p symmetry at the Fermi level of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ has been investigated as a function of x and y. Core-level excitations of oxygen 1s electrons into empty states by electron energy-loss spectroscopy has been used. Strong deviations from our calculations of the O 1s edges in the framework of a local density-functional model were observed in the superconducting and in the semiconducting compounds. They can be explained by taking into account on-site correlations of the Cu 3d electrons. The results indicate that for the superconductors, the density of states at the Fermi energy has predominantly oxygen 2p character. In this band the charge carriers in the superconducting compounds are holes. In the semiconducting compounds, the oxygen 2p band is probably completely filled.

Evidence for holes on oxygen sites in the high- T c superconductors La 2 − x Sr x CuO 4 and YBa 2 Cu 3 O 7 − y

High-energy electron energy-loss spectroscopy in transmission has been used to obtain information on the symmetry of unoccupied electronic states in the new high-T/sub c/ superconductors. In particular, for YBa/sub 2/Cu/sub 3/O/sub 7/ and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/, the O 1s and Cu 2p absorption edges have been measured for momentum transfer parallel and perpendicular to the a,b plane. Both sigma and in-plane ..pi.. holes on O, formed upon doping, are compatible with our experimental results. For YBa/sub 2/Cu/sub 3/O/sub 7/, in both cases, holes must be in the planes and in the ribbons. Out-of-plane ..pi.. holes can be excluded for YBa/sub 2/Cu/sub 3/O/sub 7/ and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/. In the undoped and doped compounds, the unoccupied states on the Cu sites have predominantly in-plane 3d/sub x//sub <2/-y/sup =/ symmetry. There is an admixture of about 10% of lower-lying states probably with 3d/sub 3//sub z//sub <2/-r/sup =/ symmetry.

Symmetry of holes in high-Tc superconductors.

A detailed x-ray absorption study of the oxygen $K$ edge of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ is presented. A preedge peak is observed for all samples with $x\ensuremath{\ge}6.4$ which we argue to be due to holes in the $\mathrm{O} 2p$ band. By comparison to ${\mathrm{Li}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ the $x$ dependence of the number $\mathrm{O} 2p$ holes in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ is determined.

X-ray absorption study of the O 2p hole concentration dependence on O stoichiometry in YBa2Cu3Ox.

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x
 is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x
 is constructed.

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

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The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x