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

The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

Synthesis and properties of epitaxial electronic oxide thin-film materials

The electronic structure of the transition-metal compounds is discussed with nickel oxide (NiO) serving as the prime example. Other systems are also discussed, especially the high-temperature superconductors. The main finding of the review from the inspection of many data is that the electronic structure of these compounds is severely influenced by correlation of the Mott-Hubbard-type. Also in the metallic (itinerant) state these systems show aspects of correlations (satellites), which can only be explained in a one-ion model. It is this simultaneous occurrence of one-ion and one-electron features which makes the investigation of this class of materials fascinating but also frustrating, because at this point there is no unified theoretical approach available that would allow one to describe the electronic properties of these systems coherently.

Electronic structure of NiO and related 3d-transition-metal compounds

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable “structural isosbestic” point and a threshold effect for the dependence of c-axis expansion (Δc) on fluence (F), with Δc/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at ∼0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.

https://science.sciencemag.org/content/sci/316/5823/425.full.pdf

Nonequilibrium phase transitions in cuprates observed by ultrafast electron crystallography.

ELECTRON-DOPED copper oxide superconductors have recently been discovered in the system Ln2–xCexCuO4–y, where Ln is Pr, Nd or Sin1,2. Here we report the growth of single crystals of Nd2–xCexCuO4–y, and measurements of their anisotropic superconducting properties. A sharp superconducting transition is observed at 22.5 K in Nd1.84Ce0.16CuO4–y. The resistivity shows a metallic temperature dependence both parallel and perpendicular to the basal plane. Applying a magnetic field perpendicular to the basal plane causes a parallel shift of the resistive transition curve to lower temperatures, as in conventional type II superconductors but unlike the hole-doped copper oxide superconductors. The Ginzburg–Landau coherence lengths estimated from the resistively defined upper critical magnetic field are 70.2 A in the basal plane and 3.4 A along the c axis, showing an anisotropy factor of 21. This value is much larger than that in La2–xSrxCuO4–y despite their apparent structural similarities. Crystallographic differences from the hole-doped systems and the concomitant changes in electronic properties may offer clues to the role of the Cu–O planes in the microscopic pairing mechanism of high-temperature superconductors.

Growth and anisotropic superconducting properties of Nd2-xCexCu04-y single crystals

The low-field Hall coefficient ${R}_{H}$, optical reflectance and transmittance of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system with various Sr concentrations from $x=0 \mathrm{to} 0.36$ are systematically studied using single-crystal thin films epitaxially grown on (100) face SrTi${\mathrm{O}}_{3}$ substrates with the $c$ axis normal to the film surface. For the $x$ range measured, ${R}_{H}$ is positive and decreases more rapidly than that expected from the Sr concentration but more slowly than reported earlier for polycrystalline specimens, indicating anisotropy of ${R}_{H}$. Furthermore, the $x$ dependence indicates deviation from that expected from a simple band model. Within the superconducting composition range, ${R}_{H}$ exhibits characteristic temperature dependence. The optical reflectance spectrum changes from that of a semiconductor at $x=0$ to a typical metallic one characterized by the Drude model for $xg0.1$, indicating the development of itinerant holes in the Cu-O planes. In the optical transmission spectra, an anomalous absorption band is seen in addition to the fundamental absorption corresponding to an energy gap of about 2 eV. This band, which develops with Sr doping, implies an enhancement of the density of states near the Fermi level. Taking these observations into account, the normal-state transport properties are explained with a qualitative consistence.

Minoru Suzuki

Papers

Hall coefficients and optical properties of La2-xSrxCuO4 single-crystal thin films.