The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

https://arxiv.org/pdf/cond-mat/0208504.pdf

Angle-resolved photoemission studies of the cuprate superconductors

Characterization of amorphous and nanocrystalline carbon films

Since the first days of high-Tc superconductivity, the materials science and the physics of grain boundaries in superconducting compounds have developed into fascinating fields of research. Unique electronic properties, different from those of the grain boundaries in conventional metallic superconductors, have made grain boundaries formed by high-Tc cuprates important tools for basic science. They are moreover a key issue for electronic and large-scale applications of high-Tc superconductivity. The aim of this review is to give a summary of this broad and dynamic field. Starting with an introduction to grain boundaries and a discussion of the techniques established to prepare them individually and in a well-defined manner, the authors present their structure and transport properties. These provide the basis for a survey of the theoretical models developed to describe grain-boundary behavior. Following these discussions, the enormous impact of grain boundaries on fundamental studies is reviewed, as well as high-power and electronic device applications.

/pdf/grain-boundaries-in-high-t-c-superconductors-2widq11ljn.pdf

Grain boundaries in high- T c superconductors

Experimental studies of the superconductive properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why ${\mathrm{A}}_{3}$${\mathrm{C}}_{60}$ are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity, are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.

/pdf/superconductivity-in-fullerides-1l7z4d92t8.pdf

Superconductivity in fullerides

NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds

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.

Electron-energy-loss spectroscopy has been used to investigate the changes of the electronic structure in ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (0\ensuremath{\le}x\ensuremath{\le}1). The O 1s absorption edges and the low-energy-loss function indicate that Pr does not fill holes in the ${\mathrm{CuO}}_{2}$ planes. This suggests, by comparison with other experiments, that Pr localizes mobile holes thereby leading to the suppression of superconductivity and the metal-insulator transition.

Evidence against hole filling by Pr in YBa2Cu3O7.

The electronic structure of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ has been studied by measuring O 1s absorption edges using high-energy electron-energy-loss spectroscopy in transmission. Upon doping the insulating compound the conduction-band states are reduced and states in the gap are formed. When going from insulating to conducting compounds there is a continuous increase of states at the Fermi level, which is at the bottom of the gap. The insulator-metal transition is probably driven by delocalization of these states.

H. Romberg

Papers

Symmetry of holes in high-Tc superconductors.

Evidence against hole filling by Pr in YBa2Cu3O7.

Electronic structure of the system La2-xSrxCuO4+ delta.

Electron energy-loss and X-ray absorption spectroscopy of cuprate superconductors and related compounds

Plasmons and interband-transitions in bi2sr2cacu2o8