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

Sol-gel chemistry of transition metal oxides

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

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

This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Invited review “sol-gel” preparation of high temperature superconducting oxides

Large area (up to 2 cm2) thin (10–100 μm) single crystals of Bi2+xSr2−xCuO6+y (“2201”) were grown using a self-flux technique. These crystals grow with the c-axis perpendicular to their platelet-like surface, with a cation composition, as determined by wavelength dispersive X-ray analysis (WDXA), strongly deficient in Sr. Their transport properties, which rank from semiconducting/insulating or metallic to superconducting, can be altered by changing the growth ambient (either N2, air or O2) as well as by choosing different annealing treatments.

Growth and properties of large area Bi2+xSr2−xCuO6+y single crystals

We have studied the oxygen dependence on the two magnetic transitions (antiferromagnetic ordering of chains and planes) in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{6+y}$ using neutron scattering. It is found that both transition temperatures increase with decreasing oxygen concentration. At $y\ensuremath{\approx}0.37$ ($\ensuremath{\equiv}{y}_{0}$) the two transition temperatures are equal, so that chains and planes order at a single transition temperature for $y\ensuremath{\le}{y}_{0}$. For $y=1$ the compound is superconducting at 60 K. Therefore, this system qualitatively exhibits the magnetic and superconducting properties of pure $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ while providing important insight on the oxygen dependence of chain site magnetic ordering. A discussion is presented which also includes results on Ni and Al substitutions.

Magnetic transitions in the system YBa2Cu2.8Co0.2O6+y.

The synthesis procedures (solid state and solution) used to produced YBa 2 Cu 3 O 7−x superconducting material in bulk or thick film form are briefly reviewed. Our results indicate that a solution technique (sol-gel process) permits lower reaction temperatures and yields dense (90%) and homogeneous ceramics with conducting properties which, in the normal states, tend toward those obtained on single crystals. However, independent of the synthesis technique used, the critical currents I c remain low probably because of the presence of carbon or carbonate at the grain boundaries. Stable, viscous gels were used to prepare superconducting thick films on various substrates and problems specifically associated with thick film processing (i.e. film adhesion and reaction with the substrate) are discussed. Solid state reactions have been used to perform doping studies and to produced 3d-metal-substituted perovskites YBa 2 Cu 3−x M x O y−y in which the oxygen content has been found to depend upon the nature and concentration of the dopant (i.e. M).

On synthesis of high Tc superconducting perovskites

The substitution of bismuth by lead was performed in the compound Bi2−xPbxSr2(Sm0.85Ce0.15)2Cu2O10+y. Its influence on the superconducting properties as well as on the structure was studied by X-ray diffraction, transmission electron microscopy (TEM) and various transport measurements. Lead was found to replace bismuth up to approximately x = 1, superconductivity occurring with a maximum Tc of 34–36 K at concentrations of x greater than 0.6. Resistivity data show a transition from semiconducting to metallic/superconducting behavior. Increasing lead content is accompanied by a decrease in the oxygen content with a corresponding decrease of the structural modulation which is directly correlated to the excess oxygen. At x = 0.9 this compound does not show a structural modulation, but still superconducts at 36 K.

G. W. Hull

Papers

Growth and properties of large area Bi2+xSr2−xCuO6+y single crystals

Magnetic transitions in the system YBa2Cu2.8Co0.2O6+y.

On synthesis of high Tc superconducting perovskites

Increase of Tc and disappearance of the structural modulation in the system Bi-Sr-Sm-Ce-Cu-O upon Pb substitution

On several new ternary molybdenum sulfide phases M3.4Mo15S19 (M = vacancy, Li, Na, K, Zn, Cd, Sn and Tl)