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

The crystal chemistry of several Ba/sub 2/RCu/sub 3/O/sub 7/ high-T/sub c/ superconductors (R = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm) has been studied by x-ray diffraction on twinned crystals. All samples except the Pr compound show superconductivity around 90 K. From the variation of the interatomic distances with different rare-earth ions, we infer the presence of a (CuO/sub 2/-BaO-CuOD'Alembertian-BaO-CuO/sub 2/) building unit.

/pdf/structural-properties-of-ba2rcu3o7-high-tc-superconductors-1tmk6hexjg.pdf

Structural properties of Ba2RCu3O7 high-Tc superconductors

The synthesis of high‐temperature superconducting oxides using solution chemistry has been investigated and physical properties are compared to ceramics obtained by conventional solid‐state reactions. We report on controlled precipitation and sol‐gel processes, both of which produce materials with particle sizes smaller than 5 μm. We find that the superconducting properties of the high Tc ceramics are affected by their manner of preparation, such that the transitions are slightly lower in temperature, but sharper, for samples made by solution rather than solid‐state chemistry. The ability to prepare stable viscous gels provides an opportunity for obtaining large areas of superconducting coatings. For thick films on alumina or silicon substrates, contamination from the substrate is shown to be a problem. Finally, we observe that the sol‐gel process lowers the synthesis temperature by 100 °C.

Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol‐gel processes

The structure and magnetic properties of the Bi/sub 2/Sr/sub 2/Cu/sub 1/minus//ital x//M/sub x/O/sub /ital y// (/ital M/=Co and Fe) materials were studied. The limits of solid solution formation are at /ital x/=0.5 for the Fe system and /ital x/=1 for the Co system. Crystals of the new Bi/sub 2/Sr/sub 2/CoO/sub /ital Y// phase were grown and the structure established by x-ray crystallography. The subcell is the same as that of the 10-K superconductor, Bi/sub 2/Sr/sub 2/CuO/sub /ital y//, but the superstructure is different, as it exhibits a commensurate modulation of periodicity 4 instead of 5. Extra oxygen is accommodated in the Bi layers, as in Bi/sub 2/Sr/sub 3/Fe/sub 2/O/sub /ital y//, and the structure of the Bi-O layers can be described as 50% rocksalt-type and 50% oxygen-deficient perovskite for /ital x/=1, but with disorder at the oxygen positions. The formal valence of Co in this compound is about 2.5+-0.2 as deduced from structural and chemical measurements, whereas Fe adopts the oxidation state +3 as deduced by M/umlt o/ssbauer measurements. Bi/sub 2/Sr/sub 2/CoO/sub /ital y// is an antiferromagnetic insulator with the spins lying within the CoO/sub 2/ sheets and the antiferromagnetic ordering temperature (/ital T//sub /ital N//) is sensitive to processingmore » conditions and composition changes. The high anisotropy of the susceptibility suggests that Bi/sub 2/Sr/sub 2/CoO/sub /ital y// may be an Ising or /ital XY/ antiferromagnet.« less

/pdf/structure-and-magnetic-properties-of-nonsuperconducting-4zfeab174o.pdf

Structure and magnetic properties of nonsuperconducting doped Co and Fe Bi2Sr2Cu1-xMxOy phases.

Single crystals of the n-type Nd-Ce-Cu-O superconducting materials have been grown via a flux technique, and their structural and physical properties characterized. Optimum crystal growth conditions were arrived at from a survey of various compositions and temperatures. We determine that the charge composition (50 mol % ${\mathrm{Nd}}_{2}$${\mathrm{O}}_{3}$--${\mathrm{CeO}}_{2}$, 50 mol % CuO) held at a temperature of 1300 \ifmmode^\circ\else\textdegree\fi{}C and followed by a slow cooling to 1000 \ifmmode^\circ\else\textdegree\fi{}C produces plateletlike crystals of the n-type phase ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$, which, after being annealed at 900 \ifmmode^\circ\else\textdegree\fi{}C in nitrogen, become superconducting. The limit of the Ce solubility is x=0.18, and only crystals having a Ce content between 0.14 and 0.17 were found to superconductors, with the maximum ${T}_{c}$ occurring at x=0.14. The oxygen uptake and loss in the Nd material is similar to that measured on ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ but occurs at higher temperatures (750 \ifmmode^\circ\else\textdegree\fi{}C instead of 500 \ifmmode^\circ\else\textdegree\fi{}C). The large Meissner fraction indicates bulk superconductivity. However, due to a complex microstructure or compositional (e.g., oxygen) inhomogeneities in the crystal, zero resistance is difficult to achieve. The resistivity temperature dependence above ${T}_{c}$ is metalliclike, and linear above 150 K, in contrast to bulk ceramics. Finally, no evidence for magnetic ordering over the temperature range 4--350 K was observed for the superconducting Ce-doped ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ phase, whereas signs of magnetic ordering were found at 340 K for the undoped material.

/pdf/growth-structural-and-physical-properties-of-superconducting-4rf9dr53a7.pdf

Growth, structural, and physical properties of superconducting Nd2-xCexCuO4 crystals.

The effect of oxygen content and cationic substitution on the structural and superconducting properties of the T\ensuremath{'} phase were studied by investigating the ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{Ce}}_{\mathrm{y}}$${\mathrm{CuO}}_{\mathrm{z}}$ and ${\mathrm{R}}_{1.85\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{R}}_{\mathrm{x}}^{\ensuremath{'}}$${\mathrm{M}}_{0.15}$${\mathrm{CuO}}_{\mathrm{z}}$ (R=Nd,Eu; R\ensuremath{'}=La,Y; M=Ce,Th; z\ensuremath{\cong}4) series. We find that solid solutions of the T\ensuremath{'} phases can be prepared, and the solubility range of La and Y depends on the sizes of the host versus the dopant ions. The transition from tetragonal T\ensuremath{'} phase to the orthorhombic T phase is quite abrupt, without evidence for the T* phase. In this system, as for the La-Sr-Cu-O system, we find that superconductivity seems to correlate with the in-plane Cu-Cu distance, and there is a critical distance (3.95 \AA{}) for which ${\mathrm{T}}_{\mathrm{c}}$ is maximum. The as-prepared Ce- or Th-doped materials have an oxygen content per unit formula greater than four, and when heated under nitrogen they lose oxygen in two steps at temperatures of 400 and 800 \ifmmode^\circ\else\textdegree\fi{}C, with the amount of oxygen lost through each step being strongly dependent on sample processing. The total amount of mobile oxygen lost decreases with increasing Ce or Th content. From thermogravimetric-analysis measurements we show the crucial importance of the reducing step at temperatures greater than 850 \ifmmode^\circ\else\textdegree\fi{}C for inducing superconductivity as well as the importance of the cooling rate on the superconducting properties of the T\ensuremath{'} phases. The importance of oxygen disorder to the normal-state properties is demonstrated.

/pdf/cationic-substitution-and-role-of-oxygen-in-the-n-type-1hw25jwknn.pdf

G. W. Hull

Papers

Structural properties of Ba2RCu3O7 high-Tc superconductors

Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol‐gel processes

Structure and magnetic properties of nonsuperconducting doped Co and Fe Bi2Sr2Cu1-xMxOy phases.

Growth, structural, and physical properties of superconducting Nd2-xCexCuO4 crystals.

Cationic substitution and role of oxygen in the n -type superconducting T ′ system Nd 2 − y Ce y CuO z