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 effects of a substitution for Cu by other 3d metals (Ni, Co, and Zn) on ${\mathit{T}}_{\mathit{c}}$ in the Nd-Ce-Cu-O system was studied and compared with effects of the same ions on ${\mathit{T}}_{\mathit{c}}$ in the La-Sr-Cu-O system. We found (1) Zn suppresses ${\mathit{T}}_{\mathit{c}}$ more slowly in the Nd than in the La systems, so the disorder produced by the nonmagnetic ions is less important in the Nd system, (2) Ni and Co depress ${\mathit{T}}_{\mathit{c}}$ more quickly in the Nd than in the La system, showing that the magnetic pair breaking is stronger in the Nd system, and (3) in the La system the magnetism of the dopant has no effect on ${\mathit{T}}_{\mathit{c}}$. Thus, within the same chemical system (cuprates) we find that as the correlation length is increased one obtains more familiar BCS-type behavior. We suggest that much of the behavior of the various high-${\mathit{T}}_{\mathit{c}}$ oxides may be simply a function of the correlation length.

/pdf/magnetic-versus-nonmagnetic-ion-substitution-effects-on-tc-2lc9sytkp2.pdf

Magnetic versus nonmagnetic ion substitution effects on Tc in the La-Sr-Cu-O and Nd-Ce-Cu-O systems.

The upper critical fields ${B}_{c2}(T)$ were measured up to 28 T for ${R}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$, where $R=\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{u},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{D}\mathrm{y},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m}$, and for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ The midpoints ${(\frac{d{B}_{c2}}{\mathrm{dT}})}_{T={T}_{c}}$ cluster about (28\ifmmode\pm\else\textpm\fi{}02) T/K, the ${T}_{c}'\mathrm{s}=(93\ifmmode\pm\else\textpm\fi{}2)$ K show a slight systematic decrease from Nd to Tm, and the extrapolated values of ${B}_{c2}(0)$ cluster about (160\ifmmode\pm\else\textpm\fi{}20) T The wide range of normal-state resistivities indicates that they are not intrinsic to the superconductor The anisotropy $A=\frac{{B}_{c2\ensuremath{\parallel}}}{{B}_{c2}\ensuremath{\perp}}$ is estimated to be less than 12 based on the data for the resistive transition Criteria for the midpoint resistive transition are also reexamined

Upper critical fields and anisotropy limits of high-Tc superconductors R1Ba2Cu3O7-y, where R=Nd, Eu, Gd, Dy, Ho, Er, and Tm, and YBa2Cu3O7-y.

Superconductivity in rare-earth-doped oxygen-defect perovskites La2−x−yLnySrxCuO4−z

Optical data for a series of La/sub 2/..sqrt../sub x/Sr/sub x/CuO/sub 4/..sqrt../sub delta/ solid solutions from x = 0 to x = 0.30 reveal several structures, including a relatively intense, dopant-induced absorption band around 0.5 eV whose oscillator strength tracks the Meissner effect and T/sub c/ measured on the same set of samples. The dopant-induced optical-transition--superconductivity correlation strongly supports an electronically driven superconductivity mechanism mediated by the 0.5-eV excitation

https://repository.ust.hk/ir/bitstream/1783.1-59048/1/PhysRevB.37.3396.pdf

Correlation of dopant-induced optical transitions with superconductivity in La2-xSrxCuO4- delta.

The upper critical fields B/sub c//sub 2/(T) of La/sub 2-//sub x/Sr/sub x/CuO/sub 4/(x = 0.15 and x = 0.20) were measured near T/sub c/ in dc fields to 23 T and at 4.2 K with pulsed fields to 45 T. Resistive midpoint transitions for the x = 0.15 sample yield (dB/sub c//sub 2//dT)/sub T //sub = //sub =/ = 2.2 T/K, T/sub c/ = 38.1 K, and B/sub c//sub 2/(0) = 58 T assuming a dirty type-II superconductor with no Pauli paramagnetic limiting. Data for the onset yield 4.5 +- 0.5 K/sup -1/, 40.1 K, and 125 +- 15 T, respectively. Pulsed-field data show that fractions of both materials are superconducting above 45 T at 4.2 K. Calculations of limits of B/sub c//sub 2/(T) are presented and experimental limitations of measurements in these high-field superconductors are indicated.

G. W. Hull

Papers

Magnetic versus nonmagnetic ion substitution effects on Tc in the La-Sr-Cu-O and Nd-Ce-Cu-O systems.

Upper critical fields and anisotropy limits of high-Tc superconductors R1Ba2Cu3O7-y, where R=Nd, Eu, Gd, Dy, Ho, Er, and Tm, and YBa2Cu3O7-y.

Superconductivity in rare-earth-doped oxygen-defect perovskites La2−x−yLnySrxCuO4−z

Correlation of dopant-induced optical transitions with superconductivity in La2-xSrxCuO4- delta.

Upper critical fields of high-Tc superconducting La2-xSrxCuO4-y: Possibility of 140 tesla.