We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.

Iron-Based Layered Superconductor La[O1-xFx]FeAs (x = 0.05−0.12) with Tc = 26 K

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

This review presents a summary and evaluation of the experimental properties of unconventional superconductivity in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ as they were known in the spring of 2002. At the same time, the paper is intended to be useful as an introduction to the physics of spin-triplet superconductivity. First, the authors show how the normal-state properties of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ are quantitatively described in terms of a quasi-two-dimensional Fermi liquid. Then they summarize its phenomenological superconducting parameters in the framework of the Ginzburg-Landau model, and discuss the existing evidence for spin-triplet pairing. After a brief introduction to the vector order parameter, they examine the most likely symmetry of the triplet state. The structure of the superconducting energy gap is discussed, as is the effect of symmetry-breaking magnetic fields on the phase diagram. The article concludes with a discussion of some outstanding issues and desirable future work. Appendixes on additional details of the normal state, difficulty in observing the bulk Fermi surface by angle-resolved photoemission, and the enhancement of superconducting transition temperature in a two-phase ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}\ensuremath{-}\mathrm{Ru}$ system are included.

The superconductivity of Sr 2 RuO 4 and the physics of spin-triplet pairing

We report superconductivity in an iron-based layered oxy-pnictide LaOFeP. LaOFeP is composed of an alternate stack of lanthanum oxide (La3+O2-) and iron pnictide (Fe2+P3-) layers. Magnetic and electrical resistivity measurements verify the occurrence of the superconducting transition at ∼4 K.

Iron-Based Layered Superconductor: LaOFeP

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

FOLLOWING the discovery of superconductivity at ∼30 K in La2−xBaxCuO4 (ref. 1), a large number of related compounds have been found that are superconducting at relatively high temperatures. The feature common to all of these materials is a layered crystal structure based on a perovskite template and containing planar networks of copper and oxygen. This raises the question of whether superconductivity can occur in layered perovskites that do not contain copper. To the best of our knowledge, no such material has been found to date, despite nearly a decade of searching. We describe here the discovery of superconductivity in Sr2RuO4, a layered perovskite isostructural with La2−xBaxCuO4 (Fig. 1). Our results demonstrate that the presence of copper is not a prerequisite for the existence of superconductivity in a layered perovskite. But the low value of the superconducting transition temperature (Tc = 0.93 K) points towards a special role for copper in the high-temperature superconductors.

Superconductivity in a layered perovskite without copper

We report a comprehensive study of magneto-oscillatory phenomena in the normal state of S${\mathrm{r}}_{2}$Ru${\mathrm{O}}_{4}$, the first layered perovskite superconductor $({T}_{c}\ensuremath{\cong}1\mathrm{K})$ not based on copper. The form of the quasiparticle spectrum observed may be interpreted in terms of an almost two-dimensional Fermi liquid model which is consistent with Luttinger's theorem and successfully predicts bulk thermodynamic and transport properties at low temperatures. A study of the spectra and transport along the c axis provides insights into the different normal state and superconducting behavior of S${\mathrm{r}}_{2}$Ru${\mathrm{O}}_{4}$ and the cuprates.

Quantum oscillations in the layered perovskite superconductor Sr2RuO4

High-temperature resistivity of Sr2RuO4: Bad metallic transport in a good metal

We report a detailed study of the Hall effect in ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$, the first layered perovskite superconductor which does not contain copper (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}1 K). The Hall coefficient (${\mathit{R}}_{\mathit{H}}$) was measured at temperatures between 20 mK and 300 K, and an unusual dependence of ${\mathit{R}}_{\mathit{H}}$ on the applied magnetic field was observed. ${\mathit{R}}_{\mathit{H}}$ has a strong temperature dependence below 25 K, but below 1 K it saturates at a value of -1.15\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{m}}^{3}$/C. The Fermi surface of ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$ is known from quantum oscillation measurements, and since it is nearly two dimensional, it is possible to derive a simple expression for ${\mathit{R}}_{\mathit{H}}$ using methods developed by Ong [Phys. Rev. B 43, 193 (1991)]. We show that if the mean free path, l, is assumed to be isotropic at low temperatures, it is possible to make an accurate quantitative calculation of ${\mathit{R}}_{\mathit{H}}$ on the basis of the known Fermi surface parameters. \textcopyright{} 1996 The American Physical Society.

Hall effect in the two-dimensional metal Sr2RuO4

We report the upper critical field H c2 of single crystalline Sr 2 RuO 4 , which is the only known layered-perovskite superconductor without copper. The ratio H c2∥ ab / H c2∥ c = 26 indicates that the ruthenate is a highly anisotropic superconductor. In addition, we found a peculiar second dissipation peak only when the flux lines, as well as their induced motions, are within the ab -planes.

/pdf/anisotropic-superconductivity-of-sr2ruo4-4aac5ztchl.pdf

S. Nishizaki

Papers

Superconductivity in a layered perovskite without copper

Quantum oscillations in the layered perovskite superconductor Sr2RuO4

High-temperature resistivity of Sr2RuO4: Bad metallic transport in a good metal

Hall effect in the two-dimensional metal Sr2RuO4

Anisotropic superconductivity of Sr2RuO4