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

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

This review considers unusual effects in superconductor-ferromagnet structures, in particular, the triplet component of the condensate generated in those systems. This component is odd in frequency and even in momentum, which makes it insensitive to nonmagnetic impurities. If the exchange field is not homogeneous in the system, the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, and formation of periodic magnetic structures due to the influence of the superconductor. Finally, theoretical predictions are compared with existing experiments.

https://arxiv.org/pdf/cond-mat/0506047.pdf

Odd triplet superconductivity and related phenomena in superconductor-ferromagnet structures

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 detailed normal state measurements of the resistivity, magnetic susceptibility and specific heat of high-quality single crystals of Sr 2 RuO 4 , the only copper-free, layered perovskite superconductor ( T c ≈1 K). It is intriguing that despite its very strong anisotropy (the ratio of the interlayer to in-plane resistivity exceeds 500) arising from the essentially two-dimensional Fermi surface, the normal state properties are consistent with a universal description for a strongly correlated Fermi liquid. Therefore, Sr 2 RuO 4 serves as an ideal reference material for the investigation of the limits of applicability of Fermi liquid theory to other highly correlated layered compounds, including high- T c cuprates.

Two-Dimensional Fermi Liquid Behavior of the Superconductor Sr2RuO4

Ru NMR and nuclear quadrupole resonance studies are reported on single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ ${(T}_{c}=$0.7 K) with the same layered perovskite structure as ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. The Pauli spin susceptibility deduced from the Ru Knight shift is found to be largely enhanced by a factor of $\ensuremath{\sim}$5.4 as compared with the value from the band calculation. In the superconducting state, the nuclear spin-lattice relaxation rate ${1/T}_{1}$ exhibits a sharp decrease with no coherence peak just below ${T}_{c}$ and the ${T}_{1}T$=constant behavior well below ${T}_{c},$ suggesting that the anisotropic pairing state is realized as in heavy-fermion and high-${T}_{c}$ superconductors.

Anisotropic pairing in superconducting Sr 2 RuO 4 : Ru NMR and NQR studies

We discuss some of the current issues on the copper-free layered perovskite superconductor Sr2RuO4, for which a sharp transition at Tc = 1.2 K has been reproducibly obtained. The normal state is characterized as an essentially twodimensional Fermi liquid, and the coherent interlayer transport is established only at low temperatures. The cylindrical Fermi surface observed by de Haas-van Alphen experiments is consistent with other thermodynamic and transport properties. Although the specific heat jump across Tcconfirms the bulk superconductivity, the large residual T-linear term which correlates with the variation in Tcis unusual and suggestive of unconventional pairing.

Normal-state and superconducting properties of 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

K. Yoshida

Papers

Superconductivity in a layered perovskite without copper

Two-Dimensional Fermi Liquid Behavior of the Superconductor Sr2RuO4

Anisotropic pairing in superconducting Sr 2 RuO 4 : Ru NMR and NQR studies

Normal-state and superconducting properties of Sr2RuO4

Anisotropic superconductivity of Sr2RuO4