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

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

https://repository.ubn.ru.nl/bitstream/handle/2066/112848/112848.pdf

Electronic properties of small metallic particles

The outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read-only memories in the form of compact discs (CD-ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write-once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared-absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto-optical data recording. An introduction to CD-ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.

Materials for Optical Data Storage

Reviewed at both a relatively elementary and more advanced level is the interpretative theory of the electron spin resonance of magnetic ions in metals. The various theoretical possibilities are illustrated by experimental examples wherever possible. Initially the development is based upon the Bloch-Hasegawa equations. The relationship of the advanced theory to this more elementary theory is emphasized. Multimagnetic-impurity experiments, the skin depth problem, the analysis of exchange, hyperfine and crystal field parameters, Redfield theory, ionic and virtual bound state models and the Kondo effect, all in relation to E.S.R. in metals, are each covered in some detail. A discussion of some outstanding problems and projections for the future are also included.

Theory of electron spin resonance of magnetic ions in metals

We observed 195 Pt NMR in UPt 3 in both the normal and superconducting states, The spectrum has a large negative shift and is quite anisotropic In 5 kOe, no changes in the negative shift were observed down to the temperature of 60 mK, where the system was in the superconducting state This means that the conduction electron spin susceptibility does not decrease in the superconducting state The result does not contradict the possibility of anisotropic p-wave superconductivity of UPt 3 

195Pt knight shift in the heavy Fermion superconductor UPt3

The nuclear magnetic relaxation rate, 1/ T 1 , of 195 Pt in a normal and superconducting state was measured. In the normal state, 1/ T 1 is proportional to T , indicating the Fermi liquid nature of the system. In the superconducting state, 1/ T 1 has no enhancement just below the transition temperature and varies in proportion to T 3 at low temperatures. These results indicate the existence of an anisotropic superconducting energy gap, which vanishes at lines on the Fermi surface.

Nuclear Magnetic Relaxation in the Heavy-Fermion Superconductor UPt3

Crystals with a stoichiometry either Ba2YCu3-xO7-y or Ba4YCu2O8-y are a high tc superconductor. The former is well known. With respect to the latter the crystal structure was determined to be tetragonal with lattice parameters a=5.78 A and c=7.99 A. It was found that both of them reacted with water. The former was slowly decomposed by warmed water at about 35°C, while the latter was destroyed at room temperature in an instant.

Two Crystal Phases in the Superconducting Ba-Y-Cu-O System and Their Reactivity to Water

An anomalous Cu NMR signal with extremely small e 2 q Q has been observed in the oxide superconductor Nd 1.85 Ce 0.15 CuO 4- y . The Knight shift decreased significantly as temperature decreased below T c . The temperature-independent T 1 of 63 Cu suggests that the relaxation is dominated by the fluctuation of the Nd-moment, and no magnetic order is achieved for this content of Ce.

NMR Observation in Nd1.85Ce0.15CuO4-y

Crystal structure of Ba2YCu2.89O6.80 sigle crystal was determined by the X-ray diffraction method at room temperature. The space group is Pmmm with lattice parameters a = 3.8699(8) A, b = 3.8667(8) A and c = 11.688(2) A. A specific structure was found in the c-plane sandwiched between two barium layers. The Cu-O bond perpendicular to the plane 1.828(6) A is shorter than the normal bond 1.95 A. Within the plane oxygen atoms show an extremely large thermal vibration.

Yasukage Oda

Papers

195Pt knight shift in the heavy Fermion superconductor UPt3

Nuclear Magnetic Relaxation in the Heavy-Fermion Superconductor UPt3

Two Crystal Phases in the Superconducting Ba-Y-Cu-O System and Their Reactivity to Water

NMR Observation in Nd1.85Ce0.15CuO4-y

Crystal Structure of Superconductor Ba2YCu2.89O6.80