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

The selective oxidation of C-H bonds and the use of O(2) as a stoichiometric oxidant represent two prominent challenges in organic chemistry. Copper(II) is a versatile oxidant, capable of promoting a wide range of oxidative coupling reactions initiated by single-electron transfer (SET) from electron-rich organic molecules. Many of these reactions can be rendered catalytic in Cu by employing molecular oxygen as a stoichiometric oxidant to regenerate the active copper(II) catalyst. Meanwhile, numerous other recently reported Cu-catalyzed C-H oxidation reactions feature substrates that are electron-deficient or appear unlikely to undergo single-electron transfer to copper(II). In some of these cases, evidence has been obtained for the involvement of organocopper(III) intermediates in the reaction mechanism. Organometallic C-H oxidation reactions of this type represent important new opportunities for the field of Cu-catalyzed aerobic oxidations.

Copper-Catalyzed Aerobic Oxidative C ? H Functionalizations: Trends and Mechanistic Insights

ion and completion of the cyclometalation reaction are feasible and produce the cyclometalated complex 205. Notably, addition of a strong acid reverts the cyclometalation and regenerates the agostic species 204. Small changes in the rhodium precursor significantly modify the product outcome and lead at fast rates to the cyclometalated complex 203, that is, the final oxidative addition product S. Similarly, alkene C-H bond activation has been observed to occur readily in a PCP ligand setting.348 Milstein and co-workers developed a procedure for preparing the cyclometalated PCP, PCN, or PCO pincer complexes 209 by rhodium-mediated cleavage of a strong Caryl-Calkyl bond (Scheme 87).32,349,350 Kinetically, Calkyl-H bond activation is competitive and complex 208 is formed as a second product at essentially similar rates. However, C-C bond cleavage appears to be the thermodynamically more favored process. Obviously, the higher energy required for Caryl-Calkyl bond activation (bond dissociation energy 427 kJ mol-1) compared to C-H bond breaking (368 kJ mol-1) is compensated by the stronger Rh-Caryl bond and the formation of two fiverather than six-membered metallacycles. Because rhodium-mediated Caryl-Calkyl and Calkyl-H bond activations in 206 are thermodynamically and kinetically very similar processes,351 subtle changes in the ligand parameters strongly affect the reaction outcome. For example, PPh2 donors induce C-H bond cleavage to afford 208d exclusively at ambient temperatures. C-C activation and formation of 209d is only induced upon heating under H2 atmosphere,352 presumably because these conditions allow the product selectivity to be controlled thermodynamically. In contrast, bulky PtBu2 donors as in 206a promote C-C bond activation already at room temperature, and the aryl complex 209a is the only product observed. Similarly, replacing one phosphine donor by a harder NEt2 group or the use of less basic phosphinite donors results in selective C-C bond cleavage at room temperature. These preferences suggest steric factors to play a more dominant role than electronic differences in determining the C-C vs C-H selectivity. Steric congestion induced by the donor substituents has been proposed to arrange the orbitals of the metal center to be directed toward the C-C bond rather than toward the C-H bond.353 Kinetic investigations have identified the coordination compound 207 as a common intermediate of Calkyl-H and the Caryl-Calkyl bond activation. On the basis of the similar rates for both cleavage processes and because of the absence of any detectable intermediate, formation of 207 and not C-H or C-C bond activation appears to be rate-limiting. While displacement of a weakly bound olefin ligand in the metal precursor may be fast, formation of the 8-membered metallacycle in 207 may be less favored, especially due to the potential for forming diand polymeric coordination compounds. It is worth noting that, upon substituting the PCP pincer ligand to a PCN donor array, the coordination complex 207c becomes stable at low temperature and has been spectroscopically characterized.354 As a consequence, the rate-determining step is shifted to a later stage on the reaction coordinate. Notably, slight modifications in the metal precursor, that is, utilization of [Rh(C2H4)(CO)(solv)2]BF4 instead of [RhCl(C2H4)2]2, alters the stability of the intermediates and produces complex 210 (Scheme 88).355 This complex may be represented by an agostic interaction of the C-C bond with the rhodium center, or alternatively by an arenium structure including an sp3-hybridized ipso carbon. Because of only weak Cipso-Rh interactions in solution (e.g., the absence of any 103Rh coupling), the agostic description is favored. The different product outcome has been presumed to originate from the reduced electron density at the cationic rhodium(I) center due to the strongly withdrawing CO ligand and the relatively weak Rh-N bond. This configuration Scheme 87 Scheme 86 Cyclometalation Using d-Block Transition Metals Chemical Reviews, 2010, Vol. 110, No. 2 603

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Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

https://iopscience.iop.org/article/10.1088/0034-4885/67/11/R01/pdf

Electronic and ionic transport properties and other physical aspects of perovskites

Correlated electron fluids can exhibit a startling array of complex phases, among which one of the more surprising is the electron nematic, a translationally invariant metallic phase with a spontaneously generated spatial anisotropy. Classicalnematicsgenerally occur in liquids of rod-like molecules; given that electrons are point like, the initial theoretical motivation for contemplating electron nematics came from thinking of the electron fluid as a quantum melted electron crystal, rather than a strongly interacting descendent of a Fermi gas. Dramatic transport experiments in ultra-clean quantum Hall systems in 1999 and in Sr3Ru2O7 in a strong magnetic field in 2007 established that such phases exist in nature. In this article, we briefly review the theoretical considerations governing nematic order, summarize the quantum Hall and Sr3Ru2O7 experiments that unambiguously establish the existence of this phase, and survey some of the current evidence for such a phase in the cuprate and Fe-based high temperature superconductors.

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Nematic Fermi Fluids in Condensed Matter Physics

We show that single-crystalline ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ grown by a floating-zone technique is an isotropic paramagnet and a quasi-two-dimensional metal, as spin-triplet superconducting ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is. The ground state is a Fermi liquid with very low residual resistivity $(\ensuremath{\approx}3 \ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$ for in-plane currents) and a nearly ferromagnetic metal with the largest Wilson ratio ${R}_{\mathrm{W}}g~10$ among paramagnets so far. This contrasts with the ferromagnetic order at ${T}_{\mathrm{c}}=104 \mathrm{K}$ reported on single crystals grown by a flux method [Cao et al., Phys. Rev. B 55, R672 (1997)]. However, we have found a dramatic changeover from paramagnetism to ferromagnetism under applied pressure. This suggests the existence of a substantial ferromagnetic instability in the Fermi-liquid state.

Ground state in Sr 3 Ru 2 O 7 : Fermi liquid close to a ferromagnetic instability

The synthesis, characterization, and X-ray structure of the first example of an air-stable silver(III) complex of an N-confused tetraphenylporphyrin (5,10,15,20-tetraphenyl-2-aza-21-carbaporphyrin argentate(III) or Ag(III)NCTPP, 7) is reported. Crystals of Ag(III)NCTPP·CH2Cl2 belong to the monoclinic space group P21/c (No. 14), with a = 9.038(4) A, b = 15.097(3) A, c = 25.578(8) A, β = 91.32(3)°, and Z = 4. The lengths (A) of the three silver−nitrogen and one silver−carbon bonds are inequivalent [Ag−N(1), 2.06(2); Ag−N(2), 2.08(2); Ag−N(3), 2.03(2); and Ag−C(17), 2.04(2)], reflecting the asymmetric structure of the NCTPP ligand. The very small distortion from a square planar geometry is shown in N−Ag−N and N−Ag−C angles (degrees):  N(1)−Ag−N(2), 89.7(8); N(1)−Ag−N(3), 179(1); N(1)−Ag−C(17), 90.3(9); N(2)−Ag−N(3), 89.6(9); N(2)−Ag−C(17), 179(1); and N(3)−Ag−C(17), 90.9(1). The diamagnetic nature of Ag(III)NCTPP was confirmed by 1H and 13C NMR and magnetic susceptibility measurements. Electrochemical and sp...

N-confused tetraphenylporphyrin-silver(iii) complex

Rare organometallic complexes can be stabilized by using an N-confused system as a carbon ligand. A stable organocopper(II) compound of N-confused calix[4]phyrin has been characterized by X-ray crystallography. Complete π conjugation was found not to be a prerequisite for the formation of the metal-carbon bond.

Metal Complexes of an N-Confused Calix[4]phyrin Derivative—The First X-ray Structure of an Organometallic Compound of Divalent Copper

We present a systematic study of the dc-resistivity, Hall effect, and magnetoresistance in the normal state of quasi 2D heavy fermion superconductors CeMIn5 (M: Rh and Co) under pressure. Here the electronic system evolves with pressure from an antiferromagnetic (AF) metal, through a highly unconventional non-Fermi liquid, and finally into a Fermi-liquid state. The amplitude of the Hall coefficient increases dramatically with decreasing T, reaching at low temperatures a value significantly larger than 1/ne. Furthermore, the magnetoresistance is characterized by T- and H-dependence which clearly violate Kohler's rule. We found that the Hall angle cot\Theta varies as T^2, and the magnetoresistance is well scaled by the Hall angle as \Delta \rho_{xx}/\rho_{xx}\propto \tan^2\Theta. These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the QCP. We lay particular emphasis on the striking resemblance of these anomalous magnetotransport with those of the high-Tc cuprates. We argue that features commonly observed in quasi 2D heavy fermion and cuprates very likely capture universal features of strongly correlated electron systems.

Non-Fermi liquid behavior in the magnetotransport of CeMIn5 (M: Co and Rh): Striking similarity between quasi 2D heavy fermion and high-Tc cuprates

A hydrostatic high pressure micro cell for measuring the magnetization in a commercial magnetometer is developed. Experiments at pressures up to 10 kbar and in the temperature range 2 K≤T≤300 K have been carried out. The sensitivity of measurements under high pressure is the same as at ambient pressure.

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Yoshiya Uwatoko

Papers

Ground state in Sr 3 Ru 2 O 7 : Fermi liquid close to a ferromagnetic instability

N-confused tetraphenylporphyrin-silver(iii) complex

Metal Complexes of an N-Confused Calix[4]phyrin Derivative—The First X-ray Structure of an Organometallic Compound of Divalent Copper

Non-Fermi liquid behavior in the magnetotransport of CeMIn5 (M: Co and Rh): Striking similarity between quasi 2D heavy fermion and high-Tc cuprates

High Pressure Apparatus for Magnetization Measurements