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Evidence is presented that high-temperature superconductivity does not necessarily originate in the cuprate-planes. In the cuprates such as YBaCuO, i is argued that the superconductivity resides in the BaO layers. This superconductivity is -wave, not -wave, in the bulk. The trio of ruthenate compounds, Cu-doped SrYRuO, GdSrCuRuO, and GdCeSrCuRuO all super conduct in their SrO layers, which is why they have almost the same ∼45 K onset temperatures for superconductivity. BaGdRuO, whether doped or not, does not superconduct, because the Gd breaks Cooper-like pairs. BiSrCaCuO YBaCuO, NdCeCuO homologues, and the PbSr(RE)CaCuO compounds that superconduct (where RE is a rare-earth) are all -wave, p-type superconductors.

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We examine the band properties of the extended Emery model, parameterized by Cu-O charge transfer energy Δ, copper-oxygen overlap , oxygen-oxygen overlap ′ and Coulomb interaction on the copper site. In the charge-transfer limit ≫ Δ > , ′, it retains realistic three band structure, unlike the usually used Hubbard model. Neglecting the ordering of spin degrees of freedom, infinite slave boson mean field approximation is used to calculate at small doping δ the renormalization trends of the effective band parameters Δ and , replacing Δ and , while ′, related exclusively to oxygens, remains unchanged. It is shown that small, negative ′ expands the range of stability of the metallic phase, changing, through the higher order corrections in ′, the thermodynamical nature of the metal-insulator transition point. In the nonperturbative limit, ′ modifies qualitatively the renormalization of Δ at zero doping, making it saturate at the value of 4|′| and keeping thus the system in the regime of strong correlations. Finite doping δ suppresses the insulating = 0 state approximately symmetrically with respect to its sign. The regime δ ≈ 4|′| fits very well the ARPES spectra of LSCO and also explains the evolution of the FS with doping accompanied by the spectral weight- transfer from the oxygen to the resonant band. Additionally, the values of the band- parameters Δ /′ and /′ are allowed to vary freely, in order to describe the ARPES data on LSCO, thus providing the direct insight in the role of oxygens, underestimated in the magnetic t-J limit of the large Emery model. The important role in this theory is played by the effective Cu-O overlap , very small in the underdoped HTS cuprates and evolving to the values of the order of ′ at larger doping.

The condensation energy for the antiferromagnetic spin fluctuations mechanism of pairing are considered. For the calculation the method of functional integrals was used. It has been shown that the condensation energy in high-Tc superconductors is highly sensitive to the doping level, and is greatly reduced in underdoped region.We consider that this effect is due to the decreasing of the number of hot quasiparticles, which are responsible for the interaction with antiferromagnetic spin fluctuations. It is in qualitative agreement with experiment.

It has long been established that disorder has profound effects on unconventional superconductors and it has been suggested repeatedly that observation and analysis of these disorder effects can help to identify the order parameter symmetry. In much of the relevant literature, including very sophisticated calculations of interference and weak localization effects, the disorder is represented by δ-function scatterers of arbitrary strength. One obvious shortcoming of this approximation is that resonant scattering resulting from the wavelength of the scattered quasiparticle matching the spatial extent of the defect is not included. We find that the mitigation of the Tc-reduction, expected when d-wave scattering is included, is very sensitive to the average strength of the scattering potential and is most effective for weak scatterers. Disorder with finite range not only has drastic effects on the predicted density of states at low energies, relevant for transport properties, but affects the spectral function at all energies up to the order parameter amplitude. The gap structure, which does not appear to be of the simplest d-wave form, should show a defect-dependent variation with temperature, which could be detected in ARPES experiments.

I propose a superconductivity model, which is based on the assumption that stripes in high- cuprates (a) exist and (b) organize themselves in a two-dimensional superstructure. The model describes hole states, which are localized either inside the stripes or in the antiferromagnetic domains between the stripes. The superconductivity in this model emerges due to the interaction, which is, presumably, mediated by the transverse fluctuations of stripes. The tunnelling density of states obtained from the mean field solution of the model is asymmetric with respect to the chemical potential, has Van Hove singularity identified as a superconducting peak, and, in one of the model regimes, has linear functional form in the vicinity of the chemical potential. The relation between the critical temperature and the zero-temperature superfluid density has “fish-like” form, which quantitatively resembles experimental data. The superconducting order parameter obtained from this model has two components exhibiting non-trivial phase and sign change under translations in real space.

We argue that a Scalapino-Schrieffer-Wilkins analysis of the elementary excitations is possible in high- cuprates. We demonstrate this by considering the resonance peak in inelastic neutron scattering (INS) experiments and the kink in angle-resolved photoemission (ARPES) data. Both properties contain characteristic features of the superconducting gap function Δ(k, ω) that reflect the pairing interaction. We can qualitatively explain the experiments by using a generalized Eliashberg theory for the one-band Hubbard model based on spin-fluctuationmediated Cooper-pairing in which Δ(k, ω) is calculated self-consistently. This gives strong evidence for Cooper-pairing due to spin excitations in the high- cuprates.

Starting from the three-band p — d Hubbard Hamiltonian we derive the effective single-correlated model Hamiltonian including electron-phonon interaction of quasiparticles with optical phonons and strong electron correlations. Within an effective Hamiltonian we analyze their influence on the dynamical spin susceptibility in layered cuprates. We find an isotope effect on resonance peak in the magnetic spin susceptibility, Im (, ), seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account beyond random phase approximation(RPA) scheme.

The carriers in the high- cuprates are found to be polaron-like “stripons” carrying charge and located in stripe-like inhomogeneities, “quasi-electrons” carrying charge and spin, and “svivons” carrying spin and some lattice distortion. The anomalous spectroscopic and transport properties of the cuprates are understood. The stripe-like inhomogeneities result from the Bose condensation of the svivon field, and the speed of their dynamics is determined by the width of the double-svivon neutron-resonance peak. The connection of this peak to the peak-dip-hump gap structure observed below emerges naturally. Pairing results from transitions between pair states of stripons and quasi-electrons through the exchange of svivons. The pairing symmetry is of the − type; however, sign reversal through the charged stripes results in features not characteristic of this symmetry. The phase diagram is determined by pairing and coherence lines within the regime of a Mott transition. Coherence without pairing results in a Fermi-liquid state, and incoherent pairing results in the pseudogap state where localized electron and electron pair states exist within the Hubbard gap. A metal-insulator-transition quantum critical point occurs between these two states at = 0 when the superconducting state is suppressed. An intrinsic heterogeneity is expected of superconducting and pseudogap nanoscale regions.

The superconducting state in the novel family of layered metalochloronitrides is provided by electronic collective excitations, the acoustic plasmons. These plasmons are a characteristic feature of layered conductors. This is the first experimentally observed case of self-supported superconductivity.