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It has been proposed by Müller and Shengelaya that underdoped ultrathin layers of copper-oxide high-temperature superconductors (HTSs) sandwiched between high-dielectric-constant insulator layers could manifest increased superconducting critical temperature T c . To check this hypothesis, we investigated structural and transport properties of YBa 2 Cu 3 O 7 - δ (YBCO) thin films sandwiched between SrTiO 3 (STO) layers. Scanning transmission electron microscopy (STEM) showed that a high-quality interface is formed between YBCO and the top STO layers. An increase of T c up to Δ T ≈ 20 K was observed at these interfaces in case of underdoped YBCO films. [ABSTRACT FROM AUTHOR]

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB2 and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps. [ABSTRACT FROM AUTHOR]

The competition between singlet and triplet superconductivity is examined in consideration of correlations on an extended Hubbard model. It is shown that the triplet superconductivity may not be included in the common Hubbard model since the strong correlation favors the singlet superconductivity, and thus the triplet superconductivity should be induced by the electron–phonon interaction and the ferromagnetic exchange interaction. We also present a superconducting qualification with which magnetism is unbeneficial to superconductivity. [ABSTRACT FROM AUTHOR]

Dynamical gravitational and geodesic equations are derived for superfluid densities of nonlocal self-coherent particles. The geometrized gravitational particle is the r−4 distribution of inertial mass that balances Ricci curvatures in the Einstein equation without the right-hand side. The spatial energy integral of such an infinite radial particle is finite and determines its nonlocal gravimechanical charge for energy-to-energy interactions with other nonlocal particles. Non-empty space of the flat material world is filled continuously by overlapping energy-flows of all nonlocal particles and their fields. [ABSTRACT FROM AUTHOR]

Within the BCS theory of superconductivity we calculate the superconducting gap at a zero-temperature for metallic hydrogen–graphene system in order to estimate the superconducting critical temperature of quasi-two-dimensional highly oriented pyrolytic graphite. The obtained results are given as a function of the hydrogen-induced density of carriers n and their effective mass m ⋆. The obtained gap shows a Maxwell-like distribution with a maximum of ∼60 K at n∼3×1014 cm−2 and m ⋆/ m=1. The theoretical results are discussed taking into account recent experimental evidence for granular superconductivity in graphite. [ABSTRACT FROM AUTHOR]

The mechanism of spin–phonon coupling (SPC) and possible consequences for the properties of high- T C copper oxides are presented. The results are based on ab initio LMTO band calculations and a nearly free-electron (NFE) model of the band near E F. Many observed properties are compatible with SPC, as for the relation between doping and $\vec{q}$ for spin excitations and their energy dependence. The main pseudogap is caused by SPC and waves along [1,0,0], but it is suggested that secondary waves, generated along [1,1,0], contribute to a ‘waterfall’ structure. Conditions for optimal T C, and the possibilities for spin enhancement at the surface are discussed. [ABSTRACT FROM AUTHOR]

In this study we focus on the magnetic properties of a system consisting of Spin Density Wave (SDW) order, d-wave singlet and π-triplet superconductivity (SC) taken on the same footing within a mean-field treatment. The competition and coexistence of these order parameters manifest the H-T phase diagrams of all antiferromagnetic superconductors. An experimental probe to such a multicomponent state can be provided by magnetic measurements such as NMR or magnetization techniques. We have calculated, both analytically and numerically, the static spin susceptibility of our multicomponent system as a function of temperature and external magnetic field under particle–hole asymmetry. Our results can serve as a way to identify these novel multicomponent states. An application to the identification of the high-field low-temperature phase in CeCoIn5 is discussed. [ABSTRACT FROM AUTHOR]

The relevance of the strong correlations in the high critical temperature superconductors (HTSC) is well experimentally documented. However, if the properties of the normal and superconducting state in HTSC oxides are interpreted in terms of the standard Eliashberg theory, which holds in low temperature superconductor systems, the Migdal–Eliashberg approach implies serious limitations in the reproduction of experimental spectroscopies whose contributions are inter-band and not intra-band. Recent angle-resolved photoemission spectroscopy (ARPES) measurements on HTSC oxides whose contributions are intra-band show a kink in the quasiparticle spectrum at characteristic phonon frequencies in the normal and superconducting state. In contrast with our theoretical discussion, we include our theoretical results for the renormalized energy E k as a function of the bare band energy ε k obtained from ARPES in a Pb sample and in a Bi2Sr2CaCu2O8+ δ optimally doped sample (Bi2212). This is clear evidence that electron-phonon is strong and involved in pairing. [ABSTRACT FROM AUTHOR]