Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely 1 . Here, we analyse the charge-and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa 2 Cu 3 O 6.6 . We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin-fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations [2][3][4][5][6][7] and 'kinks' in the fermionic band dispersions along the nodal direction [8][9][10][11][12] . The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.Looking back at conventional superconductors, the most convincing demonstration of the electron-phonon interaction as the source of electron pairing was based on the quantitative correspondence between features in the electronic tunnelling conductance and the phonon spectrum measured by inelastic neutron scattering (INS; for reviews, see the articles by Scalapino, McMillan and Rowell in ref. 13). The rigorous comparison of fermionic and bosonic spectra was made possible by the Eliashberg theory, which enabled the tunnelling conductance to be derived from the experimentally determined phonon spectrum. Various difficulties have impeded a similar approach to the origin of high-temperature superconductivity. First, the d-wave pairing state found in these materials implies a strongly momentum-dependent pairing interaction. A more elaborate analysis based on data from momentum-resolved experimental techniques such as INS and angle-resolved photoemission spectroscopy (ARPES) is thus required. These methods, in turn, impose conflicting constraints on the materials. (refs 11,12) have overcome problems related to polar surfaces and enabled the observation of superconducting gaps and band renormalization effects ('kinks') akin to those previously reported in La-and Bi-based cuprates 8 . Third, calculations based on the two-dimensional Hubbard model have demonstrated Fermi surfaces, single-particle spectral weights, antiferromagnetic spin correlations and d x 2 −y 2 pairing correlations in qualitative agreement with experimental measurements [15][16][17] . Numerically accurate solutions of this model can thus serve as a valuable guideline for a treatment of the spin-fluctuation interaction in the cuprates. This is the approach we take here.Recent quantum Monte Carlo calculations of the twodimensional Hubbard model within the dynamical cluster approximation 17 for a realistic value of the bare U /t = 8 and different doping levels ranging from un...

The penetration depth and surface resistance of a superconductor depend upon the superfluid current density. This dependence gives rise to nonlinear mixing in a superconducting microstrip resonator. Here we discuss the problem of intermodulation in which two signals at ω1 and ω2, laying within the pass band of a microstrip cavity resonance, mix and generate a signal at 2ω1−ω2. An expression relating the power generated at 2ω1−ω2 to the power transmitted at ω1 and ω2 is given. We focus on the high-Tc superconductors where it is believed that the order parameter has dx2−y2 symmetry. We find for a resonator with a large unloaded Q that intermodulation arises dominantly from the reactive nonlinear inductance of the superconducting film.

We investigate dynamical chiral symmetry breaking in unquenched QED 3 using the coupled set of Dyson-Schwinger equations for the fermion and photon propagators. For the fermion-photon interaction we employ an ansatz which satisfies its Ward-Green-Takahashi identity. We present self-consistent analytical solutions in the infrared as well as numerical results for all momenta. In Landau gauge, we find a phase transition at a critical number of flavours of N crit f ≈ 4. In the chirally symmetric phase the infrared behaviour of the propagators is described by power laws with interrelated exponents. For N f = 1 and N f = 2 we find small values for the chiral condensate in accordance with bounds from recent lattice calculations. We investigate the Dyson-Schwinger equations in other linear covariant gauges as well. A comparison of their solutions to the accordingly transformed Landau gauge solutions shows that the quenched solutions are approximately gauge covariant, but reveals a significant amount of violation of gauge covariance for the unquenched solutions.

We calculate the temperature dependence of NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T 2 at low temperatures and following a √ T behavior at high temperatures. Both results are in qualitative agreement with recent observations on β-pyrochlore oxide superconductors.PACS numbers: 74.70. Dd, 74.25.Kc The recent discovery of superconductivity in the family of pyrochlore oxides KOs 2 O 6 , RbOs 2 O 6 , and CsOs 2 O 6 has attracted great interest because of their unusual properties. Among these KOs 2 O 6 with the highest T c of 9.6 K appears to be the most unusual. The temperature dependence of its electrical resistivity shows a strong concave-downward temperature dependence [1,2], in contrast to the other two compounds, where a T 2 temperature dependence at low temperatures has been observed [3,4]. Specific heat measurements have shown a large mass enhancement, large specific heat jump at T c , and existence of low frequency Einstein modes [5,6]. Bandstructure calculations have indicated that the anomalies, in particular in KOs 2 O 6 , might be due to a highly anharmonic low frequency rattling motion of the alkali-ion inside an oversized cage formed by the Os and O ions [7,8]. This is consistent with X-ray observations of anomalously large atomic displacements for the K ions [9] and low frequency phonon structures seen in photoemission spectroscopy [10].Recent observations of NMR relaxation rates 1/T 1 T at the K site have been demonstrated to be entirely dominated by the vibrations of the K ion via coupling of the electric field gradient to the nuclear quadrupole moment [11]. Such a domination of phonons for nuclear relaxation usually occurs in diamagnetic insulators, but is extremely rare in metals and was attributed to the rattling motion of the K ions. The temperature dependence of 1/T 1 T was found to be anomalous as well, showing a peak around 12-14 K and decreasing at higher temperatures. It has been argued that such a behavior is inconsistent with the two-phonon Raman process, which usually dominates quadrupolar relaxation, and it has been interpreted in terms of the direct phonon process with a strongly temperature dependent phonon damping rate [11]. In the superconducting state 1/T 1 T exhibits a sudden decrease, suggesting a strong coupling of the phonon mode to the conduction electrons and an associated increase of the phonon lifetime in the superconducting state.Motivated by these experimental findings, in the present work we study the influence of a local, strongly anharmonic, and damped phonon mode on the NMR relaxation rate and the electrical resistivity. We find that within this model the two-phonon Raman process is expected to dominate NMR relaxati...