Superconducting instability in the Holstein-Hubbard model: A numerical renormalization group study

We have studied the d-wave pairing-instability in the two-dimensional Holstein-Hubbard model at the level of a full fluctuation exchange approximation which treats both Coulomb and electron-phonon (EP) interaction diagrammatically on an equal footing. A generalized numerical renormalization group technique has been developed to solve the resulting self-consistent field equations. The $d$-wave superconducting phase diagram shows an optimal T_c at electron concentration ~ 0.9 for the purely electronic Hubbard system. The EP interaction suppresses the d-wave T_c which drops to zero when the phonon-mediated on-site attraction $U_p$ becomes comparable to the on-site Coulomb repulsion $U$. The isotope exponent $\alpha$ is negative in this model and small compared to the classical BCS value $\alpha_{BCS} = 1/2$ or compared to typical observed values in non-optimally doped cuprate superconductors.Comment: 4 pages RevTeX + 3 PS figures include

Origin and roles of a strong electron-phonon interaction in cuprate oxide superconductors

A strong electron-phonon interaction arises from the modulation of the superexchange interaction by phonons. As is studied in Phys. Rev. B 70, 184514 (2004), Cu-O bond stretching modes can be soft around (pm pi/a, 0) and (0, pm pi/a), with a the lattice constant of CuO_2 planes. In the critical region of SDW, where antiferromagnetic spin fluctuations are developed around nesting wave numbers Q of the Fermi surface, the stretching modes can also be soft around 2Q. Almost symmetric energy dependences of the 2Q component of the density of states, which are observed in the so called stripe and checker-board states, cannot be explained by CDW with 2Q following the complete softening of the 2Q modes, but they can be explained by a second-harmonic effect of SDW with Q. The strong electron-phonon interaction can play no or only a minor role in the occurrence of superconductivity.Comment: 5 pages, 1 fugur

Bose-Einstein transition temperature in a dilute repulsive gas

We discuss certain specific features of the calculation of the critical temperature of a dilute repulsive Bose gas. Interactions modify the critical temperature in two different ways. First, for gases in traps, temperature shifts are introduced by a change of the density profile, arising itself from a modification of the equation of state of the gas (reduced compressibility); these shifts can be calculated simply within mean field theory. Second, even in the absence of a trapping potential (homogeneous gas in a box), temperature shifts are introduced by the interactions; they arise from the correlations introduced in the gas, and thus lie inherently beyond mean field theory - in fact, their evaluation requires more elaborate, non-perturbative, calculations. One illustration of this non-perturbative character is provided by the solution of self-consistent equations, which relate together non-linearly the various energy shifts of the single particle levels k. These equations predict that repulsive interactions shift the critical temperature (at constant density) by an amount which is positive, and simply proportional to the scattering length a; nevertheless, the numerical coefficient is difficult to compute. Physically, the increase of the temperature can be interpreted in terms of the reduced density fluctuations introduced by the repulsive interactions, which facilitate the propagation of large exchange cycles across the sample.Comment: two minor corrections, two refs adde

Object-based reverberation for spatial audio

Object-based audio is gaining momentum as a means for future audio content to be more immersive, interactive, and accessible. Recent standardization developments make recommendations for object formats; however, the capture, production, and reproduction of reverberation is an open issue. In this paper parametric approaches for capturing, representing, editing, and rendering reverberation over a 3D spatial audio system are reviewed. A framework is proposed for a Reverberant Spatial Audio Object (RSAO), which synthesizes reverberation inside an audio object renderer. An implementation example of an object scheme utilizing the RSAO framework is provided, and supported with listening test results, showing that: the approach correctly retains the sense of room size compared to a convolved reference; editing RSAO parameters can alter the perceived room size and source distance; and, format-agnostic rendering can be exploited to alter listener envelopment

The oxygen isotope effect in the ab-plane reflectance of underdoped YBa_2Cu_3O_{7-delta}

We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBCO. The frequency shift of the transverse optic phonons due to the substitution of O-16 by O-18 yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1. The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.Comment: 4 pages 2 figure

Analysis of repeated high-intensity running performance in professional soccer

The aims of this study conducted in a professional soccer team were two-fold: to characterise repeated high-intensity movement activity profiles in official match-play; b) to inform and verify the construct validity of tests commonly used to determine repeated-sprint ability in soccer by investigating the relationship between the results from a test of repeated-sprint ability and repeated high-intensity performance in competition. High-intensity running performance (movement at velocities >19.8 km/h for a minimum of 1-s duration) in 20 players was measured using computerised time motion analysis. Performance in 80 French League 1 matches was analysed. In addition, 12 out of the 20 players performed a repeated-sprint test on a non-motorized treadmill consisting of 6 consecutive 6s sprints separated by 20s passive recovery intervals. In all players, the majority of consecutive high-intensity actions in competition were performed after recovery durations ≥61s, recovery activity separating these efforts was generally active in nature with the major part of this spent walking, and players performed 1.1±1.1 repeated high-intensity bouts (a minimum of 3 consecutive high-intensity with a mean recovery time ≤20s separating efforts) per game. Players reporting lowest performance decrements in the repeated-sprint ability test performed more high-intensity actions interspersed by short recovery times (≤20s, p<0.01 and ≤30s, p<0.05) compared to those with higher decrements. Across positional roles, central-midfielders performed a greater number of high-intensity actions separated by short recovery times (≤20s) and spent a larger proportion of time running at higher intensities during recovery periods while fullbacks performed the most repeated high-intensity bouts (statistical differences across positional roles from p<0.05 to p<0.001). These findings have implications for repeated high-intensity testing and physical conditioning regimens

E NEUE ENTE AUS JAVA

abstract not availabl

Nonequilibrium molecular dynamics simulation of rapid directional solidification

We present the results of non-equilibrium molecular dynamics simulations for the growth of a solid binary alloy from its liquid phase. The regime of high pulling velocities, $V$, for which there is a progressive transition from solute segregation to solute trapping, is considered. In the segregation regime, we recover the exponential form of the concentration profile within the liquid phase. Solute trapping is shown to settle in progressively as $V$ is increased and our results are in good agreement with the theoretical predictions of Aziz [J. Appl. Phys. {\bf 53}, 1158 (1981)]. In addition, the fluid advection velocity is shown to remain directly proportional to $V$, even at the highest velocities considered here ($V\simeq10$ms$^{-1}$).Comment: Submitted to Phys. Rev.