Electronic dynamic Hubbard model: exact diagonalization study

A model to describe electronic correlations in energy bands is considered. The model is a generalization of the conventional Hubbard model that allows for the fact that the wavefunction for two electrons occupying the same Wannier orbital is different from the product of single electron wavefunctions. We diagonalize the Hamiltonian exactly on a four-site cluster and study its properties as function of band filling. The quasiparticle weight is found to decrease and the quasiparticle effective mass to increase as the electronic band filling increases, and spectral weight in one- and two-particle spectral functions is transfered from low to high frequencies as the band filling increases. Quasiparticles at the Fermi energy are found to be more 'dressed' when the Fermi level is in the upper half of the band (hole carriers) than when it is in the lower half of the band (electron carriers). The effective interaction between carriers is found to be strongly dependent on band filling becoming less repulsive as the band filling increases, and attractive near the top of the band in certain parameter ranges. The effective interaction is most attractive when the single hole carriers are most heavily dressed, and in the parameter regime where the effective interaction is attractive, hole carriers are found to 'undress', hence become more like electrons, when they pair. It is proposed that these are generic properties of electronic energy bands in solids that reflect a fundamental electron-hole asymmetry of condensed matter. The relation of these results to the understanding of superconductivity in solids is discussed.Comment: Small changes following referee's comment

Fully compressible simulation of low-speed premixed reacting flows

Low speed premixed combustion flows in industrial applications are generally simulated using the "incompressible" Navier-Stokes algorithms, which belong to the family of fractional step methods, or segregated methods. The approximations used for the combustion modelling in the framework of the segregated mathematical formulation, often represent important limitations for applying the combustion numerical simulation to a wider class of problems of engineering interest. Recent developments of preconditioning techniques allow to apply the same complete system of Navier-Stokes equations to a wide variety of fluid flow problems characterized by the whole range of Reynolds, Mach, Grashof, Prandtl and Damkoeler numbers. The present work describes the development of a fully "compressible" mathematical model for the simulation of low-speed turbulent premixed reactive flows. Issues on flow and fluid compressibility as well as on the two mathematical alternative formulations, are discussed. Also discussed are issues related to coupling the flamelet premixed combustion model (based on the solution of a transport equation for the progress variable) with one-equation turbulence models, instead of the classical two-equation K – ε model. In this work the model by Spalart & Allmaras is used. The several advantages brought about by the use of the fully compressible formulation are discussed based on the results obtained on a test case taken from literature

Superconducting Upper Critical Field Near a 2D Van Hove Singularity

The superconducting upper critical field $H_{c2}(T)$ of a two dimensional BCS superconductor is calculated in the vicinity of a van-Hove singularity. The zero temperature upper critical field is strongly enhanced at weak coupling when the Fermi contour coincides with van-Hove points, scaling as $H_{c2}(0) \propto T_c^{\sqrt{2}}$ compared to the usual result $H_{c2}(0) \propto T_c^{2}$. The result can be interpreted in terms of the non-Fermi liquid decay of normal state pair correlations in the vicinity of a van-Hove point.Comment: 4 pages, Latex 2.09 (revtex), 3 postscript figures appended (tar-compressed and uuencoded using `uufiles'

Exact evaluation of the nuclear form factor for new kinds of majoron emission in neutrinoless double beta decay

We have developed a formalism, based on the Fourier-Bessel expansion, that facilitates the evaluation of matrix elements involving nucleon recoil operators, such as appear in serveral exotic forms of neutrinoless double beta decay ($\beta\beta_{0\nu}$). The method is illustrated by applying it to the ``charged'' majoron model, which is one of the few that can hope to produce an observable effect. From our numerical computations within the QRPA performed for $^{76}Ge$, $^{82}Se$, $^{100} Mo$, $^{128}Te$ and $^{150}Nd$ nuclei, we test the validity of approximations made in earlier work to simplify the new matrix elements, showing that they are accurate to within 15%. Our new method is also suitable for computing other previously unevaluated $\beta\beta_{0\nu}$ nuclear matrix elements.Comment: 11pp., latex, fixed minor typographical error

Effective R-parity violation from supersymmetry breaking

We present a scenario in which Yukawa-like R-parity violating (RPV) couplings are naturally suppressed. In our framework, RPV is assumed to originate from the SUSY breaking mechanism and then transmitted into the SUSY Lagrangian only through soft SUSY breaking operators in the scalar potential. The RPV Yukawa-like operators of the superpotential, conventionally parametrized by the couplings \lambda, \lambda' and \lambda'', are then generated through loops containing the SUSY scalars, the gauginos and the soft RPV interactions and are, therefore, manifest as effective operators with a typical strength of order 10^{-3}.Comment: 4 pages, 2 figures, Revtex4. Main changes with respect to Version 1 are: improved discussions on RGE effects, discussion added on neutrino masses, a toy model added for the proposed scenario. Conclusions remain unchanged. As will appear in Phys. Rev.

Optical Sum Rule anomalies in the High-Tc Cuprates

We provide a brief summary of the observed sum rule anomalies in the high-T$_c$ cuprate materials. A recent issue has been the impact of a non-infinite frequency cutoff in the experiment. In the normal state, the observed anomalously high temperature dependence can be explained as a `cutoff effect'. The anomalous rise in the optical spectral weight below the superconducting transition, however, remains as a solid experimental observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal

Synthesizing attractors of Hindmarsh-Rose neuronal systems

In this paper a periodic parameter switching scheme is applied to the Hindmarsh-Rose neuronal system to synthesize certain attractors. Results show numerically, via computer graphic simulations, that the obtained synthesized attractor belongs to the class of all admissible attractors for the Hindmarsh-Rose neuronal system and matches the averaged attractor obtained with the control parameter replaced with the averaged switched parameter values. This feature allows us to imagine that living beings are able to maintain vital behavior while the control parameter switches so that their dynamical behavior is suitable for the given environment.Comment: published in Nonlinear Dynamic

Functional impairment and depressive symptoms in older adults: Mitigating effects of hope.

OBJECTIVES: We examined trait hopefulness and its component subscales of agency and pathways as potential moderators of the association between functional impairment and depressive symptoms, hypothesizing that hopefulness would buffer this association. DESIGN: Cross-sectional, interview-based. METHODS: Older adult, primary care patients (N= 105; 62% female) completed measures of cognitive functioning, functional impairment, medical illness burden, trait hope, and depressive symptoms. RESULTS: Functional impairment was significantly positively associated with depressive symptoms and dispositional hope total score moderated this relationship. Independently, the pathways subscale was a significant moderator and agency neared significance, yet their interaction was not significant. CONCLUSIONS: Older adults with functional impairment appear to benefit from agency and pathways; either subcomponent alone or their additive effect can activate hopefulness. Facilitation of infrastructure (pathways), primarily, and self-efficacy (agency), secondarily, may be important strategies for reducing depressive symptoms in elderly patients with functional impairment

Charged Majoron Emission in Neutrinoless Double Beta Decay

We examine in detail the predictions of the charged majoron model, introduced recently by Burgess and Cline, for 0+ --> 0+ double beta decay transitions. The relevant nuclear matrix elements are evaluated, within the quasiparticle random phase approximation, for 76Ge, 82Se, 100Mo, 128Te and 150Nd nuclei. The calculated transition rates turn out to be much smaller than the experimental upper limits on possible majoron emission, except in a small region of the model's parameter space.Comment: 9 pages, 1 encapsulated postscript figure, uses epsf.tex; reference [15] has been correcte