Band Crossing and Novel Low-Energy Behaviour in a Mean Field Theory of a Three-Band Model on a Cu--O lattice

We study correlation effects in a three-band extended Hubbard model of Cu -- O planes within the 1/N mean field approach, in the infinite U limit. We investigate the emerging phase diagram and discuss the low energy scales associated with each region. With increasing direct overlap between oxygen orbitals, $t_{pp} >0$, the solution displays a band crossing which, for an extended range of parameters, lies close to the Fermi level. In turn this leads to the nearly nested character of the Fermi surface and the resulting linear temperature dependence of the quasi-particle relaxation rate for sufficiently large T. We also discuss the effect of band crossing on the optical conductivity and comment on the possible experimental relevance of our findings.Comment: 12 pages, Latex-Revtex, 6 PostScript figures. Submitted to Phys. Rev.

The Quantum Mechanics of Hyperion

This paper is motivated by the suggestion [W. Zurek, Physica Scripta, T76, 186 (1998)] that the chaotic tumbling of the satellite Hyperion would become non-classical within 20 years, but for the effects of environmental decoherence. The dynamics of quantum and classical probability distributions are compared for a satellite rotating perpendicular to its orbital plane, driven by the gravitational gradient. The model is studied with and without environmental decoherence. Without decoherence, the maximum quantum-classical (QC) differences in its average angular momentum scale as hbar^{2/3} for chaotic states, and as hbar^2 for non-chaotic states, leading to negligible QC differences for a macroscopic object like Hyperion. The quantum probability distributions do not approach their classical limit smoothly, having an extremely fine oscillatory structure superimposed on the smooth classical background. For a macroscopic object, this oscillatory structure is too fine to be resolved by any realistic measurement. Either a small amount of smoothing (due to the finite resolution of the apparatus) or a very small amount of environmental decoherence is sufficient ensure the classical limit. Under decoherence, the QC differences in the probability distributions scale as (hbar^2/D)^{1/6}, where D is the momentum diffusion parameter. We conclude that decoherence is not essential to explain the classical behavior of macroscopic bodies.Comment: 17 pages, 24 figure

Collisional cross sections and momentum distributions in astrophysical plasmas: dynamics and statistical mechanics link

We show that, in stellar core plasmas, the one-body momentum distribution function is strongly dependent, at least in the high velocity regime, on the microscopic dynamics of ion elastic collisions and therefore on the effective collisional cross sections, if a random force field is present. We take into account two cross sections describing ion-dipole and ion-ion screened interactions. Furthermore we introduce a third unusual cross section, to link statistical distributions and a quantum effect originated by the energy-momentum uncertainty owing to many-body collisions, and propose a possible physical interpretation in terms of a tidal-like force. We show that each collisional cross section gives rise to a slight peculiar correction on the Maxwellian momentum distribution function in a well defined velocity interval. We also find a possible link between microscopical dynamics of ions and statistical mechanics interpreting our results in the framework of non-extensive statistical mechanics.Comment: 8 page

Kinetic Theory of a Dilute Gas System under Steady Heat Conduction

The velocity distribution function of the steady-state Boltzmann equation for hard-core molecules in the presence of a temperature gradient has been obtained explicitly to second order in density and the temperature gradient. Some thermodynamical quantities are calculated from the velocity distribution function for hard-core molecules and compared with those for Maxwell molecules and the steady-state Bhatnagar-Gross-Krook(BGK) equation. We have found qualitative differences between hard-core molecules and Maxwell molecules in the thermodynamical quantities, and also confirmed that the steady-state BGK equation belongs to the same universality class as Maxwell molecules.Comment: 36 pages, 4 figures, 5 table

Random paths and current fluctuations in nonequilibrium statistical mechanics

An overview is given of recent advances in nonequilibrium statistical mechanics about the statistics of random paths and current fluctuations. Although statistics is carried out in space for equilibrium statistical mechanics, statistics is considered in time or spacetime for nonequilibrium systems. In this approach, relationships have been established between nonequilibrium properties such as the transport coefficients, the thermodynamic entropy production, or the affinities, and quantities characterizing the microscopic Hamiltonian dynamics and the chaos or fluctuations it may generate. This overview presents results for classical systems in the escape-rate formalism, stochastic processes, and open quantum systems

Determination of Omega_b From Big Bang Nucleosynthesis in the Presence of Regions of Antimatter

Production of regions of antimatter in the early universe is predicted in many baryogenesis models. Small scale antimatter regions would annihilate during or soon after nucleosynthesis, affecting the abundances of the light elements. In this paper we study how the acceptable range in Omega_b changes in the presence of antimatter regions, as compared to the standard big bang nucleosynthesis. It turns out that it is possible to produce at the same time both a low 4He value (Y_p < 0.240) and a low D/H value (D/H < 4e-5), but overproduction of 7Li is unavoidable at large Omega_b.Comment: 9 pages, PRD version, ref. 6 correcte

On modified simple reacting spheres kinetic model for chemically reactive gases

Versão dos autores para esta publicação.We consider the modiffed simple reacting spheres (MSRS) kinetic model that, in addition to the conservation of energy and momentum, also preserves the angular momentum in the collisional processes. In contrast to the line-of-center models or chemical reactive models considered in [1], in the MSRS (SRS) kinetic models, the microscopic reversibility (detailed balance) can be easily shown to be satisfied, and thus all mathematical aspects of the model can be fully justi ed. In the MSRS model, the molecules behave as if they were single mass points with two internal states. Collisions may alter the internal states of the molecules, and this occurs when the kinetic energy associated with the reactive motion exceeds the activation energy. Reactive and non-reactive collision events are considered to be hard spheres-like. We consider a four component mixture A, B, A*, B*, in which the chemical reactions are of the type A + B = A* + B*, with A* and B* being distinct species from A and B. We provide fundamental physical and mathematical properties of the MSRS model, concerning the consistency of the model, the entropy inequality for the reactive system, the characterization of the equilibrium solutions, the macroscopic setting of the model and the spatially homogeneous evolution. Moreover, we show that the MSRS kinetic model reduces to the previously considered SRS model (e.g., [2], [3]) if the reduced masses of the reacting pairs are the same before and after collisions, and state in the Appendix the more important properties of the SRS system.Fundação para a Ciência e a Tecnologi

Lower limit on the neutralino mass in the general MSSM

We discuss constraints on SUSY models with non-unified gaugino masses and R_P conservation. We derive a lower bound on the neutralino mass combining the direct limits from LEP, the indirect limits from gmuon, bsgamma, Bsmumu and the relic density constraint from WMAP. The lightest neutralino (mneutralino=6GeV) is found in models with a light pseudoscalar with MA<200GeV and a large value for $tan\beta$. Models with heavy pseudoscalars lead to mneutralino>18(29)GeV for $\tan\beta=50(10)$. We show that even a very conservative bound from the muon anomalous magnetic moment can increase the lower bound on the neutralino mass in models with mu<0 and/or large values of $\tan\beta$. We then examine the potential of the Tevatron and the direct detection experiments to probe the SUSY models with the lightest neutralinos allowed in the context of light pseudoscalars with high $\tan\beta$. We also examine the potential of an e+e- collider of 500GeV to produce SUSY particles in all models with neutralinos lighter than the W. In contrast to the mSUGRA models, observation of at least one sparticle is not always guaranteed.Comment: 37 pages, LateX, 16 figures, paper with higher resolution figures available at http://wwwlapp.in2p3.fr/~boudjema/papers/bound-lsp/bound-lsp.htm

A high-resolution record of early Paleozoic climate

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygen-isotope paleothermometry. Bulk-rock δ¹⁸O datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro–Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk δ¹⁸O due to low fluid exchange. Consequently, both sequences preserve temporal trends in δ¹⁸O. Motivated by this result, we compile a global suite of bulk rock δ¹⁸O data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock δ¹⁸O with brachiopod and conodont δ¹⁸O trends through time. Given evidence that the δ¹⁸O value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely least-altered, δ¹⁸O values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early–Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events