Quantum spin chains with regularly alternating bonds and fields

We consider the spin-1/2 XY chain in a transverse field with regularly varying exchange interactions and on-site fields. In two limiting cases of the isotropic XX and extremely anisotropic (Ising) exchange interaction the thermodynamic quantities are calculated rigorously with the help of continued fractions. We discuss peculiarities of the low-temperature magnetic properties and a possibility of the spin-Peierls instability.Comment: Presented at 11-th Czech and Slovak Conference on Magnetism, Ko\v{s}ice, 20-23 August 200

The mass loss process in dwarf galaxies from 3D hydrodynamical simulations: the role of dark matter and starbursts

Theoretical $\Lambda$CDM cosmological models predict a much larger number of low mass dark matter haloes than has been observed in the Local Group of galaxies. One possible explanation is the increased difficulty of detecting these haloes if most of the visible matter is lost at early evolutionary phases through galactic winds. In this work we study the current models of triggering galactic winds in dwarf spheroidal galaxies (dSph) from supernovae, and study, based on 3D hydrodynamic numerical simulations, the correlation of the mass loss rates and important physical parameters as the dark matter halo mass and its radial profile, and the star formation rate. We find that the existence of winds is ubiquitous, independent on the gravitational potential. Our simulations revealed that the Rayleigh-Taylor Instability (RTI) may play a major role on pushing matter out of these systems, even for very massive haloes. The instability is responsible for 5 - 40% of the mass loss during the early evolution of the galaxy, being less relevant at $t > 200$Myrs. There is no significant difference in the mass loss rates obtained for the different dark matter profiles studied (NFW and logarithmic). We have also found a correlation between the mass loss rate and both the halo mass and the rate of supernovae, as already reported in previous works. Besides, the epoch in which most of the baryon galactic matter is removed from the galaxy varies depending on the SN rate and gravitational potential. The later, combined to the importance of the RTI in each model, may change our understanding about the chemical evolution of dwarf galaxies, as well as in the heavy element contamination of the intergalactic medium at high redshifts.Comment: MNRAS, accepte

Gluon saturation and the Froissart bound: a simple approach

At very high energies we expect that the hadronic cross sections satisfy the Froissart bound, which is a well-established property of the strong interactions. In this energy regime we also expect the formation of the Color Glass Condensate, characterized by gluon saturation and a typical momentum scale: the saturation scale $Q_s$. In this paper we show that if a saturation window exists between the nonperturbative and perturbative regimes of Quantum Chromodynamics (QCD), the total cross sections satisfy the Froissart bound. Furthermore, we show that our approach allows us to describe the high energy experimental data on $pp/p\bar{p}$ total cross sections.Comment: 6 pages, 5 figures. Includes additional figures, discussion and reference

Pastagens naturais pesquisas realizadas na região da fronteira Sudoeste do RS. (Fazenda experimental "Cinco Cruzes"/DNPEA - Embrapa Pecuária Sul 1954-1997).

As pesquisas na área de pastagens naturais, são de grande importância na região Sudoeste do Rio Grande do Sul, tendo em vista o predomínio do sistema extensivo de criação. As pastagens naturais são neste sistema a principal fonte de alimento dos rebanhos.bitstream/item/110369/1/PASTAGENS-NATURAIS-PESQUISAS.pd

Hymenoptera pollinator effect on environment

There are several studies reporting the evidence that pollinators are declining as a result of local and global environmental degradation. Operation Pollinator is an international biodiversity program, supported by Syngenta, to boost the number of pollinating insects on commercial farms. It works by creating specific habitats, tailored to local conditions and native insects. The aim of this work is to increase the ecological suitability of pollinator populations through improves food availability for pollinator employing strategies for surrounding landscape. Two cherry orchards, located in Fundão, Portugal, were studied: one was installed a patch meadow with flowery prairie with the purpose of increasing pollinator’s number; other with poor native biodiversity. It was identified the main groups of insects visiting the cherry blossoms and the surrounding flora, during the flowering cherry trees. This review was conducted through observation and analysis of pollen in pollinator’s nests, artificially placed in the orchards in order to evaluate the visiting flowers. There was a greater number of pollinating insects in the orchard with higher biodiversity and it was found that insects visited other flora along with the cherry blossoms. The Hymenoptera identified belong to genus: Andrena; Apis; Eucera; Tropinota; Anthophora; Osmia; Xylocopa. The orchard where haven’t been installed the patch meadow, shows a lower number of visiting insects as well as lower species variability. The increasing of pollinators protects the environment and increases the fruit production and quality. The findings of the sweeter fruit, with the increased number and diversity of insects leads to the conclusion that the environment benefits from the increase in pollinators with more balanced environment, it's a winning environment for everyone. The conservation of pollinator habitat can also enrich overall biodiversity and the ecosystem services, protect soil and water qualityinfo:eu-repo/semantics/publishedVersio