Orbital currents and charge density waves in a generalized Hubbard ladder

We study a generalized Hubbard model on the two-leg ladder at zero temperature, focusing on a parameter region with staggered flux (SF)/d-density wave (DDW) order. To guide our numerical calculations, we first investigate the location of a SF/DDW phase in the phase diagram of the half-filled weakly interacting ladder using a perturbative renormalization group (RG) and bosonization approach. For hole doping 6 away from half-filling, finite-system density-matrix renormalizationgroup (DMRG) calculations are used to study ladders with up to 200 rungs for intermediate-strength interactions. In the doped SF/DDW phase, the staggered rung current and the rung electron density both show periodic spatial oscillations, with characteristic wavelengths 2/delta and 1/delta, respectively, corresponding to ordering wavevectors 2k(F) and 4k(F) for the currents and densities, where 2k(F) = pi(1 - delta). The density minima are located at the anti-phase domain walls of the staggered current. For sufficiently large dopings, SF/DDW order is suppressed. The rung density modulation also exists in neighboring phases where currents decay exponentially. We show that most of the DMRG results can be qualitatively understood from weak-coupling RG/bosonization arguments. However, while these arguments seem to suggest a crossover from non-decaying correlations to power-law decay at a length scale of order 1/delta, the DMRG results are consistent with a true long-range order scenario for the currents and densities. (c) 2005 Elsevier Inc. All rights reserved.

Melhoria do sistema produtivo de escadotes da FUTE – Fábrica de Utilidade de Tubos, SA

Atualmente as empresas encontram-se perante uma sociedade cada vez mais competitiva e desenvolvida, sendo por isso, imperativo a procura da melhoria contínua como forma de assegurar a adaptação da organização empresarial às alterações socioeconómicas que possam surgir. Incutir esta cultura é fundamental dentro do meio empresarial. A redução dos custos e o aumento da produtividade são dois dos principais desafios que todas as empresas se confrontam diariamente, sendo para isso desenvolvidas e aplicadas várias metodologias. A presente dissertação foi realizada no âmbito do Mestrado em Engenharia Mecânica no ramo de Gestão Industrial no Instituto Superior de Engenharia do Porto. Este trabalho realizou-se no ambiente industrial da empresa Afer FUTE - Fábrica de Utilidades de Tubos, S.A, que é uma empresa dedicada, essencialmente à produção de Tábuas de Engomar e Escadotes. O objetivo deste projeto visava melhorar todo o sistema produtivo da secção dos Escadotes, englobando assim todos os procedimentos desde o momento de entrada dos materiais até à obtenção do produto acabado. Tendo em vista a resolução dos problemas inicialmente existentes na empresa, foram apresentadas propostas de melhoria do processo produtivo segundo uma visão, Lean Production ou seja baseada na eliminação de qualquer tipo de desperdícios. A implementação destas propostas permitiu a obtenção de resultados positivos no Sistema Produtivo de Escadotes, nomeadamente um aumento de produtividade em cerca de 40%.

Population ecology of the sea lamprey (Petromyzon marinus) as an invasive species in the Laurentian Great Lakes and an imperiled species in Europe.

The sea lamprey Petromyzon marinus (Linnaeus) is both an invasive non-native species in the Laurentian Great Lakes of North America and an imperiled species in much of its native range in North America and Europe. To compare and contrast how understanding of population ecology is useful for control programs in the Great Lakes and restoration programs in Europe, we review current understanding of the population ecology of the sea lamprey in its native and introduced range. Some attributes of sea lamprey population ecology are particularly useful for both control programs in the Great Lakes and restoration programs in the native range. First, traps within fish ladders are beneficial for removing sea lampreys in Great Lakes streams and passing sea lampreys in the native range. Second, attractants and repellants are suitable for luring sea lampreys into traps for control in the Great Lakes and guiding sea lamprey passage for conservation in the native range. Third, assessment methods used for targeting sea lamprey control in the Great Lakes are useful for targeting habitat protection in the native range. Last, assessment methods used to quantify numbers of all life stages of sea lampreys would be appropriate for measuring success of control in the Great Lakes and success of conservation in the native range.