Uniform semiclassical approximation in quantum statistical mechanics

We present a simple method to deal with caustics in the semiclassical approximation to the partition function of a one-dimensional quantum system. The procedure, which makes use of complex trajectories, is applied to the quartic double-well potential.Comment: 5 pages, 1 figure, Latex. Contribution to the Proceedings of the XXI Brazilian National Meeting on Particles and Fields (Sao Lourenco, October 23-27, 2000

Scaling laws for the decay of multiqubit entanglement

We investigate the decay of entanglement of generalized N-particle Greenberger-Horne-Zeilinger (GHZ) states interacting with independent reservoirs. Scaling laws for the decay of entanglement and for its finite-time extinction (sudden death) are derived for different types of reservoirs. The latter is found to increase with the number of particles. However, entanglement becomes arbitrarily small, and therefore useless as a resource, much before it completely disappears, around a time which is inversely proportional to the number of particles. We also show that the decay of multi-particle GHZ states can generate bound entangled states.Comment: Minor mistakes correcte

Suppression of Anderson localization of light and Brewster anomalies in disordered superlattices containing a dispersive metamaterial

Light propagation through 1D disordered structures composed of alternating layers, with random thicknesses, of air and a dispersive metamaterial is theoretically investigated. Both normal and oblique incidences are considered. By means of numerical simulations and an analytical theory, we have established that Anderson localization of light may be suppressed: (i) in the long wavelength limit, for a finite angle of incidence which depends on the parameters of the dispersive metamaterial; (ii) for isolated frequencies and for specific angles of incidence, corresponding to Brewster anomalies in both positive- and negative-refraction regimes of the dispersive metamaterial. These results suggest that Anderson localization of light could be explored to control and tune light propagation in disordered metamaterials.Comment: 4 two-column pages, 3 figure

Efeito de agentes de controle biológico na indução de resistência em videira.

O objetivo deste trabalho foi avaliar o efeito de agentes comerciais de controle biológico como indutores de resistência em videira para o controle do míldio

Some new results concerning the vacuum in Dirac Hole Theory

In Dirac's hole theory the vacuum state is generally believed to be the state of minimum energy. It will be shown that this is not, in fact, the case and that there must exist states in hole theory with less energy than the vacuum state. It will be shown that energy can be extracted from the hole theory vacuum state through the application of an electric field.Comment: Accepted by Physica Scripta, 19 page

A fruticultura como vetor de desenvolvimento: o caso dos municipios de Petrolina (PE) e Juazeiro (BA).

Para contrapor o fenomeno das secas, que frequentemente provocam perda da producao agricola na zona semi-arida do Nordeste, gerando grandes problemas sociais, o governo tem implantado diversos perimetros publicos irrigados e promovido outros estimulos governamentais para a iniciativa privada no setor agropecuario. Atraves dessas iniciativas, o governo tem criado importantes Polos de producao agricola no que diz respeito as pequenas, medias e grandes empresas de producao de frutas e hortalicas. A agricultura irrigada e, mais especificamente, a fruticultura irrigada, promoveu um grande dinamismo na economia do Polo na estrutura urbana tornando-se o aglomerado urbano mais prospero do Vale do Sao Francisco. As grandes mudancas dos sistemas produtivos de culturas anuais para fruteiras perenes, de exportacao, desencadeou no Polo de Petrolina (PE)/Juazeiro (BA) demanda de outros investimentos de apoio para a comercializacao de frutas, motivando o Governo Federal a financiar pesquisas, priorizando aquelas relacionadas com culturas de exportacao, promover cursos de especializacao em comercio exterior e melhorar a infra-estrutura logistica da regiao. A fruticultura irrigada e o principal vetor do desenvolvimento do Submedio Sao Francisco, notadamente das suas cidades Polo, Petrolin,a e Juazeiro, que se constituem em verdadeiros "paraisos"de desenvolvimento dentro do Semi-arido brasileiro. Isto porque com o crescimento da fruticultura se desencadeou uma sinergia de crescimento em praticamente todos os setores produtivos, como o industrial, comercio e turismo. O objetivo deste trabalho e analisar a evolucao do Polo e pontos de estrangulamentos para a ampliacao do desenvolvimento da regiao

Driving-dependent damping of Rabi oscillations in two-level semiconductor systems

We propose a mechanism to explain the nature of the damping of Rabi oscillations with increasing driving-pulse area in localized semiconductor systems, and have suggested a general approach which describes a coherently driven two-level system interacting with a dephasing reservoir. Present calculations show that the non-Markovian character of the reservoir leads to the dependence of the dephasing rate on the driving-field intensity, as observed experimentally. Moreover, we have shown that the damping of Rabi oscillations might occur as a result of different dephasing mechanisms for both stationary and non-stationary effects due to coupling to the environment. Present calculated results are found in quite good agreement with available experimental measurements

Onsager Loop-Transition and First Order Flux-Line Lattice Melting in High-TcT_c Superconductors

Monte-Carlo simulations in conjunction with finite-size scaling analysis are used to investigate the $(H,T)$-phase diagram in uniaxial anisotropic high- $T_c$ superconductors, both in zero magnetic field and in intermediate magnetic fields for various mass-anisotropies. The model we consider is the uniformly frustrated anisotropic Villain Model. In zero magnetic field, and for all anisotropies considered, we find one single second order phase transition, mediated by an Onsager vortex-loop blowout. This is the superconductor-normal metal transition.A comparison with numerical simulations and a critical scaling analysis of the zero-field loop-transition yields the same exponent of the loop distribution function at the critical point. In the intermediate magnetic field regime, we find two anomalies in the specific heat. The first anomaly at a temperature $T_m$ is associated with the melting transition of the flux-line lattice. The second anomaly at a temperature $T_z$ is one where phase coherence along the field direction is destroyed. We argue that $T_m=T_z$ in the thermodynamic and continuum limit. Hence, there is no regime where the flux line lattice melts into a disentangled flux-line liquid. The loss of phase coherence parallel to the magnetic field in the sample is argued to be due to the proliferation of closed non-field induced vortex loops on the scale of the magnetic length in the problem, resulting in flux-line cutting and recombination. In the flux-line liquid phase, therefore, flux-lines appear no longer to be well defined entities. A finite-size scaling analysis of the delta function peak specific heat anomaly at the melting transition is used to extract the discontinuity of the entropy at the melting transition.This entropy discontinuity is found to increase rapidly with mass-anisotropy.Comment: 22 pages, 11 figures included, to be published in Phys. Rev. B, 57 xxx (1998