Anisotropic superconducting properties of aligned MgB2 crystallites

Samples of aligned MgB2 crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy Hc2^{ab}/Hc2^{c}= xi_{ab}/xi_{c} ~ 1.73; with xi_{o,ab} ~ 70 A, xi_{o,c} ~ 40 A, and a mass anisotropy ratio m_{ab}/m_{c} ~ 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.Comment: 4 pages, 4 figures; Revised version to appear in Phys. Rev. Let

Entanglement dynamics via coherent-state propagators

The dynamical generation of entanglement in closed bipartite systems is investigated in the semiclassical regime. We consider a model of two particles, initially prepared in a product of coherent states, evolving in time according to a generic Hamiltonian, and derive a formula for the linear entropy of the reduced density matrix using the semiclassical propagator in the coherent-state representation. The formula is explicitly written in terms of quantities that define the stability of classical trajectories of the underlying classical system. The formalism is then applied to the problem of two nonlinearly coupled harmonic oscillators and the result is shown to be in remarkable agreement with the exact quantum measure of entanglement in the short-time regime. An important byproduct of our approach is a unified semiclassical formula which contemplates both the coherent-state propagator and its complex conjugate.Comment: 10 page

Determinação automática de parâmetros morfométricos de bacias hidrográficas no município de Campinas - SP.

A bacia hidrográfica se configura atualmente como umas das principais unidades de gerenciamento territorial que dispomos nas atividades agrossilvipastoris, sendo modelada pelas condições geológicas e climáticas locais. Os dados de sensoriamento remoto orbital atendem à necessidade de informação em diversas escalas, representando um meio viável de monitoramento da superfície terrestre através de satélites e seus sensores, e vem servindo de fonte de informações para estudos e levantamentos geológicos, agrícola cartográficos, florestais, urbanos, entre outros. Esta pesquisa tem pretende fornecer um retrato da situação atual dos canais fluviais e pluviais das bacias hidrográficas dos ribeirões Anhumas e Quilombo, e dos rios Capivari, Capivari-Mirim, Jaguari e Atibaia no Município de Campinas, a partir da confecção de um aplicativo em Arc Macro Language (AML) para automatizar a extração de parâmetros morfométricos, possibilitando uma análise temporal da evolução dos canais fluviais, a qual é fundamental para a aplicação de medidas mitigadoras e voltadas para o gerenciamento dos rios

Vacuum fluctuations of a scalar field near a reflecting boundary and their effects on the motion of a test particle

The contribution from quantum vacuum fluctuations of a real massless scalar field to the motion of a test particle that interacts with the field in the presence of a perfectly reflecting flat boundary is here investigated. There is no quantum induced dispersions on the motion of the particle when it is alone in the empty space. However, when a reflecting wall is introduced, dispersions occur with magnitude dependent on how fast the system evolves between the two scenarios. A possible way of implementing this process would be by means of an idealized sudden switching, for which the transition occurs instantaneously. Although the sudden process is a simple and mathematically convenient idealization it brings some divergences to the results, particularly at a time corresponding to a round trip of a light signal between the particle and the wall. It is shown that the use of smooth switching functions, besides regularizing such divergences, enables us to better understand the behavior of the quantum dispersions induced on the motion of the particle. Furthermore, the action of modifying the vacuum state of the system leads to a change in the particle energy that depends on how fast the transition between these states is implemented. Possible implications of these results to the similar case of an electric charge near a perfectly conducting wall are discussed.Comment: 17 pages, 8 figure

Demonstration of the Complementarity of One- and Two-Photon Interference

The visibilities of second-order (single-photon) and fourth-order (two-photon) interference have been observed in a Young's double-slit experiment using light generated by spontaneous parametric down-conversion and a photon-counting intensified CCD camera. Coherence and entanglement underlie one-and two-photon interference, respectively. As the effective source size is increased, coherence is diminished while entanglement is enhanced, so that the visibility of single-photon interference decreases while that of two-photon interference increases. This is the first experimental demonstration of the complementarity between single- and two-photon interference (coherence and entanglement) in the spatial domain.Comment: 21 pages, 7 figure