Dressing a Naked Singularity: an Example

Considering the evolution of a perfect fluid with self-similarity of the second kind, we have found that an initial naked singularity can be trapped by an event horizon due to collapsing matter. The fluid moves along time-like geodesics with a self-similar parameter $\alpha = -3$. Since the metric obtained is not asymptotically flat, we match the spacetime of the fluid with a Schwarzschild spacetime. All the energy conditions are fulfilled until the naked singularity.Comment: 14 pages, 1 figure. This version corrects an error in the calculus of the pressure and in the conclusion

Paleoambiental reconstruction based on mineralogical data of quaternary littoral deposits occuring at Cortegaça and Maceda beaches (Ovar, Portugal)

The main goal of the present work is the use of mineralogical data corresponding to sediment fine fractions (silt and clay) of Quaternary littoral deposits for the definition of a more detailed vertical zonography and to discriminate the most significant morphoclimatic changes concerned with sediment source areas and sediment deposition areas. The analysis of the available mineralogical data reveals a vertical evolution of the mineral composition. The following aspects deserve particular reference: 1) fine fractions (<38 nm) are composed of quartz and phyllosilicates associated to feldspars, prevailing over other minerals; however in certain sections iron hydroxides and evaporitic minerals occur in significant amounts; 2) clay fractions (<2 nm) show a general prevalence of illite associated with kaolinite and oscillations, in relative terms, of kaolinite and illite contents. Qualitative and quantitative lateral and vertical variations of clay and non clay minerals allow the discrimination of sedimentary sequences and the establishment of the ritmicity and periodicity of the morphoclimatic Quaternary episodes that occurred in the Cortegaça and Maceda beaches. To each one of the sedimentary sequences corresponds, in a first stage, a littoral environment that increasingly became more continental. Climate would be mild to cold, sometimes with humidity - aridity oscillations. Warmer and moister episodes alternated with cooler and dryer ones

A computationally efficient method for calculating the maximum conductance of disordered networks: Application to 1-dimensional conductors

Random networks of carbon nanotubes and metallic nanowires have shown to be very useful in the production of transparent, conducting films. The electronic transport on the film depends considerably on the network properties, and on the inter-wire coupling. Here we present a simple, computationally efficient method for the calculation of conductance on random nanostructured networks. The method is implemented on metallic nanowire networks, which are described within a single-orbital tight binding Hamiltonian, and the conductance is calculated with the Kubo formula. We show how the network conductance depends on the average number of connections per wire, and on the number of wires connected to the electrodes. We also show the effect of the inter-/intra-wire hopping ratio on the conductance through the network. Furthermore, we argue that this type of calculation is easily extendable to account for the upper conductivity of realistic films spanned by tunneling networks. When compared to experimental measurements, this quantity provides a clear indication of how much room is available for improving the film conductivity.Comment: 7 pages, 5 figure

Statistical analysis of mineralogical parameters used as lithostratigraphic markers. Application to Quaternary sediments of the littoral between Espinho and Furadouro (Portugal)

The findings of lithological, sedimentological and mineralogical studies presented in this paper are based on samples from five boreholes located in the area surrounding the Aveiro Ria (northern Portugal), cores taken from depths of nearly 40 m. The sedimentary columns are constituted by fine and medium sands containing intercalations of silt and clay. A relative homogeneity was found in the mineralogical composition of the sedimentary columns. Nevertheless, there is some vertical variation with regards to the minerals that are better represented quantitatively (phyllosilicates, quartz and feldspars). Illite dominates in the clay fraction. However, both vermiculite and kaolinite can be found in significant amounts in some layers. Using the fine/coarse detrital minerals ratio and the vertical evolution of the factor scores (principal components analysis), a possible lithostratigraphic zonation was established, as well as a correlation between drilling cores. The result consisted of the definition of guide-layers, corresponding to clear anisotropies. We believe that the methodologies adopted could be of great interest in discriminating situations where the classical parameters yield homogenous information.Se presentan los resultados correspondientes a los estudios litológicos, sedimentológicos y mineralógicos realizados en muestras provenientes de cinco sondeos efectuados en la región litoral circundante de la ría de Aveiro, que alcanzan profundidades en torno a los 40 m. Las columnas sedimentarias están constituidas por arenas de tamaño fino-medio, con intercalaciones finas de limos y arcillas. Se ha observado una relativa monotonía en la composición mineral a lo largo de las columnas, aunque con alguna variación vertical en lo referente a los minerales más abundantes (filosilicatos, cuarzo y feldespatos). En la fracción arcillosa, la ilita es el mineral predominante, junto con vermiculita y caolinita, que tienen una presencia relativamente importante en algunos niveles. Usando la razón entre minerales detríticos finos y gruesos y la evolución vertical de las factor scores (análisis de componentes principales), se procedió a establecer la posible zonación litoestratigráfica y la correlación entre los sondeos. El resultado consistió en la definición de niveles guía, caracterizados por evidentes anisotropías en la evolución vertical de estos marcadores. Por último, creemos que son de gran interés estos tipos de metodologías en situaciones de evidente monotonía de los parámetros clásicamente utilizados, tales como la descripción litológica y las variaciones de composición mineral.Instituto Español de Oceanografí

Bromeliaceae species from coastal restinga habitats, Brazilian states of Rio de Janeiro, Espírito Santo, and Bahia.

Bromeliaceae is one of the most representative plant families in restinga habitats. We analyzed the speciesrichness and composition of Bromeliaceae in 13 restinga habitats along the Brazilian coast. We found a total of 41species distributed along the restinga habitats studied. The restinga of Praia do Sul, in the state of Rio de Janeiro, hadthe highest number of species (15), whereas the restinga of Abaeté, in the state of Bahia, had the lowest (4). Our dataare suggestive that the Doce River may represent the limit of distribution for some bromeliad species, with some speciesoccurring only south of that river and others occurring only to the north of it. The differences in Bromeliaceae speciescomposition among restinga habitats probably are not only due to differences in local environmental conditions, butalso due to the geographic distribution pattern of each species and to the present degree of disturbance at each restinga

Upper bound for the conductivity of nanotube networks

Films composed of nanotube networks have their conductivities regulated by the junction resistances formed between tubes. Conductivity values are enhanced by lower junction resistances but should reach a maximum that is limited by the network morphology. By considering ideal ballistic-like contacts between nanotubes we use the Kubo formalism to calculate the upper bound for the conductivity of such films and show how it depends on the nanotube concentration as well as on their aspect ratio. Highest measured conductivities reported so far are approaching this limiting value, suggesting that further progress lies with nanowires other than nanotubes.Comment: 3 pages, 1 figure. Minor changes. Accepted for publication in Applied Physics Letter

Critical Excitation Spectrum of Quantum Chain With A Local 3-Spin Coupling

This article reports a measurement of the low-energy excitation spectrum along the critical line for a quantum spin chain having a local interaction between three Ising spins and longitudinal and transverse magnetic fields. The measured excitation spectrum agrees with that predicted by the (D$_4$, A$_4$) conformal minimal model under a nontrivial correspondence between translations at the critical line and discrete lattice translations. Under this correspondence, the measurements confirm a prediction that the critical line of this quantum spin chain and the critical point of the 2D 3-state Potts model are in the same universality class.Comment: 7 pages, 2 figure

Magnetic fabric of Red Beds from the Conraria Formation, Central Portugal

Poster apresentado ao VIII Congresso Nacional de Geologia, em Braga (9-16 de Julho 2010)O objectivo principal deste estudo consistiu em quantificar o fabric magnético em amostras da subunidade superior da Formação de Conraria, Grupo de Silves, na região de Coimbra. Foram efectuadas determinações de anisotropia da susceptibilidade magnética (ASM) em 50 amostras, obtidas em 11 sites, de forma a caracterizar o seu fabric magnético. O paralelismo geral entre um fabric achatado e a estratificação poderá indicar uma conjugação entre o arranjo sedimentar primário e outro com origem na diagénese ou mesmo em componentes de origem tectónica precoce.The main goal of this study was to quantify the magnetic fabric on rock samples of the upper subunit of the Conraria Formation, Silves Group, in the region of Coimbra. Studies of Anisotropy of Magnetic Susceptibility (AMS) were carried out on 50 samples from 11 sites in order to characterise the magnetic fabric. The general parallelism between an oblate fabric and stratification may indicate a combination between the primary sedimentary arrangement and another one with origin in the diagenesis or even in components of earlier tectonic origin