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Basto M.P.,University Of Ivora | Mira A.,University Of Ivora
Animal Biology | Year: 2011

Water is a limiting factor in Mediterranean regions, being specially important to aquatic species such as the Eurasian otter. The seasonal fluctuation of this resource is often addressed by constructing small and medium-sized water reservoirs. However, their role in the ecology and conservation of Eurasian otters is largely unknown. Our main goals were to assess the level of use of these reservoirs by otters and determine the main factors that may explain the observed levels of use. Intensity of use was determined by signs of otter presence and related to environmental variables using generalized linear models and variation partitioning techniques. Otters were present in the majority of reservoirs, both in the wet and dry seasons. Otter marking intensity was higher during the dry season, and positively associated with abundance of fish and American crayfish, existence of refuges and marking sites, number of watercourses and area of reservoir. In contrast, cattle settlements, annual crops and length of watercourses in the surrounding area negatively affected the use of the reservoirs. Otter diet reflected their opportunistic behaviour through the selection of seasonally available prey and corroborated the importance of American crayfish as a food item. Our results confirm that otters use the majority of small and medium-sized reservoirs in the study area. Despite this, reservoirs may be considered suboptimal habitats and seem to be specially relevant in the dry season, increasing water availability and acting as important feeding areas mainly when close to watercourses with good refuge conditions. Management implications are discussed. © 2011 Koninklijke Brill NV, Leiden.

Carmelo J.M.P.,University of Minho | Carmelo J.M.P.,University of Stuttgart | Araujo M.A.N.,University Of Ivora | Araujo M.A.N.,University of Lisbon | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The spectral-weight distribution in recent neutron scattering experiments on the parent compound La 2CuO 4 (LCO), which are limited in energy range to about 450 meV, is studied in the framework of the Hubbard model on the square lattice with effective nearest-neighbor transfer integral t and on-site repulsion U. Our study combines a number of numerical and theoretical approaches, including, in addition to standard treatments, density matrix renormalization group calculations for Hubbard cylinders and a suitable spinon approach for the spin excitations. The latter spin-12 spinons are the spins of the rotated electrons that singly occupy sites. These rotated electrons are mapped from the electrons by a uniquely defined unitary transformation, in which rotated-electron single and double occupancy are good quantum numbers for finite interaction values. Our results confirm that the U/8t magnitude suitable to LCO corresponds to intermediate U values smaller than the bandwidth 8t, which we estimate to be 8t2.36 eV for U/8t0.76. This confirms the unsuitability of the conventional linear spin-wave theory. Our theoretical studies provide evidence for the occurrence of ground-state d-wave spinon pairing in the half-filled Hubbard model on the square lattice. This pairing applies only to the rotated-electron spin degrees of freedom, but it could play a role in a possible electron d-wave pairing formation upon hole doping. We find that the higher-energy spin spectral weight extends to about 566 meV and is located at and near the momentum [π,π]. The continuum weight energy-integrated intensity vanishes or is extremely small at momentum [π,0]. This behavior of this intensity is consistent with that of the spin waves observed in recent high-energy neutron scattering experiments, which are damped at the momentum [π,0]. We suggest that future LCO neutron scattering experiments scan the energies between 450 and 566 meV and momenta around [π,π]. © 2012 American Physical Society.

Gross F.,College of William and Mary | Gross F.,Jefferson Lab | Stadler A.,University of Lisbon | Stadler A.,University Of Ivora
Physical Review C - Nuclear Physics | Year: 2010

We present the effective range expansions for the 1S0 and 3S1 scattering phase shifts, and the relativistic deuteron wave functions that accompany our recent high precision fits (with χ2/N data1) to the 2007 world np data below 350 MeV. The wave functions are expanded in a series of analytical functions (with the correct asymptotic behavior at both large and small arguments) that can be Fourier-transformed from momentum to coordinate space and are convenient to use in any application. A fortran subroutine to compute these wave functions can be obtained from the authors. © 2010 The American Physical Society.

Araujo M.A.N.,University Of Ivora | Araujo M.A.N.,University of Lisbon | Garcia-Garcia A.M.,University of Lisbon | Garcia-Garcia A.M.,University of Cambridge | Sacramento P.D.,University of Lisbon
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Recent experiments have shown that in agreement with previous theoretical predictions, superconductivity in nanostructures can be enhanced with respect to the bulk (L→) limit. Motivated by these results, we study finite size effects (FSEs) in iron pnictide superconductors. We employ a five-band mean-field approach that reproduces quantitatively the band structure of these materials around the Fermi energy. For realistic values of the bulk critical temperature Tcbulk∼20-50 K, we find that Tc(L) has a complicated oscillating pattern as a function of the system size L. For a simplified two-band model we show analytically that these oscillations are caused by fluctuations of the spectral density around the Fermi energy. We identify a scale L∼10 nm for which deviations from mean fields, not included in our model, are small but still Tc(L) is higher than Tcbulk. Similar results are obtained for different boundary conditions and geometries. Finally we show that the differential conductance, an experimental observable, is also very sensitive to FSE. © 2011 American Physical Society.

Lopes I.,University Of Ivora | Lopes I.,University of Lisbon | Casanellas J.,University of Lisbon | Eugenio D.,University of Lisbon
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We studied the rate at which stars capture dark matter (DM) particles, considering different assumptions regarding the DM characteristics and, in particular, investigating how the stellar physics influences the capture rate. Two scenarios were considered: first, we assumed the maximal values for the spin-dependent and spin-independent DM particle-nucleon scattering cross sections allowed by the limits from direct detection experiments. Second, we considered that both scattering cross sections are of the same order, with the aim of studying the dependencies of the capture rate on stellar elements other than hydrogen. We found that the characteristics of the capture rate are very different in the two scenarios. Furthermore, we quantified the uncertainties on the computed capture rate (Cχ) and on the ratio between the luminosities from DM annihilations and thermonuclear reactions (L χ/Lnuc) derived from an imprecise knowledge of the stellar structure and DM parameters. For instance, while an uncertainty of 10% on the typical DM velocity leads to similar errors on the computed C χ and Lχ/Lnuc, the same uncertainty on the stellar mass becomes more relevant and duplicates the errors. Our results may be used to evaluate the reliability of the computed capture rate for the hypothetical use of stars other than the Sun as DM probes. © 2011 American Physical Society.

Ramalho G.,University of Lisbon | Pena M.T.,University of Lisbon | Stadler A.,University Of Ivora | Stadler A.,University of Lisbon
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Using a covariant spectator quark model that describes the recent lattice QCD data for the Δ electromagnetic form factors and all available experimental data on γN→Δ transitions, we analyze the charge and magnetic dipole distributions of the Δ baryon and discuss its shape. We conclude that the quadrupole moment of the Δ is a good indicator of the deformation and that the Δ + charge distribution has an oblate shape. We also calculate transverse moments and find that they do not lead to unambiguous conclusions about the underlying shape. © 2012 American Physical Society.

Sacramento P.D.,University of Lisbon | Araujo M.A.N.,University of Lisbon | Araujo M.A.N.,University Of Ivora | Vieira V.R.,University of Lisbon | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We calculate the anomalous Hall conductance of superconductors with spin-orbit (SO) interaction and with either uniform or local magnetization. In the first case, we consider a uniform ferromagnetic ordering in a spin-triplet superconductor, whereas, in the second case, we consider a conventional s-wave spin-singlet superconductor with a magnetic impurity (or a diluted set of magnetic impurities). In the latter case, we show that the anomalous Hall conductance can be used to track the quantum-phase transition that occurs when the spin coupling between the impurity and the electronic spin density exceeds a certain critical value. In both cases, we find that, for large SO coupling, the superconductivity is destroyed and the Hall conductance oscillates strongly. © 2012 American Physical Society.

Rodriguez J.P.,California State University, Los Angeles | Araujo M.A.N.,University Of Ivora | Araujo M.A.N.,University of Lisbon | Sacramento P.D.,University of Lisbon
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

A two-orbital t-J model over the square lattice that describes low-energy electronic excitations in iron-pnictide high-T c superconductors is analyzed with Schwinger-boson-slave-fermion mean-field theory and by exact numerical diagonalization on a finite system. When interorbital hole hopping is suppressed, a quantum critical point (QCP) is identified that separates a commensurate spin-density wave (cSDW) state at strong Hund's rule coupling from a hidden half metal state at weak Hund's rule coupling. Low-energy spin waves that disperse anisotropically from cSDW momenta are predicted at the QCP. Nested Fermi surfaces similar to those observed experimentally in iron-pnictide materials are also predicted in such case. © 2011 American Physical Society.

Sacramento P.D.,University of Lisbon | Fernandes Silva L.C.,University of Lisbon | Nunes G.S.,University of Lisbon | Araujo M.A.N.,University of Lisbon | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed to, a conventional superconductor, via the passage of a spin polarized current through a ferromagnet-superconductor-ferromagnet junction. Solving the Landau-Lifshitz-Gilbert equation of motion for the magnetic moments, we calculate the velocity of the domain wall and compare it with the case of a ferromagnet-normal-ferromagnet junction. We find that in several regimes the domain wall velocity is higher when it is driven by a supercurrent. © 2011 American Physical Society.

Pogorelov Y.G.,University of Porto | Silva H.G.,University of Ivora | Polido J.F.,Ecole Polytechnique Federale de Lausanne
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Tunnel transport processes are considered in a square lattice of metallic nanogranules embedded into insulating host to model tunnel conduction in real metal/insulator granular layers. Based on a simple model with three possible charging states (± or 0) of a granule and three kinetic processes (creation or recombination of a ± pair, and charge transfer) between neighbor granules, the mean-field kinetic theory is developed. It describes the interplay between charging energy and temperature and between the applied electric field and the Coulomb fields by the noncompensated charge density. The resulting charge and current distributions are found to essentially differ in the free area (FA), between the metallic contacts, or in the contact areas (CA) beneath those contacts. Thus, the steady-state dc transport is compatible only with zero charge density and ohmic resistivity in FA, but charge accumulation and nonohmic behavior are necessary for conduction over CA. The approximate analytic solutions are obtained for characteristic regimes (low or high charge density) of such conduction. The comparison is done with the measurement data on tunnel transport in related experimental systems. © 2011 American Physical Society.

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