http://faraday-in.fc.up.pt/cfp Articles in international journals with referees. daily 1 2005-11-25T19:50:50Z http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-03-09.2029013982 We report on a four-fold anisotropy of the in-plane ferromagnetic resonance (FMR) field $H_r$ in a patterned square lattice of circular Permalloy dots when the interdot distance $a$ is comparable to the dot diameter $d$. The difference between the minimum of $H_r$, along the lattice [11] (or [$\bar{1}$1]) axes, and the maximum, along the [10] (or [01]) axes, is $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy is not expected in uniformly magnetized dots and can be explained by a non-uniform magnetization $\bm(\br)$ in a dot in response to dipolar forces from neighboring dots. Using a variational approach, the non-uniformity is described by a non-linear equation which can be iteratively solved, starting from the uniform zeroth iteration: $\bm^{(0)}(\br) = M_s{\bf{H}}/H$. A good agreement with the observed FMR behavior is found after a single iteration. No publisher mcosta 2007-05-31T16:18:25Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/optimized We propose a new recursive procedure to estimate the microcanonical density of states in multicanonical Monte Carlo simulations which relies only on measurements of moments of the energy distribution, avoiding entirely the need for energy histograms. This method yields directly a piecewise analytical approximation to the microcanonical inverse temperature, $\beta(E)$, and allows improved control over the statistics and efficiency of the simulations. We demonstrate its utility in connection with recently proposed schemes for improving the efficiency of multicanonical sampling, either with adjustment of the asymptotic energy distribution or with the replacement of single spin flip dynamics with collective updates. No publisher mcosta 2006-10-06T12:02:30Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Pereira2005b We consider the electronic structure near vacancies in the half-filled honeycomb lattice. It is shown that vacancies induce the formation of localized states. When particle-hole symmetry is broken, localized states become resonances close to the Fermi level. We also study the problem of a finite density of vacancies, obtaining the electronic density of states, and discussing the issue of electronic localization in these systems. Our results also have relevance for the problem of disorder in d-wave superconductors. No publisher vpereira 2007-05-15T18:10:17Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Pereira2005c Substituting Eu by Ca in ferromagnetic EuB6 leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the Eu1-xCaxB6 series, based on measurements of the reflectivity R(omega) from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of R(omega) we extract the complete absorption spectra of samples with different values of x. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double-exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing Ca content at low temperatures. No publisher vpereira 2006-01-31T15:21:59Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2007-05-31.9373752498 We probed for radio ... No publisher airesff 2007-05-31T16:14:46Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/copy_of_Castro-2006 No publisher vpereira 2007-12-10T20:29:45Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-19.0604830401 We discuss necessary conditions for a network of cosmic domain walls to have a chance of providing the dark energy that might explain the recent acceleration of the universe. We derive a strong bound on the curvature of the walls, which shows that viable candidate networks must be fine-tuned and non-standard. We also discuss various requirements that any stable lattice of frustrated walls must obey. We conjecture that, even though one can build (by hand) lattices that would be stable, no such lattices will ever come out of realistic domain wall forming cosmological phase transitions. We provide some simple numerical simulations that illustrate our results and correct some misconceptions in the published literature, but a detailed numerical analysis is left for a companion paper. No publisher jeolivei 2006-12-19T13:06:04Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-19.6820859873 We study a number of domain wall forming models where various types of defect junctions can exist. These illustrate some of the mechanisms that will determine the evolution of defect networks with junctions. Understanding these mechanisms is vital for a proper assessment of a number of cosmological scenarios: we will focus on the issue of whether or not cosmological frustrated domain wall networks can exist at all, but our results are also relevant for the dynamics of cosmic (super)strings, where junctions are expected to be ubiquitous. We also define and discuss the properties that would make up the ideal model in terms of hypothetical frustrated wall networks, and provide an explicit construction for such a model. We carry out a number of numerical simulations of the evolution of these networks, analyze and contrast their results, and discuss their implications for our no-frustration conjecture. No publisher jeolivei 2006-12-19T13:10:21Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-08-29.7718627634 We construct a class of toric Kahler manifolds, M_4, of real dimension four, a subset of which corresponds to the Kahler bases of all known 5D asymptotically AdS(5) supersymmetric black-holes. In a certain limit, these Kahler spaces take the form of cones over Sasaki spaces, which, in turn, are fibrations over toric manifolds of real dimension two. The metric on M_4 is completely determined by a single function H(x), which is the conformal factor of the two dimensional space. We study the solutions of minimal five dimensional gauged supergravity having this class of Kahler spaces as base and show that in order to generate a five dimensional solution H(x) must obey a simple sixth order differential equation. We discuss the solutions in detail, which include all known asymptotically AdS(5) black holes as well as other spacetimes with non-compact horizons. Moreover we find an infinite number of supersymmetric deformations of these spacetimes with less spatial isometries than the base space. These deformations vanish at the horizon, but become relevant asymptotically. No publisher paccetti 2006-11-18T12:41:03Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-08-30.4536645071 We study the inflationary scenarios driven by a Wilson line field - the fifth component of a 5D gauge field and corresponding modulus field, within S(1)/Z2 orbifold supergravity (SUGRA). We use our off shell superfield formulation and give a detailed description of the issue of SUSY breaking by the F-component of the radion superfield. By a suitably gauged U(1)R symmetry and including couplings with compensator supermultiplets and a linear multiplet, we achieve a self consistent radion mediated SUSY breaking of no scale type. The inflaton 1-loop effective potential has attractive features needed for successful inflation. An interesting feature of both presented inflationary scenarios are the red tilted spectra with ns~0.96. For gauge inflation we obtain a significant tensor to scalar ratio r~0.1 of the density perturbations, while for the modulus inflation r is strongly suppressed. No publisher paccetti 2006-08-30T14:07:12Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2007-01-12.2344605585 We present results from a detailed numerical study of the small-scale and loop production properties of cosmic string networks, based on the largest and highest resolution string simulations to date. We investigate the nontrivial fractal properties of cosmic strings, in particular, the fractal dimension and renormalized string mass per unit length, and we also study velocity correlations. We demonstrate important differences between string networks in flat (Minkowski) spacetime and the two very similar expanding cases. For high resolution matter era network simulations, we provide strong evidence that small-scale structure has converged to "scaling" on all dynamical length scales, without the need for other radiative damping mechanisms. We also discuss preliminary evidence that the dominant loop production size is also approaching scaling. No publisher cmartins 2007-05-31T16:20:47Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Pogorelov2005d No publisher vpereira 2006-10-11T10:34:40Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Cornalba2005b No publisher vpereira 2006-10-11T10:41:45Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/copy_of_Cornalba2005b No publisher vpereira 2006-10-11T10:40:37Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-20.2563465271 Can entanglement and the quantum behavior in physical systems survive at arbitrary high temperatures? In this Letter we show that this is the case for a electromagnetic field mode in an optical cavity with a movable mirror in a thermal state. We also identify two different dynamical regimes of generation of entanglement separated by a critical coupling strength. No publisher airesff 2006-12-20T18:50:58Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/smallbh We give evidence in favour of a string/black hole transition in the case of BPS fundamental string states of the Heterotic string. Our analysis goes beyond the counting of degrees of freedom and considers the evolution of dynamical quantities in the process. As the coupling increases, the string states decrease their size up to the string scale when a small black hole is formed. We compute the absorption cross section for several fields in both the black hole and the perturbative string phases. At zero frequency, these cross sections can be seen as order parameters for the transition. In particular, for the scalars fixed at the horizon the cross section evolves to zero when the black hole is formed. No publisher jpenedones 2007-05-31T16:04:57Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-22.1686919855 No publisher crherdei 2006-12-22T11:17:32Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-22.5074864999 We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized $\chi^2$ probability distribution function parametrized by the number of degrees of freedom, $\nu$. We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to $z=6$, as required by the measurement of the Gunn--Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, $\tau=0.09 \pm 0.03$, weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in mini-halos, which increases dramatically as we decrease $\nu$. No publisher ppavelin 2007-05-31T15:18:03Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-22.0433706136 We revisit the idea of using varying couplings to probe the nature of dark energy, in particular by reconstructing its equation of state. We show that this method can be far superior to the standard methods (using type Ia supernovae or weak lensing). We also show that the simultaneous use of measurements of the fine-structure constant $\alpha$ and the electron-to-proton mass ratio $\mu$ allows a direct probe of grand unification scenarios. We present forecasts for the sensitivity of this method, both for the near future and for the next generation of spectrographs -- for the latter we focus on the planned CODEX instrument for ESO's Extremely Large Telescope (formerly known as OWL). A high-accuracy reconstruction of the equation of state may be possible all the way up to redshift $z\sim4$. No publisher ppavelin 2006-12-22T17:06:31Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-22.2857780900 We study the evolution of the fine-structure constant, $\alpha$, induced by non-linear density perturbations in the context of the simplest class of quintessence models with a non-minimal coupling to the electromagnetic field, in which the two available free functions (potential and gauge kinetic function) are Taylor-expanded up to linear order. We show that the results obtained using the spherical infall model for an infinite wavelength inhomogeneity are inconsistent with the results of a local linearized gravity study and we argue in favour of the second approach. We also discuss recent claims that the value of $\alpha$ inside virialised regions could be significantly different from the background one on the basis of these findings. No publisher ppavelin 2006-12-22T17:11:00Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-12-22.7107179054 We study spatial variations of the fine-structure constant in the presence of static straight cosmic strings in the weak-field approximation in Einstein gravity. We work in the context of a generic Bekenstein-type model and consider a gauge kinetic function linear in the scalar field. We determine an analytical form for the scalar field and the string metric at large distances from the core. We show that the gravitational effects of $\alpha$-varying strings can be seen as a combination of the gravitational effects of global and local strings. We also verify that at large distances to the core the space-time metric is similar to that of a global string. We study the motion of test particles approaching from infinity and show that photons are scattered to infinity while massive particles are trapped in bounded trajectories. We also calculate an overall limit on the magnitude of the variation of $\alpha$ for a GUT string, by considering suitable cosmological constraints coming from the Equivalence Principle. No publisher ppavelin 2006-12-22T17:17:31Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Lopes2006 We report an atomic-scale study on the ferromagnetic insulator manganite LaMnO3.12 using gamma-gamma perturbed angular correlation spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller (JT) distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (rhombohedral-orthorhombic). Two distinct regimes of JT distortions were found: a high-temperature regime where uncorrelated polaron clusters with severe distortions of the Mn3+O6 octahedra survive up to T[approximate]800 K and a low-temperature regime where correlated regions have a weaker JT-distorted symmetry. No publisher vpereira 2006-10-11T09:28:50Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/articlereference.2006-08-03.6825564522 We present a detailed study of the reduction to 4D of 5D supergravity compactified on the S^1/Z_2 orbifold. For this purpose we develop and employ a recently proposed N=1 conformal superfield description of the 5D supergravity couplings to abelian vector and hypermultiplets. In particular, we obtain a unique relation of the "radion" to chiral superfields as in global 5D SUSY and we can embed the universal hypermultiplet into this formalism. In our approach, it is transparent how the superconformal structure of the effective 4D actions is inherited from the one of the original 5D supergravity. We consider both ungauged and gauged 5D supergravities. This includes compactifications in unwarped geometries, generalizations of the supersymmetric Randall-Sundrum (RS) model as well as 5D heterotic M-theory. In the unwarped case, after obtaining the effective Kaehler potentials and superpotentials, we demonstrate that the tree-level 4D potentials have flat and/or tachyonic directions. One-loop corrections to the Kaehler potential and gaugino condensation are presented as suitable tools for moduli stabilization to be discussed in subsequent work. Turning to the RS-like models, we obtain a master formula for the Kaehler potential for an arbitrary number of vector and hyper moduli, which we evaluate exactly for special cases. Finally, we formulate the superfield description of 5D heterotic M-theory and obtain its effective 4D description for the universal (h^(1,1)=1) case, in the presence of an arbitrary number of bulk 5-branes. We present, as a check of our expressions, time-dependent solutions of 4D heterotic M-theory, which uplift to 5D solutions generalizing the ones recently found in hep-th/0502077. No publisher paccetti 2006-08-10T11:04:11Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/copy_of_Castro2005 We consider the effect of random site dilution on a honeycomb lattice of quantum spins described by the antiferromagnetic Heisenberg spin-S model. Using linear spin-wave theory, we compute the zero-temperature magnetization and density of states as a function of dilution up to the classical percolation threshold. Several subtle issues regarding the treatment of quasidivergent zero-energy modes, which appear in the real-space formulation of the spin-wave problem, are clarified. For S 1/2, the spin-wave theory is well defined in the sense that results at all dilution concentrations are consistent with the underlying assumptions of the theory. For S=1/2, however, the approximation breaks down. In this case, we have studied the effect of dilution on the staggered magnetization using the stochastic series expansion Monte Carlo method. Two main results are to be stressed from the Monte Carlo calculation: i an improved estimate for the staggered magnetization of the undiluted system mav L =0.2677 6 and ii a finite value of the staggered magnetization of the percolating cluster at the classical percolation threshold, showing that there is no quantum critical transition driven by dilution in the Heisenberg model. We have used the computed staggered magnetization and density of states to analyze neutron scattering experiments and Néel temperature measurements of two quasi-two-dimensional diluted honeycomb systems: i MnpZn1 pPS3 a diluted S=5/2 system and ii Ba NipMg1 p 2V2O8 a diluted S=1 system . We have found that our calculations are in good agreement with the experimental data. No publisher vpereira 2007-12-10T20:31:16Z Article Reference http://faraday-in.fc.up.pt/cfp/research/publications/2006/articles/Herdeiro2005a No publisher vpereira 2006-04-11T18:14:02Z Article Reference