Phase diagram and quasiparticles of a lattice SU(2) scalar-fermion model in 2+1 dimensions

The phase diagram at zero temperature of a lattice SU(2) scalar-fermion model in 211 dimensions is studied numerically and with mean-field methods. Special attention is devoted to the strong coupling regime. We have developed a new method to adapt the hybrid Monte Carlo algorithm to the O(3) non-linear σ model constraint. The charged excitations in the various phases are studied at the mean-field level. Bound states of two charged fermions are found in a strongly coupled paramagnetic phase. On the other hand, in the strongly coupled antiferromagnetic phase fermionic excitations around momenta (±π/2, ±π/2, ±π/2) emerge

Evaluation of fatigue life data by normalising procedures

13th European Conference on Fracture, San Sebastian (Spain)Structures and mechanical components are frequently submitted to loads of variable amplitudes and of a random nature. The corresponding fatigue life prediction has to be analysed by means of damage accumulation models which utilise as basic information the S-N field of the material, determined from fatigue life tests conducted at various different constant stress ranges. Thus, the reliability of the life prediction under variable amplitude loading depends to a great extent on the quality of the estimation of the parameters related to the S-N field. Accordingly, a statistical non-linear regression analysis of the fatigue results is needed on account of the limited number of fatigue results spread over several stress ranges and of the considerable scatter of the results within each stress range. Since two random variables have to be considered − the stress range ∆σ or the stress level σ, depending on the material tested, and the number of cycles to failure N − two different statistical distributions, F(N; ∆σ), representing the number of cycles to failure given the stress range ∆σ, or else E(∆σ; N), representing the stress range given the number of cycles to failure, could be envisaged. Both distributions must fulfil physical and statistical conditions for the statistical model to be valid. In this paper, a consistent statistical model for analysing the S-N field is presented as well as methods for estimating the model parameters, based on normalising test data. Additionally, damage indices identified as the normalised variables are defined and their interpretation discusse

Characterizing the Optical Variability of Bright Blazars: Variability-based Selection of Fermi Active Galactic Nuclei

We investigate the use of optical photometric variability to select and identify blazars in large-scale time-domain surveys, in part to aid in the identification of blazar counterparts to the ∼30% of γ -ray sources in the Fermi 2FGL catalog still lacking reliable associations. Using data from the optical LINEAR asteroid survey, we characterize the optical variability of blazars by fitting a damped random walk model to individual light curves with two main model parameters, the characteristic timescales of variability τ , and driving amplitudes on short timescales σ . Imposing cuts on minimum τ and σ allows for blazar selection with high efficiency E and completeness C. To test the efficacy of this approach, we apply this method to optically variable LINEAR objects that fall within the several arcminute error ellipses of γ -ray sources in the Fermi 2FGL catalog. Despite the extreme stellar contamination at the shallow depth of the LINEAR survey, we are able to recover previously associated optical counterparts to Fermi active galactic nuclei with E ≥ 88% and C = 88% in Fermi 95% confidence error ellipses having semimajor axis r < 8'. We find that the suggested radio counterpart to Fermi source 2FGL J1649.6+5238 has optical variability consistent with other γ -ray blazars and is likely to be the γ -ray source. Our results suggest that the variability of the non-thermal jet emission in blazars is stochastic in nature, with unique variability properties due to the effects of relativistic beaming. After correcting for beaming, we estimate that the characteristic timescale of blazar variability is ∼3 years in the rest frame of the jet, in contrast with the ∼320 day disk flux timescale observed in quasars. The variability-based selection method presented will be useful for blazar identification in time-domain optical surveys and is also a probe of jet physics

Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measure-ment have been performed as a function of temperature (50 K to 325 K) and under applied mag-netic fields perpendicular to the nanowire axis, with magnitudes up to 15 % at room tempera-ture. A linear relation between thermopower S and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs. σ and the Mott formula, the energy derivative of the resistivity has been determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the non-diffusive thermopower contributions of the nanowire can be neglected, it was found that the magnetic field induced changes of thermopower and re-sistivity are equivalent. The main emphasis in the present paper is put on a comparison of the magnetoresistance and magnetothermopower results and it was found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various indi-vidual nanowires

The generalized Lasso with non-linear observations

We study the problem of signal estimation from non-linear observations when the signal belongs to a low-dimensional set buried in a high-dimensional space. A rough heuristic often used in practice postulates that non-linear observations may be treated as noisy linear observations, and thus the signal may be estimated using the generalized Lasso. This is appealing because of the abundance of efficient, specialized solvers for this program. Just as noise may be diminished by projecting onto the lower dimensional space, the error from modeling non-linear observations with linear observations will be greatly reduced when using the signal structure in the reconstruction. We allow general signal structure, only assuming that the signal belongs to some set K in R^n. We consider the single-index model of non-linearity. Our theory allows the non-linearity to be discontinuous, not one-to-one and even unknown. We assume a random Gaussian model for the measurement matrix, but allow the rows to have an unknown covariance matrix. As special cases of our results, we recover near-optimal theory for noisy linear observations, and also give the first theoretical accuracy guarantee for 1-bit compressed sensing with unknown covariance matrix of the measurement vectors.Comment: 21 page

Model Checking Linear Logic Specifications

The overall goal of this paper is to investigate the theoretical foundations of algorithmic verification techniques for first order linear logic specifications. The fragment of linear logic we consider in this paper is based on the linear logic programming language called LO enriched with universally quantified goal formulas. Although LO was originally introduced as a theoretical foundation for extensions of logic programming languages, it can also be viewed as a very general language to specify a wide range of infinite-state concurrent systems. Our approach is based on the relation between backward reachability and provability highlighted in our previous work on propositional LO programs. Following this line of research, we define here a general framework for the bottom-up evaluation of first order linear logic specifications. The evaluation procedure is based on an effective fixpoint operator working on a symbolic representation of infinite collections of first order linear logic formulas. The theory of well quasi-orderings can be used to provide sufficient conditions for the termination of the evaluation of non trivial fragments of first order linear logic.Comment: 53 pages, 12 figures "Under consideration for publication in Theory and Practice of Logic Programming