High-spin polaron in lightly doped CuO2_2 planes

We device and investigate numerically a minimal yet detailed spin polaron model that describes lightly doped CuO$_2$ layers. The low-energy physics of a hole is studied by total-spin-resolved exact diagonalization on clusters of up to 32 CuO$_2$ unit cells, revealing features missed by previous studies. In particular, spin-polaron states with total spin 3/2 are the lowest eigenstates in several regions of the Brillouin zone. In these regions, and also at other points the quasiparticle weight is identically zero, indicating orthogonal states to those represented in the one electron Green's function. This highlights the importance of proper treatment of spin fluctuations in the many-body background.Comment: To appear in Phys. Rev. Lett. Final version and Supplementary Materials will be available at the journal's websit

Generating Factor Variables for Asymmetry, Non-independence and Skew-symmetry Models in Square Contingency Tables using SAS

In this paper, a SAS program (macro) is written to generate factor and regression variables required for implementing asymmetry, non-independence, non-symmetry + independence models as well as skew-symmetry models in discussed in square a x a contingency tables having nominal or ordinal categories. While several authors have developed similar factor variables for use with GLIM, we have extended this to the non-independence and the non-symmetry+independence models. The former includes both the fixed and variable distance models as well as the quasi-ordinal symmetry model. Further, our implementation of the asymmetry model in terms of the required factor variable is different from those defined for implementation of same in GLIM. Most of the models described in this paper however assume ordinal categories for the contingency table. The SAS macro developed can be applied to any square table of dimension a. We apply the models discussed in this paper to the 5 x 5 Danish mobility data that have been widely analyzed in various literatures.

A homogeneous analysis of disks around brown dwarfs

We re-analyzed the Herschel/PACS data of a sample of 55 brown dwarfs (BDs) and very low mass stars with spectral types ranging from M5.5 to L0. We investigated the dependence of disk structure on the mass of the central object in the substellar regime based on a homogeneous analysis of Herschel data from flux density measurements to spectral energy distribution (SED) modeling. A systematic comparison between the derived disk properties and those of sun-like stars shows that the disk flaring of BDs and very low mass stars is generally smaller than that of their higher mass counterparts, the disk mass is orders of magnitude lower than the typical value found in T Tauri stars, and the disk scale heights are comparable in both sun-like stars and BDs. We further divided our sample into an early-type brown dwarf (ETBD) group and a late-type brown dwarf (LTBD) group by using spectral type (=M8) as the border criterion. We systematically compared the modeling results from Bayesian analysis between these two groups, and found the trends of flaring index as a function of spectral type also present in the substellar regime. The spectral type independence of the scale height is also seen between high-mass and very low-mass BDs. However, both the ETBD and LTBD groups feature a similar median disk mass of 10^{-5}Msun and no clear trend is visible in the distribution, probably due to the uncertainty in translating the far-IR photometry into disk mass, the detection bias and the age difference among the sample. Unlike previous studies, our analysis is completely homogeneous in Herschel/PACS data reduction and modeling with a statistically significant sample. Therefore, we present evidence of stellar-mass-dependent disk structure down to the substellar mass regime, which is important for planet formation models. (Abridged Version)Comment: Accepted for publication in A&

IR AND UV-VISIBLE SPECTRA OF IRON(II) PHTHALOCYANINE COMPLEXES WITH PHOSPHINE OR PHOSPHITE

We have prepared and studied by vibrational and electronic spectrometry of a series of ferrophthalocyanine complexes with phosphine or phosphite axial ligands [PMe3, P(OMe)3, PPh3, P(OPh)3, PPh(OMe)3 and POH(OEt)2]. With PMe3, P(OMe)3 as ligands, only hexacoordinated complexes are obtained but with PPh3, P(OPh)3 and POH(OEt)2 as ligands, the coordination number depends on the FePc:ligand ratio in the reaction mixture. In the ratio 1:1 the complexes thus formed are pentacoordinated and in the ratio 1:2 hexacoordinated complexes are obtained. In the electronic spectra of hexacoordinated complexes, two charge-transfer transition bands, one at ~375 nm and the other at ~421 nm can be observed. We attribute the band at 375 nm to charge-transfer from axial ligand to macrocycle (CT Lax ® Pc) and the band at 421 nm to charge-transfer from metal to axial ligand (CT Fe ® Lax). In the IR spectra, the position of the n FeN4 band is linked to the coordination number; in the spectra of pentacoordinated complexes, its frequency is almost the same as that in the FePc spectrum but in hexacoordinated complexes, it moves to high frequencies. (Received September 10, 2001; revised February 26, 2002) Bull. Chem. Soc. Ethiop. 2002, 16(1), 73-79

3D-Mesomechanical analysis of external sulfate attack in concrete

The present study focuses on degradation of concrete by external sulfate attack. The numerical model developed by the MECMAT/UPC group, incorporates coupled C-M analysis using a meso-mechanical approach with discrete cracking, using the MEF and zero thickness interface elements with a constitutive law based on nonlinear fracture mechanics concepts. Examples of application are run on 2D and 3D samples, with geometries and FE meshes generated with a code developed also in-house. The numerical analysis is carried out using two independent codes and a “staggered” procedure. The first code performs the mechanical analysis and the second the diffusive/reaction chemical problem. 2D uncoupled and coupled analysis are presented and discussed. Preliminary coupled 3D results are also presented and compared with equivalent 2D results, and the differences are detected and analyzed

Search for associations containing young stars (SACY). V. Is multiplicity universal? Tight multiple systems

Context: Dynamically undisrupted, young populations of stars are crucial to study the role of multiplicity in relation to star formation. Loose nearby associations provide us with a great sample of close ($<$150 pc) Pre-Main Sequence (PMS) stars across the very important age range ($\approx$5-70 Myr) to conduct such research. Aims: We characterize the short period multiplicity fraction of the SACY (Search for Associations Containing Young stars) accounting for any identifiable bias in our techniques and present the role of multiplicity fractions of the SACY sample in the context of star formation. Methods: Using the cross-correlation technique we identified double-lined spectroscopic systems (SB2), in addition to this we computed Radial Velocity (RV) values for our subsample of SACY targets using several epochs of FEROS and UVES data. These values were used to revise the membership of each association then combined with archival data to determine significant RV variations across different data epochs characteristic of multiplicity; single-lined multiple systems (SB1). Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no significant difference between the short period multiplicity fraction ($F_\mathrm{m}$) of the SACY sample and that of nearby star forming regions ($\approx$1-2 Myr) and the field ($F_\mathrm{m}\leq$10%) both as a function of age and as a function of primary mass, $M_1$, in the ranges $P$ [1:200 day] and $M_2$ [0.08 $M_{\odot}$-$M_1$]. Conclusions: Our results are consistent with the picture of universal star formation, when compared to the field and nearby star forming regions (SFRs). We comment on the implications of the relationship between increasing multiplicity fraction with primary mass, within the close companion range, in relation to star formation.Comment: 14 pages, 18 figures, published, A&A http://dx.doi.org/10.1051/0004-6361/20142385

Environmental Risk Assessment of Agrochemicals — A Critical Appraisal of Current Approaches

This chapter provides insights into the difficulties and challenges of performing risk evaluations of agrochemicals. It is a critical review of the current methodologies used in ecological risk assessment of these chemicals, not their risks to humans. After an introduction to the topic, the current framework for ecological risk assessment is outlined. Two types of assessments are typically carried out depending on the purpose: i) regulatory assessments for registration of a chemical product; and ii) ecological assessments, for the protection of both terrestrial and aquatic ecosystems, which are usually site-specific. Although the general framework is well established, the methodologies used in each of the steps of the assessment are fraught with a number of shortcomings. Notwithstanding the subjectivity implicit in the evaluation of risks, there is scepticism in scientific circles about the appropriateness of the current methodologies because, after so many years of evaluations, we are still incapable of foreseeing the negative consequences that some agrochemicals have in the environment. A critical appraisal of such methodologies is imperative if we are to improve the current assessment process and fix the problems we face today. The chapter reviews first the toxicity assessment methods, pointing to the gaps in knowledge about this essential part of the process and suggesting avenues for further improvement. Deficiencies in the current regulations regarding toxicity testing are discussed, in particular the effect of the time factor on toxicity and the issue of complex mixtures. Other matters of concern are the extrapolation of toxicity data from the individual to the population and community levels, and the sub-lethal effects. The exposure assessment methods are dealt with in a second place. These rely on modelling and actual measurements of chemical residues in the environment. Various techniques employed to determine to exposure and bioavailability of agrochemicals to the various organisms in both aquatic and terrestrial ecosystems are reviewed. Again, the shortcomings and gaps in knowledge are addressed and suggestions for improvement are pointed out. Then, the process of putting together the information from the toxicity and exposure assessments to evaluate risks is discussed. Tiers I and II of the risk assessment are reviewed. The challenge here is to keep objectivity in the evaluations; this may require the introduction of new methods of risk assessment. Finally, the risk assessment implies establishing a management strategy that aims at reducing or minimising the impacts of agrochemicals under normal agricultural scenarios. Recommendations are often case-specific and need to be based on sound science as well as common sense principles. The chapter concludes with a summary of issues that need to be considered for improving risk assessments of agrochemicals