Necessary symmetry conditions for the rotation of light

Two conditions on symmetries are identified as necessary for a linear scattering system to be able to rotate the linear polarisation of light: Lack of at least one mirror plane of symmetry and electromagnetic duality symmetry. Duality symmetry is equivalent to the conservation of the helicity of light in the same way that rotational symmetry is equivalent to the conservation of angular momentum. When the system is a solution of a single species of particles, the lack of at least one mirror plane of symmetry leads to the familiar requirement of chirality of the individual particle. With respect to helicity preservation, according to the analytical and numerical evidence presented in this paper, the solution preserves helicity if and only if the individual particle itself preserves helicity. However, only in the particular case of forward scattering the helicity preservation condition on the particle is relaxed: We show that the random orientation of the molecules endows the solution with an effective rotational symmetry; at its turn, this leads to helicity preservation in the forward scattering direction independently of any property of the particle. This is not the case for a general scattering direction. These results advance the current understanding of the phenomena of molecular optical activity and provide insight for the design of polarisation control devices at the nanoscale.Comment: 17 pages, 3 figure

Far-field measurements of vortex beams interacting with nanoholes

We measure the far-field intensity of vortex beams going through nanoholes. The process is analyzed in terms of helicity and total angular momentum. It is seen that the total angular momentum is preserved in the process, and helicity is not. We compute the ratio between the two transmitted helicity components, $\gamma_{m,p}$. We observe that this ratio is highly dependent on the helicity ($p$) and the angular momentum ($m$) of the incident vortex beam in consideration. Due to the mirror symmetry of the nanoholes, we are able to relate the transmission properties of vortex beams with a certain helicity and angular momentum, with the ones with opposite helicity and angular momentum. Interestingly, vortex beams enhance the $\gamma_{m,p}$ ratio as compared to those obtained by Gaussian beams

Highlights of the expansion of SN1993J

SN 1993J in M 81, a powerful radio supernova high in the northern sky, has been carefully monitored with VLBI throughout its lifetime. Its extremely circular shell-like radio structure has expanded over 15 years in a rather self-similar way in reasonable agreement with Chevalier’s model. An extension of Chevalier’s model simultaneously accounts for all the light-curve and VLBI results. Beyond the first year a single deceleration parameter, mtrue = 0.87± 0.02, characterizes the shock expansion. However, at short wavelengths and beyond 5 years, an enhanced deceleration, mshortwavelengths = 0.79±0.01, is measured. This enhancement is interpreted as due to a combination of effects ranging from varying free-free opacity in the supernova ejecta at the short wavelengths to the radial shape of the intensity of the magnetic fields in the emitting region. The final radio-loud stage is characterized by an abrupt decrease of radio emission. This large flux-density decay rate can be explained as due to the supernova shock surpassing the outer boundary of the circumstellar medium generated by the supernova progenitor. Presently, the supernova expansion beyond the circumstellar/interstellar boundary is rather radio silent, although there are indications that the supernova remnant could be studied by LOFAR and eMERLIN

A theory of reversibility for Erlang

[EN] In a reversible language, any forward computation can be undone by a finite sequence of backward steps. Reversible computing has been studied in the context of different programming languages and formalisms, where it has been used for testing and verification, among others. In this paper, we consider a subset of Erlang, a functional and concurrent programming language based on the actor model. We present a formal semantics for reversible computation in this language and prove its main properties, including its causal consistency. We also build on top of it a rollback operator that can be used to undo the actions of a process up to a given checkpoint. (C) 2018 Elsevier Inc. All rights reserved.This work has been partially supported by MINECO/AEI/FEDER (EU) under grants TIN2013-44742-C4-1-R and TIN2016-76843-C4-1-R, by the Generalitat Valenciana under grant PROMETEO-II/2015/013 (SmartLogic), by the COST Action IC1405 on Reversible Computation-extending horizons of computing, and by JSPS KAKENHI Grant Number JP17H01722. Adrian Palacios was partially supported by the EU (FEDER) and the Spanish Ayudas para contratos predoctorales para la formacian de doctores and Ayudas a la movilidad predoctoral para la realtzacion de estancias breves en centros de I+D, MINECO (SEIDI), under FPI grants BES-2014-069749 and EEBB-I-16-11469. Ivan Lanese was partially supported by INdAM as a member of GNCS (Gruppo Nazionale per il Calcolo Scientifico). Part of this research was done while the third and fourth authors were visiting Nagoya and Bologna Universities; they gratefully acknowledge their hospitality. Finally, we thank Salvador Tamarit and the anonymous reviewers for their helpful suggestions and comments.Lanese, I.; Nishida, N.; Palacios, A.; Vidal, G. (2018). A theory of reversibility for Erlang. Journal of Logical and Algebraic Methods in Programming. 100:71-97. https://doi.org/10.1016/j.jlamp.2018.06.004S719710

Evaluation of multiple cation/anion perovskite solar cells through life cycle assessment

After the great initiation of perovskite as a photovoltaic material, laboratory efficiencies similar to other photovoltaic technologies already commercialised have been reached. Consequently, recent research interests on perovskite solar cells try to address the stability improvement as well as make its industrialisation possible. Record efficiencies in perovskite solar cells (PSCs) have been achieved using as active material a multiple cation/anion perovskite by combining methylammonium (MA) and formamidinium (FA), but also Cs cation and I and Br as anions, materials that also have demonstrated a superior stability. Herein, the environmental performance of the production of such perovskite films was evaluated via life cycle assessment. Our study points out that multiple cation/anion perovskite films show major detrimental environmental impacts for all categories assessed, except for abiotic depletion potential, when they are compared with a canonical perovskite MAPbI3. In addition, a closer analysis of the materials utilised for the synthesis of the different multiple cation perovskites compositions revealed that lead halide reagents and chlorobenzene were the most adverse compounds in terms of impact. However, the former is used in all the perovskite compositions and the later can be avoided by the use of alternative fabrication methods to anti-solvent. To this extent, FAI, with the current synthesis procedures, is the most determining compound as it increases significantly the impacts and the cost in comparison with MAI. A further economic analysis, exposed that multiple cation perovskites need a significantly higher photoconversion efficiency to produce the same payback times than canonical perovskite

UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data

Context. The analysis of astronomical interferometric data is often performed on the images obtained after deconvolving the interferometer's point spread function. This strategy can be understood (especially for cases of sparse arrays) as fitting models to models, since the deconvolved images are already non-unique model representations of the actual data (i.e., the visibilities). Indeed, the interferometric images may be affected by visibility gridding, weighting schemes (e. g., natural vs. uniform), and the particulars of the (non-linear) deconvolution algorithms. Fitting models to the direct interferometric observables (i.e., the visibilities) is preferable in the cases of simple (analytical) sky intensity distributions. Aims. We present UVMULTIFIT, a versatile library for fitting visibility data, implemented in a Python-based framework. Our software is currently based on the CASA package, but can be easily adapted to other analysis packages, provided they have a Python API. Methods. The user can simultaneously fit an indefinite number of source components to the data, each of which depend on any algebraic combination of fitting parameters. Fits to individual spectral-line channels or simultaneous fits to all frequency channels are allowed. Results. We have tested the software with synthetic data and with real observations. In some cases (e. g., sources with sizes smaller than the diffraction limit of the interferometer), the results from the fit to the visibilities (e. g., spectra of close by sources) are far superior to the output obtained from the mere analysis of the deconvolved images. Conclusions. UVMULTIFIT is a powerful improvement of existing tasks to extract the maximum amount of information from visibility data, especially in cases close to the sensitivity/resolution limits of interferometric observations