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1The Fe-vacancy ordering patterns in the superconducting K x Fe 2-y Se 2 and non-superconducting K x (Fe,Co) 2-y Se 2 samples have been investigated by electron diffraction and high angle annular dark field scanning transmission electron microscopy. The Fe-vacancy ordering occurs in the ab plane of the parent ThCr 2 Si 2 -type structure, demonstrating two types of patterns. The superstructure I retains the tetragonal symmetry and can be described with the a I = b I = a s √5 (a s is the unit cell parameter of the parent ThCr 2 Si 2 -type structure) supercell and I4/m space group. The superstructure II reduces the symmetry to orthorhombic with the a II = a s √2, b II = 2a s √2 supercell and the Ibam space group. This type of superstructure is observed for the first time in K x Fe 2-y Se 2 . The Fe-vacancy ordering is inhomogeneous: the disordered areas interleave with the superstructures I and II in the same crystallite.The observed superstructures represent the compositionally-dependent uniform ordering patterns of two species (the Fe atoms and vacancies) on a square lattice. More complex uniform ordered configurations, including compositional stripes, can be predicted for different chemical compositions of the K x Fe 2-y Se 2 (0 < y < 0.5) solid solutions.

The structure of pyrrhotite (Fe(1 - x)S with 0.05 < or = x < or = 0.125) has been reinvestigated in the framework of the superspace formalism. A common model with a centrosymmetric superspace group is proposed for the whole family. The atomic domains in the internal space representing the Fe atoms are parametrized as crenel functions that fulfil the closeness condition. The proposed model explains the x-dependent space groups observed and the basic features of the structures reported up to now. Our model yields for any x value a well defined ordered distribution of Fe vacancies in contrast to some of the structural models proposed in the literature. A new (3 + 1)-dimensional refinement of Fe(0.91)S using the deposited dataset [Yamamoto & Nakazawa (1982). Acta Cryst. A38, 79-86] has been performed as a benchmark of the model. The consistency of the proposed superspace symmetry and its validity for other compositions has been further checked by means of ab initio calculations of both atomic forces and equilibrium atomic positions in non-relaxed and relaxed structures, respectively.

It was recently proved that solitons embedded in the spectrum of linear waves may exist in discrete systems, and explicit solutions for isolated unstable embedded lattice solitons (ELS) of a differentialdifference version of a higher-order NLS equation were found [Physica D 197 (2004) 86]. The discovery of these ELS gives rise to relevant questions such as the following: are there continuous families of ELS?, can ELS be stable?, is it possible for ELS to move along the lattice?, how do ELS interact?. The present work addresses these questions by showing that a novel equation (a discrete version of a complex modified KdV equation which includes next-nearest-neighbor couplings) has a two-parameter continuous family of exact ELS. These solitons can move with arbitrary velocities across the lattice, and the numerical simulations demonstrate that these ELS are completely stable. Moreover, the numerical tests show that these ELS are robust enough to withstand collisions, and the result of a collision is only a shift in the positions of the solitons. The model may apply to the description of a Bose-Einstein condensate with dipole-dipole interactions between the atoms, trapped in a deep optical-lattice potential. In nonlinear systems where solitons can exist, the propagation of small-amplitude linear waves, which obey the linearized version of the nonlinear equations, is possible too. However, for a soliton to exist, it is absolutely necessary that no resonances occur between the soliton and these linear waves. Otherwise, the soliton would decay due to an energy transfer towards the linear waves. Based on this no-resonance argument, it was frequently assumed that the solitons' internal frequencies could not be contained in the linear spectrum of the system, i.e., they could not lie within the band of frequencies permitted to linear waves. However, at the end of the nineties exceptions to this rule were found, and a special type of solitons were discovered, which do not resonate with linear waves, in spite of having frequencies immersed in the spectrum of these waves. In 1999 these peculiar solitary waves were given the name of embedded solitons (ES), and in the following years a number of models supporting ES were identified. Most of these models describe continuous systems. However, some examples of discrete ES were recently found too. These embedded lattice solitons (ELS) are isolated solutions which are stable against small perturbations in the linear approximation, but are nonlinearly unstable. The discovery of these isolated unstable ELS triggered the search for models admitting continuous families of stable ELS. In this article we present a novel differential-difference equation which has a two-parameter continuous family of exact ELS. These solitons can move with arbitrary velocities across the lattice, and the numerical simulations show that they are completely stable solutions.

Este documento presenta el primer análisis exhaustivo para la región acerca las características y costos de la congestión urbana en América Latina y el Caribe, con resultados para las áreas metropolitanas de Bogotá (Colombia), Buenos Aires (Argentina), Ciudad de México (México), Lima (Perú), Montevideo (Uruguay), Río de Janeiro (Brasil), San Salvador (El Salvador), Santiago (Chile), Santo Domingo (República Dominicana) y Sao Paulo (Brasil). Utiliza big data y ciencia de datos para elucidar la dinámica de la congestión en cada ciudad y sus costos directos e indirectos para la sociedad. Con el fin de mitigar estos impactos, el documento reúne cinco grupos de medidas de política pública: (i) instrumentos de gestión de tráfico; (ii) políticas que restringen el uso del vehículo particular; (iii) políticas que promueven el uso del transporte público, el transporte activo y el transporte compartido; (iv) planificación integrada de la movilidad y el uso del suelo; y (v) políticas para la gestión de la logística urbana. Concluye con recomendaciones sobre las medidas más adecuadas y la secuenciación de las mismas para reducir la congestión en las grandes ciudades y las megaciudades de la región.