Ces dernieres annees, la thematique des metasurfaces est devenue un sujet de recherche en pleine expansion dans plusieurs domaines de l’ingenierie et de la physique appliquee, en raison de leur capacite a manipuler a la fois la phase et l’amplitude des champs electromagnetiques. Ces materiaux artificiels bi-dimensionnels, generalement composes d’elements metalliques imprimes sur des substrats dielectriques, ont l’avantage d’etre de tres faible epaisseur, legers et faciles a fabriquer et a integrer avec les circuits imprimes. Cet article passe en revue les dernieres avancees dans la conception d’antennes a metasurface, ou les metasurfaces sont utilisees pour minimiser l’epaisseur, augmenter la bande passante et controler le diagramme de rayonnement en champ proche et en champ lointain.

Cities are systems with a large number of constituents and agents interacting with each other and can be considered as emblematic of complex systems. Modeling these systems is a real challenge and triggered the interest of many disciplines such as quantitative geography, spatial economics, geomatics and urbanism, and more recently physics. (Statistical) Physics plays a major role by bringing tools and concepts able to bridge theory and empirical results, and we will illustrate this on some fundamental aspects of cities: the growth of their surface area and their population, their spatial organization, and the spatial distribution of activities. We will present state-of-the-art results and models but also open problems for which we still have a partial understanding and where physics approaches could be particularly helpful. We will end this short review with a discussion about the possibility of constructing a science of cities.

Cities are systems with a large number of constituents and agents interacting with each other and can be considered as emblematic of complex systems. Modeling these systems is a real challenge and triggered the interest of many disciplines such as quantitative geography, spatial economics, geomatics and urbanism, and more recently physics. (Statistical) Physics plays a major role by bringing tools and concepts able to bridge theory and empirical results, and we will illustrate this on some fundamental aspects of cities: the growth of their surface area and their population, their spatial organization, and the spatial distribution of activities. We will present state-of-the-art results and models but also open problems for which we still have a partial understanding and where physics approaches could be particularly helpful. We will end this short review with a discussion about the possibility of constructing a science of cities.

I give an overview of the topic of this special issue, the applications of (statistical) physics to social sciences at large. I discuss several examples of simple social models put forward by physicists and discuss their interest. I argue that while they may be conceptually useful to correct our intuitive models of social mechanisms, their relevance for real social systems is moot. What is more, since physicists have always needed to tame the world inside laboratories to make their models relevant, I suggest that social modeling might be linked to human taming, a smashing political project.

After the discovery of graphene and its many fascinating properties, there has been a growing interest for the study of "artificial graphenes". These are totally different and novel systems which bear exciting similarities with graphene. Among them are lattices of ultracold atoms, microwave or photonic lattices, "molecular graphene" or new compounds like phosphorene. The advantage of these structures is that they serve as new playgrounds for measuring and testing physical phenomena which may not be reachable in graphene, in particular: the possibility of controlling the existence of Dirac points (or Dirac cones) existing in the electronic spectrum of graphene, of performing interference experiments in reciprocal space, of probing geometrical properties of the wave functions, of manipulating edge states, etc. These cones, which describe the band structure in the vicinity of the two connected energy bands, are characterized by a topological "charge". They can be moved in reciprocal space by appropriate modification of external parameters (pressure, twist, sliding, stress, etc.). They can be manipulated, created or suppressed under the condition that the total topological charge be conserved. In this short review, I discuss several aspects of the scenarios of merging or emergence of Dirac points as well as the experimental investigations of these scenarios in condensed matter and beyond.

Abstract In February 1851, Leon Foucault published in the Comptes rendus his famous pendulum experiment performed at the “Observatoire de Paris”. This ended two centuries of quest for an experimental demonstration of Earth rotation. One month later, the experiment was reproduced at larger scale in the Pantheon and, as early as the summer of 1851, it was being repeated in many places across the world. The next year, Foucault invented the gyroscope to get a still more direct proof of Earth rotation. The theory relied on the masterpiece treatise of Laplace on celestial mechanics, published in 1805, which already contained the mathematical expression of the force that would be discovered by Gustave Coriolis 30 years later. The idea of a fictitious inertial force proposed by Coriolis prevailed by the end of 19th century, as it was conceptually simpler than Laplace's approach. The full theory of the Foucault pendulum, taking into account its unavoidable imperfections, was not obtained until three decades later by Kamerlingh Onnes, the future discoverer of liquid helium and superconductivity. Today, Foucault's exceptional creativity is still a source of inspiration for research and the promotion of science through experimental proofs widely available to the public.

We present the time scales elaborated at the International Bureau of Weights and Measures (BIPM), review their present status, and discuss the transition in frequency performance from the present 10−16 to the future 10−17–10−18, and its impact on time and frequency metrology. We focus our attention on future developments in the calculation of Coordinated Universal Time (UTC), on the evolution of time links and algorithms, on improving the access to the time reference and on possible changes in the definition of the timescales. Nous présentons les échelles de temps élaborées par le Bureau international des poids et mesures (BIPM) et évaluons leurs performances présentes. Nous discutons la transition en cours pour passer du niveau actuel de 10−16 sur l'incertitude de fréquence au niveau futur de 10−17–10−18, et de l'impact de ce changement sur la métrologie temps–fréquence. Nous concentrons notre attention sur les développements futurs pour le calcul du temps universel coordonné (UTC), sur l'évolution des techniques de comparaisons d'horloges et des algorithmes, sur l'amélioration de l'accès à la référence de temps et sur les changements possibles dans la définition des échelles de temps.

Abstract What mechanism governs slow flows of granular media? Microscopically, the grains experience enduring frictional contacts in these flows. However, a straightforward translation to a macroscopic frictional rheology, where the shear stresses are proportional to the normal stresses with a rate-independent friction coefficient, fails to capture important aspects of slow granular flows. There is now overwhelming evidence that agitations, tiny fluctuations of the grain positions, associated with large fluctuation of their contact forces, play a central role for slow granular flows. These agitations are generated in flowing regions, but travel deep inside the quiescent zones, and lead to a nonlocal rheology.

Abstract This paper discusses strain localisation in granular media by presenting experimental, full-field analysis of mechanical tests on sand, both at a continuum level, as well as at the grain scale . At the continuum level, the development of structures of localised strain can be studied. Even at this scale, the characteristic size of the phenomena observed is in the order of a few grains. In the second part of this paper, therefore, the development of shear bands within specimen of different sands is studied at the level of the individual grains, measuring grains kinematics with x-ray tomography. The link between grain angularity and grain rotation within shear bands is shown, allowing a grain-scale explanation of the difference in macroscopic residual stresses for materials with different grain shapes. Finally, rarely described precursors of localisation, emerging well before the stress peak are observed and commented.

Abstract The features associated with modern synchrotron radiation machines (intense and coherent beams) result in a substantial extension of X-ray imaging capabilities in terms of spatial and temporal resolution, phase contrast and 3D images. This allows crystal growth-related information to be obtained which is not available otherwise. After briefly describing the main synchrotron radiation based imaging techniques of interest, we give original examples illustrating the new capabilities for crystal growth: characterisation of crystals grown for applications, such as ice tri-crystals produced for mechanical deformation studies; SiC; crystalline silicon for solar photovoltaic cells; in situ and in real time studies of quasicrystal growth (AlPdMn); and ultrafast tomography for the study of the growth of dendrites in metallic alloys.