A microscopic model for solidification

We present a novel picture of a non isothermal solidification process starting from a molecular level, where the microscopic origin of the basic mechanisms and of the instabilities characterizing the approach to equilibrium is rendered more apparent than in existing approaches based on coarse grained free energy functionals \`a la Landau. The system is composed by a lattice of Potts spins, which change their state according to the stochastic dynamics proposed some time ago by Creutz. Such a method is extended to include the presence of latent heat and thermal conduction. Not only the model agrees with previous continuum treatments, but it allows to introduce in a consistent fashion the microscopic stochastic fluctuations. These play an important role in nucleating the growing solid phase in the melt. The approach is also very satisfactory from the quantitative point of view since the relevant growth regimes are fully characterized in terms of scaling exponents.Comment: 7 pages Latex +3 figures.p

Homogenization in magnetic-shape-memory polymer composites

Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a large change of shape to the presence of an external magnetic field. As an alternative for the difficult to manifacture single crystal of these alloys we study composite materials in which small magnetic-shape-memory particles are embedded in a polymer matrix. The macroscopic properties of the composite depend strongly on the geometry of the microstructure and on the characteristics of the particles and the polymer. We present a variational model based on micromagnetism and elasticity, and derive via homogenization an effective macroscopic model under the assumption that the microstructure is periodic. We then study numerically the resulting cell problem, and discuss the effect of the microstructure on the macroscopic material behavior. Our results may be used to optimize the shape of the particles and the microstructure.Comment: 17 pages, 4 figure

The Jurassic pleurotomarioidean gastropod Laevitomaria and its palaeobiogeographical history

The genus Laevitomaria is reviewed and its palaeobiogeographical history is reconstructed based on the re-examination of its type species L. problematica, the study of material stored at the National Natural History Museum of Luxembourg, and an extensive review of the literature. The systematic study allows ascribing to Laevitomaria a number of Jurassic species from the western European region formerly included in other pleurotomariid genera. The following new combinations are proposed: Laevitomaria allionta, Laevitomaria amyntas, Laevitomaria angulba, Laevitomaria asurai, Laevitomaria daityai, Laevitomaria fasciata, Laevitomaria gyroplata, Laevitomaria isarensis, Laevitomaria joannis, Laevitomaria repeliniana, Laevitomaria stoddarti, Laevitomaria subplatyspira, and Laevitomaria zonata. The genus, which was once considered as endemic of the central part of the western Tethys, shows an evolutionary and palaeogeographical history considerably more complex than previously assumed. It first appeared in the Late Sinemurian in the northern belt of the central western Tethys involved in the Neotethyan rifting, where it experienced a first radiation followed by an abrupt decline of diversity in the Toarcian. Species diversity increased again during Toarcian\u2013Aalenian times in the southernmost part of western European shelf and a major radiation occurred during the Middle Aalenian to Early Bajocian in the northern Paris Basin and southern England. After a latest Bajocian collapse of diversity, Laevitomaria disappeared from both the central part of western Tethys and the European shelf. In the Bathonian, the genus appeared in the south-eastern margin of the Tethys where it lasted until the Oxfordian

On the generation and the nonlinear dynamics of X-waves of the Schroedinger equation

The generation of finite energy packets of X-waves is analysed in normally dispersive cubic media by using an X-wave expansion. The 3D nonlinear Schroedinger model is reduced to a 1D equation with anomalous dispersion. Pulse splitting and beam replenishment as observed in experiments with water and Kerr media are explained in terms of a higher order breathing soliton. The results presented also hold in periodic media and Bose-condensed gases.Comment: 18 pages, 6 figures, corrected version to be published in Physical Review

Big images of two-dimensional pseudorepresentations

Bella\"iche has recently applied Pink-Lie theory to prove that, under mild conditions, the image of a continuous 2-dimensional pseudorepresentation $\rho$ of a profinite group on a local pro-$p$ domain $A$ contains a nontrivial congruence subgroup of ${\rm SL}_2(B)$ for a certain subring $B$ of $A$. We enlarge Bella\"iche's ring and give this new $B$ a conceptual interpretation in terms of conjugate self-twists of $\rho$, symmetries that naturally constrain its image. As a corollary, this new $B$ is optimal among congruence subgroups contained in the image. We also interpret the new $B$ vis-a-vis the adjoint trace ring of $\rho$, which we show is a more natural ring for these questions in general. Finally, we use our purely algebraic result to recover and extend a variety of arithmetic big-image results for ${\rm GL}_2$ Galois representations arising from elliptic, Hilbert, and Bianchi modular forms and $p$-adic Hida or Coleman families of elliptic and Hilbert modular forms

A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini near-Infrared spectroscopy

We present H- and K-band spectroscopy of OB and Wolf-Rayet (WR) members of the Milky Way cluster 1806-20 (G10.0-0.3), to obtain a revised cluster distance of relevance to the 2004 giant flare from the SGR 1806-20 magnetar. From GNIRS spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous mid WN and late WC classifications for two WR stars. Based upon an absolute Ks-band magnitude calibration for B supergiants and WR stars, and near-IR photometry from NIRI at Gemini North plus archival VLT/ISAAC datasets, we obtain a cluster distance modulus of 14.7+/-0.35 mag. The known stellar content of the 1806-20 cluster suggests an age of 3-5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7+/-0.7 mag. Together, our results favour a distance modulus of 14.7+/-0.4 mag (8.7^+1.8_-1.5 kpc) to the 1806-20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the December 2004 giant flare is reduced by a factor of three to 7 X 10^46 erg/s, such that the contamination of BATSE short gamma ray bursts (GRB's) from giant flares of extragalactic magnetars is reduced to a few percent. We infer a magnetar progenitor mass of ~48^+20_-8 Msun, in close agreement with that obtained recently for the magnetar in Westerlund 1.Comment: 6 pages, 4 figures, accepted for MNRAS Letter

Shaken Granular Lasers

Granular materials have been studied for decades, also driven by industrial and technological applications. These very simple systems, composed by agglomerations of mesoscopic particles, are characterized, in specific regimes, by a large number of metastable states and an extreme sensitivity (e.g., in sound transmission) on the arrangement of grains; they are not substantially affected by thermal phenomena, but can be controlled by mechanical solicitations. Laser emission from shaken granular matter is so far unexplored; here we provide experimental evidence that it can be affected and controlled by the status of motion of the granular, we also find that competitive random lasers can be observed. We hence demonstrate the potentialities of gravity affected moving disordered materials for optical applications, and open the road to a variety of novel interdisciplinary investigations, involving modern statistical mechanics and disordered photonics.Comment: 4 pages, 3 figures. To be published in Physical Review Letter