Quasicrystalline Order in Binary Dipolar Systems

Motivated by recent experimental findings, we investigate the possible occurrence and characteristics of quasicrystalline order in two-dimensional mixtures of point dipoles with two sorts of dipole moments. Despite the fact that the dipolar interaction potential does not exhibit an intrinsic length scale and cannot be tuned a priori to support the formation of quasicrystalline order, we find that configurations with long--range quasicrystallinity yield minima in the potential energy surface of the many particle system. These configurations emanate from an ideal or perturbed ideal decoration of a binary tiling by steepest descent relaxation. Ground state energy calculations of alternative ordered states and parallel tempering Monte-Carlo simulations reveal that the quasicrystalline configurations do not correspond to a thermodynamically stable state. On the other hand, steepest descent relaxations and conventional Monte-Carlo simulations suggest that they are rather robust against fluctuations. Local quasicrystalline order in the disordered equilibrium states can be strong.Comment: 10 pages, 7 figure

The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS3_3

Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS$_3$, a quasi-two dimensional antiferromagnet with spin $S = 1$ on a honeycomb lattice. The spectra show clear, dispersive magnons with a $\sim 7$ meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS$_3$ has an easy axis anisotropy with $\Delta = 0.3$ meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of $J_3 = -6.90$ meV. The data can be fitted reasonably well with either $J_1 < 0$ or $J_1 > 0$, however the best quantitative agreement with high-resolution data indicate that the nearest-neighbour interaction is ferromagnetic with $J_1 = 1.9$ meV and that the second nearest-neighbour exchange is small and antiferromagnetic with $J_2 = -0.1$ meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS$_3$ is close to being unstable.Comment: 21 pages, 7 figures, 33 reference

The X-shooter Spectral Library (XSL): I. DR1. Near-ultraviolet through optical spectra from the first year of the survey

We present the first release of XSL, the X-Shooter Spectral Library. This release contains 237 stars spanning the wavelengths 3000--10200 \AA\ observed at a resolving power $R \equiv \lambda / \Delta\lambda \sim 10000$. The spectra were obtained at ESO's 8-m Very Large Telescope (VLT). The sample contains O -- M, long-period variable (LPV), C and S stars. The spectra are flux-calibrated and telluric-corrected. We describe a new technique for the telluric correction. The wavelength coverage, spectral resolution and spectral type of this library make it well suited to stellar population synthesis of galaxies and clusters, kinematical investigation of stellar systems and studying the physics of cool stars.Comment: 41 pages, 38 figures, 5 tables. Accepted for publication in A&A. Webpage: http://xsl.u-strasbg.fr

Fick's law and Fokker-Planck Equation in inhomogeneous environments

In inhomogeneous environments, the correct expression of the diffusive flux is often not given by the Fick's law $\Gamma = - D \nabla n$. The most general hydrodynamic equation modelling diffusion is indeed the Fokker-Planck Equation (FPE). The microscopic dynamics of each specific system may affect the form of the FPE, either establishing connections between the diffusion and the convection term, as well as providing supplementary terms. In particular, the Fick's form for the Diffusion Equation may arise only in consequence of a specific kind of microscopic dynamics. It is also shown how, in the presence of sharp inhomogeneities, even the hydrodynamic FPE limit may becomes inaccurate and mask some features of the true solution, as computed from the Master Equation.Comment: V2: English amended. V3: final version accepted by Physics Letters

Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective

This work provides a ground for a quantitative interpretation of experiments on step bunching during sublimation of crystals with a pronounced Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step bunching instability takes place when the kinetic length is larger than the average distance between the steps on the vicinal surface. In the opposite limit the instability is weak and step bunching can occur only when the magnitude of step-step repulsion is small. The central result are power law relations of the between the width, the height, and the minimum interstep distance of a bunch. These relations are obtained from a continuum evolution equation for the surface profile, which is derived from the discrete step dynamical equations for. The analysis of the continuum equation reveals the existence of two types of stationary bunch profiles with different scaling properties. Through a mathematical equivalence on the level of the discrete step equations as well as on the continuum level, our results carry over to the problems of step bunching induced by growth with a strong inverse ES effect, and by electromigration in the attachment/detachment limited regime. Thus our work provides support for the existence of universality classes of step bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103 (2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure

Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)

We report on the structural properties of Ge_(1-x)Mn_x layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure

Near-IR spectra of ISOGAL sources in the Inner Galactic Bulge

In this work we present near-IR spectra (HK-band) of a sample of 107 sources with mid-IR excesses at 7 and 15 $\rm \mu$m detected during the ISOGAL survey. Making use of the DENIS interstellar extinction map from Schultheis et al. (1999) we derive luminosities and find that the $\rm M_{bol}$ vs.~$\rm ^{12}CO$ and $\rm M_{bol} vs. H_{2}O$ diagrams are powerful tools for identifying supergiants, AGB stars, giants and young stellar objects. The majority of our sample are AGB stars (~ 80%) while we find four good supergiant candidates, nine young stellar objects and 12 RGB candidates. We have used the most recent $\rm K_{0}-[15]$ relation by Jeong et al. (2002) based on recent theoretical modeling of dust formation of AGB stars to determine mass-loss rates. However, the uncertainties in the mass-loss rates are rather large. The mass-loss rates of the supergiants are comparable with those in the solar neighbourhood while the long-period Variables cover a mass-loss range from $\rm -5 < log \dot{\it{M}} < -7$. The red giant candidateslie at the lower end of the mass-loss rate range between $\rm -6.5 < log \dot{{\it{M}}} < -9$. We used the equivalent width of the CO bandhead at 2.3 $\rm \mu m$, the NaI doublet and the CaI triplet to estimate metallicities using the relation by Ram\'{\i}rez et al. (\cite{Ramirez2000}). The metallicity distribution of the ISOGAL objects shows a mean [Fe/H] $\sim$ -0.25 dex with a dispersion of $\rm \pm 0.40 dex$ which is in agreement with the values of Ram\'{i}rez et al. (\cite{Ramirez2000}) for Galactic Bulge fields between $\rm b = -4^{o}$ and $\rm b = -1.3^{o}$. A comparison with the solar neighbourhood sample of Lan\c{c}on & Wood (LW) shows that our sample is ~ 0.5 dex more metal-rich on average.Comment: 23 pages, 9 figures, 2 appendix with IR spectra. accepted for A&

Dynamics of Viscoplastic Deformation in Amorphous Solids

We propose a dynamical theory of low-temperature shear deformation in amorphous solids. Our analysis is based on molecular-dynamics simulations of a two-dimensional, two-component noncrystalline system. These numerical simulations reveal behavior typical of metallic glasses and other viscoplastic materials, specifically, reversible elastic deformation at small applied stresses, irreversible plastic deformation at larger stresses, a stress threshold above which unbounded plastic flow occurs, and a strong dependence of the state of the system on the history of past deformations. Microscopic observations suggest that a dynamically complete description of the macroscopic state of this deforming body requires specifying, in addition to stress and strain, certain average features of a population of two-state shear transformation zones. Our introduction of these new state variables into the constitutive equations for this system is an extension of earlier models of creep in metallic glasses. In the treatment presented here, we specialize to temperatures far below the glass transition, and postulate that irreversible motions are governed by local entropic fluctuations in the volumes of the transformation zones. In most respects, our theory is in good quantitative agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure