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A group of high order Gautschi-type exponential wave integrators (EWIs) Fourier pseudospectral method are proposed and analyzed for solving the nonlinear Klein-Gordon equation (KGE) in the nonrelativistic limit regime, where a parameter $0, Comment: 24 pages, 2 figures more...

In this paper, we propose efficient and accurate numerical methods for computing the ground state and dynamics of the dipolar Bose-Einstein condensates utilising a newly developed dipole-dipole interaction (DDI) solver that is implemented with the non-uniform fast Fourier transform (NUFFT) algorithm. We begin with the three-dimensional (3D) Gross-Pitaevskii equation (GPE) with a DDI term and present the corresponding two-dimensional (2D) model under a strongly anisotropic confining potential. Different from existing methods, the NUFFT based DDI solver removes the singularity by adopting the spherical/polar coordinates in Fourier space in 3D/2D, respectively, thus it can achieve spectral accuracy in space and simultaneously maintain high efficiency by making full use of FFT and NUFFT whenever it is necessary and/or needed. Then, we incorporate this solver into existing successful methods for computing the ground state and dynamics of GPE with a DDI for dipolar BEC. Extensive numerical comparisons with existing methods are carried out for computing the DDI, ground states and dynamics of the dipolar BEC. Numerical results show that our new methods outperform existing methods in terms of both accuracy and efficiency., 26 pages, 5 figures more...

We develop the idea that a natural link between Boltzmann schemes and finite volumes exists naturally: the conserved mass and momentum during the collision phase of the Boltzmann scheme induces general expressions for mass and momentum fluxes. We treat a unidimensional case and focus our development in two dimensions on possible flux boundary conditions. Several test cases show that a high level of accuracy can be achieved with this scheme. more...

We study numerically the Toner-Tu field theory where the density field is maintained constant, a limit case of "Malthusian" flocks for which the asymptotic scaling of correlation functions in the ordered phase is known exactly. While we confirm these scaling laws, we also show that such constant-density flocks are metastable to the nucleation of a specific defect configuration, and are replaced by a globally disordered phase consisting of asters surrounded by shock lines that constantly evolves and remodels itself. We demonstrate that the main source of disorder lies along shock lines, rendering this active foam fundamentally different from the corresponding equilibrium system. We thus show that in the context of active matter also, a result obtained at all orders of perturbation theory can be superseded by non-perturbative effects, calling for a different approach., 6 pages, 4 figures more...

International audience; In this contribution, we recall the derivation of the anti bounce back boundary condition for the D2Q9 lattice Boltzmann scheme. We recall various elements of the state of the art for anti bounce back applied to linear heat and acoustics equations and in particular the possibility to take into account curved boundaries. We present an asymptotic analysis that allows an expansion of all the fields in the boundary cells. This analysis based on the Taylor expansion method confirms the well known behaviour of anti bounce back boundary for the heat equation. The analysis puts also in evidence a hidden differential boundary condition in the case of linear acoustics. Indeed, we observe discrepancies in the first layers near the boundary. To reduce these discrepancies, we propose a new boundary condition mixing bounce back for the oblique links and anti bounce back for the normal link. This boundary condition is able to enforce both pressure and tangential velocity on the boundary. Numerical tests for the Poiseuille flow illustrate our theoretical analysis and show improvements in the quality of the flow. more...

International audience; Solid wetting-layers in inorganic nano-reactors: the water in imogolite nanotube case In 2016, 'isolated' water molecules trapped inside beryl nanochannels provided the fi rst fi rm experimental evidence of the dipole ordering of water molecules and a new 'quantum tunneling' state was evidenced. We have discovered a new phase of 'isolated' water molecules at the inner surface of inorganic nanotubes and we have investigated their associated dynamical properties. Our fi ndings are signifi cant to the emerging fi eld of 'isolated' nanoconfi ned water and to the areas of water science, nanoreactors and nanofl uidics. Inelastic neutron scattering and DFT-MD techniques are used in association with new methods of analysis which may be of interest to other fi elds of physics and nanosciences. By combined use of wide-angle X-ray scattering, thermo-gravimetric analysis, inelastic neutron scattering, density functional theory and density functional theory molecular dynamics simulations, we investigate the structure, dynamics and stability of the water wetting-layer in single-walled aluminogermanate imogolite nanotubes (SW Ge-INTs): an archetypal system for synthetically controllable and monodisperse nano-reactors. We demonstrate that the water wetting-layer is strongly bound and solid-like up to 300 K under atmospheric pressure, with dynamics markedly different from that of bulk water. Atomic-scale characterisation of the wetting-layer reveals organisation of the H 2 O molecules in a curved triangular sublattice stabilised by the formation of three H-bonds to the nanotube's inner surface, with covalent interactions sufficiently strong to promote energetically favourable decoupling of the H 2 O molecules in the adlayer. The evidenced changes in the local composition, structure, electrostatics and dynamics of the Ge-INT's inner surface upon the formation of the solid wetting-layer demonstrate solvent-mediated functionalisation of the nanotube's cavity at room temperature and pressure, suggesting new strategies for the design of nano-rectors towards potential control of chemical reactivity in nano-confined volumes. more...

Structural heterogeneity of amorphous solids present difficult challenges that stymie the prediction of plastic events, which are intimately connected to their mechanical behavior. Based on a perturbation analysis of the potential energy landscape, we derive the atomic nonaffinity as an indicator with intrinsic orientation, which quantifies the contribution of an individual atom to the total nonaffine modulus of the system. We find that the atomic nonaffinity can efficiently characterize the locations of the shear transformation zones, with a predicative capacity comparable to the best indicators. More importantly, the atomic nonaffinity, combining the sign of third order derivative of energy with respect to coordinates, reveals an intrinsic softest shear orientation. By analyzing the angle between this orientation and the shear loading direction, it is possible to predict the protocol-dependent response of plastic events. Employing the new method, the distribution of orientations of shear transformation zones in a model two-dimensional amorphous solids can be measured. The resulting plastic events can be understood from a simple model of independent plastic events occurring at variously oriented shear transformation zones. These results shed light on the characterization and prediction of the mechanical response of amorphous solids., 3 figures more...

International audience; Après avoir rappelé des éléments fondamentaux sur l'équation de Boltzmann discrète, l'analyse des schémas de Boltzmann sur réseau est proposée au second ordre à l'aide de la méthode du développement de Taylor. Cette approche est ensuite utilisée pour tenter d'approcher les équations de Navier-Stokes compressibles à l'aide d'une seule distribution de particules dans le cas de deux et trois dimensions d'espace more...

International audience; In this work, we study numerically the convergence of the scalar D2Q9 lattice Boltzmann scheme with multiple relaxation times when the time step is proportional to the space step and tends to zero. We do this by a combination of theory and numerical experiment. The classical formal analysis when all the relaxation parameters are fixed and the time step tends to zero shows that the numerical solution converges to solutions of the heat equation, with a constraint connecting the diffusivity, the space step and the coefficient of relaxation of the momentum. If the diffusivity is fixed and the space step tends to zero, the relaxation parameter for the momentum is very small, causing a discrepency between the previous analysis and the numerical results. We propose a new analysis of the method for this specific situation of evanescent relaxation, based on the dispersion equation of the lattice Boltzmann scheme. A new asymptotic partial differential equation, the damped acoustic system, is emergent as a result of this formal analysis. Complementary numerical experiments establish the convergence of the scalar D2Q9 lattice Boltzmann scheme with multiple relaxation times and acoustic scaling in this specific case of evanescent relaxation towards the numerical solution of the damped acoustic system. more...

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green's functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives. Fil: Hourahine, B.. University of Strathclyde; Reino Unido Fil: Aradi, B.. Universitat Bremen; Alemania Fil: Blum, V.. University of Duke; Estados Unidos Fil: Bonafé, F.. Max Planck Institute For The Structure And Dynamics Of Matter; Alemania Fil: Buccheri, A.. University Of Bristol; Reino Unido Fil: Camacho, C.. Universidad de Costa Rica; Costa Rica Fil: Cevallos, C.. Universidad de Costa Rica; Costa Rica Fil: Deshaye, M. Y.. Western Washington University; Estados Unidos Fil: Dumitric, T.. University of Minnesota; Estados Unidos Fil: Dominguez, A.. Universitat Bremen; Alemania Fil: Ehlert, S.. Universitat Bonn; Alemania Fil: Elstner, M.. Karlsruhe Institute Of Technology; Alemania Fil: Van Der Heide, T.. Universitat Bremen; Alemania Fil: Hermann, J.. Freie Universität Berlin; Alemania Fil: Irle, S.. Oak Ridge National Laboratory; Estados Unidos Fil: Kranz, J.J.. Karlsruhe Institute Of Technology; Alemania Fil: Köhler, C.. Universitat Bremen; Alemania Fil: Kowalczyk, T.. Western Washington University.; Estados Unidos Fil: Kubar, T.. Karlsruhe Institute Of Technology; Alemania Fil: Lee, I.S.. Ulsan National Institute Of Science And Technology; Estados Unidos Fil: Lutsker, V.. Universitat Regensburg; Alemania Fil: Maurer, R.J.. University of Warwick; Reino Unido Fil: Min, S.K.. Ulsan National Institute Of Science And Technology; Corea del Sur Fil: Mitchell, I.. Institute For Basic Science, Daejeon; Corea del Sur Fil: Negre, C.. Los Alamos National Laboratory Theoretical Division; Estados Unidos Fil: Niehaus, T.A.. Universite Lyon 2; Francia Fil: Niklasson, A.M.N.. Los Alamos National Laboratory Theoretical Division; Estados Unidos Fil: Page, A.J.. School Of Environmental And Life Sciences; Australia Fil: Pecchia, A.. Centre National de la Recherche Scientifique; Francia Fil: Penazzi, G.. Universitat Bremen; Alemania Fil: Persson, M.P.. No especifíca; Fil: Rezac, J.. Institute of Organic Chemistry and Biochemistry; República Checa Fil: Sanchez, Cristian Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; Argentina Fil: Sternberg, M.. Argonne National Laboratory; Estados Unidos Fil: Stöhr, M.. University Of Luxembourg; Luxemburgo Fil: Stuckenberg, F.. Universitat Bremen; Alemania Fil: Tkatchenko, A.. University Of Luxembourg; Luxemburgo Fil: Yu, V.W.Z.. University of Duke; Estados Unidos Fil: Frauenheim, T.. Universitat Bremen; Alemania more...

Amorphous solids lack long-range order. Therefore identifying structural defects - akin to dislocations in crystalline solids - that carry plastic flow in these systems remains a daunting challenge. By comparing many different structural indicators in computational models of glasses, under a variety of conditions we carefully assess which of these indicators are able to robustly identify the structural defects responsible for plastic flow in amorphous solids. We further demonstrate that the density of defects changes as a function of material preparation and strain in a manner that is highly correlated with the macroscopic material response. Our work represents an important step towards predicting how and when an amorphous solid will fail from its microscopic structure. more...

Metallic transition metal dichalcogenides (TMDs)1–8 are good catalysts for the hydrogen evolution reaction (HER). The overpotential and Tafel slope values of metallic phases and edges9 of two-dimensional (2D) TMDs approach those of Pt. However, the overall current density of 2D TMD catalysts remains orders of magnitude lower (~10–100 mA cm−2) than industrial Pt and Ir electrolysers (>1,000 mA cm−2)10,11. Here, we report the synthesis of the metallic 2H phase of niobium disulfide with additional niobium (2H Nb1+xS2, where x is ~0.35)12 as a HER catalyst with current densities of >5,000 mA cm−2 at ~420 mV versus a reversible hydrogen electrode. We find the exchange current density at 0 V for 2H Nb1.35S2 to be ~0.8 mA cm−2, corresponding to a turnover frequency of ~0.2 s−1. We demonstrate an electrolyser based on a 2H Nb1+xS2 cathode that can generate current densities of 1,000 mA cm−2. Our theoretical results reveal that 2H Nb1+xS2 with Nb-terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER. Therefore, 2H Nb1+xS2 could be a viable catalyst for practical electrolysers. Metallic transition metal dichalcogenides are promising catalysts for hydrogen evolution reactions but their performances are still lower than industrial Pt and Ir electrolysers. The metallic 2H phase of niobium disulfide now exhibits enhanced current densities versus a reversible hydrogen electrode. more...

International audience; In a previous work, we have proposed a method for the analysis of the bounce back boundary condition with the Taylor expansion method in the linear case. In this work two new schemes of modified bounce back are proposed. The first one is based on the expansion of the iteration of the internal scheme of the lattice Boltzmann method. The analysis puts in evidence some defects and a generalized version is proposed with a set of essentially four possible parameters to adjust. We propose to reduce this number to two with the elimination of spurious density first order terms. Thus a new scheme for bounce back is found exact up to second order and allows an accurate simulation of the Poiseuille flow for a specific combination of the relaxation and boundary coefficients. We have validated the general expansion of the value in the first cell in terms of given values on the boundary for a stationary ''accordion'' test case. more...

We analyze rigorously error estimates and compare numerically spatial/temporal resolution of various numerical methods for the discretization of the Dirac equation in the nonrelativistic limit regime, involving a small dimensionless parameter $0, Comment: 34 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:1511.01192 more...

International audience; Amorphous materials have a rich relaxation spectrum, which is usually described in terms of a hierarchy of relaxation mechanisms. In this work, we investigate the local dynamic modulus spectra in a model glass just above the glass transition temperature by performing a mechanical spectroscopy analysis with molecular dynamics simulations. We find that the spectra, at the local as well as on the global scale, can be well described by the Cole-Davidson formula in the frequency range explored with simulations. Surprisingly, the Cole-Davidson stretching exponent does not change with the size of the local region that is probed. The local relaxation time displays a broad distribution, as expected based on dynamic heterogeneity concepts, but the stretching is obtained independently of this distribution. We find that the size dependence of the local relaxation time and moduli can be well explained by the elastic shoving model. more...

International audience; In this paper, we introduce a pore-scale model to study the interaction between biofilm growth and non-aqueous-phase-liquid (NAPL) dissolution. Liquid flow and dissolved NAPL transport are coupled with a biofilm growth model to correctly describe the complex dynamics of the processes including fluid flow, NAPL dissolution/biodegradation and biofilm growth. Fluid flow is simulated using an immersed boundary-lattice Boltzmann (IB-LB) model; while solute transport is solved by a cut-cell finite volume method (FVM). A uniform dissolution approach is also adopted to capture the temporal evolution of trapped blobs. Spatio-temporal distributions of the biomass are investigated using a cellular automaton algorithm combined with the immersed boundary method (IBM). Simulations focused on NAPL dissolution in both abiotic and biotic conditions are conducted to assess the capability of the model. In abiotic conditions, we analyze the effects of the hydrodynamic regimes and the spatial distribution of NAPL blobs on the dissolution rate under different assumptions (i.e., blob size and Péclet number). In biotic conditions, a series of impact factors are also investigated (i.e., spatial distribution, reaction kinetics and NAPL-induced toxicity). Finally, the current model is used to evaluate the pore scale relevance of a local equilibrium assumption between fluid phase and biofilm phase in the vicinity of the NAPL source. more...

cited By 1; The corrosion behavior of 2205 DSS in HCl solutions containing sulfide were investigated using mass loss test, electrochemical measurements, scanning Kelvin probe, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that Na2S had significant effect on corrosion behavior of 2205 DSS in dilute HCl solutions. Slight Na2S can prevent the passive film from localized attacking of Cl− in HCl solution with a concentration lower than 0.1 mol/L. However, when the concentration of HCl solution higher than 0.137 mol/L, Na 2 S addition will tremendously promote corrosion. The intergranular corrosion combined surficial active dissolution of 2205 DSS could happen in HCl + Na 2 S solution. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. more...

International audience; The aim of this paper is to derive an efficient scheme for solving one-dimensional time-fractional nonlinear Schrödinger equations set in unbounded domains. We first derive some absorbing boundary conditions for the fractional system by using the unified approach introduced in [57,58] and a linearization procedure. Then, the initial boundary-value problem for the fractional system with ABCs is discretized and the error estimate O(h 2 + τ) is stated. To accelerate the scheme in time, the fractional derivative is approximated through a linearized L1-scheme. Finally, we end the paper by some numerical simulations to validate the properties (accuracy and efficiency) of the derived scheme. In addition, we illustrate the behavior of the solution by reporting a few simulations for various parameter values of the fractional order 0 < α < 1, nonlinearities and potentials. more...

International audience; Cryogenic rejuvenation in metallic glasses reported in Ketov et al 's experiment (2015 Nature 524 200) has attracted much attention, both in experiments and numerical studies. The atomic mechanism of rejuvenation has been conjectured to be related to the heterogeneity of the glassy state, but the quantitative evidence is still elusive. Here we use molecular dynamics simulations of a model metallic glass to investigate the heterogeneity in the local thermal expansion. We then combine the resulting spatial distribution of thermal expansion with a continuum mechanics calculation to infer the internal stresses caused by a thermal cycle. Comparing the internal stress with the local yield stress, we prove that the heterogeneity in thermo mechanical response has the potential to trigger local shear transformations, and therefore to induce rejuvenation during a cryogenic thermal cycling. more...

International audience; This paper is concerned with inverse source problems for the time-dependent Lamé system and the recovery of initial data in an unbounded domain corresponding to the exterior of a bounded cavity or the full space R 3. If the time and spatial variables of the source term can be separated with compact support, we prove that the vector valued spatial source term can be uniquely determined by boundary Dirichlet data in the exterior of a given cavity. If the cavity is absent, uniqueness and stability for recovering source terms depending on the time variable and two spatial variables in the whole space are also obtained using partial Dirichlet boundary data. more...

We are concerned with time-dependent inverse source problems in elastodynamics. The source term is supposed to be the product of a spatial function and a temporal function with compact support. We present frequency-domain and time-domain approaches to show uniqueness in determining the spatial function from wave fields on a large sphere over a finite interval. Stability estimate of the temporal function from the data of one receiver and uniqueness result using partial boundary data are proved. Our arguments rely heavily on the use of the Fourier transform, which motivated inversion schemes that can be easily implemented. A Landweber iterative algorithm for recovering the spatial function and a non-iterative inversion scheme based on the uniqueness proof for recovering the temporal function are proposed. Numerical examples are demonstrated in both two and three dimensions., 28 pages, 14 figures more...

International audience; We show that the asymptotic properties of the link-wise artificial com-pressibility method are not compatible with a correct approximation of fluid properties. We propose to adapt the previous method through a framework suggested by the Tay-lor expansion method and to replace first order terms in the expansion by appropriate three or five points finite differences and to add non linear terms. The " FD-LBM " scheme obtained by this method is tested in two dimensions for shear wave, Stokes modes and Poiseuille flow. The results are compared with the usual lattice Boltzmann method in the framework of multiple relaxation times. more...

International audience; The Ground State Dominance Approximation(GSDA) has been extensively used to study the assembly of viral shells. In this work we employ the self-consistent field theory (SCFT) to investigate the adsorption of RNA onto positively charged spherical viral shells and examine the conditions when GSDA does not apply and SCFT has to be used to obtain a reliable solution. We find that there are two regimes in which GSDA does work. First, when the genomic RNA length is long enough compared to the capsid radius, and second, when the interaction between the genome and capsid is so strong that the genome is basically localized next to the wall. We find that for the case in which RNA is more or less distributed uniformly in the shell, regardless of the length of RNA, GSDA is not a good approximation. We observe that as the polymer-shell interaction becomes stronger, the energy gap between the ground state and first excited state increases and thus GSDA becomes a better approximation. We also present our results corresponding to the genome persistence length obtained through the tangent-tangent correlation length and show that it is zero in case of GSDA but is equal to the inverse of the energy gap when using SCFT. more...

Determination of the atomic structure of inorganic single-walled nanotubes with complex stoichiometry remains elusive due to the too many atomic coordinates to be fitted with respect to X-ray diffractograms inherently exhibiting rather broad features. Here we introduce a methodology to reduce the number of fitted variables and enable resolution of the atomic structure for inorganic nanotubes with complex stoichiometry. We apply it to recently synthesized methylated aluminosilicate and aluminogermanate imogolite nanotubes of nominal composition (OH)3Al2O3Si(Ge)CH3. Fitting of X-ray scattering diagrams, supported by Density Functional Theory simulations, reveals an unexpected rolling mode for these systems. The transferability of the approach opens up for improved understanding of structure–property relationships of inorganic nanotubes to the benefit of fundamental and applicative research in these systems., Structural determination of inorganic nanotubes has lagged far behind that of their carbon-based counterparts. Here, the authors present a transferable methodology, combining wide angle X-ray scattering and computation, to quantitatively resolve the atomic structure of inorganic nanotubes with complex stoichiometry. more...

The dielectric constant of ionic solutions is known to reduce with increasing ionic concentrations. However, the origin of this effect has not been thoroughly explored. In this paper we study two such possible sources: long-range Coulombic correlations and solvent excluded volume. Correlations originate from fluctuations of the electrostatic potential beyond the mean-field Poisson-Boltzmann theory, evaluated by employing a field-theoretical loop expansion of the free energy. The solvent excluded-volume, on the other hand, stems from the finite ion size, accounted for via a lattice-gas model. We show that both correlations and excluded volume are required in order to capture the important features of the dielectric behavior. For highly polar solvents, such as water, the dielectric constant is given by the product of the solvent volume fraction and a concentration-dependent susceptibility per volume fraction. The available solvent volume decreases as function of ionic strength due the increasing volume fraction of ions. A similar decrease occurs for the susceptibility due to correlations between the ions and solvent, reducing the dielectric response even further. Our predictions for the dielectric constant fit well with experiments for a wide range of concentrations for different salts in different temperatures, using a single fit parameter related to the ion size., Comment: 12 pages, 4 figures more...

International audience; We consider the D1Q3 lattice Boltzmann scheme with an acoustic scale for the simulation of diffusive processes. When the mesh is refined while holding the diffusivity constant, we first obtain asymptotic convergence. When the mesh size tends to zero, however, this convergence breaks down in a curious fashion, and we observe qualitative discrepancies from analytical solutions of the heat equation. In this work, a new asymptotic analysis is derived to explain this phenomenon using the Taylor expansion method, and a partial differential equation of acoustic type is obtained in the asymptotic limit. We show that the error between the D1Q3 numerical solution and a finite-difference approximation of this acoustic-type partial differential equation tends to zero in the asymptotic limit. In addition, a wave vector analysis of this asymptotic regime demonstrates that the dispersion equation has nontrivial complex eigenvalues, a sign of underlying propagation phenomena, and a portent of the unusual convergence properties mentioned above. more...

International audience; We consider multi relaxation times lattice Boltzmann scheme with two particle distributions for the thermal Navier Stokes equations formulated with conservation of mass and momentum and dissipation of volumic entropy.Linear stability is taken into consideration to determine a coupling between two coefficients of dissipation.We present interesting numerical results for one-dimensional strong nonlinear acoustic waves with shocks. more...

We study the problem of determining uniquely a time-dependent singular potential \begin{document}$q$\end{document} , appearing in the wave equation \begin{document}$\partial_t^2u-Δ_x u+q(t,x)u = 0$\end{document} in \begin{document}$Q = (0,T)×Ω$\end{document} with \begin{document}$T>0$\end{document} and \begin{document}$Ω$\end{document} a \begin{document}$ \mathcal C^2$\end{document} bounded domain of \begin{document}$\mathbb{R}^n$\end{document} , \begin{document}$n≥2$\end{document} . We start by considering the unique determination of some general singular time-dependent coefficients. Then, by weakening the singularities of the set of admissible coefficients, we manage to reduce the set of data that still guaranties unique recovery of such a coefficient. To our best knowledge, this paper is the first claiming unique determination of unbounded time-dependent coefficients, which is motivated by the problem of determining general nonlinear terms appearing in nonlinear wave equations. more...

International audience; In this work, tungsten carbide (WC) reinforced maraging steel matrix composites were in-situ manufactured by selective laser melting (SLM) from powder mixture. The SLM processed samples presented high relative density (over 99%) with a homogenous distribution of WC. The as-fabricated surface quality of SLM processed samples was improved significantly by the addition of WC. Focused ion beam and transmission electron microscopy were employed to characterize the interfacial properties between tungsten carbide and steel matrix. The elemental analysis indicates that metallurgical bonding appears at interfacial region due to the diffusion. Tensile behavior of SLM processed maraging steel was different from their composite with several WC contents. more...

International audience; Neisseria meningitidis, a bacterium responsible for meningitis and septicemia, proliferates and eventually fills the lumen of blood capillaries with multicellular aggregates. The impact of this aggregation process and its specific properties are unknown. We first show that aggregative properties are necessary for efficient infection and study their underlying physical mechanisms. Micropipette aspiration and single-cell tracking unravel unique features of an atypical fluidized phase, with single-cell diffusion exceeding that of isolated cells. A quantitative description of the bacterial pair interactions combined with active matter physics-based modeling show that this behavior relies on type IV pili active dynamics that mediate alternating phases of bacteria fast mutual approach, contact, and release. These peculiar fluid properties proved necessary to adjust to the geometry of capillaries upon bacterial proliferation. Intermittent attractive forces thus generate a fluidized phase that allows for efficient colonization of the blood capillary network during infection. more...

International audience; We introduce and study in two dimensions a new class of dry, aligning active matter that exhibits a direct transition to orientational order, without the phase-separation phenomenology usually observed in this context. Characterized by self-propelled particles with velocity reversals and a ferromagnetic alignment of polarities, systems in this class display quasi-long-range polar order with continuously varying scaling exponents, yet a numerical study of the transition leads to conclude that it does not belong to the Berezinskii-Kosterlitz-Thouless universality class but is best described as a standard critical point with an algebraic divergence of correlations. We rationalize these findings by showing that the interplay between order and density changes the role of defects. more...

International audience; The purpose of this paper is to discuss some recent developments concerning the numerical simulation of space and time fractional Schrödinger and Gross-Pitaevskii equations. In particular, we address some questions related to the discretization of the models (order of accuracy and fast implementation) and clarify some of their dynamical properties. Some numerical simulations illustrate these points. more...

We study the dynamic spin structure factor of the spin-$1/2$ square-lattice Heisenberg antiferromagnet and of the $J$-$Q$ model (with 4-spin interactions $Q$ and Heisenberg exchange $J$). Using an improved method for stochastic analytic continuation of imaginary-time correlation functions computed with QMC simulations, we can treat the sharp ($\delta$-function) contribution from spinwaves (magnons) and a continuum at higher energy. The results for the Heisenberg model agree with neutron scattering experiments on Cu(DCOO)$_2$$\cdot$4D$_2$O, where a broad spectral-weight continuum at $q=(\pi,0)$ was interpreted as deconfined spinons. Our results at $(\pi,0)$ show a similar reduction of the magnon weight and a large continuum, while the continuum is much smaller at $q=(\pi/2,\pi/2)$ (as also seen experimentally). Turning on $Q$, we observe a rapid reduction of the $(\pi,0)$ magnon weight to zero, well before the deconfined quantum phase transition into a spontaneously dimerized state. We re-interpret the picture of deconfined spinons at $(\pi,0)$ in the experiments as nearly deconfined spinons---a precursor to deconfined quantum criticality. To further elucidate the picture of a fragile $(\pi,0)$-magnon in the Heisenberg model and its depletion in the $J$-$Q$ model, we introduce an effective model in which a magnon can split into two spinons that do not separate but fluctuate in and out of the magnon space (in analogy with the resonance between a photon and a particle-hole pair in the exciton-polariton problem). The model reproduces the $(\pi,0)$ and $(\pi/2,\pi/2)$ features of the Heisenberg model. It can also account for the rapid loss of the $(\pi,0)$ magnon with increasing $Q$ and a remarkable persistence of a large magnon pole at $q=(\pi/2,\pi/2)$ even at the deconfined critical point., Comment: 25 pages, 24 figures, some additional discussion in version 3, to be appeared in PRX more...

International audience; Many simple RNA viruses enclose their genetic material by a protein shell called the capsid. While the capsid structures are well characterized for most viruses, the structure of RNA inside the shells and the factors contributing to it remain poorly understood. We study the impact of base pairing on the conformations of RNA and find that it undergoes a swollen coil to globule continuous transition as a function of the strength of the pairing interaction. We also observe a first order transition and kink profile as a function of RNA length. All these transitions could explain the different RNA profiles observed inside viral shells. more...

We propose a stochastic method to generate exactly the overdamped Langevin dynamics of semi-flexible Gaussian chains, conditioned to evolve between given initial and final conformations in a preassigned time. The initial and final conformations have no restrictions, and hence can be in any knotted state. Our method allows the generation of statistically independent paths in a computationally efficient manner. We show that these conditioned paths can be exactly generated by a set of local stochastic differential equations. The method is used to analyze the transition routes between various knots in crossable filamentous structures, thus mimicking topological reconnections occurring in soft matter systems or those introduced in DNA by topoisomerase enzymes. We find that the average number of crossings, writhe and unknotting number are not necessarily monotonic in time and that more complex topologies than the initial and final ones can be visited along the route., 12 pages, 3 figures more...

International audience; We propose a novel stochastic method to generate Brownian paths conditioned to start at an initial point and end at a given final point during a fixed time t f under a given potential U (x). These paths are sampled with a probability given by the overdamped Langevin dynamics. We show that these paths can be exactly generated by a local Stochastic Partial Differential Equation (SPDE). This equation cannot be solved in general but we present several approximations that are valid either in the low temperature regime or in the presence of barrier crossing. We show that this method warrants the generation of statistically independent transition paths. It is computationally very efficient. We illustrate the method first on two simple potentials, the two dimensional Mueller potential as well as on the Mexican hat potential, and then on the multi-dimensional problem of conformational transitions in proteins using the "Mixed Elastic Network Model" as a benchmark. more...

We show that fore-aft asymmetry, a generic feature of living organisms and some active matter systems, can have a strong influence on the collective properties of even the simplest flocking models. Specifically, an arbitrarily weak asymmetry favoring front neighbors changes qualitatively the phase diagram of the Vicsek model. A region where many sharp traveling band solutions coexist is present at low noise strength, below the Toner-Tu liquid, at odds with the phase-separation scenario well describing the usual isotropic model. Inside this region, a `banded liquid' phase with algebraic density distribution coexists with band solutions. Linear stability analysis at the hydrodynamic level suggests that these results are generic and not specific to the Vicsek model., Comment: 6 pages, 5 figures more...

International audience; Collective oscillatory behaviour is ubiquitous in nature1, having a vital role in many biological processes from embryogenesis2 and organ development3 to pace-making in neuron networks4. Elucidating the mechanisms that give rise to synchronization is essential to the understanding of biological self-organization. Collective oscillations in biological multicellular systems often arise from long-range coupling mediated by diffusive chemicals2, 5, 6, 7, 8, 9, by electrochemical mechanisms4, 10, or by biomechanical interaction between cells and their physical environment11. In these examples, the phase of some oscillatory intracellular degree of freedom is synchronized. Here, in contrast, we report the discovery of a weak synchronization mechanism that does not require long-range coupling or inherent oscillation of individual cells. We find that millions of motile cells in dense bacterial suspensions can self-organize into highly robust collective oscillatory motion, while individual cells move in an erratic manner, without obvious periodic motion but with frequent, abrupt and random directional changes. So erratic are individual trajectories that uncovering the collective oscillations of our micrometre-sized cells requires individual velocities to be averaged over tens or hundreds of micrometres. On such large scales, the oscillations appear to be in phase and the mean position of cells typically describes a regular elliptic trajectory. We found that the phase of the oscillations is organized into a centimetre-scale travelling wave. We present a model of noisy self-propelled particles with strictly local interactions that accounts faithfully for our observations, suggesting that self-organized collective oscillatory motion results from spontaneous chiral and rotational symmetry breaking. These findings reveal a previously unseen type of long-range order in active matter systems (those in which energy is spent locally to produce non-random motion)12, 13. This mechanism of collective oscillation may inspire new strategies to control the self-organization of active matter14, 15, 16, 17, 18 and swarming robots more...

We propose a novel stochastic method to generate Brownian paths conditioned to start at an initial point and end at a given final point during a fixed time $t_f$ under a given potential $U(x)$. These paths are sampled with a probability given by the overdamped Langevin dynamics. We show that these paths can be exactly generated by a local $Stochastic$ $Partial$ $Differential$ $Equation$ $(SPDE)$. This equation cannot be solved in general. We present several approximations that are valid either in the low temperature regime or in the presence of barrier crossing. We show that this method warrants the generation of statistically independent transition paths. It is computationally very efficient. We illustrate the method on the two dimensional Mueller potential as well as on the Mexican hat potential. more...

We study the collective dynamics of elongated swimmers in a very thin fluid layer by devising long, filamentous, non-tumbling bacteria. The strong confinement induces weak nematic alignment upon collision, which, for large enough density of cells, gives rise to global nematic order. This homogeneous but fluctuating phase, observed on the largest experimentally-accessible scale of millimeters, exhibits the properties predicted by standard models for flocking such as the Vicsek-style model of polar particles with nematic alignment: true long-range nematic order and non-trivial giant number fluctuations., 6 pages, 4 figures. Supplemental Material: 6 pages, 3 figures more...

We derive hydrodynamic equations from Vicsek-style dry active matter models in three dimensions (3D), building on our experience on the 2D case using the Boltzmann-Ginzburg-Landau approach. The hydrodynamic equations are obtained from a Boltzmann equation expressed in terms of an expansion in spherical harmonics. All their transport coefficients are given with explicit dependences on particle-level parameters. The linear stability analysis of their spatially-homogeneous solutions is presented. While the equations derived for the polar case (original Vicsek model with ferromagnetic alignment) and their solutions do not differ much from their 2D counterparts, the active nematics case exhibits remarkable differences: we find a true discontinuous transition to order with a bistability region, and cholesteric solutions whose stability we discuss., Comment: 40 pages, 14 figures, comments are welcome more...

In a seminal paper Ginzburg and Adler analyzed the bounce-back boundary conditions for the lattice Boltzmann scheme and showed that it could be made exact to second order for the Poiseuille flow if some expressions depending upon the parameters of the method were satisfied, thus defining so-called "magic parameters". Using the Taylor expansion method that one of us developed, we analyze a series of simple situations (1D and 2D) for diffusion and for linear fluid problems using bounce-back and "anti bounce-back" numerical boundary conditions. The result is that "magic parameters" depend upon the detailed choice of the moments and of their equilibrium values. They may also depend upon the way the flow is driven., Comment: 14 pages more...

We study in detail the hydrodynamic theories describing the transition to collective motion in polar active matter, exemplified by the Vicsek and active Ising models. Using a simple phenomenological theory, we show the existence of an infinity of propagative solutions, describing both phase and microphase separation, that we fully characterize. We also show that the same results hold specifically in the hydrodynamic equations derived in the literature for the active Ising model and for a simplified version of the Vicsek model. We then study numerically the linear stability of these solutions. We show that stable ones constitute only a small fraction of them, which however includes all existing types. We further argue that in practice, a coarsening mechanism leads towards phase-separated solutions. Finally, we construct the phase diagrams of the hydrodynamic equations proposed to qualitatively describe the Vicsek and active Ising models and connect our results to the phenomenology of the corresponding microscopic models., Comment: 20 pages, 20 figures more...

We propose and rigourously analyze a multiscale time integrator Fourier pseudospectral (MTI-FP) method for the Dirac equation with a dimensionless parameter $\varepsilon\in(0,1]$ which is inversely proportional to the speed of light. In the nonrelativistic limit regime, i.e. $0, Comment: 25 pages, 1 figure more...

We show that memory, in the form of underdamped angular dynamics, is a crucial ingredient for the collective properties of self-propelled particles. Using Vicsek-style models with an Ornstein-Uhlenbeck process acting on angular velocity, we uncover a rich variety of collective phases not observed in usual overdamped systems, including vortex lattices and active foams. In a model with strictly nematic interactions the smectic arrangement of Vicsek waves giving rise to global polar order is observed. We also provide a calculation of the effective interaction between vortices in the case where a telegraphic noise process is at play, explaining thus the emergence and structure of the vortex lattices observed here and in motility assay experiments., 5 pages, 4 figures more...

We revisit the Kuramoto model to explore the finite-size scaling (FSS) of the order parameter and its dynamic fluctuations near the onset of the synchronization transition, paying particular attention to effects induced by the randomness of the intrinsic frequencies of oscillators. For a population of size $N$, we study two ways of sampling the intrinsic frequencies according to the {\it same} given unimodal distribution $g(\omega)$. In the `{\em random}' case, frequencies are generated independently in accordance with $g(\omega)$, which gives rise to oscillator number fluctuation within any given frequency interval. In the `{\em regular}' case, the $N$ frequencies are generated in a deterministic manner that minimizes the oscillator number fluctuations, leading to quasi-uniformly spaced frequencies in the population. We find that the two samplings yield substantially different finite-size properties with clearly distinct scaling exponents. Moreover, the hyperscaling relation between the order parameter and its fluctuations is valid in the regular case, but is violated in the random case. In this last case, a self-consistent mean-field theory that completely ignores dynamic fluctuations correctly predicts the FSS exponent of the order parameter but not its critical amplitude. more...

International audience; Organic molecules with a higher spin state such as quintet ground state have been observed at low temperatures but they could not exist at higher temperature due to chemical instability. [ 10 ] However, Kekulé diradicals are expected to provide the necessary stability. [ 2 ] Organic stable radicals have been studied extensively for decades. [ 1b ] Among them, 1,3-diaryl-1,2,4-benzotriazinyl radicals have attracted our attention due to their interesting magnetic, as well as electronic properties, and more importantly , their exceptional air and moisture stability. [ 11 ] In the past, we have been interested in linking two of the 1,3-diaryl-1,2,4-benzo triazinyl radical moieties in a single molecule. We designed a hexa-azaanthracene with the goal of generating a stable triplet molecule as shown in 1T (Figure 1), based on the notion that the aromatic sextet would dominate the stability of the molecule and that the two unpaired spins would not be able to pair due to their location relative to each other (1,3-aromatic substitution). Instead, very surprisingly, 1ZI was formed (Figure 1). In this case cyanine stabilization over aromaticity was the driving force. [ 12 ] This unexpected result, coupled with the observation that QDM derivatives have, in some cases, produced diradical contributions to the overall structure (diradicaloids), [ 2,8 ] led us to design 6,6-bis(2-ethylhexyl)-1,1′,3,3′-tetraphenyl-4,6-dihydro-1H-fl uoreno[2,3,8,9]-1,2,4-ditriazin-4-yl (FDT). Here, we demonstrate the fi rst example of synthesis and characterization of an organic diradicaloid based on stable benzotri-azinyls. We further show that electron spin resonance (ESR) spectra of a polycrystalline sample of FDT resulted in the discovery of a, so far unprecedented, intermolecular quintet state at room temperature, supported by the following observations: i) recording of the Δ M s = ±2 forbidden transition by ESR spectros-copy; ii) essentially perfectly superimposable simulation of the ESR spectrum for a quintet and NOT for a triplet; and iii) density functional theory calculations. Variable-temperature (VT) NMR, bond-length analyses, and DFT calculations indicated that the molecule has contributions from a closed shell, covalent structure (FDT (QI)) and an open shell diradical canonical form (FDT) (Figure 1). Static magnetic susceptibility studies showed that the effective magnetic moment continuously decreased with decreasing temperature, and surprisingly, increasing magnetic fi eld strength. A paramagnetic susceptibility was observable only at very weak fi eld strengths (100–1000 Oe). At higher fi eld strengths, the solid is diamagnetic. The same material can be fabricated into a near-infrared (NIR) photodetector with detectivity over 10 11 cm Hz 1/2 W –1 at 1200 nm. The synthetic route to FDT followed the familiar procedure [ 13 ] for the formation of benzotriazinyls, shown in Obtaining high-spin-state organic solids at ambient conditions has been the goal of organic materials scientists, chemists, and physicists because they would enable, not only a fundamental understanding of the nature of spin–spin correlations , but also provide a wide range of advanced applications, quantum information processing systems, and organic spin-tronics. [ 1 ] Molecules with two unpaired electrons have been heavily investigated in the recent past. [ 2 ] Delocalized diradicals can be divided into Kekulé and non-Kekulé structures. Typical examples of non-Kekulé molecules are meta-quinodimethane (m-QDM), [ 3 ] trimethylenemethanes, [ 4 ] and nitrogen-centered (aminyl) radicals. [ 5 ] Only a few molecules with stable open shell structures at room temperature have been studied, such as a suitably substituted corannulene, [ 6 ] bis(triarylamine) dicat-ions, [ 7 ] p-quinodimethanes [ 8 ] and specifi cally designed poly-cyclic aromatic hydrocarbons. [ 9 ] An essential component of all these molecules is the proper substitution to stabilize the unpaired electrons. more...

In this contribution, we propose a new method of analysis for the lattice Boltzmann scheme's boundary conditions. We develop this approach for bounce-back with a given velocity between two mesh points. We develop a "accordion" test case for linear stationary Stokes problem with an analytic expression. Numerical experiment show a good agreement at first and second order accuracy. This study has been also applied to the computation of momentum transfer. Resume. Dans cette contribution, nous proposons une nouvelle methode d'analyse des conditions aux limites pour les schemas de Boltzmann sur reseau. Nous developpons cette approche pour une condition de rebond avec une donnee de vitesse imposee. Nous developpons un cas test "accordeon" pour le probleme de Stokes stationnaire et en explicitons la solution analytique. Nos experiences numeriques montrent un bon accord entre solution approchee numerique et solution analytique, a l'ordre un et a l'ordre deux. L'etude precedente est egalement appliquee pour le calcul du transfert d'impulsion a la paroi. more...

This paper is dedicated to a detailed analysis of the symmetry properties of the cubical Stokes eigenmodes and to the numerical verification of its predictions. These modes are computed numerically by using two different Stokes solvers, the Projection-Diffusion Chebyshev spectral method and a lattice Boltzmann approach. They distribute themselves into 10 symmetry families, 2 families of singlets, 2 families of doublets and 6 families of triplets. The existence of the singlets, doublets and triplets is directly related to the fact that two different bases can be constructed for describing the cyclic-permutation state operator. The singlets and doublets are associated with one of these bases, and they are therefore generated together, the triplets being associated with the other basis. more...

We propose the derivation of acoustic-type isotropic partial differential equations that are equivalent to linear lattice Boltzmann schemes with a density scalar field and a momentum vector field as conserved moments. The corresponding linear equivalent partial differential equations are generated with a new "Berliner version" of the Taylor expansion method. The details of the implementation are presented. These ideas are applied for the D2Q9, D2Q13, D3Q19 and D3Q27 lattice Boltzmann schemes. Some limitations associated with necessary stability conditions are also presented., 24 pages more...