arXiv subjects

Luis Vázquez

Publications and source records attributed to Luis Vázquez.

7 recordsLinked to original sources

arXiv2022

A modified EM method and its fast implementation for multi-term Riemann-Liouville stochastic fractional differential equations

In this paper, a modified Euler-Maruyama (EM) method is constructed for a kind of multi-term Riemann-Liouville stochastic fractional differential equations and the strong convergence order min{1-α_m, 0.5} of the proposed method is proved with Riemann-Liouville fractional derivatives' orders 0<α_1<α_2<...<α_m <1. Then, based on the sum-of-exponentials approximation, a fast implementation of the modified EM method which is called a fast EM method is derived to greatly improve the computational efficiency. Finally, some numerical examples are carried out to support the theoretical results and show the powerful computational performance of the fast EM method.

math.NA↗

arXiv2020

Silicon and hydrogen chemistry under laboratory conditions mimicking the atmosphere of evolved stars

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H$_2$ efficiently produces silane (SiH$_4$), disilane (Si$_2$H$_6$) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC\,+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H$_2$ are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH$_4$ abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH$_4$ and Si$_2$H$_6$ at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

astro-ph.GA↗

arXiv2018

Non-universality of front fluctuations for compact colonies of non-motile bacteria

The front of a compact bacterial colony growing on a Petri dish is a paradigmatic instance of non-equilibrium fluctuations in the celebrated Eden, or Kardar-Parisi-Zhang (KPZ), universality class. While in many experiments the scaling exponents crucially differ from the expected KPZ values, the source of this disagreement has remained poorly understood. We have performed growth experiments with B. subtilis 168 and E. coli ATCC 25922 under conditions leading to compact colonies in the classically-alleged Eden regime, where individual motility is suppressed. Non-KPZ scaling is indeed observed for all accessible times, KPZ asymptotics being ruled out for our experiments due to the monotonic increase of front branching with time. Simulations of an effective model suggest the occurrence of transient non-universal scaling due to diffusive morphological instabilities, agreeing with expectations from detailed models of the relevant biological reaction-diffusion processes.

cond-mat.stat-mech↗

arXiv2012

Strong anisotropy in surface kinetic roughening: analysis and experiments

We report an experimental assessment of surface kinetic roughening properties that are anisotropic in space. Working for two specific instances of silicon surfaces irradiated by ion-beam sputtering under diverse conditions (with and without concurrent metallic impurity codeposition), we verify the predictions and consistency of a recently proposed scaling Ansatz for surface observables like the two-dimensional (2D) height Power Spectral Density (PSD). In contrast with other formulations, this Ansatz is naturally tailored to the study of two-dimensional surfaces, and allows to readily explore the implications of anisotropic scaling for other observables, such as real-space correlation functions and PSD functions for 1D profiles of the surface. Our results confirm that there are indeed actual experimental systems whose kinetic roughening is strongly anisotropic, as consistently described by this scaling analysis. In the light of our work, some types of experimental measurements are seen to be more affected by issues like finite space resolution effects, etc. that may hinder a clear-cut assessment of strongly anisotropic scaling in the present and other practical contexts.

cond-mat.mtrl-sci↗

arXiv2012

Universality of cauliflower-like fronts: from nanoscale thin films to macroscopic plants

Chemical vapor deposition (CVD) is a widely used technique to grow solid materials with accurate control of layer thickness and composition. Under mass-transport-limited conditions, the surface of thin films thus produced grows in an unstable fashion, developing a typical motif that resembles the familiar surface of a cauliflower plant. Through experiments on CVD production of amorphous hydrogenated carbon films leading to cauliflower-like fronts, we provide a quantitative assessment of a continuum description of CVD interface growth. As a result, we identify non-locality, non-conservation, and randomness as the main general mechanisms controlling the formation of these ubiquitous shapes. We also show that the surfaces of actual cauliflower plants and combustion fronts obey the same scaling laws, proving the validity of the theory over seven orders of magnitude in length scales. Thus, a theoretical justification is provided, that had remained elusive thus far, for the remarkable similarity between the textures of surfaces found for systems that differ widely in physical nature and typical scales.

cond-mat.stat-mech↗

arXiv2007

Self-organized surface nanopatterning by ion beam sputtering

The production of nanopatterns on the surfaces of targets irradiated by ion beams at low and intermediate energies has developed during the present decade to a salient degree of control over the main pattern features. However, there is still a wide experimental scatter in the type and relevance of various dynamic interfacial properties induced by this technique as a function of system type and parameters. In parallel, diverse theoretical models exist that differ in their capabilities to reproduce such a wide range of experimental features. We provide an overview of the most recent studies of nanoripple and dot production by ion-beam sputtering, with special attention to the comparison between experiments and (continuum) models, and with a focus on those issues that remain open or, at least, ambiguous. These are perhaps more evident for the case of nanodot patterns on amorphizable targets than for the case of nanoripples due, possibly, to the relative novelty of the former. The pattern properties to be considered are those of potential increased technological importance, such as the variation of nanodot size with parameters like ion energy, target temperature and sputtering time (i.e., fluence), as well as the conditions under which the quality of the pattern order is enhanced, issues such as wavelength coarsening and order enhancement becoming relevant.

cond-mat.mtrl-sci↗

arXiv2003

Diffusion of intrinsic localised modes by attractor hopping

Propagating intrinsic localised modes (discrete breathers) exist in the dissipative driven sine-Gordon chain as attractors of the dynamics. Thermal fluctuations induce random transitions between attracting configurations corresponding to opposite signs of the propagation velocity, effecting a diffusive motion of the breather. Results for the mean value and the variance of the position from numerical simulations (Heun method) are compared with predictions from a two-parameter model developed in this paper.

nlin.PS↗