Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height

The formation of mounded surfaces in epitaxial growth is attributed to the presence of barriers against interlayer diffusion in the terrace edges, known as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth using a ES barrier explicitly dependent on the step height. Our model has an intrinsic topological step barrier even in the absence of an explicit ES barrier. We show that mounded morphologies can be obtained even for a small barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma equation, is observed in absence of an explicit step barrier. The mounded surfaces are described by a super-roughness dynamical scaling characterized by locally smooth (faceted) surfaces and a global roughness exponent $\alpha>1$. The thin film limit is featured by surfaces with self-assembled three-dimensional structures having an aspect ratio (height/width) that may increase or decrease with temperature depending on the strength of step barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary materia

Building analytical three-field cosmological models

A difficult task to deal with is the analytical treatment of models composed by three real scalar fields, once their equations of motion are in general coupled and hard to be integrated. In order to overcome this problem we introduce a methodology to construct three-field models based on the so-called "extension method". The fundamental idea of the procedure is to combine three one-field systems in a non-trivial way, to construct an effective three scalar field model. An interesting scenario where the method can be implemented is within inflationary models, where the Einstein-Hilbert Lagrangian is coupled with the scalar field Lagrangian. We exemplify how a new model constructed from our method can lead to non-trivial behaviors for cosmological parameters.Comment: 11 pages, and 3 figures, updated version published in EPJ

Time-Driven Activity-Based Costing: designing a model in a Portuguese production environment

Purpose: By designing a pilot Time-Driven Activity-Based Costing (TDABC) model, this study seeks to examine in depth the suitability and the complexity of TDABC in a manufacturing company. Design/Methodology/Approach: To obtain a deeper understanding on the matters to analyse, this research adopts an interventionist approach. The host organisation is GP, a Portuguese company in the frozen food sector. Findings: Our experience allows us to assert that TDABC is suitable for a manufacturing company and it is able to deal with the variability of the industrial processes. Nonetheless, through a comparison with the models presented in the literature, TDABC appears to be more complex for manufacturing. We argue that this happens for two reasons: First, the two types of resources (human labour and machinery) used in production areas create a need to split tasks and to create two equations for each process, something that does not happen in service companies. Second, times are difficult to individualise for certain highly automated procedures, which could also give rise to some errors. Research limitations/implications: The designed model is compared to other models presented in the literature. Practical implications: This study shows a real example of TDABC in manufacturing and the procedural innovation of the time equations. Originality/Value: Since the TDABC literature has been mostly focused on examples of service companies, we examine the technical suitability and the complexity of TDABC in manufacturing companies.info:eu-repo/semantics/acceptedVersio

Are Magnetic Wind-Driving Disks Inherently Unstable?

There have been claims in the literature that accretion disks in which a centrifugally driven wind is the dominant mode of angular momentum transport are inherently unstable. This issue is considered here by applying an equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated, magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves for this class of models typically exhibit two distinct branches. It is argued that only one of these branches represents unstable equilibria and that a real disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20). Uses emulateapj.cl

Carbon nanotube: a low-loss spin-current waveguide

We demonstrate with a quantum-mechanical approach that carbon nanotubes are excellent spin-current waveguides and are able to carry information stored in a precessing magnetic moment for long distances with very little dispersion and with tunable degrees of attenuation. Pulsed magnetic excitations are predicted to travel with the nanotube Fermi velocity and are able to induce similar excitations in remote locations. Such an efficient way of transporting magnetic information suggests that nanotubes are promising candidates for memory devices with fast magnetization switchings