592 research outputs found
Reaction-diffusion fronts with inhomogeneous initial conditions
Properties of reaction zones resulting from A+B -> C type reaction-diffusion
processes are investigated by analytical and numerical methods. The reagents A
and B are separated initially and, in addition, there is an initial macroscopic
inhomogeneity in the distribution of the B species. For simple two-dimensional
geometries, exact analytical results are presented for the time-evolution of
the geometric shape of the front. We also show using cellular automata
simulations that the fluctuations can be neglected both in the shape and in the
width of the front.Comment: 11 pages, 3 figures, submitted to J. Phys.
Luz e cores: relato de atividade educacional usando uma abordagem dialógica problematizadora
Anais do II Seminário Seminário Estadual PIBID do Paraná: tecendo saberes / organizado por Dulcyene Maria Ribeiro e Catarina Costa Fernandes — Foz do Iguaçu: Unioeste; Unila, 2014O Programa Institucional de Bolsa de Iniciação à Docência (PIBID) oportuniza aos
bolsistas uma aproximação do espaço escolar, permitindo que estes futuros professores
estejam mais bem preparados para a tarefa docente. Dentro do ensino de física, a utilização de
práticas experimentais ou demonstrativas pode ser uma forma muito eficaz de substituir o uso
do Método Bancário, uma vez que os alunos têm a oportunidade de observar e chegar às suas
conclusões, e não apenas aceitar aquilo que o professor diz. Infelizmente, a realidade do
ensino público muitas vezes impede essas práticas, devido à falta de laboratórios e/ou
aparatos experimentais. Nesse trabalho é apresentada uma atividade educacional abordando o
tema luz e cores, executada como ação do PIBID, que utiliza um experimento simples e de
baixo custo
Decay Process for Three - Species Reaction - Diffusion System
We propose the deterministic rate equation of three-species in the reaction -
diffusion system. For this case, our purpose is to carry out the decay process
in our three-species reaction-diffusion model of the form . The
particle density and the global reaction rate are also shown analytically and
numerically on a two-dimensional square lattice with the periodic boundary
conditions. Especially, the crossover of the global reaction rate is discussed
in both early-time and long-time regimes.Comment: 6 pages, 3 figures, Late
Non-equilibrium thermodynamic description of junctions in semiconductor devices
The methods of non-equilibrium thermodynamics of systems with an interface
have been applied to the study of transport processes in semiconductor
junctions. A complete phenomenological model for drift-diffusion processes in a
junction has been derived, which includes, from first principles, both surface
equations and boundary conditions, together with the usual drift-diffusion
equations for the bulks. In this way a self-consistent characterisation of the
whole system, bulks and interface, has been obtained in a common framework. The
completeness of the model has been shown and a simple application to
metal-semiconductor junctions developed.Comment: 12 pages, RevTex. Submitted to Phys. Rev. B minor LaTex errors
correcte
Liesegang patterns: Effect of dissociation of the invading electrolyte
The effect of dissociation of the invading electrolyte on the formation of
Liesegang bands is investigated. We find, using organic compounds with known
dissociation constants, that the spacing coefficient, 1+p, that characterizes
the position of the n-th band as x_n ~ (1+p)^n, decreases with increasing
dissociation constant, K_d. Theoretical arguments are developed to explain
these experimental findings and to calculate explicitly the K_d dependence of
1+p.Comment: RevTex, 8 pages, 3 eps figure
Porous silicon formation and electropolishing
Electrochemical etching of silicon in hydrofluoride containing electrolytes
leads to pore formation for low and to electropolishing for high applied
current. The transition between pore formation and polishing is accompanied by
a change of the valence of the electrochemical dissolution reaction. The local
etching rate at the interface between the semiconductor and the electrolyte is
determined by the local current density. We model the transport of reactants
and reaction products and thus the current density in both, the semiconductor
and the electrolyte. Basic features of the chemical reaction at the interface
are summarized in law of mass action type boundary conditions for the transport
equations at the interface. We investigate the linear stability of a planar and
flat interface. Upon increasing the current density the stability flips either
through a change of the valence of the dissolution reaction or by a nonlinear
boundary conditions at the interface.Comment: 18 pages, 8 figure
Diffusion-Limited Annihilation with Initially Separated Reactants
A diffusion-limited annihilation process, A+B->0, with species initially
separated in space is investigated. A heuristic argument suggests the form of
the reaction rate in dimensions less or equal to the upper critical dimension
. Using this reaction rate we find that the width of the reaction front
grows as in one dimension and as in two
dimensions.Comment: 9 pages, Plain Te
Formation of Liesegang patterns: A spinodal decomposition scenario
Spinodal decomposition in the presence of a moving particle source is
proposed as a mechanism for the formation of Liesegang bands. This mechanism
yields a sequence of band positions x_n that obeys the spacing law
x_n~Q(1+p)^n. The dependence of the parameters p and Q on the initial
concentration of the reagents is determined and we find that the functional
form of p is in agreement with the experimentally observed Matalon-Packter law.Comment: RevTex, 4 pages, 4 eps figure
Theoretical Principles of Single-Molecule Electronics: A Chemical and Mesoscopic View
Exploring the use of individual molecules as active components in electronic
devices has been at the forefront of nanoelectronics research in recent years.
Compared to semiconductor microelectronics, modeling transport in
single-molecule devices is much more difficult due to the necessity of
including the effects of the device electronic structure and the interface to
the external contacts at the microscopic level. Theoretical formulation of the
problem therefore requires integrating the knowledge base in surface science,
electronic structure theory, quantum transport and device modeling into a
single unified framework starting from the first-principles. In this paper, we
introduce the theoretical framework for modeling single-molecule electronics
and present a simple conceptual picture for interpreting the results of
numerical computation. We model the device using a self-consistent matrix
Green's function method that combines Non-Equilibrium Green's function theory
of quantum transport with atomic-scale description of the device electronic
structure. We view the single-molecule device as "heterostructures" composed of
chemically well-defined atomic groups, and analyze the device characteristics
in terms of the charge and potential response of these atomic groups to
perturbation induced by the metal-molecule coupling and the applied bias
voltage. We demonstrate the power of this approach using as examples devices
formed by attaching benzene-based molecules of different size and internal
structure to the gold electrodes through sulfur end atoms.Comment: To appear in International Journal of Quantum Chemistry, Special
Issue in memory of J.A. Pople. 13 pages, 9 figure
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