10,119 research outputs found
Accurate polarization within a unified Wannier function formalism
We present an alternative formalism for calculating the maximally localized
Wannier functions in crystalline solids, obtaining an expression which is
extremely simple and general. In particular, our scheme is exactly invariant
under Brillouin zone folding, and therefore it extends trivially to the
Gamma-point case. We study the convergence properties of the Wannier functions,
their quadratic spread and centers as obtained by our simplified technique. We
show how this convergence can be drastically improved by a simple and
inexpensive ``refinement'' step, which allows for very efficient and accurate
calculations of the polarization in zero external field.Comment: 9 pages, 4 figure
The Inverse Amplitude Method and Adler Zeros
The Inverse Amplitude Method is a powerful unitarization technique to enlarge
the energy applicability region of Effective Lagrangians. It has been widely
used to describe resonances from Chiral Perturbation Theory as well as for the
Strongly Interacting Symmetry Breaking Sector. In this work we show how it can
be slightly modified to account also for the sub-threshold region,
incorporating correctly the Adler zeros required by chiral symmetry and
eliminating spurious poles. These improvements produce negligible effects on
the physical region.Comment: 17 pages, 4 figure
Ab-initio theory of metal-insulator interfaces in a finite electric field
We present a novel technique for calculating the dielectric response of
metal/insulator heterostructures. This scheme allows, for the first time, the
fully first-principles calculation of the microscopic properties of thin-film
capacitors at finite bias potential. The method can be readily applied to pure
insulators, where it provides an interesting alternative to conventional
finite-field techniques based on the Berry-phase formalism. We demonstrate the
effectiveness of our method by performing comprehensive numerical tests on a
model Ag/MgO/Ag heterostructure.Comment: 10 pages, 5 figures, major revisio
Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite
The dependencies on strain and oxygen vacancies of the ferroelectric
polarization and the weak ferromagnetic magnetization in the multiferroic
material bismuth ferrite, BiFeO_3, are investigated using first principles
density functional theory calculations. The electric polarization is found to
be rather independent of strain, in striking contrast to most conventional
perovskite ferroelectrics. It is also not significantly affected by oxygen
vacancies, or by the combined presence of strain and oxygen vacancies. The
magnetization is also unaffected by strain, however the incorporation of oxygen
vacancies can alter the magnetization slightly, and also leads to the formation
of Fe^{2+}. These results are discussed in light of recent experiments on
epitaxial films of BiFeO_3 which reported a strong thickness dependence of both
magnetization and polarization.Comment: 9 pages, 3 figure
Self modulated dynamics of a relativistic charged particle beam in plasma wake field excitation
Self modulated dynamics of a relativistic charged particle beam is reviewed
within the context of the theory of plasma wake field excitation. The
self-consistent description of the beam dynamics is provided by coupling the
Vlasov equation with a Poisson-type equation relating the plasma wake potential
to the beam density. An analysis of the beam envelope self-modulation is then
carried out and the criteria for the occurrence of the instability are
discussed thereby.Comment: This is a 10 pages manuscript which contain 4 figures. This
manuscript is recently submitted in 'Nuclear Instruments and Methods in
Physics Research Section A' as a proceeding of the conference 'EAAC 2015
Classical and Quantum-like approaches to Charged-Particle Fluids in a Quadrupole
A classical description of the dynamics of a dissipative charged-particle
fluid in a quadrupole-like device is developed. It is shown that the set of the
classical fluid equations contains the same information as a complex function
satisfying a Schrodinger-like equation in which Planck's constant is replaced
by the time-varying emittance, which is related to the time-varying temperature
of the fluid. The squared modulus and the gradient of the phase of this complex
function are proportional to the fluid density and to the current velocity,
respectively. Within this framework, the dynamics of an electron bunch in a
storage ring in the presence of radiation damping and quantum-excitation is
recovered. Furthermore, both standard and generalized (including dissipation)
coherent states that may be associated with the classical particle fluids are
fully described in terms of the above formalism.Comment: LaTex, to appear in Physica Script
Mass spectrum from stochastic Levy-Schroedinger relativistic equations: possible qualitative predictions in QCD
Starting from the relation between the kinetic energy of a free
Levy-Schroedinger particle and the logarithmic characteristic of the underlying
stochastic process, we show that it is possible to get a precise relation
between renormalizable field theories and a specific Levy process. This
subsequently leads to a particular cut-off in the perturbative diagrams and can
produce a phenomenological mass spectrum that allows an interpretation of
quarks and leptons distributed in the three families of the standard model.Comment: 8 pages, no figures. arXiv admin note: substantial text overlap with
arXiv:1008.425
Specifying and Analysing SOC Applications with COWS
COWS is a recently defined process calculus for specifying and combining service-oriented applications, while modelling their dynamic behaviour. Since its introduction, a number of methods and tools have been devised to analyse COWS specifications, like e.g. a type system to check confidentiality properties, a logic and a model checker to express and check functional properties of services. In this paper, by means of a case study in the area of automotive systems, we demonstrate that COWS, with some mild linguistic additions, can model all the phases of the life cycle of service-oriented applications, such as publication, discovery, negotiation, orchestration, deployment, reconfiguration and execution. We also provide a flavour of the properties that can be analysed by using the tools mentioned above
The Sensoria Approach Applied to the Finance Case Study
This chapter provides an effective implementation of (part of) the Sensoria approach, specifically modelling and formal analysis of service-oriented software based on mathematically founded techniques. The ‘Finance case study’
is used as a test bed for demonstrating the feasibility and effectiveness of the use of the process calculus COWS and some of its related analysis techniques and tools. In particular, we report the results of an application of a temporal logic and its model checker for expressing and checking functional properties of services and a type system for guaranteeing confidentiality properties of services
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