Search results for "approximation"
showing 10 items of 818 documents
Light Quark Masses from Lattice Quark Propagators at Large Momenta
1999
We compute non-perturbatively the average up-down and strange quark masses from the large momentum (short-distance) behaviour of the quark propagator in the Landau gauge. This method, which has never been applied so far, does not require the explicit calculation of the quark mass renormalization constant. Calculations were performed in the quenched approximation, by using O(a)-improved Wilson fermions. The main results of this study are ml^RI(2GeV)=5.8(6)MeV and ms^RI(2GeV)=136(11)MeV. Using the relations between different schemes, obtained from the available four-loop anomalous dimensions, we also find ml^RGI=7.6(8)MeV and ms^RGI=177(14)MeV, and the MSbar-masses, ml^MS(2GeV)=4.8(5)MeV and …
Effective kink-kink interaction in a one-dimensional model mediated by phonon exchange
1994
The general 1D double-well model with anharmonic interaction is considered in the displacive limit. Expansion of the Hamiltonian around a multikink state results in a phonon-kink Hamiltonian. It is shown that at rather low temperatures and short wave lengths the phonon-kink interaction can be treated in Born approximation, leading to a decomposition of the multikink-phonon Hamiltionian. Elimination of the phonons results in an effective potential for the kink-kink interaction, which corresponds to the one-dimensional analog of the RKKY interaction. This long-range interaction is inherent only for models with anharmonic on-site potentials and not in case of a double-parabola model.
Scattering of Particles by Potentials
2013
The three prototypes of spectra of self-adjoint operators, the discrete spectrum, with or without degeneracy, the continuous spectrum, and the mixed spectrum, as well as the corresponding wave functions, contain important information about the physical systems that they describe
Evaluation using m.c.n.p. code of the bremsstrahlung energy spectrum produced by interactions between structural materials and accelerated electrons
2004
Abstract In order to design the biological shield of industrial accelerator plants, it is needed to have a good knowledge of the bremsstrahlung energy spectrum and the intensity of the bremsstrahlung produced by electron interactions with both products (usually water equivalent) and structural materials such as concrete, iron, aluminium. Using the MCNP code, a normalized bremsstrahlung energy spectrum was obtained for materials with average atomic number lower than or equal to 13 and irradiated with 5 and 10 Mev electrons, respectively; multiplying the spectrum by suitable coefficients, it was possible to obtain the real spectrum for materials such as water, concrete, aluminium and iron. Th…
Stochastic models for heterogeneous relaxation: Application to inhomogeneous optical lineshapes
2001
Dynamic heterogeneity has often been modeled by assuming that a single-particle observable, fluctuating at a molecular scale, is influenced by its coupling to environmental variables fluctuating on a second, perhaps slower, time scale. Starting from the most simple Gaussian Markov process we model the exchange between 'slow' and 'fast' environments by treating the fluctuating single-particle variable as a projection from a higher-dimensional Markov process. The moments of the resulting stochastic process are calculated from the corresponding Master equations or Langevin equations, depending on the model. The calculations show the importance of the way to treat exchange processes. The result…
Numerical study on the limit of quasi-static approximation for plasmonic nanosphere
2019
Plasmonic nanospheres are often employed as resonant substrates in many nanophotonic applications, like in enhanced spectroscopy, near-field microscopy, photovoltaics, and sensing. Accurate calculation and tuning of optical responses of such nanospheres are essential to achieve optimal performance. Mie theory is widely used to calculate optical properties of spherical particles. Although, an approximated version of Mie approach, the quasi-static approximation (QSA) can also be used to determine the very same properties of those spheres with a lot simpler formulations. In this work, we report our numerical study on the limit and accuracy of QSA with respect to the rigorous Mie approach. We c…
Analytical investigation of solitary waves in nonlinear Kerr medium
2004
Abstract We study analytically the solution of nonlinear equation which result from the propagation of electromagnetic waves within a nonlinear Kerr medium. The medium is characterized by a dielectric constant which varies periodically and depends on the local field intensity. As a first step, we detail the resolution of the nonlinear equations with a quadratic nonlinearity. After that, we apply the slowly varying envelope approximation to obtain a Sine–Gordon equation. In this kind of nonlinearity, a gap solitons occurs. Moreover we verify that the solutions of the nonlinear equation for all frequencies within the gap are solitons solutions. After that we study the conditions of apparition…
Scattering from concentration fluctuations in polymer blends: A monte carlo investigation
1989
The collective scattering function Scoll( $$\vec q$$ ), which describes light (neutron-, x-ray) scattering under wavevector $$\vec q$$ , is obtained from Monte Carlo simulations for a symmetrical polymer mixture. The polymers are modelled by self-avoiding walks ofN A=NB=N steps on a simple cubic lattice, where a fractionφ V of sites is left vacant, and an attractive energye occurs if two neighboring sites are taken by the same kind of monomer. Spinodal curves are estimated from linear extrapolation of S coll −1 (0) vs.e/k B T, whereT is the temperature. Also the single chain structure factor is obtained and the de Gennes random phase approximation (RPA) can thus be tested. Unexpectedly, str…
Correlation effects in bistability at the nanoscale: Steady state and beyond
2012
The possibility of finding multistability in the density and current of an interacting nanoscale junction coupled to semi-infinite leads is studied at various levels of approximation. The system is driven out of equilibrium by an external bias and the nonequilibrium properties are determined by real-time propagation using both time-dependent density functional theory (TDDFT) and many-body perturbation theory (MBPT). In TDDFT the exchange-correlation effects are described within a recently proposed adiabatic local density approximation (ALDA). In MBPT the electron-electron interaction is incorporated in a many-body self-energy which is then approximated at the Hartree-Fock (HF), second-Born,…
Density Functional Theory of Multicomponent Quantum Dots
2004
Quantum dots with conduction electrons or holes originating from several bands are considered. We assume the particles are confined in a harmonic potential and assume the electrons (or holes) belonging to different bands to be different types of fermions with isotropic effective masses. The density functional method with the local density approximation is used. The increased number of internal (Kohn-Sham) states leads to a generalisation of Hund's first rule at high densities. At low densitites the formation of Wigner molecules is favored by the increased internal freedom.