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Field Approximation (field + approximation)

Distribution by Scientific Domains
Chemistry45%
Physics and Astronomy36%

Kinds of Field Approximation

  • mean field approximation
    		•

    Selected Abstracts

    Parallelization of the quantum dynamics code for cluster architecture and its applications to the Gross,Pitaevskii equation

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2006
    Adam Gorecki
    Abstract We present a parallel version of a quantum dynamics (QD) algorithm in a wave function representation. The algorithm has been optimized for a Linux cluster as well as for Cray T3E, using a parallel version of a three-dimensional FFT library. We have applied this algorithm for solving the nonlinear Gross,Pitaevskii equation (GPE), which describes the evolution of Bose,Einstein condensates (BEC) in the mean field approximation. Our method reduces the computational time and allows the study of nonlinear quantum systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

    A new method to locate saddle points for reactions in solution by using the free-energy gradient method and the mean field approximation

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2004
    I. Fdez.
    Abstract A new method for calculating saddle points of reactions in solution is presented. The main characteristics of the method are: (1) the solute,solvent system is described by the averaged solvent electrostatic potential/molecular dynamics method (ASEP/MD). This is a quantum mechanics/molecular mechanics method (QM/MM) that makes use of the mean field approximation (MFA) and that permits one to simultaneously optimize the electronic structure and geometry of the solute molecule and the solvent structure around it. (2) The transition state is located by the joint use of the free-energy gradient method and the mean field approximation. An application to the study of the Menshutkin reaction between NH3 and CH3Cl in aqueous solution is discussed. The accuracy and usefulness of the proposed method is checked through comparison with other methods. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1227,1233, 2004 [source]

    A two-scale model for liquid-phase epitaxy

    MATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 1 2009
    Ch. Eck
    Abstract We study a model for liquid-phase epitaxy that is based on a continuum description of the transport processes in the liquid and a Burton,Cabrera,Frank (BCF) model for the growth of the solid by epitaxy. In order to develop a model that is capable to incorporate structures of a very small scale in the solid phase within a computation for a technically relevant macroscopic length scale, we apply homogenization methods. The result of the homogenization procedure is a two-scale model that consists of macroscopic equations for fluid flow and solute diffusion in the fluid volume, coupled to microscopic BCF models for the evolution of the microstructure in the solid phase. The obtained two-scale model is justified by an estimate for the model error that is valid under appropriate assumptions on the regularity of the solutions. This estimate is proved for a phase field approximation of the BCF model. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Basic aspects of geopotential field approximation from satellite-to-satellite tracking data

    MATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 11 2001
    W. Freeden
    Abstract The satellite-to-satellite tracking (SST) problems are characterized from mathematical point of view. Uniqueness results are formulated. Moreover, the basic relations are developed between (scalar) approximation of the earth's gravitational potential by ,scalar basis systems' and (vectorial) approximation of the gravitational field by ,vectorial basis systems'. Finally, the mathematical justification is given for approximating the external geopotential field by finite linear combinations of certain gradient fields (for example, gradient fields of multi-poles) consistent to a given set of SST data. Copyright © 2001 John Wiley & Sons, Ltd. [source]

    Magnetic impurity transition in a (d,+,s)-wave superconductor

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2010
    L. S. BorkowskiArticle first published online: 8 FEB 2010
    Abstract We consider the superconducting state of symmetry with finite concentration of Anderson impurities in the limit . The model consists of a BCS-like term in the Hamiltonian and the Anderson impurity treated in the self-consistent large- N mean field approximation. Increasing impurity concentration or lowering the ratio drives the system through a transition from a state with two sharp peaks at low energies and exponentially small density of states at the Fermi level to one with . This transition is discontinuous if the energy of the impurity resonance is the smallest energy scale in the problem. [source]

    Theory of the cubic-tetragonal phase transition in PbTiO3

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2006
    Es-saïd El-frikhe
    Abstract The phase transition in PbTiO3 is discussed quantitatively from the microscopic free energy based upon the mean field approximation, where the Ti ions shift is considered as the order parameter in the lattice having the spontaneous deformation. From our calculation of the microscopic free energy, the ionic shift of the Ti ion and the coefficient of the expanded free energy, we concluded that the transition temperature and the order of cubic-tetragonal transition are well explained by this model. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Magnetization and FMR studies of [Fe/Cr]n structures with ultrathin iron layers

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2006
    A. B. Drovosekov
    Abstract Magnetic multilayer structures [Fe(tFe)/Cr(tCr)]n with ultrathin (tFe < 5 Å) iron layers prepared by a molecular beam epitaxy were studied using SQUID-magnetometry and ferromagnetic resonance technique. For the samples with tFe , 3 Å, the obtained magnetization curves and resonance spectra are typical for regular magnetic superlattices. On the contrary, the samples with tFe , 3 Å demonstrate superparamagnetic-like behavior. These samples show a strong temperature dependence of their static and resonance magnetic properties. To explain the obtained results, we propose a theoretical model considering a cluster structure of thin iron layers. Magnetization curves and resonance spectra are calculated using an effective field approximation taking into account an interlayer interaction and cluster size distribution. The calculated dependencies show a qualitative agreement with the experimental data. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    X-ray atomic orbital analysis of 4f and 5d electron configuration of SmB6 at 100, 165, 230 and 298,K

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
    Shiro Funahashi
    Accurate electron-density measurement of SmB6 at 100, 165, 230 and 298,K, and X-ray atomic orbital (XAO) analysis were carried out. The 4f -electron density around Sm and 5d electron density at ,,1,Å from Sm were analysed by XAO analysis. The 5d electron density is due to the electrons of the 5dJ = 5/2,8 orbitals which stem from the eg orbitals in the strong field approximation. The change in electron populations of the 5d5/2,8 orbitals with temperature is similar to that of the resistivity. Since the conduction band consists of 5d5/2,8 and B-2p orbitals according to band theory, this indicates that the larger populations of the 5d5/2,8 orbitals correspond to the larger number of localized electrons and are correlated to the resistivity of SmB6. The occupation of the bulky 5d5/2,8 orbitals may be the reason for the elongation of the lattice parameter below 150,K. The 4f7/2,6 orbitals are obviously occupied except at 100,K, which seems to be caused by the energy gap between 4f5/2 and 4f7/2 states, which begins to exist between 100 and 150,K, and may represent one of the properties of a Kondo insulator. [source]

    Baryons as Fock states of 3,5, ... quarks ,

    ANNALEN DER PHYSIK, Issue 11-12 2004
    D. Diakonov
    Abstract We present a generating functional producing quark wave functions of all Fock states in the octet, decuplet and antidecuplet baryons in the mean field approximation, both in the rest and infinite momentum frames. In particular, for the usual octet and decuplet baryons we get the SU(6)-symmetric wave functions for their 3-quark component but with specific corrections from relativism and from additional quark-antiquark pairs. For the exotic antidecuplet baryons we obtain the 5-quark wave function. [source]

    Tuning the Magnetic Moment of [Ru2(DPhF)3(O2CMe)L]+ Complexes (DPhF=N,N, -Diphenylformamidinate): A Theoretical Explanation of the Axial Ligand Influence

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2010
    Carmen Barral Dr.
    Abstract The magnetic behaviour of the compounds containing the [Ru2(DPhF)3(O2CMe)]+ ion (DPhF,=N,N, -diphenylformamidinate) shows a strong dependence on the nature of the ligand bonded to the axial position. The new complexes [Ru2(DPhF)3(O2CMe)(OPMe3)][BF4],0.5,CH2Cl2 (1, 0.5,CH2Cl2) and [Ru2(DPhF)3(O2CMe)(4-pic)][BF4] (2) (4-pic=4-methylpyridine) clearly display this influence. Complex 1,0.5,CH2Cl2 shows a magnetic moment corresponding to a S=3/2 system affected by the common zero-field splitting (ZFS) and a weak antiferromagnetic interaction, whereas complex 2 displays an intermediate behaviour between S=3/2 and S=1/2 systems. The experimental data of complex 1 are fitted with a model that considers the ZFS effect using the Hamiltonian ,D=S,D,S. The weak antiferromagnetic coupling is introduced as a perturbation, using the molecular field approximation. DFT calculations demonstrate that, in the [Ru2(O2CMe)(DPhF)3(L)]+ complexes, the energy level of the metal,metal molecular orbitals is strongly dependent on the nature of the axial ligand (L). This study reveals that the increase in the ,-acceptor character of L leads to a greater split between the ,* and ,* HOMO orbitals. The influence of the axial ligand in the relative energy between the doublet and quartet states in this type of complexes was also analysed. This study was performed on the new complexes 1,0.5,CH2Cl2 and 2. The previously isolated [Ru2(DPhF)3(O2CMe)(OH2)][BF4],0.5,CH2Cl2 (3, 0.5,CH2Cl2) and [Ru2(DPhF)3(O2CMe)(CO)][BF4],CH2Cl2 (4,CH2Cl2) complexes were also included in this study as representative examples of spin-admixed and low-spin configurations, respectively. The [Ru2(DPhF)3(O2CMe)]+ (5) unit was used as a reference compound. These theoretical studies are in accordance with the different magnetic behaviour experimentally observed. [source]