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Phase Approximation (phase + approximation)

Distribution by Scientific Domains
Physics and Astronomy71%
Chemistry28%

Kinds of Phase Approximation

  • random phase approximation
    		•

    Selected Abstracts

    Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

    ANNALEN DER PHYSIK, Issue 10-11 2009
    B. Vogt
    Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source]

    Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

    ANNALEN DER PHYSIK, Issue 10-11 2009
    B. Vogt
    Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source]

    Electron invariants and excited state structural analysis for electronic transitions within CIS, RPA, and TDDFT models

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2010
    A. V. Luzanov
    Abstract We revisit the interpretative scheme (Luzanov et al., Theor Exp Chem 1974, 10, 354) of singly excited configuration interaction (CIS) model given earlier at semiempirical level. Detailed computations and spectral (natural orbital) treatment of the CIS density matrices of various types are presented. The corresponding hole-particle densities and related excitation localization indices are described. All the quantities are extended to the excited states calculated in the random phase approximation and closely related time-dependent density functional theory (TDDFT). The localization indices and charge transfer numbers which are invoked to describe interfragment interactions provide a basis for our scheme which is referred to as the excited state structural analysis for electronic transitions. The proposed analysis is exemplified by various moderate and large-size conjugated molecules treated within ab initio TDDFT and the Parizer,Parr,Pople approximation. Finally, we propose a possible generalization to the electronic transitions between CIS-like states followed by applications to singlet organic biradicals treated within the ,-electron spin-flip CIS. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

    Ab initio study of spinodal decomposition in (Zn, Cr)Te

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2006
    T. Fukushima
    Abstract The spinodal decomposition in (Zn, Cr)Te is simulated by using first principles calculations and Monte Carlo simulation. It is found that the chemical pair interaction between Cr atoms in (Zn, Cr)Te is attractive interaction and leads to spinodal decomposition. Curie temperatures in decomposed situation are estimated by the random phase approximation with taking the magnetic percolation effect into account. This decomposed phase makes the random pattern of high concentration regions which connect each other and have possibility to realize high Curie temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Thermodynamic properties of the Ising and Heisenberg S = 1 ferromagnet with biquadratic exchange and uniaxial anisotropy

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2006
    M. Manojlovi
    Abstract We study the thermodynamic properties of S = 1 Ising and Heisenberg ferromagnets with both bilinear and biquadratic exchange and/or uniaxial anisotropy of both easy axis/plane character. Using the mean-field (MF) approximation we evaluated the free energy enabling us to study in detail the behavior of the order parameters and the dependence of the critical point on the anisotropy. We show that in the presence of biquadratic interaction, there is a difference in the behavior between the Ising and the Heisenberg model even in the MF approximation, which is not the case for the bilinear interaction. Combining the equations of motion for Green's functions with identities particular to S = 1, we managed to perform the random phase approximation without the decoupling of the operators at the same site, avoiding the peculiarities of the Callen,Anderson decoupling. This allowed us to improve the phase diagram for the Heisenberg model. The important result is the demonstration of the effect of anisotropy to nonvanishing of the quadrupolar order parameter at the Curie point. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Light scattering in an electron-hole double quantum well in the presence of spin-orbit interaction

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2007
    C. H. Yang
    Abstract In this work, we study theoretically the lineshape and intensity of light scattering in an electron-hole double quantum well structure in the presence of spin-orbit interaction (SOI). Using the random phase approximation and Green function approach, the dispersion and the excitation spectra in this structure have been calculated. It is shown that scattering intensity can be tuned by the spin-orbit (SO) couple via the applied electrical field. The peaks shift as the polarization of electron or hole increases. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Theoretical CD spectrum calculations of the crown-ether aralkyl-ammonium salt complex,

    CHIRALITY, Issue 5 2002
    Armand Lázár
    Abstract Rotatory strengths of the ,-(1-naphtyl)-ethylammonium perchlorate (NEA)-phenazino-18-crown-6 ether molecular complex is determined theoretically by the coupled oscillator model and using ab initio random phase approximation (RPA) to describe local excitations on the chromophores. The computational results are compared to the experimental circular dichroism (CD) spectrum published previously. The good qualitative agreement between calculated and measured optical rotatory strengths allows one to assign the CD bands of the complex in a unique manner. Chirality 14:377,385, 2002. © 2002 Wiley-Liss, Inc. [source]