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1993 (5)
1Author    Michael SpringborgRequires cookie*
 Title    A Comparison between Polyatecylene and Polycarbonitrile  
 Abstract    Results of a theoretical, comparative study of the electronic properties of trans-polyacetylene and polycarbonitrile are reported. Polyacetylene consists of zigzag chains of CH units, whereas poly-carbonitrile has every second CH unit replaced by an N atom. Ground-state properties (structure, electronic bonds and bands, densities of states, momentum distributions, and reciprocal form factors) of the periodic, infinite, isolated chains are studied by means of first-principles, density-functional calculations. It is demonstrated how the presence of the (nitrogen) heteroatoms in the backbone of polycarbonitrile leads to a partial localization of the electrons. In order to investigate charged chains, model calculations are subsequently performed. These indicate solitons but not polarons to be stable. In total, the analysis demonstrates how the combination of information that can be obtained from various experiments provides a detailed description of the compounds. 
  Reference    Z. Naturforsch. 48a, 159—164 (1993); received November 26 1991 
  Published    1993 
  Keywords    Conjugated polymers, Structure, Electronic structure, Momentum densities, Solitons 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0159.pdf 
 Identifier    ZNA-1993-48a-0159 
 Volume    48 
2Author    JohnE. HarrimanRequires cookie*
 Title    Electron Densities, Momentum Densities, and Density Matrices  
 Abstract    Relationships among electron coordinate-space and momentum densities and the one-electron charge density matrix or Wigner function are examined. A knowledge of either or both densities places constraints on possible density matrices. Questions are approached in the context of a finite-basis-set model problem in which density matrices are elements in a Euclidean vector space of Hermitian operators or matrices, and densities are elements of other vector spaces. The maps (called "collapse") of the operator space to the density spaces define a decomposition of the operator space into orthogonal subspaces. The component of a density matrix in a given subspace is deter-mined by one density, both densities, or neither. Linear dependencies among products of basis functions play a fundamental role. Algorithms are discussed for finding the subspaces and construct-ing an orthonormal set of functions spanning the same space as a linearly dependent set. Examples are presented and additional investigations suggested. 
  Reference    Z. Naturforsch. 48a, 203—210 (1993); received October 8 1991 
  Published    1993 
  Keywords    Electron density, Momentum density, Density matrix 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0203.pdf 
 Identifier    ZNA-1993-48a-0203 
 Volume    48 
3Author    Teiji Kobayasi, Hisashi NaraRequires cookie*
 Title    Core-Orthogonalization Effect in Pseudopotential Theory of the Charge Density Distribution of Valence Electrons in Semiconductors with Comments on the Effects in Momentum Space  
 Abstract    A pseudopotential calculation has been performed for the charge density distribution in Si and Ge, with the core-orthogonalization terms fully included. The state of a positron subject to the Coulomb field of the resultant electron distribution in Si is also calculated. A possible improvement of the calculated distributions of electrons and positrons in momentum space is discussed. 
  Reference    Z. Naturforsch. 48a, 193—197 (1993); received January 3 1992 
  Published    1993 
  Keywords    Charge density, Momentum density, Pseudopotential theory, Core orthogonalization, Semiconductors 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0193.pdf 
 Identifier    ZNA-1993-48a-0193 
 Volume    48 
4Author    Jean-Louis CalaisRequires cookie*
 Title    Momentum Distribution and Charge Density in Solid-State Theory  
 Abstract    We present a discussion of a number of conceptual and methodological aspects associated with the theoretical characterization and computation of charge densities and momentum distributions in solids. The main ambition has been to stress properties that both exact and approximate quantities must possess. We have also attempted to point out conceptual and computational trends which would seem to be of importance for the future of the subject. 
  Reference    Z. Naturforsch. 48a, 151—158 (1993); received May 14 1992 
  Published    1993 
  Keywords    Solid-state theory, Charge density, Momentum density, Density matrices, Hartree-Fock approximation, Density functional theory 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0151.pdf 
 Identifier    ZNA-1993-48a-0151 
 Volume    48 
5Author    A. BansilRequires cookie*
 Title    Modern Band Theory of Disordered Alloys: Basic Concepts Including a Discussion of Momentum Densities  
 Abstract    An overview of some of the basic concepts in the modern first-principles band theory of disordered alloys is given. The question of how the notion of Bloch energy bands and Fermi surfaces generalizes to the case of the disordered system, insofar as the average electronic spectrum is concerned, is stressed. The theory is illustrated with examples chosen from the work on binary alloys; a few examples of the very recent studies of disordered phases of the high-T c superconductors are also included. The application of the alloy theory to obtain electron and electron-positron momentum densities involved in the analysis of Compton scattering and positron annihilation (angular correla-tion) experiments in alloys is discussed, with a selection of relevant theoretical and experimental studies. 
  Reference    Z. Naturforsch. 48a, 165—179 (1993); received January 25 1992 
  Published    1993 
  Keywords    Disordered alloys, KKR-CPA, Momentum densities, Compton scattering, Positron annihilation (ACAR) 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0165.pdf 
 Identifier    ZNA-1993-48a-0165 
 Volume    48