 1  Author
 Michael Springborg  Requires cookie*   Title
 A Comparison between Polyatecylene and Polycarbonitrile    Abstract
 Results of a theoretical, comparative study of the electronic properties of transpolyacetylene and polycarbonitrile are reported. Polyacetylene consists of zigzag chains of CH units, whereas polycarbonitrile has every second CH unit replaced by an N atom. Groundstate 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 firstprinciples, densityfunctional 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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2  Author
 JohnE. Harriman  Requires cookie*   Title
 Electron Densities, Momentum Densities, and Density Matrices    Abstract
 Relationships among electron coordinatespace and momentum densities and the oneelectron 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 finitebasisset 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 determined 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 constructing 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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5  Author
 A. Bansil  Requires 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 firstprinciples 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 highT c superconductors are also included. The application of the alloy theory to obtain electron and electronpositron momentum densities involved in the analysis of Compton scattering and positron annihilation (angular correlation) 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, KKRCPA, Momentum densities, Compton scattering, Positron annihilation (ACAR)   
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