Showing papers on "Coherent potential approximation published in 1984"

Chapter 2 The Optical Absorption Edge of a-Si: H

Abstract: Publisher Summary This chapter explores the effect of temperature, isochronal annealing, and preparation conditions on the optical absorption edge of a-Si:H x films The chapter describes the Tauc, Abe, and Toyazawa models The Tauc model defines the absorption edge of an amorphous semiconductor through the concept of a nondirect optical transition in which electron momentum is not conserved (total disorder) By utilizing the random phase approximation and parabolic bands, Tauc defined an optical gap E G that is widely used by experimentalists to define the position and the shape of the high-energy side of the absorption edge for a-Si:H x and other amorphous semiconductors The quantity E G is not a significant feature of the absorption edge but can be successfully correlated with the magnitude of the mobility gap deduced from transport measurements Abe and Toyazawa generalized the Tauc model to include a variable degree of Gaussian site disorder on the basis of the coherent potential approximation From this generalization, the Tauc optical gap E G becomes an additive function of structural and thermal disorder, in agreement with experiment

Theory of electronic structure

Abstract: Revue des differents modeles et methodes recemment developpes pour la comprehension des etats electroniques des systemes amorphes. Etude particuliere du silicium amorphe hydrogene

Electron propagation in a Markovian time-fluctuating medium: A dynamical generalization of the coherent-potential approximation

Abstract: We present a generalization of the coherent-potential approximation which allows one to describe the average propagation in a disordered medium in which the on-site potential fluctuates in time according to a Markov process. This method appears useful to discuss the dynamics of some manybody problems. By extending the Kubo treatment of the motional narrowing, the averages over histories are performed with use of projection techniques which rely on the augmented-space formalism. Both the binary noise and the Gauss-Markov noise are treated. The one-particle and two-particle average Green functions are derived in each case.

Towards a Systematic Description of Short Range Order: Diagonal Disorder in a Substitutional Alloy

Abstract: Typical methods used in the theory of substitutional disorder are single-site approximations like the coherent potential approximation, which cannot properly describe effects of short range order. We propose a new approximation for the configurationally averaged single particle Green's function, which should allow for a systematic description of such effects.

The effect of the excited impurity states in heavily doped semiconductors

Abstract: The role of the excited impurity orbitals in doped semiconductors is discussed. It is shown that they are crucial elements for the case of n-doped Si or Ge at intermediate concentrations near the metal-non-metal transition. This is due to the fact that, for this range of concentrations, the states with energies near the Fermi level have mainly an 'excited' character. The system is described by a tight-binding Hamiltonian with a basis composed by two orbital per site. Only the local electron repulsion is considered. The correlation is treated in the coherent potential approximation. To calculate the configurational averages a diagrammatic scheme is developed. It represents an extension of the Matsubara and Toyozawa method for the case of two hybridised bands in the presence of electronic correlation.

Self-consistent phonons in configurationally disordered quantum crystals. I. Theory

Abstract: A generalization of the coherent potential approximation (CPA) to strongly anharmonic systems is presented that takes into account the static relaxations in the vicinity of each defect and the changes in the averaged mean lattice constant and in the mean square fluctuations. In the limits of a perturbed classical, harmonic crystal or an ideal, anharmonic crystal the theory reduces to the conventional CPA and the self-consistent phonon results, respectively. For the numerical solution an approximation is used that starts from the renormalized harmonic phonons in ideal crystals. As an example, some characteristic results are discussed for the systems ofoD2 orpH2 in Ne.

The coherent potential approximation and the diagrammatic 1/Z expansion for an anisotropic spin-1 paramagnet

Abstract: The magnetic Green functions in the paramagnetic phase or a dilute of a binary alloy system are calculated for arbitrary concentrations of magnetic ions. The calculation employs a diagrammatic perturbation formalism based on the use of semi-invariants. Specifically, the theory is applied to S=1 systems in both the easy-axis and the easy-planar cases. The approximation made in the theory is basically to consider the ions as separate entities embedded in an effective medium, which is established in a self-consistent fashion. The calculated self-energy of the Green functions contains both the CPA contributions, due to the inhomogeneities, and the contributions from the fourth-order semi-invariants of the single spins appearing in order 1/Z in the high-density expansion. The behaviour of the effective single-site Green function is analysed in detail. The spurious divergence of this quantity in the order (1/Z)0, which leads to unphysical results in the dilute limit, is found to be much suppressed.

Polarons by the coherent potential method

Abstract: La methode du potentiel coherent dans la formulation du locateur est reformulee pour le calcul de la fonction de Green du polaron du reseau du a l'interaction locale d'un electron (exciton) avec des phonons de largeur de bande nulle. Le resultat est une bonne interpolation entre les limites resolubles. Consequences pour le spectre d'absorption des excitons dans les cristaux moleculaires

On the Feasibility of AB Initio Calculations of Ordering Alloy Phase Diagrams

Abstract: Many stable or metastable intermetallic phases useful to the alloy designer have crystal structures which are ordered superstructures of a parent disordered phase. A highly reliable statistical mechanical method (CVM) has now been developed for calculating such superstructure phase equilibria derived from say, the fcc parent lattice. To obtain phase diagrams, one needs certain physical parameters, such as effective pair interaction ratios. It is possible, in principle, to extract these parameters from band structure calculations in the coherent potential approximation (CPA), particularly from recently developed cluster-CPA techniques. If sufficient accuracy can be achieved, truly first-principles phase diagram calculations may soon become feasible.

Self-consistent phonons in configurationally disordered quantum crystals. II. Group-theoretic considerations

Abstract: In a foregoing publication a self-consistent phonon theory for configurationally disordered crystals was discussed by W. Biem and the author. In this article some simplifications concerning the coherent potential approximation (CPA) equations and the static relaxations are derived with the help of group-theoretic methods. Besides the familiar reduction of the relevant matrices to block-diagonal form, a very fast Fourier transformation between thek space and the space spanned by symmetry-adapted coordinates is discussed. This allows a solution of the CPA equations without using the real lattice.