Coherent potential approximation

About: Coherent potential approximation is a research topic. Over the lifetime, 1930 publications have been published within this topic receiving 36805 citations.

On the lattice dynamics of disordered surfaces

Abstract: The local frequency distribution functions of the surface layers of a semi-infinite simple cubic crystal with randomly adsorbed impurities have been calculated for the (001) surface. A method based on the coherent potential approximation was used. The results are displayed for various concentrations and masses of adsorbed impurities.

Local approach to the one-band hubbard-model - extension of the coherent-potential approximation

Abstract: We present a simple scheme to extend the earlier single-site coherent-potential-approximation (CPA) theories by utilizing connections of the CPA solution with the exact solution of the Falicov-Kimball model in infinite dimensions, and with the single-impurity Anderson-type models. We study the local spectral density of the model at n =1; in the metallic regime, this exhibits a narrow Abrikosov-Suhl resonance and satellite peaks, which correspond, respectively, to the quasiparticle and Hubbard subband structures. This collective resonance disappears in the split-band (insulating) regime, where the CPA is found to be a good approximation. Comparisons are made with numerical works on finite-sized twodimensional lattices, and good agreement is obtained. The physics of strongly correlated electrons in solids still contains many open questions. In this context, the limit of infinite dimensionality considered by several authors provided great insights. In this limit, due to the freezing of spatial fluctuations, a certain self-consistent mean-field theory becomes exact. ' In this limit, the action becomes purely local, and the bare Green's function of the local dynamics contains information about all other sites which have been integrated out. The momentum conservation is actually trivial in infinite dimensions, and only frequency conservation needs to be ensured in the skeleton expansion. The essential simplification in this limit is that the single-particle (sp) properties can be understood by looking at a single inixed lattice site. This has enabled workers to make an exact mapping of this model to a single-impurity Anderson model with a selfconsistency condition. The earlier coherent-potential-approximation (CPA)like theories reproduce the Hubbard subband structures correctly as U is raised, but are expected to do a bad job in the low-energy region, especially in the metallic state. Indeed, in the CPA, electronlike quasiparticles are unstable for all finite U; this is somewhat counterintuitive, since for small U, we expect a Fermi liquid with Z (1. The drawback of the CPA is that it neglects the propagation of the opposite spin species; it freezes the down-spin configuration while looking at the propagation of an up spin. Strictly speaking, this is not true; it is a good approximation only in the split-band regime, where the local spin-fluctuation time z,f «h /6, the mean hopping time, but is not good at longer time scales. Here, 6 is the bandwidth of noninteracting electrons in our model. In the language of the alloy analogy, the CPA replaces dynamical, annealed disorder by quenched, static disorder. In this paper, we try to rectify the above-mentioned deficiency of the CPA. We make use of our earlier results: (1) the CPA solution for the Hubbard model is identical to the exact solution of the Falicov-Kimball model in infinite dimensions, (2) results identical to (1) above are obtained by looking at the local propagator of a single d impurity hybridized with a conduction electron sea. These connections enable us to look at the local dynamics of a O.-spin electron while the opposite spin species is also allo~ed to hop. In particular, we focus on the impurity density of states (DOS), which is just the full interacting density of states for the lattice, as is required by self-consistency. We study the local spectral density p(co) as a function of the ratio U/b for the particle-hole symmetric case of n =1. We compare our results with those obtained by Tan, Li, and Callaway using Lanczos numerical technique for finite-sized 3 X 3 [twodimensional (2D)] lattices. Our calculation reproduces all the essential features seen in their numerical work; a central "Kondo" peak at small and intermediate U with some weight transferred to the Hubbard subbands, evolving smoothly into the split-band picture in the large-U limit. The CPA results are obtained as the first truncation of our self-consistent scheme, and provide a good agreement with the numerical work in the large-U limit, as expected. The paper is organized as follows. Section II deals with the actual calculation of the DOS, and Sec. III is devoted to a short discussion of our results.

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd 1− x

Abstract: The Knight shift of Pd in Ag x Pd1−x has been determined for concentrationsx≦0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag x Pd1−x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.