Generalized inclusion of short-range ordering effects in the coherent potential approximation for complex-unit-cell materials

Abstract

The coherent potential approximation has historically allowed the efficient study of disorder effects over a variety of solid state systems. Its original formulation is however limited to a single-site or uncorrelated model of local substitutions. This neglects the effects of correlation and short range ordering, often found in realistic materials. Recent theoretical work has shown how to systematically address such shortcomings, for simple materials with only one element per unit cell. We briefly review the basic ideas of these developments within the framework of multiple scattering theory, and suggest their generalization to materials with complex lattices and possibly different types of disorder. We illustrate this extension with an example of local environment effects in the exotic Hapkeite F e − F e 37.5% Si 62.5% compound.

Improvement of magnetic hardness at finite temperatures:Ab initiodisordered local-moment approach forYCo5

“…Indeed we will see below some inherent underestimate near the Curie point in our calculations which might indicate some fragile nature of the local moments in YCo 5 . This problem can be cured by extending the calculation to that based on the non-local CPA 37,38 . For now we are mostly concerned with the lowest temperature properties and the middle-temperature range of T < ∼ 600 [K] which is well below the Curie temperature at T Curie = 920 [K] 26 .…”

“…Up to 600 [K] therefore the DLM theory as applied is adequate and we can examine the effects of transverse fluctuations on the MAE that it describes. For higher temperatures however, the effects of shortranged correlations among theê orientation (via the use of the non-local CPA for example 37,38 ) and longitudinal fluctuations 40 need to be addressed for a more complete picture. Comprehensive analyses on bcc-Fe, where the robustness of the local moments are established, can be found in a recent work 41 .…”

Short-range ordering effects on the electronic Bloch spectral function of real materials in the nonlocal coherent-potential approximation

“…Its goal is to improve the above picture with the inclusion of local environment effects. To accomplish the task, a larger-than-single-site portion of the sample, or "cavity" [8,10,26], is considered in direct space. Different forms of disorder and SRO can then be simply set up through assignment of N tot different probabilistic weight to all ways to populate such N c × N sub 1 lattice sites, at positions…”

“…In such a case it has first been shown, again in models [25] as well as full SCF-DFT KKR implementations of the scheme [26], how the corresponding effective medium leads to identical site-diagonal observables such as the DOS, in the limit of appropriate test cases. Simple examples for this testing have been a N sub = 2, N c = 1 CsCl-like unit cell, with both positions occupied either by Cu or Zn, that hence realize the same physics as a N sub = 1 bcc alloy studied for N c = 2; or similarly, a N sub = 4, N c = 1 Cu 3 Au-like unit cell, compared with a f cc N sub = 1 f cc alloy with N c = 4 setup.…”

Elastic anomalies and long/short-range ordering effects: A first-principles investigation of the AgcPd1−csolid solution

“…The goal of this development is the self-consistent construction of a richer effective medium description for the partially disordered system, where the condition of on-average no extra scattering from any portion of the bulk is generalized to multisite substitutions involving more than an atom at the time. 9,10 The corresponding scattering path operator that replaces Eq. (2) reads, in particular:…”