An augmented space approach to the study of phonons in disordered alloys : comparison between the itinerant coherent-potential approximation and the augmented space recursion

TL;DR

This paper compares two advanced theoretical formalisms, ASR and ICPA, combined with density functional perturbation theory, to study phonons in disordered alloys, highlighting their improvements over earlier models and applying them to FePd alloys.

Contribution

It introduces and compares DFPT-augmented space recursion and DFPT-ITINERANT CPA for lattice dynamics in disordered alloys, including non-local correlations.

Findings

Both formalisms effectively incorporate disorder fluctuations.

The methods provide insights into the interplay of force constants in alloys.

Application to FePd demonstrates practical utility.

Abstract

A first principles density functional based linear response theory (the so called Density Functional Perturbation theory \cite{dfpt}) has been combined separately with two recently developed formalism for a systematic study of the lattice dynamics in disordered binary alloys. The two formalisms are the Augmented space recursion (ASR) and the Itinerant coherent potential approximation (ICPA). The two different theories (DFPT-ASR and DFPT-ICPA) systematically provides a hierarchy of improvements upon the earlier single site based theories (like CPA etc.) and includes non-local correlations in the disorder configurations. The formalisms explicitly take into account fluctuations in masses, force constants and scattering lengths. The combination of DFPT with these formulation helps in understanding the actual interplay of force constants in alloys. We illustrate the methods by applying to a fcc Fe$_{50}$Pd$_{50}$ alloy.