Augmented Space Recursion Method for the Calculation of Electronic Structure of Random Alloys

Abstract

Tailoring materials with desirable set of properties for specific applications is one of the important aspects in the present day materials research. Atomistic simulations of physical properties of materials from first principles have become matured with the advent of powerful computers and can be exploited to predict the behaviour of materials without actually synthesizing them. Among the various materials the disordered alloys are used in a variety of applications. The design of alloys to meet specific engineering requirement demands at the microscopic level the first principles electronic structure calculations of these systems. In contrast to the ordered solids, the calculation of most physical properties of disordered alloys require configurational averaging over all realizations of the random variable characterizing the disorder. Among the various methods proposed for this configuration averaging, the most successful and popular method is the coherent potential approximation (CPA). The CPA coupled with the first principles band structure methods in the framework of LDA provides a starting point for the ab-initio electronic structure calculation of disordered alloys¹. Though CPA provides reliable results and useful understanding of various alloy properties but there are many physical situations where the single site approximation inherent in CPA begins to fail. Examples include alloys where clustering become important (in impurity bands of the split band alloys like Zn band in Cu rich CuZn alloys), alloys with short range order (SRO) where the occupation of a particular site is not independent but dictated by its local environment, alloys where there is local lattice distortion because of size mismatch of the constituents leading to off-diagonal disorder. All these phenomena demands a theory which can take us beyond single site approximation.