Elements: Cyberinfrastructure for spin and charge transport calculation of partially disordered alloys

Metallic alloys are ubiquitous in high technology, in industry, and even in our households. Alloys, which form when two or more chemical species are combined to create a single metallic phase, offer the chance to improve upon the properties of pure metallic elements. Some of the properties that can be altered through alloying include mechanical strength, magnetism, melting temperature, and oxidation resistance.

This project focuses on the property of electrical conductivity, by developing computer codes to evaluate the quantum mechanical scattering of electrons off of atoms. To accurately predict the scattering we must know where the atoms are located, and in an alloy that means understanding how the different chemical species arrange in space. Often these arrangements are random, but even if the elements are randomly distributed, there will be correlations in the positions of certain species relative to others as a result of chemical bonding preferences.

The code will contain features that enable it to predict these correlations and reveal how the correlations influence the conductivity. In addition to developing computer codes, this project will develop a user base of scientists interested and able to run the code and to contribute to its further development. Outreach to high school students and their teachers will enhance the pipeline of prospective scientists.

Inclusion of scattering theory in college and graduate level courses taught by the PIs will prepare Physics and Materials Science students to understand and apply the codes. Presentations at scientific society conferences will inform the existing community of the capabilities, while workshops and webinars and webinars will provide specific training for active users.

Electronic density functional theory (DFT) has flourished as a practical tool for calculating energies, forces and electronic structure; its use is now widespread both in basic science and in engineering. Charge and spin transport calculation is a capability that has not yet reached the broader user community, partly because codes that incorporate these effects are not widely available and partly because these properties are highly sensitive to the degree of crystalline order.

Basic knowledge of the degree of order is often lacking, as it can be temperature dependent, and thermal effects are not captured by most DFT codes. To address this need, a code will be developed that is easy to use (capable of running on a desktop computer), that can predict the degree of chemical order or disorder as a function of temperature, and that can calculate the resulting charge and spin conductivities.

This will be achieved by building upon innovations in electronic structure calculation, coupled with methods of statistical mechanics to address thermal disorder. Specifically, we will modify the Coherent Potential Approximation (CPA) to incorporate the effects of short range order by unifying the resulting total energies with the Cluster Variation Method (CVM) to predict temperature dependent disorder.

The modified CPA will express the total energy as a function of interatomic correlation functions, while the CVM will express the entropy in the same terms, allowing the determination of correlations that balance the energy against the entropy. This approach to density functional theory employs multiple scattering as implemented in the public domain code MuST.

This method determines the electronic Green’s functions, and consequently it integrates naturally with the Kubo and Greenwood formulas for charge and spin conductivity. This internally consistent combination of approximations will achieve both high accuracy and high performance.

This project is funded by the Office of Advanced Cyberinfrastructure in the Directorate for Computer and Information Science and Engineering, with the Division of Materials Research in the Directorate for Mathematical and Physical Sciences also contributing funds.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.