Development and application of a massively parallel KKR Green function method for large scale systems

In this thesis we present the development of the self-consistent, full-potential Korringa-Kohn-Rostoker (KKR) Green function method KKRnano for calculating the electronic properties, magnetic interactions, and total energy including all electrons on the basis of the density functional theory (DFT) on high-end massively parallelized high-performance computers for supercells containing thousands of atoms without sacrifice of accuracy. In KKRnano the computation of large-scale systems becomes possible because the method's underlying multiple scattering of the electrons can be presented by sparse matrices, which is achieved by constructing the Green functions starting from screened reference systems. The key element in KKRnano's algorithm is the iterative solution of the sparse Dyson equation, which connects the reference and real system, in conjunction with elaborate algorithmic procedures to optimize its convergence. By exploiting this scheme, quadratic or in the absence of long-range interaction even linear scaling of KKRnano can be achieved. This advantageous scaling together with the locality of the numerical operations of the KKR method facilitates a consequent mapping on massively parallel supercomputers and leads to the ability to treat supercells with the size of up to ten thousand atoms on up to hundred thousand processors. The quantitative change of atoms treatable from first-principles translates into qualitative new physics as it becomes now feasible to investigate the properties of defects, alloys and the electronic properties of disordered systems in general. Thereby, the downscaling of electronic devices, where the role of single defects and defect cluster increase in relevance, starts matching the upscaling KKRnano. In this context KKRnano was used for the following two applications. The first application is centered in the field of dilute magnetic semiconductors. In this field a new promising material combination was identified: gadolinium doped gallium nitride which shows ferromagnetic ordering of colossal magnetic moments above room temperature. It quickly turned out that additional extrinsic defects are inducing the striking properties. However, the question which kind of extrinsic defects are present in experimental samples is still unresolved. In order to shed light on this open question, we perform extensive studies of the most promising candidates: interstitial nitrogen and oxygen, as well as gallium vacancies. By analyzing the pairwise magnetic coupling amongt defects it is shown that nitrogen and oxygen interstitials cannot support thermally stable ferromagnetic order. Gallium vacancies, on the other hand, facilitate an important coupling mechanism. The vacancies are found to induce large magnetic moments on all surrounding nitrogen sites, which then couple ferromagnetically both among themselves and with the gadolinium dopants. Based on a statistical evaluation it can be concluded that already small concentrations of gallium vacancies can lead to a distinct long-range ferromagnetic ordering. Beyond this important finding we present further indications, from which we infer that gallium vacancies likely cause the striking ferromagnetic coupling of colossal magnetic moments in GaN:Gd. The second application deals with the phase-change material germanium antimony tellurium (Ge1Sb2Te4), where we apply KKRnano to study the influence of disorder on the electronic states in this material. Our work is motivated by two recent experimental findings: Firstly, the observation of a pronounced localization of the electronic states in this material and, second, a striking dependency of this localization on annealing temperatures. We gain insight into both observed phenomena, which we extract from calculations in supercells of thousands of atoms. Our results lead to the general conclusion that all fundamental local properties can be well understood by taking into account the chemical configuration of few neighboring shells. This observation remains valid for the localization of the states, for which we emphasize the particular importance of the distribution and ordering of the vacancies: we reveal that in the vicinity of regions with high vacancy density the Ge-Te and Sb-Te bonding states are pushed into a pseudo band-gap and become thus highly localized. Moreover, we reveal a clear connection of vacancy complexes to electronic transport. By combining these findings with an analysis of the temperature effects by means of simulated annealing, we are able to link the temperature dependence of the localization and transport properties to the dissolution of vacancy complexes.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2011

Author dc:creator

• Thieß, Alexander R.

Contributors dc:contributor

• Blügel, Stefan

• info:eu-repo/classification/ddc/530
• Dichtefunktional
• KKR-Methode
• Supercomputer
• Festkörperphysik
• Verdünnte magnetische Legierung
• Phase-Change-Technologie
• Physik
• density functional theory
• Korring Kohn Rostoker Green functions
• supercomputing
• dilute magnetic semiconductors
• phase change materials

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124478%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:63009