Unit:
Κατεύθυνση Πυρηνική Φυσική και Φυσική Στοιχειωδών ΣωματιδίωνLibrary of the School of Science
Supervisors info:
Φοίβος Μαυρόπουλος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Original Title:
Comparison of models of disorder in Magnetism and Superconductivity
Translated title:
Comparison of models of disorder in Magnetism and Superconductivity
Summary:
In the present thesis, a comparison of methods for calculating the magnetization and magnetic exchange interactions in alloys of the form AxB1-x, and the order parameter in enriched superconductors is presented within the framework of Density Functional Theory (DFT).
In the first part, we start by comparing two computational techniques for their response to disordered system: the Special Quasi-random Structures (SQS) method and the Coherent Potential Approximation (CPA) method. The SQS technique employs a super-cell approach to create a pseudo-random arrangement that accurately represents the alloy’s atomic concentration x and correlation functions. In contrast, the CPA method simplifies the complexity of disordered systems by replacing the weighted average of the Green's functions of the complex system with an effective medium Green's function, which on average captures the effects of the disorder. Our study centers on an alloy of Fe0.5Co0.5. By utilizing the Korringa-Kohn-Rostoker (KKR) Green's function method and DFT, we calculated quantities such as spin magnetic moments and exchange constants through a self-consistent cycle.
The second part of our research explores the superconducting properties in the case of Nb. Here, for the Nb bulk, we examine key properties like the order parameter of superconductivity, essential for the transition to the superconducting phase, also known as anomalous density, and identify the superconducting gap appearing during the superconducting phase transition, also in the context of KKR and DFT methods, but extended to incorporate the Bogoliubov-de Gennes equations. We concentrate on illustrating how these properties vary with the interaction parameter λ that controls the electron pairing. Finally, we construct a super-cell, entirely consisting of Nb atoms, and selectively eliminate superconducting interactions in certain atomic positions in the configuration in order to simulate a random alloy enriched with a non-superconducting metal.
Main subject category:
Science
Keywords:
disorder, magnetism, superconductivity, KKR, CPA, SQS, DFT, Green's functions, spin magnetic moment, coupling constants, anomalous density, superconducting gap
