High Temperature Electron-Phonon and Magnon-Phonon Interactions

Citation

Yang, Fred Chae-Reem (2019) High Temperature Electron-Phonon and Magnon-Phonon Interactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1KP5-CJ98. https://resolver.caltech.edu/CaltechTHESIS:03272019-174351662

Abstract

Computational materials discovery and design has emerged in order to meet the surge in demand for new materials for applications ranging from clean alternative energy to human welfare. This acceleration of materials discovery is exhilarating, but the applications of new advanced materials can be limited by their thermodynamic stability. Accurate calculations of the Gibbs free energy, a measure of thermodynamic stability, require a deep understanding of atomic vibrations, a main source of entropy in materials. This deep understanding of atomic vibrations requires us to treat phonons (quantized lattice vibrations) beyond the harmonic model by considering their interactions with various excitations. In this thesis, I present the effects of high temperature interactions of phonons with electrons and magnetic excitations on the thermodynamics of FeTi, vanadium, and Pd₃Fe.

A combination of ab initio calculations, inelastic neutron scattering (INS), and nuclear resonant inelastic x-ray scattering (NRIXS) showed an anomalous thermal softening of the M₅⁻ phonon mode in B2-ordered FeTi and a thermal stiffening of the longitudinal acoustic N phonon mode in body-centered-cubic vanadium. Computational investigations involving electronic band unfolding were performed to identify the nesting features on Fermi surfaces crucial to high temperature electron-phonon interactions in FeTi and vanadium. These investigations showed that the Fermi surface of FeTi undergoes a novel thermally driven electronic topological transition (ETT), in which new features of the Fermi surface arise at elevated temperatures. This ETT was also observed in vanadium, but the effects were overtaken by the thermal smearing of the Fermi surface that decreased the rate of electron-phonon scattering.

Iron phonon partial densities of states of Pd₃Fe were measured with NRIXS from room temperature through the Curie transition at 500 K. The experimental results were compared to ab initio spin-polarized calculations that modeled the finite-temperature thermodynamic properties of Pd₃Fe with magnetic special quasirandom structures (SQSs) of magnetic moments. The scattering measurements and first-principles calculations showed that the iron partial vibrational entropy is close to what is predicted by the quasiharmonic approximation owing to a cancellation of effects: phonon-phonon and magnon-phonon interactions approximately cancel a ferromagnetic optical phonon stiffening.

Thesis Files