Imaging the breakdown of phonon quasiparticle theory in MgO and BaTiO3 with atomistic simulations
File(s)
Author(s)
Coiana, Gabriele
Type
Thesis or dissertation
Abstract
The anharmonic effects in crystals have been commonly studied in the assumption that the phonon-phonon interactions are weak. However, unlike strong electronic interactions, which have been studied for decades, strong phononic interactions have not gained the same attention.
In this thesis, I study phonon-phonon interactions that are so strong that the quasiparticle picture of the phonons breaks down, and I call this event the melting of phonons.
After performing molecular dynamics simulations, I calculate the distribution of kinetic energy among frequencies (w) and wavevectors (k), E(w, k), from the Fourier transform in space and time of the atomic velocity-velocity correlation function.
This method, which is known but has not been used extensively in the literature, is fully anharmonic and fully non-perturbative, and hence is possibly the only method that allows to compute phonon spectra at extreme conditions of temperature or for very anharmonic materials in any phase they might be, even liquid.
The melting of the phonon bands and its physical consequences are investigated in two oxides, magnesium oxide (MgO) and barium titanate (BaTiO3).
In MgO, the departure from a band structure is progressive, and the quasiparticle picture of the phonons holds until temperatures close to its melting point.
In BaTiO3, band melting happens suddenly in approaching the phase transition temperature. I show that one of the transverse optical (TO) modes of BaTiO3, the Slater mode, melts because its energy is widely spread out both in w-space and in k-space.
The broadening in w-space suggests the presence of a hopping motion of Ti-O atoms between wells of the potential energy surface.
The broadening of the Slater mode in k-space creates a flat band over an area in reciprocal space, and implies linear correlations of the atomic motions.
The results strongly support the conclusion that the phase transitions in BaTiO3 are of the order-disorder type.
In this thesis, I study phonon-phonon interactions that are so strong that the quasiparticle picture of the phonons breaks down, and I call this event the melting of phonons.
After performing molecular dynamics simulations, I calculate the distribution of kinetic energy among frequencies (w) and wavevectors (k), E(w, k), from the Fourier transform in space and time of the atomic velocity-velocity correlation function.
This method, which is known but has not been used extensively in the literature, is fully anharmonic and fully non-perturbative, and hence is possibly the only method that allows to compute phonon spectra at extreme conditions of temperature or for very anharmonic materials in any phase they might be, even liquid.
The melting of the phonon bands and its physical consequences are investigated in two oxides, magnesium oxide (MgO) and barium titanate (BaTiO3).
In MgO, the departure from a band structure is progressive, and the quasiparticle picture of the phonons holds until temperatures close to its melting point.
In BaTiO3, band melting happens suddenly in approaching the phase transition temperature. I show that one of the transverse optical (TO) modes of BaTiO3, the Slater mode, melts because its energy is widely spread out both in w-space and in k-space.
The broadening in w-space suggests the presence of a hopping motion of Ti-O atoms between wells of the potential energy surface.
The broadening of the Slater mode in k-space creates a flat band over an area in reciprocal space, and implies linear correlations of the atomic motions.
The results strongly support the conclusion that the phase transitions in BaTiO3 are of the order-disorder type.
Version
Open Access
Date Issued
2023-08
Date Awarded
2023-11
Copyright Statement
Creative Commons Attribution NonCommercial Licence
Advisor
Tangney, Paul
Lischner, Johannes
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)