Theses Doctoral

Phonon Quasiparticle Studies of Anharmonic Properties of Solids

Zhang, Zhen

At the high-temperature conditions of the Earth's interior, lattice anharmonic effects in crystalline mineral phases can become pronounced. Anharmonicity, i.e., deviations of vibrations from harmonic oscillations, is caused by phonon-phonon interactions. Knowledge of lattice anharmonicity is essential to elucidate distinctive thermal properties in solids.

Yet, accurate investigations of anharmonicity encounter difficulties owing to cumbersome computations. Here we present anharmonic property calculations with the phonon quasiparticle approach for various solids. The phonon quasiparticle approach efficiently and reliably addresses lattice anharmonicity by combining molecular dynamics and lattice dynamics calculations. It characterizes anharmonic phonons by extracting renormalized frequency and phonon lifetime from the mode-projected velocity autocorrelation function without explicitly computing higher-order interatomic force constants. In principle, it accounts for full anharmonic effects and overcomes finite-size effects typical of molecular dynamics. The validity and effectiveness of the current approach are demonstrated in computations of temperature-induced frequency shifts, anharmonic thermodynamics, phase boundaries, and lattice thermal conductivities of both weakly and strongly anharmonic, both insulating and metallic, and both simple and complex systems.

These materials include a simple model crystal, Si with diamond structure, minerals of geophysical significance, MgSiO₃ perovskite and postperovskite, cubic CaSiO₃ perovskite, and B8 and B2 phases of FeO. Accurate anharmonic thermodynamic properties, phase boundaries, and lattice thermal conductivities presented in this thesis are important for geodynamic modeling. The theoretical framework validated in this thesis also enables predictive studies of various anharmonic materials which could not be previously addressed by conventional approaches, such as quasiharmonic approximation for thermodynamics calculations and finite displacement method for anharmonic lattice dynamics calculations.


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More About This Work

Academic Units
Applied Physics and Applied Mathematics
Thesis Advisors
Wentzcovitch, Renata M.
Ph.D., Columbia University
Published Here
March 29, 2023