Ab initio theory of polarons

Thesis

The polaron is a quasiparticle formed by collectively dressing an electron with a phonon cloud. It emerges in a crystal lattice when an extra electron is trapped at the lattice distortion induced by itself. Due to its unique role and technological implications, this subject has consistently received attention for many years. To date, first-principle studies of polarons in materials are still based on direct calculations with large simulation cells. In this thesis we develop a theoretical and computational framework that allows for the modelling of lattice polarons in different materials of interest, but using unit-cell calculations of electronic, vibrational properties and electron-phonon couplings. We explain the architecture of the implementation in detail, and propose a self-interaction corrected DFT scheme for the validation of our methodology and implementation. We present extensive tests and benchmark for the code. We apply this new approach to investigate polaron properties in LiF and Li₂O₂ as prototypical bulk materials that host large and small polarons. We demonstrate that our method can describe both large and small polarons on the same footing, and capture both Fr ̈ohlich-type polar electron-phonon coupling and non-Fr ̈ohlich couplings to acoustic and optical phonons. Furthermore, we report cal- culations of formation energy, excitation energy, and wavefunction of polarons. To establish a quantitative understanding of which KS states and phonon modes con- tribute to the formation of polarons, we analysis the polaron spectral decomposition in a spirit similar to the Eliashberg spectral function. The last part of the thesis is devoted to explore the dimensionality effect on the polaron properties in h-BN. In this case, we compare the polaron energies for a localised hole between the h-BN in bulk and in monolayer, and show that the two-dimensional hole polaron trapped in the h-BN monolayer exhibits a layer formation energy in comparison to the bulk one. This work will open the door to predictive calculations of polarons in more complicated systems, such as van der Waals heterostructure and interfacial systems, and constitutes as a first step toward complete ab initio many-body calculations of polarons in real materials.

Authors: Sio, WH

• DOI: 10.5287/ora-z6pkzwgjw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Electron-phonon coupling; Polarons; Electronic structure; First-principles calculations
• Subjects: Computational materials science; Condensed matter physics