UBC Theses and Dissertations
Peierls coupling in multi-orbital metal oxides Yam, Yau Chuen
In this thesis we study the effects of the Peierls electron-phonon coupling in multi-band metal oxides systems. In contrast to the more commonly employed Holstein coupling, which is used in single-band models and is momentum-independent, the momentum-dependent Peierls coupling can explicitly treat coupling to multiple bands. We demonstrate the importance of using the Peierls coupling in modelling complex systems by looking at two examples. First, we investigate single polaron physics on a perovskite lattice inspired by BaBiO₃. We find that with Peierls coupling, the ground state momentum of the polaron jumps between high-symmetry points in the Brillouin zone as the coupling strength is increased. Because such sharp transitions are not possible in the Holstein model, it follows that it is not always feasible to map the more complex Peierls model onto the simpler Holstein model. Then, we add the Peierls coupling to Emery's three-band model for cuprate layers and study its effect on the polaron effective mass. We show that although the hole-phonon coupling strength is moderate to strong, it only causes a negligible increase in the effective mass, indicating that the effective coupling to the magnon-dressed quasiparticle is much reduced by the dressing. We explain the reason for this and describe how to treat the difference between lattice coupling to bare holes versus to correlations-dressed quasiparticles. Our results prove that it is essential to choose the proper coupling when describing multi-orbital systems, in order to deduce their properties appropriately. We propose to investigate more systematically the generic nontrivial behaviour brought by the momentum-dependent Peierls coupling by studying a diatomic 1D chain, to reinforce our conclusion.
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