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UBC Theses and Dissertations

A first principle study of the electronic structure of the bismuthates Khazraie Zamanpour, Arash


Motivated by the recently renewed interest in the High Tc superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds ABiO₃ (A = Sr, Ba) using ab initio methods and tight-binding modelling. We use the density functional theory in the local density approximation to understand the role of various contributions in shaping the ABiO₃ band structure. It is established that hybridization involving Bi-6s and O-2p orbitals plays the most important role. The opening of a gap with the onset of the breathing distortion is associated with condensation of holes onto a₁g-symmetric molecular orbitals formed by the O-2pσ orbitals on the collapsed BiO₆ octahedra. The primary importance of oxygen p states is thus revealed, in contrast to a popular picture of a purely ionic Bi³⁺/Bi⁵⁺ charge-disproportionation. A single band model involving an extended molecular orbital of both Bi-6s and a linear combination of six O-2p orbitals is derived which provides a good description of the low energy scale bands straddling the chemical potential. In addition, a parameter-based phase diagram associated with materials incorporating “skipped valence” ions is developed. A crossover from a bond disproportionated (BD) to a charge-disproportionated (CD) system in addition to the presence of a new metallic phase is observed. We argue that three parameters determine the underlying physics of the BD-CD crossover when electron correlation effects are small: the hybridization between O-2pσ and s orbitals of the B cation in ABO₃, their charge-transfer energy (∆), and the width of the oxygen sub-lattice band (W ). In the BD system, we estimate an effective attractive interaction U between holes on the same O-a₁g molecular orbital. Later, we show the possibility of surface electron doping of the bismuthates via adatom. Finally, we propose a new class of materials, namely heterostructure composed of LaLuO₃ and SrBiO₃, that can host coexisting electron and hole gases and potentially high-temperature superconductivity at their two opposite interfaces. We argue that electronic reconstruction is the dominant mechanism for solving the diverging potential. The electronic structure of this system suggests the electron-hole gas interactions can be tuned with the potential of obtaining excitonic insulating phases.

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