Final PhD Oral Examination (Thesis Title: “Experimental and Theoretical Study of The Electronic Structure of Single-crystal BaBiO3”)

Abstract:
Hole doped bismuth perovskite is one of the rare examples of a three-dimensional high transition temperature superconducting oxide (Tc = 34K) without a transition metal cation. The undoped compound, BaBiO3, also shows closely interlinked electronic and structural phase transitions and a controversial insulating mechanism. Understanding the electronic structure of the parent compound, BaBiO3, can give valuable insight into both its superconducting mechanism, in particular, and into the physics of the perovskites family, in general.

In this work, we first grow high-quality single crystals of BaBiO3 by congruent melting technique and characterize the crystals with x-ray diffraction, x-ray photoemission, and transport properties measurements. We then investigate the electronic structure of the material from both theoretical and experimental perspectives. Experimentally, we study the crystals through x-ray absorption, x-ray emission, and photoelectron spectroscopies. X-ray spectroscopy varies the results of density functional theory (DFT) regarding the overall band structure featuring strong O 2p character of the empty anti-bonding combination of the hybridized Bi 6s and O 2p states. We also develop a routine to successfully clean the sample's surface in order to achieve the intrinsic O 1s x-ray photoemission spectrum.

Finally, we employ the "exact diagonalization single cluster configuration interaction" method to investigate the ground state electronic structure of the material and to understand the measured O 1s x-ray photoemission result. From the analysis of the core level line shapes, we conclude that the dominant O 2p-Bi 6s hybridization energy scale determines the low energy scale electronic structure. We also nd that for a wide range of electronic parameters, the holes reside primarily on the ligand oxygen anions rather than the bismuth cation consistent with the band structure calculation but contrary to what is traditionally assumed for this material. This analysis provides further insight into the importance of self-doped oxygen 2p states in this high Tc family of oxides.