Suppression of charge density wave in BaBiO3 thin films via controlling thickness and oxygen stoichiometry

Abstract

The charge disproportionated charge density wave (CDW) in BaBiO3 is the periodical modulation of lattice and charge-density due to the valence skipping behavior in Bi ion. The insulating ground state and the pseudogap in the hole-doped bismuth oxide superconductors have been explained by this type of charge-density instability. Recently, in high Tc copper oxide superconductors, the presence of CDW in the ground state and competition between CDW and superconductivity attracted much attentions as a part of an effort to figure out the superconducting mechanism at high temperature. Also in bismuthate superconductors, it has been assumed that the pairing mechanism of the cooper pair is deeply related to the CDW ground state. Therefore, studying the nature of CDW in BaBiO3 could be an excellent way to improve the understandings of high temperature superconductivity. In this thesis, several parameters are tuned to manipulate the CDW, namely film thickness and oxygen stoichiometry, using pulsed laser deposition, spectroscopic ellipsometry, Raman spectroscopy, and x-ray diffraction. We found out that the charge-density instability is suppressed by reducing film thickness under critical thickness and generating oxygen vacancies. Furthermore, the oxygen vacancies are known to introduce extra electrons to the system, therefore the electronic structure of electron-doped BaBiO3, which was predicted to be a topological insulator, is discussed with the help of optical spectrum.