| Crynodeb: | Since its discovery in 1986, high-temperature superconductivity in copper oxides has puzzled researchers. Its intricate phase diagram, which unveils unusual physical properties, is particularly challenging to understand because of its strongly correlated nature. Despite numerous efforts, a consensus on the mechanism that forms electron-pair condensate underpinning the high-temperature superconductivity remains elusive. The research presented in this thesis endeavours to shed new light on the electron-pairing mechanism.
In this thesis, I use advanced scanning tunnelling microscopy (STM) techniques to study the mechanisms behind the high-temperature superconductor Bi₂Sr₂CaCu₂O₈₊ₓ (Bi-2212). Chapter 1 presents an introduction to conventional superconductivity which leads to a review of the cuprate superconductivity from both theory and experiment. Chapter 2 is devoted to introducing two novel STM techniques that are instrumental to the scientific findings presented in this thesis. In Chapter 3, I present the development of a next-generation STM, Gemini, that I built and operated during my DPhil. This home-built STM is designed to function at milli-kelvin temperatures with a 14 Tesla superconducting magnet. An in-depth examination of the design details and various testing results are presented. In Chapter 4, using the innovative STM techniques, an experimental discovery of an exotic quantum state in optimally doped Bi-2212 called the nematic pair-density wave (PDW) state is presented. Towards identifying the electron-pairing mechanism in Bi-2212, Chapter 5 first introduces a modern numerical technique called the dynamical mean-field theory (DMFT) that predicts the paring mechanism as the charge-transfer superexchange interaction. Then, this chapter presents an analogue isotope effect experiment that identifies the distance between the Cu atom and its apical O atom as the tuning parameter that alters the pairing strength. The anti-correlation relationship between the charge-transfer energy and the pairing amplitude is established whose slope conforms to the predictions of DMFT, which indicates that the charge-transfer superexchange interaction is key to the electron-pairing mechanism in optimally doped Bi-2212. Lastly, in Chapter 6, I present my recent STM experiments on candidate excitonic insulator 1T-TiSe₂. The results reveal directly the charge-transfer process between the Ti and Se atoms which is responsible for exciton formation. Furthermore, visualisation of the excitonic energy gap reveals a highly heterogeneous spatial pattern inconsistent with a conventional CDW but which indicates strong electron-electron interactions.
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