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Thesis advisor: Hong Ding / High temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on electronic structures of single layer high temperature supconductor Bi2Sr2CuO6+[delta] by angle-resolved photoemission spectroscopy. A high binding energy band anomaly is observed in PbxBi2−xSr2CuO6+[delta]. Comparing with LDA calculation, the band is highly remonetized by a factor of 3, the incoherent part coexists and forms a band anomaly as a non-dispersive dive structure. Systematic studies are performed on Bi2+xSr2−xCuO6+[delta] with a wide doping range, revealing a linear doping nature of Bi substitution. An unusual Coulomb gap is observed in the heavily substituted samples. Our results reveal the dual role of off-plane chemical substitution in high-TC cuprates and elucidate the nature of the quantum electronic nature due to strong correlation and disorder. High resolution ARPES on LaxBi2Sr2−xCuO6+[delta] observes a large gap and a small gap coexisting at antinodal region below TC. The small gap is d-wave like and attributed to superconductivity, while the large gap is attributed to a CDW order. A strong short-ranged correlation between the small and large gap magnitude suggesting that superconductivity and charge ordering are driven by similar physical mechanism. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

This thesis is composed of three projects including the AC magnetic susceptibility study of high-temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$, the ionic-liquid gating study of the Mott insulator Sr$_2$IrO$_4$, and the single-electron study of quantum dot device with self-assembled nanocrystal PbS. Chapter 1 covers a general introduction to all three projects. The basic background and the motivation for each project are presented. Project I is covered in Chapter 2, Chapter 3, and Chapter 4. The first part of Chapter 2 is a theoretical introduction to the Bardeen-Cooper-Schrieffer theory of superconductivity with its main conclusions presented. This chapter builds a basis for the use of high pressure technique to YBa$_2$Cu$_3$O$_{7-\delta}$ in the later chapters. The rest of Chapter 2 reviews the work in the study of high-temperature superconductors, especially on YBa$_2$Cu$_3$O$_{7-\delta}$, on both experiments and theories and the possible applications of high-temperature superconductors. Chapter 3 introduces the YBa$_2$Cu$_3$O$_{7-\delta}$ sample preparation process and the characterisation. A dry cryomagnetic equipment was employed for the measurement. The results and the discussion are presented in Chapter 4. Project II is described in Chapter 5, Chapter 6, and Chapter 7. Chapter 5 firstly introduces the background knowledge of the gated material SrTiO$_3$ and the technical details of the ionic-liquid gating technique. Then the sample growth and the characterisation are presented. The fabrication process of Sr$_2$IrO$_4$ and SrTiO$_3$ (material for a control experiment) are described in Chapter 6. Chapter 7 covers the measurement and the result of the fabricated devices and related discussion. Project III ranges from Chapter 8, and Chapter 9. A literature review of quantum-dot devices and self-assembled nanocrystals is presented in Chapter 8. The experimental design of this nanocrystal quantum dot device is also included. Following it, the fabrication process of quantum-dot devices and the techniques used for fabrication are introduced in the start of Chapter 9. Chapter 9 also gives a description of the probe-station for measurements. The results and discussion of the measurements are covered in the last section of Chapter 9. Chapter 10 summarises and concludes the three projects stated above and gives some suggestions about the directions for future work.

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Since the discovery of cuprate superconductors, PrBa2Cu3O7 (Pr123) has attracted much attention due to its nonsuperconductivity and other anomalies. The very recent reports on superconducting Pr123 instead of putting an end of this issue, virtually stimulate more controversies. One of the proposed explanations for the recently observed superconductivity in Pr123 is that the samples could be Ba-rich Pr123 to investigate this possibility, we prepared Pr1-xBa2+xCu3O7 (x=1~0.3) and Pr1-xCaxBa2Cu3O7 (x=0~0.6) as comparison. X-ray absorption near edge spectroscopy (XANES) indicates an increase in carrier concentration with Ba and Ca doping, which is consistent with resistivity and thermoelectric power(TEP) data. However, carriers are introduced in a peculiar why rather than simply add into CuO2 plane.

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