Ever since high-temperature cuprate superconductors were discovered in 1986, they have been a source of many mysteries and controversies among scientists. The biggest mystery is the mechanism of high-temperature superconductivity. While it is known that electrons bind together in Cooper pairs, it is not known why. A second mystery is the origin of the pseudogap, a state that exists above the superconducting critical temperature. To address these problems, experimentalists have been looking for new techniques to provide new perspectives.
In this dissertation, we study cuprate superconductors using the recently-developed technique of time- and angle-resolved photoemission spectroscopy (TARPES). This technique uses infrared laser pulses to excite materials out of equilibrium, and to probe their electronic structure. TARPES has matured to the point that we understand the technique, but have not fully explored the scientific possibilities. This work seeks to push the boundaries by finding and characterizing unusual phenomena in the TARPES data, and developing frameworks to understand them. In the process, we address the central mysteries of cuprate superconductors.
In Chapter 1, we outline the theory of conventional superconductors, including many technical details of electron-boson coupling. We then discuss cuprate superconductors, establishing the basic properties of their electronic structure and related controversies. In Chapter 2, we briefly discuss our experimental technique and what it measures. We also review the literature of TARPES studies on cuprates. Chapters 3, 4, and 5 describe three different studies on cuprates, each looking at a different phenomenon. Chapter 3 looks at the chemical potential, and finds a connection to the pseudogap state. Chapter 4 looks at the buildup time of non-equilibrium quasiparticles, and finds a connection to the stimulated recombination of Cooper pairs. Chapter 5 looks at the signatures of electron-boson coupling, and proposes a way to determine if it contributes to Cooper pairing. In Chapter 6, we briefly summarize the main findings and offer some concluding remarks.