UC Berkeley

The origin of high-temperature superconductivity in cuprate ceramics is one of the great outstanding mysteries in physics. Discovered by Bednorz and Muller in 1986, the phenomenon has continued to both fascinate and baffle scientists for almost 30 years. At the heart of the cuprate problem lies the microscopic mechanism responsible for binding together Cooper pairs, which are a superconductor's lossless charge carriers. For most low-temperature superconductors, phonons are responsible, but Cooper pairs in the high- temperature superconductors may be bound together by magnetic interactions or even more exotic phenomena.

Although this most fundamental question remains unresolved, important insights continue to be gained in the field of high-temperature superconductivity, and many of them have originated from improved experimental methods and instrumentation. This thesis details progress in understanding the cuprate problem through the development and implementation of a high-resolution apparatus for conducting time- and angle-resolved photoemission spectroscopy (time-resolved ARPES). The technique works by using a near-infrared optical pump pulse to drive an electronic system out of equilibrium, and using an ultraviolet probe pulse to measure the subsequent dynamics of the nonequilibrium state using photoemission. Sub-picosecond time resolution is obtained by varying the path length of the pump pulse, and the net result is a movie of the relaxation dynamics of the band structure. Experimental results on the bi-layered cuprate superconductor Bi2212 are presented and discussed in detail.

Content is organized into eight chapters. Chapter 1 gives an introduction and overview of superconductivity research. Chapter 2 summarizes basic theoretical underpinnings of the ARPES technique, and reviews analysis techniques that are relevant to ARPES and time- resolved ARPES as well as simple models of quasiparticle recombination. Chapter 3 contains a detailed overview of the time-resolved ARPES system constructed in the Lanzara research group. Chapters 4, 5, 6, and 7 present scientific results obtained using time-resolved ARPES to study Bi2212. Finally, conclusions and future directions are discussed in Chapter 8.