Author: Daniel Steven Wulin

Publisher:

Published: 2012

Total Pages: 264

ISBN-13: 9781267835550

Central to the field of high temperature superconductivity is understanding the pseudogap phase. This phase is associated with an anomalous normal state excitation gap. In BCS theory which describes classical superconductors, an excitation gap is only found at and below the transition temperature Tc. A central debate in the field is whether this normal state excitation gap in the cuprates is related to superconductivity or whether it reflects a new order parameter. The work in this thesis subscribes to the first of these alternatives, but proceeds in a general way to look at the implications of a small number of assumptions. The challenge is to elucidate the origin of the pseudogap by (i) addressing a large variety of different experiments (including both transport and scattering) and (ii) by studying the interrelated behavior above and below Tc. The challenges that are associated with understanding the pseudogap phase of high-Tc cuprates become particularly evident in the context of transport experiments. Constraints such as gauge invariance and related sum rules must be enforced and these can be seen to aid in the construction of a minimalistic theory, albeit adding in some ways to the complexity. This thesis starts from the point of view that only a small number of physical assumptions are needed in order to construct a theory of scattering and transport in the pseudogap phase and that its implications are of a general nature. Later chapters of the thesis show that the minimalistic theory belongs to a class of preformed pair theories and is, effectively, equivalent to the BCS-BEC crossover theory. Such a theory is constructed and applied to a wide range of scattering and transport phenomena, both in the context of cuprates and cold Fermi gases. The experiments that are analyzed are chosen based on the importance of their results to either elucidating the pseudogap state or raising crucial questions or challenges to theories of the pseudogap phase. A number of key themes emerge regarding the interplay of superconductivity and the pseudogap and how the related energy scales manifest themselves in experiments.