The Nature of Chain-Plane Coupling in YBa₂Cu₃O₇

Abstract

<p>The work in this thesis revolves around a simple model of the high temperature superconductor YBa₂Cu₃O₇_δ. Of all of the high temperature material YBa₂Cu₃O₇_δ is by far the most widely studied. Despite this, relatively little is understood about it. Although the structure of the unit cell has been known since the discovery of the material, its electronic structure is still a subject of intense controversy. Theories of the electronic structure range from the conventional theories of metallic systems with complicated band structure, to exotic theories of strongly correlated electron systems. Exotic theories are necessarily, difficult to work with, and we adopt the attitude that it is more fruitful to understand the strengths and weaknesses of the simplest models before pursuing more complicated approaches. For this reason, no attempt is made to provide a comprehensive theory of high temperature superconductivity. Instead a model is chosen which captures the few features of the structure of YBa₂Cu₃O₇_δ which we wish to study.</p> <p>The main focus of the model is the link between two structural elements within the unit cell-the so called copper-oxide "planes" and "chains". Both the chains and planes are believed to play an active role in determining the highly unusual properties of YBa₂Cu₃O₇_δ. At the moment, relatively little is known about the chains, since the planes are suspected to be the more interesting of the two elements and have been more carefully studied. Recently, though, attention has begun to shift towards the chains.</p> <p>In the chapters which follow, several physical properties of the model arc calculated. The calculations allow us to interpret a number of experiments, such as penetration depth, and YBa₂Cu₃O₇/Pb Josephson junction experiments. As a result of comparing the theory with experiment, we are able to make a few fairly broad statements about the electronic structure of YBa₂Cu₃O₇ in both the normal and superconducting states.</p>