The algebraic spin liquid of a possible model description for the normal state of underdoped high temperature superconductors

Abstract

16 years and counting ... In 1986 Bednorz and Muller discovered the layered perovskite structure (La - Ba)2Cu04 which showed the phenomenon of superconductivity at the unprecedented high temperature of Tc = 33 K. In the ensuing months and years it became more and more apparent that the cuprates, as the materials came to be known, show very peculiar correlations in their "normal" state at temperatures T > Tc. The majority of this thesis is concerned with this abnormal "normal" state, attempting to extract a coherent picture for the strange phenomenology. The underlying theoretical framework is a slave particle description of the tJ model proposed by Wen and Lee. The conceptual background is rooted in Anderson's proposal of spin charge separation as the key emergent phenomenon in cuprate physics. After a brief motivation we look at single particle tunneling into the cuprate's superconducting state from the perspective of both d-wave BCS and the SU(2) slave boson theory of Wen and Lee. Both approaches work well close to zero tunneling bias. The slave particle formulation however also naturally incorporates the particle/hole asymmetric background that is seen in experiments.

(cont.) The question of single particle correlations studied experimentally via angle resolved photo emission experiments motivates the analysis of the next chapter. The broad spectral line-shapes seen experimentally imply the absence of well defined quasi-particles in the Fermi liquid sense. We study how gauge fluctuations arising from our use of slave particle coordinates affect the physical hole spectral function. The primary influence of gauge fluctuations turns out to be through their confining tendency on the vertex rather than as a scattering field for the slave particles. The last chapter discusses the effect of gauge fluctuations on the spin susceptibility and shows that they play a vital role in restoring Neel correlations. This allows us to give a natural explanation for the spin related phenomenology of underdoped cuprates.