pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
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20 CHAPTER 1. INTRODUCTION<br />
local Coulomb correlations. To incorporate electronic correlations, one has to<br />
map the complicated band structure onto effective microscopic models which<br />
treat in a better way the electron-electron interactions.<br />
The importance of the strong correlations in these materials is corroborated<br />
by the vicinity of the metal-insulator transitions, of the antiferromagnetism instability,<br />
and also by the presence of superconductivity in this phase diagram. These<br />
phases are frequently attributed to electron interaction effects in the context of<br />
doped Mott insulators.<br />
Indeed, the low-energy charge and spin dynamics of the cuprates displays<br />
many features which defy an interpretation in terms of normal Fermi-liquid theory.<br />
In the compounds, most noteworthy are the unconventional temperature<br />
and frequency dependencies of various scattering rates and cross sections, which<br />
allow to think that strong correlation might be responsible for these unconventional<br />
electronic behaviors. Various theoretical scenari have been proposed which<br />
speculate on the complete breakdown of Fermi-liquid theory including new and<br />
exotic quantum ground states of the spin and charge carriers.<br />
Nevertheless, despite many unexpected features and unconventional behaviors<br />
in the high T c compounds, a number of electronic properties remain which are<br />
quite conventional and thereby constrain possible theories. In particular, the<br />
angular resolved photo-emission (ARPES) reveals the presence of a normal Fermi<br />
surface in the metallic compounds [5]. Moreover, photoemission data identifies<br />
dispersive single-particle states which seem related to the predictions of the LDA<br />
although they exhibit significant mass enhancement [6].<br />
In conclusion, the compounds are expected to be highly correlated, with an<br />
effective bandwidth roughly equal to the effective local Coulomb interaction. The<br />
undoped materials are antiferromagnetic charge transfer insulators at half-filling,<br />
and upon doping the antiferromagnetism is destroyed and the system becomes<br />
superconducting. At small doping, in the proximity of the antiferromagnetic<br />
phase, the normal state physics cannot be described in terms of Fermi liquid<br />
theory and is characterized by the presence of a pseudo-gap.<br />
An essential requirement of any successful theory is to capture all these fundamental<br />
features at the same time.<br />
1.4 Microscopic models for the cuprates<br />
1.4.1 Three band Hubbard model<br />
The electronic properties of oxide high-Tc superconductors have been extensively<br />
investigated over the last decade. The mechanism of superconductivity (SC) has<br />
been studied using various two-dimensional (2D) models of electronic interactions.<br />
One of the models proposed to describe the physics of high Tc materials was the<br />
so-called three-band Hubbard model introduced by Varma et al. [7] and Emery