Large Ud theory for metallic high-Tc cuprates

TL;DR

This paper develops a large-Ud theoretical framework for metallic high-Tc cuprates, revealing band narrowing, incoherent fluctuations, and the interplay between magnetic coherence and valence fluctuations.

Contribution

It introduces a novel large-Ud slave fermion approach to the Emery model, capturing incoherent fluctuations and breaking the conventional Luttinger sum rule.

Findings

Band narrowing without mean-field approximations

Incoherent Cu-O2 valence fluctuations below Fermi level

Broken Luttinger sum rule for coherent bands

Abstract

A large-Ud theory is constructed for the metallic state of high-Tc cuprates. The Emery three-band model, extended with Ox-Oy hopping tpp, and with Ud tending to infinity, is mapped on slave fermions. The Dyson time-dependent diagrammatic theory in terms of the Cu-O hopping tpd, which starts from the nondegenerate unperturbed ground state, is translationally and asymptotically locally gauge invariant. The small parameter of the theory is nd. The lowest order of the theory generates the single particle propagators of the hybridized pdp- and dpd-fermions with the exact three band structure. The leading many-body effect is band narrowing, obtained without mean-field slave particle approximations. Damping found in the single particle propagation is related to incoherent Cu-O2 intracell "mixed valence" fluctuations. The corresponding continuum falls below the Fermi level. The conventional Luttinger sum rule for coherent bands is thus broken. Due to local gauge invariance, these results are insensitive to the replacement of the Cu-O anticommutations by commutations. This replacement affects the effective local kinematic repulsion Ud\mu\ between the hybridized pdp hole propagators. The a posteriori antisymmetrization of Ud\mu\ removes the repulsion between the pdp particles in the triplet configuration, keeping their singlet repulsion intact. Such a Ud\mu\ is moderate, and close to optimal doping favors the coherent, antinodal, low energy SDW correlations. Resonant valence bonds which involve Jpd appear as incoherent perturbative corrections to these correlations. In function of doping and/or frequency, the incoherent "mixed valence" fluctuations compete with magnetic coherence. This dichotomy, between properties local in the direct and reciprocal space, is the fundamental feature of the Emery model in the large Ud metallic limit.