Non-perturbative conserving approximations and Luttinger's sum rule

Details Non-perturbative conserving approximations and Luttinger's sum rule Non-perturbative conserving approximations and Luttinger's sum rule

2007-04-02

arxiv.org/abs/0704.0249v2

Physics

Condensed Matter

Strongly Correlated Electrons

13 pages, 10 figures, revised version with slightly extended discussion, accepted by EPJB

Scientific paper

10.1140/epjb/e2007-00203-7

Weak-coupling conserving approximations can be constructed by truncations of the Luttinger-Ward functional and are well known as thermodynamically consistent approaches which respect macroscopic conservation laws as well as certain sum rules at zero temperature. These properties can also be shown for variational approximations that are generated within the framework of the self-energy-functional theory without a truncation of the diagram series. Luttinger's sum rule represents an exception. We analyze the conditions under which the sum rule holds within a non-perturbative conserving approximation. Numerical examples are given for a simple but non-trivial dynamical two-site approximation. The validity of the sum rule for finite Hubbard clusters and the consequences for cluster extensions of the dynamical mean-field theory are discussed.

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