Nonexistence of the Luttinger-Ward Functional and Misleading Convergence of Skeleton Diagrammatic Series for Hubbard-Like Models

作者：Kozik Evgeny^*; Ferrero Michel; Georges Antoine

来源：Physical Review Letters, 2015, 114(15): 156402.

DOI：10.1103/PhysRevLett.114.156402

The Luttinger-Ward functional Phi[G], which expresses the thermodynamic grand potential in terms of the interacting single-particle Green's function G, is found to be ill defined for fermionic models with the Hubbard on-site interaction. In particular, we show that the self-energy Sigma[G] alpha delta Phi[G]/delta G is not a single-valued functional of G: in addition to the physical solution for Sigma[G], there exists at least one qualitatively distinct unphysical branch. This result is demonstrated for several models: the Hubbard atom, the Anderson impurity model, and the full two-dimensional Hubbard model. Despite this pathology, the skeleton Feynman diagrammatic series for Sigma in terms of G is found to converge at least for moderately low temperatures. However, at strong interactions, its convergence is to the unphysical branch. This reveals a new scenario of breaking down of diagrammatic expansions. In contrast, the bare series in terms of the noninteracting Green's function G(0) converges to the correct physical branch of Sigma in all cases currently accessible by diagrammatic Monte Carlo calculations. In addition to their conceptual importance, these observations have important implications for techniques based on the explicit summation of the diagrammatic series.

• 出版日期2015-4-15