Dynamical vertex approximation for the attractive Hubbard model

TL;DR

This paper extends the dynamical vertex approximation to the attractive Hubbard model, enabling the study of non-local correlations and phase transitions in systems with attractive interactions.

Contribution

The authors adapt and validate the D$\Gamma$A formalism for the attractive Hubbard model, including derivation of equations and application to phase diagram analysis.

Findings

Validated the ladder approximation for the attractive Hubbard model.

Calculated phase diagrams near superconducting and charge-density wave transitions.

Analyzed the impact of non-local correlations on single-particle properties.

Abstract

In this work, we adapt the formalism of the dynamical vertex approximation (D$\Gamma$A), a diagrammatic approach including many-body correlations beyond the dynamical mean-field theory, to the case of attractive onsite interactions. We start by exploiting the ladder approximation of the D$\Gamma$A scheme, in order to derive the corresponding equations for the non-local self-energy and vertex functions of the attractive Hubbard model. Second, we prove the validity of our derivation by showing that the results obtained in the particle-hole symmetric case fully preserve the exact mapping between the attractive and the repulsive models. It will be shown, how this property can be related to the structure of the ladders, which makes our derivation applicable for any approximation scheme based on ladder diagrams. Finally, we apply our D$\Gamma$A algorithm to the attractive Hubbard model in three dimensions, for different fillings and interaction values. Specifically, we focus on the parameters region in the proximity of the second-order transition to the superconducting and charge-density wave phases, respectively, and calculate (i) their phase-diagrams, (ii) their critical behavior, as well as (iii) the effects of the strong non-local correlations on the single-particle properties.