Failure of the Baym-Kadanoff construction to match consistently quantum dynamics with thermodynamic critical behavior

TL;DR

This paper reveals fundamental issues in the Baym-Kadanoff approach, showing that its two vertices diverge differently at critical points, leading to inconsistent descriptions of phase instabilities in correlated electron systems.

Contribution

It demonstrates that the standard Baym-Kadanoff construction cannot reliably predict critical behavior due to divergent vertices and proposes that consistent critical behavior requires simultaneous divergence of both vertices.

Findings

Divergences in the dynamical and conserving vertices differ at critical points.

The dynamical vertex violates the Ward identity and cannot be continued beyond singularities.

The conserving vertex's divergence may be spurious in low-dimensional systems.

Abstract

We disclose a serious deficiency of the Baym-Kadanoff construction of thermodynamically consistent conserving approximations. There are two vertices in this scheme: dynamical and conserving. The divergence of each indicates a phase instability. We show that each leads to incomplete and qualitatively different behavior at different critical points. The diagrammatically controlled dynamical vertex from the Schwinger-Dyson equation does not obey the Ward identity and cannot be continued beyond its singularity. The standardly used dynamical vertex alone cannot, hence, conclusively decide about the stability of the high-temperature phase. On the other hand, the divergence in the conserving vertex, obeying the conservation laws, does not invoke critical behavior of the spectral function and the specific heat. Moreover, the critical behavior of the conserving vertex may become spurious in low-dimensional systems. Consequently, the description of the critical behavior of correlated electrons becomes consistent and reliable only if the fluctuations of the order parameter in the conserving vertex lead to a divergence coinciding with that of the dynamical one.