Effective interactions and fluctuation effects in spin-singlet superfluids

TL;DR

This paper develops a functional renormalization group approach to study effective interactions and fluctuations in spin-singlet superfluids, providing detailed insights into the momentum, frequency dependence, and fluctuation effects on the superfluid gap.

Contribution

It introduces a one-loop flow equation framework with a fermionic cutoff and external pairing field, capturing fluctuation effects beyond mean-field in superfluid models.

Findings

Fluctuations reduce the superfluid gap, especially at weaker interactions.

The formalism accurately describes the momentum and frequency dependence of effective interactions.

The approach enforces charge conservation via Ward identity projection.

Abstract

We derive and evaluate one-loop functional flow equations for the effective interactions, self-energy and gap function in spin-singlet superfluids. The flow is generated by a fermionic frequency cutoff, which is supplemented by an external pairing field to treat divergencies associated with the Goldstone boson. To parametrize the singular momentum and frequency dependences of the effective interactions, the Nambu interaction vertex is decomposed in charge, magnetic, and normal and anomalous pairing channels. The one-loop flow solves reduced (mean-field) models for superfluidity exactly, and captures also important fluctuation effects. The Ward identity from charge conservation is generally violated, but can be enforced by projecting the flow. Applying the general formalism to the two-dimensional attractive Hubbard model, we obtain detailed results on the momentum and frequency dependences of the effective interactions for weak and moderate bare interactions. The gap is reduced by fluctuations, with a stronger reduction at weaker interactions, as expected.