Spin-Imbalanced Pairing and Fermi Surface Deformation in Flat Bands

TL;DR

This paper investigates how flat bands influence spin-imbalanced superfluid phases in the attractive Hubbard model, revealing novel multiband pairing mechanisms and Fermi surface deformations confirmed by mean-field and DMFT analyses.

Contribution

It uncovers the rich nature of pairing in flat band systems, demonstrating nontrivial superfluid phases and Fermi surface deformations driven by multiband Cooper pairing.

Findings

Discovery of exotic superfluid phases similar to FFLO states.

Confirmation of phase stability via dynamical mean-field theory.

Identification of flat band effects on Fermi surface deformation.

Abstract

We study the attractive Hubbard model with spin imbalance on two lattices featuring a flat band: the Lieb and kagome lattices. We present mean-field phase diagrams featuring exotic superfluid phases, similar to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, whose stability is confirmed by dynamical mean-field theory (DMFT). The nature of the pairing is found to be richer than just the Fermi surface shift responsible for the usual FFLO state. The presence of a flat band allows for changes in the particle momentum distributions at null energy cost. This facilitates formation of nontrivial superfluid phases via multiband Cooper pair formation: the momentum distribution of the spin component in the flat band deforms to mimic the Fermi surface of the other spin component residing in a dispersive band. The Fermi surface of the unpaired particles that are typical for gapless superfluids becomes deformed as well. The results highlight the profound effect of flat dispersions on Fermi surface instabilities, and provide a potential route for observing spin-imbalanced superfluidity and superconductivity.