Novel dynamical excitations and roton-based measurement of Cooper-pair momentum in a two-dimensional Fulde-Ferrell-Larkin-Ovchinnikov superfluid on optical lattices

TL;DR

This paper investigates dynamical excitations in a 2D FFLO superfluid on optical lattices, revealing roton modes and anisotropic spectra that enable measurement of Cooper-pair momentum, aiding experimental identification.

Contribution

It introduces a roton-based method to measure Cooper-pair momentum in 2D FFLO superfluids via dynamical excitation spectra analysis.

Findings

Emergence of a low-energy bogolon mode in the spin channel.

Pronounced anisotropy in dynamical excitations due to finite COM momentum.

Roton mode around [π,π] evolves into a ring structure, enabling COM momentum extraction.

Abstract

Determining the center-of-mass (COM) momentum of Cooper pairs in unconventional superconductors or superfluids is a topic of great interest in condensed matter physics and ultracold atomic gases. Theoretically, we investigate the dynamical excitations of a two-dimensional spin-polarized attractive Hubbard model on a square optical lattice under an effective Zeeman field by computing the density and spin dynamical structure factors, focusing on phase transition from a Bardeen-Cooper-Schrieffer (BCS) superfluid to an Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid. In the FFLO superfluid, besides the phonon mode in the density channel, a low-energy bogolon mode emerges in the spin channel, which is associated with Bogoliubov quasiparticles on a Bogoliubov Fermi surface. Moreover, the dynamical excitations exhibit pronounced anisotropy in momentum space due to the finite COM momentum. At half filling, the roton mode around $[\pi,\pi]$ evolves from a point-like minimum into a ring structure shifted by the COM momentum across the BCS-FFLO transition, providing a roton-based protocol to extract the COM momentum. These predictions provide key insights for confirming the existence of FFLO superfluids and understanding their dynamical excitation spectra.