# Critical behaviour in one dimension: unconventional pairing, phase separation, BEC-BCS crossover and magnetic Lifshitz transition

## TL;DR

This paper explores the complex superconducting phases, including FFLO and η phases, in one-dimensional ultracold Fermi mixtures under magnetic fields, analyzing their stability and evolution from BCS to BEC regimes.

## Contribution

It provides a detailed mean-field phase diagram for 1D imbalanced Fermi gases, identifying regimes of unconventional pairing and the persistence of superconductivity across Lifshitz transitions.

## Key findings

- Identification of stable FFLO and η phases in 1D systems.
- Superconductivity persists beyond Lifshitz transition points.
- Evolution from BCS to BEC regimes with increasing attraction.

## Abstract

We study the superconducting properties of population-imbalanced ultracold Fermi mixtures in one-dimensional (1D) optical lattices that can be effectively described by the spin-imbalanced attractive Hubbard model (AHM) in the presence of a Zeeman magnetic field. We use the mean-field theory approach to obtain the ground state phase diagrams including some unconventional superconducting phases such as the Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) phase, and the $\eta$ phase (an extremal case of the FFLO phase), both for the case of a fixed chemical potential and for a fixed number of particles. It allows to determine optimal regimes for the FFLO phase as well as $\eta$-pairing stability. We also investigate the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs (BEC) with increasing attraction, at $T=0$. Finally, the obtained results show that despite of the occurrence of the Lifshitz transition induced by an external magnetic field, the superconducting state can still exist in the system, at higher magnetic field values.