Spin-Imbalance in a One-Dimensional Fermi Gas

TL;DR

This paper reports experimental evidence of FFLO-like phases in a one-dimensional ultracold Fermi gas with spin imbalance, revealing unique phase separation behavior and advancing the understanding of exotic superconducting states.

Contribution

First direct experimental observation of FFLO-like phases in a 1D ultracold Fermi gas with spin imbalance, confirming theoretical predictions.

Findings

Partially polarized core observed in 1D system

Phase separation pattern differs from 3D case

Quantitative agreement with FFLO state theory

Abstract

Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov proposed an exotic pairing mechanism (FFLO) where magnetism is accommodated by formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive. Here we report experimental measurements of density profiles of a two spin mixture of ultracold 6Li atoms trapped in an array of one dimensional (1D) tubes, a system analogous to electrons in 1D wires. At finite spin imbalance, the system phase separates with an inverted phase profile in comparison to the three-dimensional case. In 1D we find a partially polarized core surrounded by wings composed of either a completely paired BCS superfluid or a fully polarized Fermi gas, depending on the degree of polarization. Our observations are in quantitative agreement with theoretical calculations in which the partially polarized phase is found to be a 1D analogue of the FFLO state. This study demonstrates how ultracold atomic gases in 1D may be used to create non-trivial new phases of matter, and also paves the way for direct observation and further study of the FFLO phase.