Observation of Anomalous Spin Segregation in a Trapped Fermi Gas

TL;DR

This study observes long-lived spin segregation in a trapped ultracold Fermi gas of lithium-6, revealing effects of Fermi statistics and precisely determining the zero crossing of the scattering length.

Contribution

It provides the first experimental observation of spin segregation in a Fermi gas and links it to spin-wave theory, highlighting the role of Fermi statistics in the phenomenon.

Findings

Spin segregation persists for about 5 seconds in the trap.

The sign of the scattering length affects the spin density profiles.

The zero crossing of the scattering length is measured at 527.5 G.

Abstract

We report the observation of spin segregation, i.e., separation of spin density profiles, in a trapped ultracold Fermi gas of $^6$Li with a magnetically tunable scattering length close to zero. The roles of the spin components are inverted when the sign of the scattering length is reversed. The observed density profiles are in qualitative agreement with the spin-wave theory applied previously to explain spin segregation in a Bose gas, but disagree in amplitude by two orders of magnitude. The observed atomic density profiles are far from equilibrium, yet they persist for $\simeq$ 5 seconds in a trap with an axial frequency of $\simeq$ 150 Hz. We attribute this long lifetime to Fermi statistics: The scattering amplitude is nonzero only for atoms in opposite states, and vanishes for atoms in the same state. By measuring the magnetic field at which spin segregation ceases, we precisely determine the zero crossing in the scattering length of $^6$Li as $527.5\pm0.2$ G.