Observation of quantum-limited spin transport in strongly interacting two-dimensional Fermi gases

TL;DR

This study investigates spin transport in 2D ultracold Fermi gases, revealing a quantum-limited minimum spin diffusivity and linking demagnetization dynamics to contact growth, highlighting bounds on relaxation rates in strongly interacting systems.

Contribution

It provides the first measurement of quantum-limited spin diffusivity in 2D Fermi gases and explores the role of the Leggett-Rice effect in demagnetization.

Findings

Minimum spin diffusivity of 1.7(6)ħ/m at strong interactions.

Growth of s-wave contact indicating breaking of scaling symmetry.

Demagnetization rate correlates with contact dynamics.

Abstract

We measure the transport properties of two-dimensional ultracold Fermi gases during transverse demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to distinguish bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of spin current around the local magnetization. When the two-dimensional scattering length is tuned to be comparable to the inverse Fermi wave vector $k_F^{-1}$, we find that the bare transverse spin diffusivity reaches a minimum of $1.7(6)\hbar/m$, where $m$ is the bare particle mass. The rate of demagnetization is also reflected in the growth rate of the s-wave contact, observed using time-resolved spectroscopy. At unitarity, the contact rises to $0.28(3) k_F^2$ per particle, measuring the breaking of scaling symmetry. Our observations support the conjecture that in systems with strong scattering, the local relaxation rate is bounded from above by $k_B T/\hbar$.