Universal Spin Transport in a Strongly Interacting Fermi Gas

TL;DR

This paper investigates spin transport in a strongly interacting ultracold Fermi gas, revealing universal behaviors, damping of spin currents, and insights into magnetic states, with implications for understanding non-equilibrium fermionic matter.

Contribution

It provides the first detailed measurement of spin transport properties in a strongly interacting Fermi gas, showing universal laws and ruling out certain magnetic phases.

Findings

Spin currents are maximally damped and can be reversed by strong interactions.

Spin diffusivity approaches a quantum limit at low temperatures.

Measurements exclude a metastable ferromagnetic state in the system.

Abstract

Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics [4, 5]. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity [3, 6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coeffcient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by the ratio of the reduced Planck's constant to the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7-9].