Universal Spin Transport and Quantum Bounds for Unitary Fermions

TL;DR

This paper reviews recent experimental and theoretical progress in understanding spin transport in strongly interacting Fermi gases, emphasizing a universal lower bound on spin diffusivity and its broader implications.

Contribution

It introduces the concept of a quantum bound on spin diffusivity in unitary fermions and discusses the formalism of spin hydrodynamics and related phenomena.

Findings

Observation of a lower bound on spin diffusivity in strongly interacting regimes

Analysis of spin transport formalism and effects of confinement

Discussion of quantum critical transport and open questions

Abstract

We review recent advances in experimental and theoretical understanding of spin transport in strongly interacting Fermi gases. The central new phenomenon is the observation of a lower bound on the (bare) spin diffusivity in the strongly interacting regime. Transport bounds are of broad interest for the condensed matter community, with a conceptual similarity to observed bounds in shear viscosity and charge conductivity. We discuss the formalism of spin hydrodynamics, how dynamics are parameterized by transport coefficients, the effect of confinement, the role of scale invariance, the quasi-particle picture, and quantum critical transport. We conclude by highlighting open questions, such as precise theoretical bounds, relevance to other phases of matter, and extensions to lattice systems.