Optical spin conductivity in ultracold quantum gases

TL;DR

This paper proposes measuring optical spin conductivity in ultracold atomic gases to gain insights into quantum states, quasiparticle excitations, and non-Fermi liquid properties, providing a new probe for clean quantum systems.

Contribution

It introduces a formalism for measuring optical spin conductivity in ultracold gases and demonstrates its ability to reveal rich quantum information in various systems.

Findings

Spectra reflect quasiparticle excitations and non-Fermi liquid behavior.

Accessible quantities include superfluid gap, contact, and spin excitations.

Method applicable to disorder-free atomic gases and extendable to other systems.

Abstract

We show that the optical spin conductivity being a small AC response of a bulk spin current and elusive in condensed matter systems can be measured in ultracold atoms. We demonstrate that this conductivity contains rich information on quantum states by analyzing experimentally achievable systems such as a spin-1/2 superfluid Fermi gas, a spin-1 Bose-Einstein condensate, and a Tomonaga-Luttinger liquid. The obtained conductivity spectra being absent in the Drude conductivity reflect quasiparticle excitations and non-Fermi liquid properties. Accessible physical quantities include the superfluid gap and the contact for the superfluid Fermi gas, gapped and gapless spin excitations as well as quantum depletion for the Bose-Einstein condensate, and the spin part of the Tomonaga-Luttinger liquid parameter elusive in cold-atom experiments. Unlike its mass transport counterpart, the spin conductivity serves as a probe applicable to clean atomic gases without disorder and lattice potentials. Our formalism can be generalized to various systems such as spin-orbit coupled and nonequilibrium systems.