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arXiv:2309.07226 · Physical Review A

Energy-resolved spin correlation measurements: Decoding transverse spin dynamics in weakly interacting Fermi gases

J. Huang, John E. Thomas

Published Apr 10, 2024• 4 citations
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Abstract

The authors measure transverse spin correlations in energy space to uncover hidden spin dynamics in a weakly interacting Fermi gas. The correlation functions reveal the microscopic structure of a demagnetizing or magnetizing synthetic spin lattice, which models a collective Heisenberg Hamiltonian, and provide new observables for studies of transitions between dynamical phases.

DOI
10.1103/physreva.109.l041301

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The authors measure transverse spin correlations in energy space to uncover hidden spin dynamics in a weakly interacting Fermi gas. The correlation functions reveal the microscopic structure of a demagnetizing or magnetizing synthetic spin lattice, which models a collective Heisenberg Hamiltonian, and provide new observables for studies of transitions between dynamical phases.
Recent experiments with dilute trapped Fermi gases observed that weak interactions can drastically modify spin transport dynamics and give rise to robust collective effects including global demagnetization, macroscopic spin waves, spin segregation, and spin self-rephasing. In this Letter, we develop a framework for studying the dynamics of weakly interacting fermionic gases following a spin-dependent change of the trapping potential which illuminates the interplay between spin, motion, Fermi statistics, and interactions. The key idea is the projection of the state of the system onto a set of lattice spin models defined on the single-particle mode space. Collective phenomena, including the global spreading of quantum correlations in real space, arise as a consequence of the long-ranged character of the spin model couplings. This approach achieves good agreement with prior measurements and suggests a number of directions for future experiments.
Understanding the quantum dynamics of strongly interacting fermions is a problem relevant to diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasma. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions. Here, we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We observe the demagnetization of the gas, caused by diffusive spin transport. At low temperatures, the diffusion constant saturates to the conjectured quantum-mechanical lower bound ≃ ħ/m, where m is the particle mass. The development of pair correlations, indicating the transformation of the initially noninteracting gas toward a unitary spin mixture, is observed by measuring Tan's contact parameter.
The time evolution of spin-energy correlations in a very weakly interacting Fermi gas of 6{}^{6}6Li produces complex spin-density profiles, which are observed and quantitatively explained by a one-dimensional mean-field model. This system exhibits rich collective dynamics for exploring the interplay between spin, motion, Fermi statistics, and interactions in many-body systems.
This thesis summarizes experimental work investigating strongly interacting Fermi gases in two and three dimensions. Radio-frequency spin manipulation is used to initialize and probe strongly interacting Fermi gases dynamically to measure relaxation to a steady-state. Time-resolved spectroscopy is used to discover the p-wave contacts by observing how correlations in the system develop after quenching the atoms into an interacting state. By combining quasi-steady state measurements with new contact relations, an attractive p-wave interaction energy as large as half the Fermi energy is inferred. Spin echo measurements of the magnetization dynamics are used to observe spin transport in two and three dimensions in the presence of strong interactions. The bare spin diffusivity is distinguished from the Leggett-Rice effect, in which demagnetization is slowed by precession of a spin current around the local magnetization. Observations of the demagnetization dynamics support the conjecture that a general quantum bound on spin transport is obeyed where the diffusivity D greater than or similar to ħ/m, where m is the particle mass.