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arXiv:1310.5140 · Science

Transverse Demagnetization Dynamics of a Unitary Fermi Gas

Alma Bardon, Scott Beattie, C. Luciuk, William B. Cairncross, Daniel Fine, Nathan Cheng, Graham J. A. Edge, Edward Taylor, Shizhong Zhang, Stefan Trotzky, Joseph H. Thywissen

Published May 15, 2014• 114 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1126/science.1247425

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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.
This thesis investigates the dynamics of a degenerate Fermi gas in the unitary limit of strong interactions, in particular, the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. Short-range interactions between ultracold fermionic potassium atoms are tuned to their quantum mechanical limit by employing a Feshbach resonance. Spin-echo measurements of magnetization dynamics show that demagnetization is caused by diffusive spin transport with a diffusion constant that saturates at low temperatures to the conjectured quantum-mechanical lower bound ~/m, where m is the particle mass. A prediction for the increase in temperature due to demagnetization based on universal thermodynamics is compared to measured momentum distributions. The development of pair correlations is observed by time-resolved radiofrequency spectroscopy of Tan's contact parameter.
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We compute spin transport in the unitary Fermi gas using the strong-coupling Luttinger-Ward theory. In the quantum degenerate regime the spin diffusivity attains a minimum value of ${D}_{s}\ensuremath{\simeq}1.3\ensuremath{\hbar}/m$ approaching the quantum limit of diffusion for a particle of mass mmm. Conversely, the spin drag rate reaches a maximum value of ${\ensuremath{\Gamma}}_{\mathrm{sd}}\ensuremath{\simeq}1.2{k}_{B}{T}_{F}/\ensuremath{\hbar}$ in terms of the Fermi temperature TF{T}_{F}TF​. The frequency-dependent spin conductivity ${\ensuremath{\sigma}}_{s}(\ensuremath{\omega})$ exhibits a broad Drude peak, with spectral weight transferred to a universal high-frequency tail ${\ensuremath{\sigma}}_{s}(\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\infty})={\ensuremath{\hbar}}^{1/2}C/3\ensuremath{\pi}(m\ensuremath{\omega}{)}^{3/2}$ proportional to the Tan contact density CCC. For the spin susceptibility ${\ensuremath{\chi}}_{s}(T)$ we find no downturn in the normal phase.
Transport of strongly interacting fermions is crucial for the properties of modern materials, nuclear fission, the merging of neutron stars, and the expansion of the early Universe. Here, we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting atomic Fermi gas by studying sound propagation and its attenuation through the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature, in contrast to the diverging behavior of weakly interacting Fermi liquids. Below the superfluid transition temperature, D attains a universal value set by the ratio of Planck's constant and the particle mass. Our findings inform theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons, and quarks.
© 2020 American Association for the Advancement of Science. All rights reserved. Transport of strongly interacting fermions is crucial for the properties of modern materials, nuclear fission, the merging of neutron stars, and the expansion of the early Universe. Here, we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting atomic Fermi gas by studying sound propagation and its attenuation through the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature, in contrast to the diverging behavior of weakly interacting Fermi liquids. Below the superfluid transition temperature, D attains a universal value set by the ratio of Planck's constant and the particle mass. Our findings inform theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons, and quarks.