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arXiv:1505.00738 · Physical Review Letters

Decoherence of Impurities in a Fermi Sea of Ultracold Atoms

Marko Cetina, Michael Jag, Rianne S. Lous, Jook T. M. Walraven, Rudolf Grimm, Rasmus Søgaard Christensen, Georg M. Bruun

Published Sep 22, 2015• 124 citations
arXivHTMLPDF

Abstract

We investigate the decoherence of ^{40}K impurities interacting with a three-dimensional Fermi sea of ^{6}Li across an interspecies Feshbach resonance. The decoherence is measured as a function of the interaction strength and temperature using a spin-echo atom interferometry method. For weak to moderate interaction strengths, we interpret our measurements in terms of scattering of K quasiparticles by the Fermi sea and find very good agreement with a Fermi liquid calculation. For strong interactions, we observe significant enhancement of the decoherence rate, which is largely independent of temperature, pointing to behavior that is beyond the scattering of quasiparticles in the Fermi liquid picture.

DOI
10.1103/physrevlett.115.135302

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We report on the observation of Feshbach resonances in ultracold $^{6}\mathrm{Li}\text{\ensuremath{-}}^{164}\mathrm{Dy}$ mixtures, where 6Li^{6}\mathrm{Li}6Li atoms are respectively prepared in their three lowest spin states and 164Dy^{164}\mathrm{Dy}164Dy atoms are prepared in their lowest-energy state. We observe 21 interspecies scattering resonances over a magnetic-field range from 0 to 702 G using atom loss spectroscopy, three of which exhibit relatively broad widths. These broad resonances provide precise control over the interspecies interaction strength, enabling the study of strongly interacting effects in $^{6}\mathrm{Li}\text{\ensuremath{-}}^{164}\mathrm{Dy}$ mixtures. Additionally, we observe a well-isolated interspecies resonance at 700.1 G, offering a unique platform to explore novel impurity physics, where heavy dipolar 164Dy^{164}\mathrm{Dy}164Dy atoms are immersed in a strongly interacting Fermi superfluid of 6Li^{6}\mathrm{Li}6Li atoms.
We analyze the scattering rate of an impurity atom in a Fermi sea as a function of momentum and temperature in the BCS-BEC crossover. The cross section is calculated using a microscopic multichannel theory for the Feshbach resonance scattering, including finite range and medium effects. We show that pair correlations significantly increase the cross section for strong interactions close to the unitarity regime. They give rise to a molecule pole of the cross section at negative energy on the BEC side of the resonance, which smoothly evolves into a resonance at positive scattering energy with a nonzero imaginary part on the BCS side. The resonance is the analog of superfluid pairing for the corresponding population balanced system. Using Fermi liquid theory, we show that the low temperature scattering rate of the impurity atom is significantly increased due to these pair correlations for low momenta. We demonstrate that finite range and mass imbalance effects are significant for the experimentally relevant $^{6}\mathrm{Li}\ensuremath{-}^{40}\mathrm{K}$ mixture, and we finally discuss how the scattering rate can be measured using radio-frequency spectroscopy and Bose-Fermi mixtures.
Feshbach spectroscopy of ultracold mixtures of 6Li^{6}{\rm Li}6Li and 164Dy^{164}{\rm Dy}164Dy atoms
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