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Anisotropic Relaxation Dynamics in a Dipolar Fermi Gas Driven Out of Equilibrium

K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, J. L. Bohn, D. S. Jin, G. M. Bruun, and F. Ferlaino
Phys. Rev. Lett. 113, 263201 – Published 23 December 2014
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    Abstract

    We report on the observation of a large anisotropy in the rethermalization dynamics of an ultracold dipolar Fermi gas driven out of equilibrium. Our system consists of an ultracold sample of strongly magnetic Er167 fermions, spin polarized in the lowest Zeeman sublevel. In this system, elastic collisions arise purely from universal dipolar scattering. Based on cross-dimensional rethermalization experiments, we observe a strong anisotropy of the scattering, which manifests itself in a large angular dependence of the thermal relaxation dynamics. Our result is in good agreement with recent theoretical predictions. Furthermore, we measure the rethermalization rate as a function of temperature for different angles and find that the suppression of collisions by Pauli blocking is not influenced by the dipole orientation.

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    • Received 6 May 2014

    DOI:https://doi.org/10.1103/PhysRevLett.113.263201

    © 2014 American Physical Society

    Authors & Affiliations

    K. Aikawa1, A. Frisch1, M. Mark1, S. Baier1, R. Grimm1,2, J. L. Bohn3, D. S. Jin3, G. M. Bruun4, and F. Ferlaino1,2

    • 1Institut für Experimentalphysik and Zentrum für Quantenphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
    • 2Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria
    • 3JILA, NIST, and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
    • 4Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark

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    Vol. 113, Iss. 26 — 31 December 2014

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    • Figure 1
      Figure 1

      Time-of-flight absorption image of a degenerate Fermi gas of 3.0×104  Er atoms at T/TF=0.11(1) after 12 ms of expansion (a) and its density distribution integrated along the x direction (upper panel) and z direction (lower panel) (b). The observed profiles (circles) are well described by fitting the Fermi-Dirac distribution to the data (solid lines), while they substantially deviate from a fit using a Gaussian distribution to the outer wings of the cloud (dashed lines), i.e., outside the disk with radius w, where w is the 1/e radius of the Gaussian fit to the entire cloud. The absorption image is averaged over 20 individual measurements.

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    • Figure 2
      Figure 2

      Typical cross-dimensional thermalization measurements for three dipole orientations: β=90° (squares), β=109° (circles), and β=138° (triangles). The time evolution of the temperature in the z direction Tz is plotted as a function of holding time after the cloud is excited in the y direction. The geometry of the cigar-shaped trap and the coordinates are indicated in the inset. The yellow arrow represents the dipole orientation. After the equilibration, the Fermi gas is at T/TF≃0.75 with TF≃710  nK.

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    • Figure 3
      Figure 3

      Angle dependence of the number of collisions α required to rethermalize ultracold dipolar fermions. The experimental data (circles) are compared with the parameter-free theoretical prediction calculated under our trapping and excitation conditions (solid line).

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    • Figure 4
      Figure 4

      The Pauli suppression factor η as a function of the temperature. The experimental data are taken for two different orientations of the dipoles, i.e., for β=90° (squares) and for β=110° (circles). The experimental values are compared with the theoretical predictions on Pauli blocking (solid line); see text.

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