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Surface tension in population imbalanced unitary Fermi gases

Theja N. De Silva, Erich J. Mueller

Published Apr 28, 2006• 3 citations
OpenAlex

Abstract

We study the effects of surface tension between normal and superfluid regions of a trapped Fermi gas at unitarity. We find that surface tension causes notable distortions in the shape of large aspect ratio clouds. Including these distortions in our theories resolves many of the apparent discrepancies among different experiments and between theory and experiments.

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Theory of the normal-superfluid interface in population-imbalanced Fermi gases

Stefan K. Baur, Sourish Basu, Theja N. De Silva +1 more

We present a series of theoretical studies of the boundary between a superfluid and a normal region in a partially polarized gas of strongly interacting fermions. We present mean-field estimates of the surface energy in this boundary as a function of temperature and scattering length. We discuss the structure of the domain wall, and use a previously introduced phenomonological model to study its influence on experimental observables. Our microscopic mean-field calculations are not consistent with the magnitude of the surface tension found from our phenomonological modeling of data from the Rice University experiments. We conclude that one must search for novel mechanisms to explain the experiments.

arXiv:0901.2945•Physical Review A•Jun 24, 2009•22 citations
We study the effects of surface tension between normal and superfluid regions of a trapped Fermi gas at unitarity. We find that surface tension causes notable distortions in the shape of large aspect ratio clouds. Including these distortions in our theories resolves many of the apparent discrepancies among different experiments and between theory and experiments.
We show that the maximum population imbalance ratio PCCP_\mathrm{CC}PCC​ for a two-component Fermi gas near the unitarity limit to condense does not increase with the trap aspect ratio λλλ, by two methods of 1) solving the Bogoliubov-de Gennes equations with coupling-constant renormalization, and 2) studying the pairing susceptibility by the real-space self-consistent TTT-matrix approximation. The deviation of the cloud shape from what is expected from the trap shape increases but stays minor with increasing λλλ up to 50. This finding indicates that despite the apparent discrepancy between the MIT and Rice experiments over the value of PCCP_\mathrm{CC}PCC​ and the validity of local density approximation, the equilibrium state of the system for the aspect ratio in the Rice experiment should be consistent with that of MIT.
We present a general approach for describing trapped Fermi gases, when the cloud shape is distorted with respect to the trap shape. Our approach provides a consistent way to explore physics beyond the local density approximation, if this is necessary due to the distortion. We illustrate this by analyzing in detail experimentally observed distortions in a trapped imbalanced Fermi mixture. In particular, we demonstrate in that case dramatic deviations from ellipsoidal cloud shapes arising from the competition between surface and bulk energies.
We present a series of theoretical studies of the boundary between a superfluid and a normal region in a partially polarized gas of strongly interacting fermions. We present mean-field estimates of the surface energy in this boundary as a function of temperature and scattering length. We discuss the structure of the domain wall, and use a previously introduced phenomonological model to study its influence on experimental observables. Our microscopic mean-field calculations are not consistent with the magnitude of the surface tension found from our phenomonological modeling of data from the Rice University experiments. We conclude that one must search for novel mechanisms to explain the experiments.