1. Pairing in Imbalanced Fermi Mixtures: Cold Atom Clouds and Neutron Stars
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Utrecht University

2. The Quantum Fluids and Solids Group
Students: Koos Gubbels
Mathijs Romans
Arnoud Koetsier
Jeroen Diederix
Postdoc’s: Masud Haque (Dresden)
Pietro Massignan
(Barcelona)
Staff: Henk Stoof
Special thanks:
Randy Hulet

3. Imbalanced Fermi Mixture (I)
Atomic Fermi mixtures in an optical trap:
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4. Imbalanced Fermi Mixture (II)
This means that for
BCS gap parameter
FFLO
DFS
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5. Zero Temperature Theory (I)
Using the BCS ansatz we find for the homogeneous case:
BCS
Normal
Phase diagram P
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6. Zero Temperature Theory (II)
Local-density theory and BCS ansatz:
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7. Nonzero Temperature Theory (I)
Proposed phase diagram:
Superfluid Phase
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[M.M. Parish et al., Nature Physics 3, 124 (2007);
K.B. Gubbels et al., PRL 97, (2006).]

8. Nonzero Temperature Theory (II)
Phases are distinguished in a trap by:
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9. Extreme Imbalanced Fermi Gas
What happens if we add one particle to the majority Fermi sea:
[Lobo et al. , PRL 97, (2006).]
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10. MIT experiment Uses in-situ imaging:
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[Y. Shin et al., Nature 451, 689 (2008).]

11. Homogeneous Phase Diagram
State-of-the-art knowledge based on MIT experiments and RG theory:
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[Y. Shin et al., cond-mat/ ;
K.B. Gubbels et al., cond-mat/ ]

12. Bizarre Technique: RG Theory for imbalanced Fermi gas
Renormalization of chemical potentials:
Renormalization of interaction
by ‘ladder diagram’ (scattering
of particles) and ‘bubble diagram’
(screening by particle-hole excitations).
Phase transition: diverges (Thouless
criterion). Momentum/frequency dependence
important!

13. Conclusions and Outlook
Story is actually more interesting because of the trap, and there presently still exists a discrepancy between MIT and Rice experiments.
Remaining problem is maybe the microscopic details of the superfluid-normal interface. We are working on this!
Imbalanced Fermi gases in neutron stars!
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