Fermi Condensates

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Fermi Condensates
Deborah Jin
JILA, NIST and the University of Colorado
NSF, NASA, NIST
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Bose-Einstein condensation
macroscopic occupation of a single quantum
state
BEC 1995
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Quantum Particles
Photons in a laser 87Rb atoms in a BEC
The basic constituents of visible matter!
Protons Neutrons Electrons Ultracold 40K atoms
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Fermion Fermion Boson
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BEC, more generally
BEC shows up in condensed matter, nuclear
physics, elementary particle physics,
astrophysics, and atomic physics.
Cooper pairs of electrons in superconductors
4He atoms in superfluid liquid He

Excitons, biexcitons in semiconductors
Alkali atoms in ultracold atom gases
3He atom pairs in superfluid 3He-A,B
Neutron pairs, proton pairs in nuclei And
neutron stars
Mesons in neutron star matter
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Fermi condensate
Can we see this in a gas?

• amenable to theoretical understanding
• unique experimental control

Why?
The Plan
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Fermi gas of atoms
1999 40K JILA 6Li - Rice, Duke, ENS,
MIT, Innsbruck 40K - LENS, ETH Zurich
EF kBTF
spin ?
spin ?
spin degree of freedom is frozen
Fermi sea of atoms
T 0.1 TF
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Interactions

• Interactions are characterized by the
• s-wave scattering length, a
• In an ultracold atomic gas, we can control a!

a gt 0 repulsive, a lt 0 attractive Large a ?
strong interactions
?
0
scattering length
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Magnetic-field Feshbach resonance
V(R)
repulsive
Ebinding
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Magnetic-field Feshbach resonance
s-wave scattering length, a
repulsive
free atoms
DB
gt
Ebinding
attractive
molecules
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Magnetic-field Feshbach resonance

• spectroscopic measurement of the mean-field
  energy shift

repulsive
attractive
C. A. Regal and D. S. Jin, PRL 90, 230404 (2003)
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Bosonic molecules

• We can create lots of ultracold molecules!
• They are extremely weakly bound !
• They can survive many collisions !

Interesting regime
rf photodissociation
C. Regal et al. Nature 424, 47 (2003)
Theory D.S. Petrov et al., PRA 71, 012708
(2005), Expts Rice, ENS, Innsbruck, JILA
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Making condensates with fermions

• BEC of diatomic molecules
• BCS superconductivity/superfluidity
• Something in between?

1. Bind fermions together. 2. BEC
Condensation of Cooper pairs of atoms
(pairing in momentum space, near the Fermi
surface)

Resonance superfluidity M. Holland et al.,
PRL 87, 120406 (2001)
BCS-BEC crossover (generalized Cooper pairs)
For example Eagles, Leggett, Nozieres and
Schmitt-Rink, Randeria, Strinati, Zwerger,
Holland, Timmermans, Griffin, Levin
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BCS-BEC landscape
M. Holland et al., PRL 87, 120406 (2001)
BEC
transition temperature
BCS
interactions
energy to break fermion pair
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Changing the interaction strength in real time
FAST
repulsive
2 ms/G
DB
gt
attractive
molecules
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Changing the interaction strength in real time
FAST
40 ms/G
DB
gt
attractive
molecules
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Changing the interaction strength in real time
SLOW
4000 ms/G
initial T/TF0.19 0.06
DB
gt
attractive
molecules
M. Greiner, C. Regal, and D. S. Jin, Nature 426,
537 (2003)
Cubizolles et al., PRL 91, 240401 (2003) L. Carr
et al., PRL 92, 150404 (2004)
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Observing a Fermi condensate
repulsive
40 ms/G
DB
gt
attractive
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Observing a Fermi condensate
repulsive
40 ms/G
DB
gt
T/TF0.08
attractive
DB 0.12 G DB 0.25 G DB0.55 G
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BCS-BEC Crossover
Condensate fraction
0
? BEC (molecules) BCS
(atoms) ?
C. Regal, M. Greiner, and D. S. Jin, PRL 92,
040403 (2004)
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Current group members
J. Stewart, M. Greiner, J. Goldwin, T. Nicholson,
C. Regal, J. Lang, M. Phelps, M. Olsen