Transcript Slide 1

Bose-Fermi Degeneracy
in a
Micro-Magnetic Trap
Seth A. M. Aubin
University of Toronto / Thywissen Group
February 25, 2006
CIAR Ultra-cold Matter Workshop, Banff.
Work supported by NSERC, CFI, OIT, PRO and Research Corporation.
Outline
 Motivation
 Micro-magnetic traps and apparatus
 Boson and Fermion degeneracy
 Surprises in Rb-K scattering
 Future experiments
Why ultra-cold bosons and fermions?
Objectives:
 Condensed matter physics.
 Boson-fermion mixtures.
 Atom interferometry.
Why on a chip?
Advantages:
 Short experimental cycle.
 Single UHV chamber.
 Complex multi-trap geometries.
Micro-Magnetic Trap
Technology:
 Electroplated gold wires on a silicon substrate.
 Manufactured by J. Estève (Aspect/Orsay).
Z-trap current
Trap Potential: Z-wire trap
Iz
defects
Evaporated Ag and Au (B. Cieslak and S. Myrskog)
RF for evaporation
Light-Induced Atom Desorption (LIAD)
Conflicting pressure requirements:
• Large Alkali partial pressure  large MOT.
• UHV vacuum  long magnetic trap lifetime.
Solution: Use LIAD to control pressure dynamically !
 405nm LEDs (power=600 mW) in a pyrex cell.
Rapid
High Efficiency
Bose-Fermi Degeneracy
High Efficiency Evaporation of 87Rb
10-13
thermal
atoms
10-6
MOT
magnetic
trapping
105
1
evap.
cooling
PSD
BEC
Evaporation Efficiency

d ln(PSD)
 3.95  0.1
d ln(N)
87Rb
BEC
[email protected] MHz:
[email protected] MHz:
[email protected] MHz:
N = 7.3x105, T>Tc
N = 6.4x105, T~Tc
N=1.4x105, T<Tc
Surprise! Reach Tc with
only a 30x loss in number.
(trap loaded with 2x107 atoms)
 Experimental cycle = 5 - 15 seconds
Sympathetic Cooling
of fermionic 40K with bosonic 87Rb
104
Phase Space Density
102
100
105
106
107
Cooling Efficiency
10-2

10-4
10-6
10-8
Atom Number
 ln(PSD)
8
 ln(N)
Non-Gaussian Distribution
Fit:
EF
Optical Density
1st signature of Fermi Degeneracy
N = 4104
TF = 960 nK
T/TF = 0.14(2)
0
z = 1.4103
2 |Fermi  0.9
Fit Residuals
Residuals:
200
400
Radial distance (m)
2 |Gaussian  2.2
0
200
400
Radial distance (m)
Non-Thermal
Distribution
Pauli Pressure -- 2
nd
signature of Fermi Degeneracy
EK,release/EF
EF
Fermi
Boltzmann
Gaussian Fit
kTRb/EF
Surprises
with Rb-K
cold collisions
Naïve Scattering Theory
Collision Rates
Rb-Rb
Rb-K
 RbRb  nRb RbRb vRbRb
 RbK  nRb RbK vRbK
2
4aRbK
2
8aRbRb
aRbRb  5.238nm
aRbK  10.8 nm
 RbK
 2.7
 RbRb
Sympathetic cooling
should work really well !!!
Sympathetic cooling 1st try:
 “Should just work !” -- Anonymous
 Add 40K to 87Rb BEC  No sympathetic cooling observed !
Experiment:
Sympathetic cooling only works
for slow evaporation
Evaporation 3 times slower
than for BEC
4
10
Phase Space Density
102
100
105
106
10-2
10-4
10-6
10-8
Atom Number
107
Cross-Section Measurement
TK40 (K)
Thermalization of 40K with 87Rb
Rb-K cross-section (nm2)
What’s happening?
Future Experiments … come see the poster
Pauli Blocking of light
scattering:
 Fermi sea reduces number of
states an excited atom can recoil into.
 Atomic lifetime increases,
linewidth decreases.
B. DeMarco and D. Jin, Phys. Rev. A 58, R4267 (1998).
Species-specific
trapping potentials ?
 Bosons and fermions in different
trapping potentials.
 Isothermal “cooling” of fermions
with bosons.
 Boson-mediated interaction of
fermions in an optical lattice.
… or use a “magic” wavelength for Rb and K.
C. Precilla and R. Onofrio, Phys. Rev. Lett.90, 030404 (2003).
Summary

87Rb
BEC with up to 2105 atoms.
 cycle time as short as 5 s.
 40K Fermi degeneracy: T/TF~0.1
with 4104 atoms.
 Sympathetic cooling to 0.1TF in 6 s.
 cycle time of 30 s.
 Observation of severe reduction of Rb-K
scattering cross-section at high T.
 Bose-Fermi degeneracy in a chip trap.
First time on a chip !
arXiv: cond-mat/0512518
EF
Thywissen Group
S. Aubin
D. McKay
B. Cieslak
M. H. T. Extavour
S. Myrskog
A. Stummer
Colors:
Staff/Faculty
Postdoc
Grad Student
Undergraduate
L. J. LeBlanc
J. H. Thywissen