Diapositive 1

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Recent results with ultra cold chromium atoms

• de Paz (PhD), B. Naylor, A. Sharma (post-doc), A.
  Chotia (post doc), J. Huckans (visitor), O.
  Gorceix , E. Maréchal,
• L. Vernac , P. Pedri, B. Laburthe-Tolra

Collaborations L. Santos (Hannover) Students
Antoine Reigue, Ariane
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Outline
Quantum Magnetism with ultracold bosons
Production of a chromium Fermi sea
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Quantum Magnetism what is it about?
What is (are) the (quantum) phase(s) of a given
crystal at "low" T ?
Heisenberg Hamiltonian
ferromagnetic
anti ferromagnetic
Magnetism ie quantum phases not set by ddi but by
exchange interactions
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Quantum Magnetism with cold atoms
tunneling assisted super exchange
U
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Quantum Magnetism with a dipolar species in a 3D
lattice
Vdd
Exchange term
Ising term
magnetic dipole moment
dipole-dipole interactions
direct spin-spin interaction
real spin
S3
quantum regime, high filling factor
long range beyond the next neighbor
T lt 1 nK
Vdd 10-20 Hz
to reach ground state
Spin dynamics in an out of equilibrium system
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Quantum Magnetism with a chromium BEC in a 3D
lattice
S3
Cr BEC loaded in a 3D lattice a Mott state
spin preparation, measurement of the evolution of
the Zeeman states populations
different spin dynamics induced by dipole-dipole
interactions
spin exchange
constant magnetization
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dipolar relaxation
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change of the magnetization
1
0
-1
-2
magnetization
-3
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Dipolar relaxation in a 3D lattice - observation
of resonances
nx , ny , nz
kHz
(Larmor frequency)
1 mG 2.8 kHz
width of the resonances tunnel effect B field,
lattice fluctuations
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Spin exchange dynamics in a 3D lattice
10 mG
B
0
first resonance
dipolar relaxation suppressed evolution at
constant magnetization
spin exchange from -2
experimental sequence
-1
-2
Load optical lattice
-3
state preparation in -2
vary time
Stern Gerlach analysis
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Different Spin exchange dynamics in a 3D lattice
Contact interaction (intrasite)
expected Mott distribution
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Different Spin exchange dynamics in a 3D lattice
Contact interaction (intrasite)
Dipole-dipole interaction (intersite)
without spin changing term
dipolar relaxation with
doublons removed only singlons
expected Mott distribution
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Spin exchange dynamics in a 3D lattice with only
singlons
the spin populations change!
E(ms) q mS2
comparison with a plaquette model (Pedri,
Santos) 33 sites , 8 sites containing one atom
1 hole quadratic light shift and tunneling taken
into account
measured with interferometry
Proof of intersite dipolar coupling Many Body
system
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Spin exchange dynamics in a 3D lattice with
doublons at long time scale
intersite dipolar coupling
result of a two site model
two sites with two atoms dipolar rate
raised (quadratic sum of all couplings)
our experiment allows the study of molecular Cr2
magnets with larger magnetic moments than
Cr atoms, without the use of a Feshbach resonance
not fast enough the system is many body
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Dipolar Spin exchange dynamics with a new
playground a double well trap
idea direct observation of spin exchange with
giant spins, "two body physics"
compensating the increase in R by the number of
atoms
realization load a Cr BEC in a double well
trap selective spin filp
R
-3
3
N atoms
N atoms
frequency of the exchange precession of one spin
in the B field created by N spins at R
R 4 µm
j 3
N 5000
Hz
B field created by one atom
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Spin exchange dynamics in a double well trap
realization
realizing a double well
spin preparation
RF spin flip in a non homogeneous B field
3
-3
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Spin exchange dynamics in a double well trap
results
No spin exchange dynamics
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Inhibition of Spin exchange dynamics in a double
well trap interpretation (1)
What happens for classical magnets?
evolution in a constant external B field
evolution of two coupled magnetic moments
q
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Inhibition of Spin exchange dynamics in a double
well trap interpretation (2)
What happens for quantum magnets in presence of
an external B field when S increases?
Evolution of two coupled magnetic moments in
presence of an external B field
no spin changing terms
2S1 states Ising contribution gives
different diagonal terms
Ising term
Exchange term
"half period" of the exchange grows exponentially
if
no more exchange possible
no complete exchange
It is as if we had two giant spins interacting
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Contact Spin exchange dynamics from a double well
trap after merging
after merging
without merging
Spin exchange dynamics due to contact interactions
Fit of the data with theory gives an estimate of
a0 the unknown scattering length of chromium
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Production of a degenerate quantum gas of
fermionic chromium
Two very different quantum statistics
or TltltTF
T gt Tc
T lt Tc
a quantum gas at TltltTF
a quantum gas at TltTc
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Production of a degenerate quantum gas of
fermionic chromium
A quantum gas ?
3D harmonic trap
Degeneracy criteria
Chemical Potential
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Production of a degenerate quantum gas of
fermionic chromium
So many lasers
7P4
53Cr MOT Trapping beams sketch
53Cr MOT laser frequencies production
7S3
Lock of TiSa 2 is done with an ultrastable cavity
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Production of a degenerate quantum gas of
fermionic chromium
Loading a one beam Optical Trap with ultra cold
chromium atoms
direct accumulation of atoms from the MOT in
mestastable states
RF sweep to cancel the magnetic force of the MOT
coils
crossed dipole trap
for 53Cr finding repumping lines
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Production of a degenerate quantum gas of
fermionic chromium
Spectroscopy and isotopic shifts
isotopic shifts unknown
5D J3 ?7P J3 for the 52    //  5D J3 F9/2
?7P J3 F9/2   for the 53
Shift between the 53 and the 52 line 1244 /-10
MHz
Deduced value for the isotopic shift Center
value 1244 -156.7 8 1095.3 MHz
Uncertainty /-(1010) MHz (our experiment) /-8
MHz (HFS of 7P3)

• isotopic shift
• mass term
• orbital term

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Production of a degenerate quantum gas of
fermionic chromium
Strategy to start sympathetic cooling
make a fermionic MOT, load the IR trap with 53Cr
more than 105 53Cr
about 106 52Cr
make a bosonic MOT, load the IR trap with 52Cr
6.105 52Cr
3.104 53Cr
inelastic interspecies collisions limits to
not great, we tried anyway
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Production of a degenerate quantum gas of
fermionic chromium
Evaporation
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Production of a degenerate quantum gas of
fermionic chromium
Why such a good surprise?
Maybe we reach the hydrodynamic regime for the
fermions
then fermions are trapped by collisions
If collisions with bosons set the mean free path
of fermions below the trap radius
How to measure Fermion-Boson collision cross
section? By heating selectively and quickly the
bosons and then measure fermions thermalization
very preliminary measurements analysis
support this interpretation
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Production of a degenerate quantum gas of
fermionic chromium
Results
In situ images
Expansion analysis
Nat
parametric excitation of the trap
trap frequencies
Temperature
slightly degenerated
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Production of a degenerate quantum gas of
fermionic chromium
What can we study with our gas?
9/2
Fermionic magnetism
7/2
5/2
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Phase separation
very different from bosonic magnetism !
3/2
2
1/2
Picture at T 0 and no interactions
1
-1/2
0
-3/2
-1
-5/2
-2
-7/2
-3
-9/2
Boltzmann
T10 nK
Population in mF-9/2
requires good in situ imaging
T50 nK
Fermi T0
T200 nK
minimize Etot
Larmor frequency (kHz)
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thank you for your attention!
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Dipolar Quantum gases
van-der-Waals Interactions
dipole dipole interactions
BEC
Tc few 100 nK
Anisotropic Long Range
Isotropic Short range
comparison of the interaction strength
for
the BEC can become unstable
polar molecules
alcaline
chromium
dysprosium
for 87Rb
erbium
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Preparation in an atomic excited state
mS -2
-3
energy
creation of a quadratic light shift
Raman transition
s-
p
-1
A s- polarized laser Close to a J?J
transition (100 mW 427.8 nm)
-2
-1
-3
quadratic effect (laser power)
-2
-3
laser power
-3
-2
transfer adiabatic
transfer in -2 80
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Dipolar Relaxation in a 3D lattice
kinetic energy gain
Ec is quantized
dipolar relaxation is possible if
selection rules
If
the atoms in doubly occupied sites are expelled
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Spin exchange dynamics in a 3D lattice with
doublons at short time scale
initial spin state
onsite contact interaction
spin oscillations with the expected period strong
damping
contact spin exchange in 3D lattice Bloch PRL
2005, Sengstock Nature Physics 2012
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Different Spin exchange dynamics with a dipolar
quantum gas in a 3D lattice
intrasite contact
intersite dipolar
expected Mott distribution
Heisenberg like hamiltonian
quantum magnetism with S3 bosons and
true dipole-dipole interactions
doublons removed only singlons
intersite dipolar
de Paz et al, Arxiv (2013)
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Spin changing collisions
V'
V
-V
-V'
from the ground state
from the highest energy Zeeman state
-1
3
dipolar relaxation
-2
2
-3
1
after an RF transfer to ms3 study of the
transfer to the others mS
spin changing collisions become possible at low B
field
dipole-dipole interactions induce a spin-orbit
coupling
rotation induced
the Cr BEC can depolarize at low B fields
At low B field the Cr BEC is a S3 spinor BEC
Cr BEC in a 3D optical lattice coupling between
magnetic and band excitations
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Spin changing collisions
V'
V
-V
-V'
from the ground state
-1
1 mG
(a)
-2
0.5 mG
(b)
0.25 mG
-3
(c)
 0 mG 
(d)
spin changing collisions become possible at low B
field
-3
-2
-1
0
1
2
3
As a6 gt a4 , it costs no energy at Bc to go from
mS-3 to mS-2 stabilization in interaction
energy compensates for the Zeeman excitation
the Cr BEC can depolarize at low B fields
At low B field the Cr BEC is a S3 spinor BEC