Spin 3 dynamics study in a chromium bec
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Spin-3 dynamics study in a chromium BEC. CLEO/Europe-EQEC Conference Munich – 23 May 2011. Olivier GORCEIX. Laboratoire de Physique des Lasers Université Paris Nord Villetaneuse - France. Interactions in BECs. Van der Waals / contact interactions :

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Spin 3 dynamics study in a chromium bec l.jpg

Spin-3 dynamicsstudy in a chromium BEC

CLEO/Europe-EQEC Conference

Munich – 23 May 2011

Olivier GORCEIX

Laboratoire de Physique des Lasers

Université Paris Nord

Villetaneuse - France


Slide2 l.jpg

Interactions in BECs

Van der Waals / contact interactions :

short range and isotropicat low T

Effective potential aSd(R), with aS = scattering length in channel S,

aS is magnetically tunable through Feshbach resonances

Dipole-dipole interactions : long range and anisotropic

magnetic atoms Cr,Er, Dy; dipolar molecules; Rydberg atoms

Chromium atoms carry a magnetic moment of 6µB

MDDI are 36 times bigger than in alkali BECs

We produce and study52CrBose-Einstein Condensates


Slide3 l.jpg

R

Chromium (S=3): contact AND dipole-dipole interactions

Dipole-dipole interaction potential

Anisotropic

Non local anisotropic mean-field

Coupling between spin and rotation


Slide4 l.jpg

PART ONE OF THE TALK

DIPOLAR RELAXATION INHIBITION

SPIN DYNAMICS IN AN OPTICAL LATTICE

How a 2D lattice can stabilize an unstable BEC ?


Slide5 l.jpg

3

Inelastic collisions - dipolar relaxation DR

2

1

0

-1

-2

-3

DR causes heating then losses

and the magnetic field strength

controls DR

Zeeman energy

ladder

in B


Slide6 l.jpg

Inelastic collisions - dipolar relaxation DR

3

B

Two allowed channels for ground state atoms

2

1

0

-1

-2

-3

Angular momentum conservation

Induces rotation in the BEC ?

Spontaneous creation of vortices ?

Einstein-de-Haas effect

K. Gawryluk et al. PRL 106, 140403 (2011)

M. Gajda, PRL 99, 130401 (2007)

B. Sun and L. You, PRL 99, 150402 (2007)

Plus many other…


Slide7 l.jpg

Dipolar relaxation in optical lattices – Experimental procedure

Adiabatic loading of the ground state M = - 3 BEC in a 1D or a 2D lattice

Rf induced transition to the metastable state M = +3

Hold time while DR takes place

RF sweep back to M = -3

Detection

3

3

2

2

1

1

0

0

Rf sweep 1

Rf sweep 2

-1

-1

Load optical lattice

-2

-2

detect m=-3

get the DR rate

-3

-3

BEC m=+3, hold time

Produce BEC m=-3

Band mapping

adiabatic


Dipolar relaxation in optical lattices l.jpg
Dipolar relaxation in optical lattices

Optical lattices:

periodic potential = ac-Stark shift in a far-detuned standing wave

Latticesprovidetightlyconfinedgeometries (tubes or pancakes)

Theyintroduce a new energyscaleand they change the initial and final states space.

B

Above Bc , DR releases energy and creates a « mini-vortex » in a lattice site

m=3

m=2


B 40 mg l.jpg

3

2

1

B ≈ 40 mG

Spin relaxation and band excitation in optical lattices

Spin-flipped atoms get promoted

from the lowest band to the excited bands

when B is over the threshold set by

B


Slide10 l.jpg

What do we measure in 1D ? (band mapping for B above threshold)

3

2

Velocity distribution

along the tubes

1

0

-1

-2

-3

m=3

m=2

Population in different bands

due to dipolar relaxation

Heating due to collisional de-excitation from excited band


Slide11 l.jpg

Dipolar relaxation inhibition in 1D (belowBc)

Conversely (almost) complete suppression of dipolar relaxation in 1D

at low field in 2D lattices

B. Pasquiou et al., Phys. Rev. Lett. 106, 015301 (2011)


Slide12 l.jpg

PRA 81,

042716 (2010)

3D trap

Log scale !!

pancakes

Phys. Rev. Lett.

106, 015301 (2011)

tubes

Above threshold: Vortices are expected to pop up

(EdH effect) but

they fade away by tunneling

Below threshold:

a (spin-excited) metastable 1D quantum gas (>100ms) ;

Interest for spinor physics, spin excitations in 1D…


Slide13 l.jpg

PART TWO OF THE TALK

SPONTANEOUS DEMAGNETIZATION OF A SPINOR BEC

What happens at extremely low magnetic fields ?

ie when

This happens for B <few 0.1 mG


Slide14 l.jpg

S=3 Spinor physics with free magnetization

  • - To date, spinorstudies have been restricted to S=1 and S=2

  • Up to now, all spinordynamicsstudieswererestricted to

  • constant magnetization

  • As required by contact interactions

  • eg in Rb a pair of collidingatomsstays in the M = mS1+mS2 = 0 multiplicity

1

0

-1

3

  • New featureswith Cr

  • First S=3spinor

  • Dipole-dipole interactions freethe magnetization

  • Possible investigation

  • of the truemany-body ground state of the system

  • (whichrequires stable and verysmallmagneticfields)

2

1

0

-1

-2

-3


Slide15 l.jpg

3

2

1

0

-1

-2

-3

Ferromagnetic phase of the spinor condensate

-1

when B ≈4 mG the chemical potential

is much smaller than the Zeeman splittings

Starting point :

BEC in the m= -3 single particle ground state

Procedure:

lower B for example to 1mG

Detection TOF + Stern-Gerlach

-2

Ferromagnetic / polarized phase

-3

Above threshold


When the final b 0 4 mg then the chemical potential becomes on the order of zeeman splittings l.jpg

3

2

1

0

-1

-2

-3

when the final B ≈0.4 mG then the chemical potential becomes on the order of Zeeman splittings

Spontaneous demagnetization of the spinor condensate

Starting point : BEC in the m= -3 single particle ground state

Procedure: lower the magnetic field

Detection TOF + Stern-Gerlach

3

2

1

0

-1

-2

-3

…spin-flipped atoms gain energy

Above threshold

Below threshold


Slide17 l.jpg

S=3 multi-component BEC with free magnetization

7 Zeeman states; all trapped

four scattering lengths: a6, a4, a2, a0

3

Phase transitions can occur between:

Ferromagnetic / Polar/ Cyclic phases

2

3

1

2

ferromagnetic

i.e. polarized in the lowest energy single particle state

0

1

0

-1

-1

-2

-2

-3

-3

Magnetic field

Santos PRL 96,

190404 (2006)

Ho PRL. 96,

190405 (2006)

a0/a6

Spontaneous demagnetization : the reason why it happens !

at Bc, it costs no energy to go from m= -3 to m= -2 : loss in interaction energy compensates for the gain in Zeeman energy)

Contact interactions decide which spin configuration has the lowest energy

Phasesset by contact interactions

differ by their magnetization


Slide18 l.jpg

Mean-field effect: when does the transition take place ?

Magnetic field control below 0.5 mG (actively stabilized)

(0.1mG stability)

(no magnetic shield…)

Critical field for depolarization depends on the density (exp check for linearity)


Slide19 l.jpg

Dynamics of the depolarization

Spontaneous demagnetization :

how it happens ? What triggers the transistion ?

Remaining atoms in m=-3

Phases are set by contact interactions,

while (de)magnetizationdynamics is due to and set by

dipole-dipole interactions

« quantum magnetism »

Time (ms)

  • B. Pasquiou et al., Phys. Rev. Lett.

  • (in the press) and arXiv:1103.4819

(a few in 3D to 10 ms in the 2D lattice)

Timescale for depolarization:


Slide20 l.jpg

Conclusion

Dipolar relaxation in bulk BECs and in reduced dimensions

Spontaneous demagnetization in a quantum gas

-phase transition

-first steps towards evidence for spinor S=3 ground state structure -spinor thermodynamics with free magnetization

Outlook

Towards a demonstration of Einstein-de-Haas effect - rotation in 3D lattice sites

Excitation spectrum (poster tomorrow and PRL 105, 040404 (2010))

Project : quantum gas of dipolar fermionic 53Cr


Acknowledgements l.jpg
Acknowledgements

Financial support:

  • Conseil Régional d’Ile de France (Contrat Sésame)

  • Ministère de l’Enseignement Supérieur et de la Recherche (CPER, FNS and ANR)

  • European Union (FEDER)

  • IFRAF

  • CNRS

  • Université Paris Nord


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Group members : The Cold Atom Group in Paris Nord

Ph.D students:

Gabriel Bismut

Benjamin Pasquiou

www-lpl.univ-paris13.fr:8082

Permanent staff:

Bruno Laburthe-Tolra, Etienne Maréchal, Paolo Pedri, Laurent Vernac and O. G.

Former members

Arnaud Pouderous, Radu Chicireanu, Quentin Beaufils, Thomas Zanon,

Jean-ClaudeKeller, René Barbé


Slide23 l.jpg

Post-doc position available (Q4 2011) applynow!!

www-lpl.univ-paris13.fr:8082

E.Maréchal, OG, P. Pedri, Q. Beaufils (PhD), B. Laburthe, L. Vernac, B. Pasquiou (PhD), G. Bismut (PhD), D. Ciampini (invited), M. Champion, JP Alvarez (trainees)


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