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Spin-3 dynamics study in a chromium BEC

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

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  1. 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

  2. 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

  3. R Chromium (S=3): contact AND dipole-dipole interactions Dipole-dipole interaction potential Anisotropic Non local anisotropic mean-field Coupling between spin and rotation

  4. PART ONE OF THE TALK DIPOLAR RELAXATION INHIBITION SPIN DYNAMICS IN AN OPTICAL LATTICE How a 2D lattice can stabilize an unstable BEC ?

  5. 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

  6. 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…

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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)

  12. 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…

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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)

  19. 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:

  20. 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

  21. 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

  22. 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

  23. 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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