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Elastic and inelastic dipolar effects in chromium BECs

Elastic and inelastic dipolar effects in chromium BECs. Laboratoire de Physique des Lasers Université Paris Nord Villetaneuse - France. B. Laburthe-Tolra. B. Pasquiou. P. Pedri. E. Maréchal. O. Gorceix . G. Bismut. L. Vernac. A. Crubellier (LAC- Orsay).

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Elastic and inelastic dipolar effects in chromium BECs

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  1. Elastic and inelasticdipolareffects in chromiumBECs Laboratoire de Physique des Lasers Université Paris Nord Villetaneuse - France B. Laburthe-Tolra B. Pasquiou P. Pedri E. Maréchal O. Gorceix G. Bismut L. Vernac A. Crubellier (LAC- Orsay) Former PhD students and post-docs:Q. Beaufils, T. Zanon, R. Chicireanu, A. Pouderous Former members of the group: J. C. Keller, R. Barbé

  2. Chromium : S=3 Dipole-dipole interactions Long range Anisotropic Non local anisotropic meanfield • Static and dynamic properties of BECs

  3. Inelastic dipolar effects Anisotropic dipole-dipole interactions Spin degree of freedom coupled to orbital degree of freedom - Dipolar relaxation - Spinor physics and magnetization dynamics

  4. Outline 1 Elastic Collective excitations in a Cr BEC 2 Inelastic Control of dipolar relaxation in opticallattices Spontaneousdemagnetizationdynamics in opticallattices

  5. Modification of BEC expansion due to dipole-dipole interactions TF profile Striction of BEC (non local effect) Parabolic ansatz is still a good ansatz Eberlein, PRL 92, 250401 (2004) Pfau,PRL 95, 150406 (2005)

  6. Collective excitations of a dipolar BEC Due to the anisotropy of dipole-dipole interactions, their effects on the BEC depend on the relative orientation of the magnetic field and the axis of the trap Parametric excitations: Repeat the experiment for two directions of the magnetic field (differential measurement)

  7. Trap geometry dependence of the measured frequency shift BEC always stretches along B Sign of quadrupole shift depends on trap geometry Shift of the quadrupole mode frequency (%) Shift of the aspect ratio (%) • Related to the trap anisotropy Eberlein, PRL 92, 250401 (2004) Good agreement with Thomas-Fermi predictions Large sensitivity of the collective mode to trap geometry unlike the striction of the BEC

  8. Outline 1 Elastic Collective excitations in a Cr BEC 2 Inelastic Control of dipolar relaxation in opticallattices Spontaneousdemagnetizationdynamics in opticallattices

  9. Angle between dipoles Dipolar relaxation 3 2 1 0 -1 Angular momentum conservation -2 -3 - Two channels for dipolar relaxation in m=3 (no DR in m=-3): Rotate the BEC ? Spontaneous creation of vortices ? (Einstein-de-Haas effect) Need of an extremely good control of B close to 0 Santos, PRL 96, 190404 (2006) See also Ueda, PRL 96, 080405 (2006)

  10. How to observe the Einstein-de Haas effect ? Ideas to ease the magnetic field control requirements Create a gap in the system: B now needs to be controlled around a finite non-zero value • Go to very tightly confined geometries (BEC in 2D optical lattices) Energy to nucleate a « mini-vortex » in a lattice site (~120 kHz) A gain of two orders of magnitude on the magnetic field requirements !

  11. Dipolar relaxation in a Cr BEC 3 2 1 Rf sweep 1 Rf sweep 2 0 -1 -2 Produce BEC m=-3 -3 BEC m=+3, vary time detect BEC m=-3 Fit of decay givesb Born approximation Pfau, Appl. Phys. B, 77, 765 (2003) Determines scattering lengths See also Shlyapnikov PRL 73, 3247 (1994) Never observed up to now

  12. Reduction of dipolar relaxation in optical lattices Load the BEC in a 1D or 2D Lattice (retro-reflected Verdi laser) Rf sweep 1 Rf sweep 2 Load optical lattice BEC m=+3, vary time Produce BEC m=-3 detect m=-3 Look at heating; deduce b One expects a reduction of dipolar relaxation, as a result of the reduction of the density of states in the lattice

  13. 3D 2D Dipolar relaxation rate parameter 10-19 m3 s-1 1D Strong reduction of dipolar relaxation when Almost complete suppression below a threshold at 1D

  14. Suppression of dipolar relaxation in 1D: the result of cylindrical symmetry (angular momentum conservation) q Dipolar relaxation rate (u.a.) • Below threshold, suppresion of DR is most efficient if : • Lattice sites are cylindrical • - The magnetic field is parallel to the 1D gases 0.8 0.6 Dipolar relaxation rate (u.a.) 0.4 Above threshold : should produce vortices in each lattice site (EdH effect) … in progress … (problem of tunneling) 0.2 q

  15. Outline 1 Elastic Collective excitations in a Cr BEC 2 Inelastic Control of dipolar relaxation in opticallattices Spontaneousdemagnetizationdynamics in 1D gases

  16. Spinor ground state at low magnetic field S=3 7 Zeeman states; all trapped four scattering lengths, a6, a4, a2, a0 A rich spinor diagram at low magnetic field Different quantum phases at relatively « large » magnetic fields (mG) due to very different scattering lengths Santos and Pfau PRL 96, 190404 (2006) Diener and Ho PRL. 96, 190405 (2006) 3 3 2 2 1 1 0 0 -1 -1 -2 -2 -3 -3 Magnetizationdynamics set by dipole-dipole interactions

  17. At VERY low magnetic fields, spontaneous depolarization of 1D quantum gases Load optical lattice vary time Produce BEC m=-3 Rapidly lower magnetic field Stern Gerlach experiments

  18. A quench through a phase transition ? Fraction in m=-3 Magnetic field (kHz) 3 2 1 0 -1 -2 -3

  19. Magnetization dynamics in an increased meanfield « Critical » field depends on chemical potential Fraction in m=-3 Magnetic field (kHz) Td Strong heating in lattice, unrelated to depolarization, still unaccounted for …in progresss… Remains below Td Td Temperature (mK) Time (ms)

  20. (many more?) open questions: Tensor light-shift: 3 -3 2 -2 1 -1 0 Effect of non zero temperature ? Effects on dynamics, phase diagram… Santos PRA 75, 053606 (2007) In which timescale will we reach the new phase ? How to describe dipolar relaxation ? Effects of 1D ?

  21. Conclusion Collective excitations – good agreement with theory Dipolar relaxation in reduced dimensions - towards Einstein-de-Haas Spontaneous demagnetization in a quantum gas – first steps towards spinor ground state (BECs in strong rf fields) (rf-assisted relaxation) (rf association) (d-wave Feshbach resonance) (MOT of 53Cr) Production of a (slightly) dipolar Fermi sea Load into optical lattices – superfluidity ? Perspectives

  22. L. Vernac E. Maréchal J. C. Keller G. Bismut Paolo Pedri B. Laburthe B. Pasquiou Q. Beaufils O. Gorceix Have left: Q. Beaufils, J. C. Keller, T. Zanon, R. Barbé, A. Pouderous, R. Chicireanu Collaboration:Anne Crubellier (Laboratoire Aimé Cotton)

  23. Influence of the BEC atom number • In our experiment, MDDI is not much larger than quantum kinetic energy Simulations with Gaussian anzatz It takes three times more atoms for the frequency shift of the collective mode to reach the TF predictions than for the striction of the BEC

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