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The anisotropic excitation spectrum of a chromium Bose-Einstein Condensate

The anisotropic excitation spectrum of a chromium Bose-Einstein Condensate. 44th EGAS Göteborg – July 11, 2012. Olivier GORCEIX. Laboratoire de Physique des Lasers Université Sorbonne Paris Cité Villetaneuse - France. Interactions within a BEC.

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The anisotropic excitation spectrum of a chromium Bose-Einstein Condensate

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  1. The anisotropic excitation spectrum of a chromium Bose-Einstein Condensate 44th EGAS Göteborg – July 11, 2012 Olivier GORCEIX Laboratoire de Physique des Lasers Université Sorbonne Paris Cité Villetaneuse - France

  2. Interactions within a BEC Van der Waals / contact interactions : isotropic and short ranged Effective potentiel proportionnal to aSd(R), with aS = scattering length, aS adjustable thanks to Feshbach resonances Dipole-dipole interactions: anisotropic and long-ranged highly magnetic atomsCr,Er, Dy, dipolar molecules; Rydberg atoms Chromium atoms carry a permanent magnetic dipole of 6µB MDDI are 36 times bigger than in alkali BECs but still edd (Cr)=0.159 while edd (Rb)=0.0044 where edd quantifies the ratio dipolar / contact interactions

  3. R Chrome (S=3): both contact AND dipolar interactions Dipole-dipole interaction potential Anisotropy Mean field becomes non local and anisotropic Spin and rotation are coupled is equal to 0.16

  4. First reported effects of DDIs on BECs with Cr BECs Striction of the BEC (non local effect) Vdd adds a non local anisotropic mean-field DDIs Eberlein et al, PRL 92, 250401 (2004) Anisotropy in the BEC expansion Pfau et al,PRL 95, 150406 (2005) The effects of DDIs are experimentally evidenced by differential measurements, for two orthogonal orientations of the B field DDIs change in the few % range the physics of a ground state BEC Bismut et al., PRL 105, 040404 (2010)

  5. attraction repulsion 1st PART COLLECTIVE OSCILLATIONS Impact of the dipolar interactions on their frequencies A small correction induced by dipolar interactions

  6. Collective excitations of a dipolar BEC MDDI are anisotropic, they impact on the q-pole excitation eigenfrequencies of a trapped BEC In this mode, oscillations along y and z are in opposition Parametric excitation We repeat the experiment for two orthogonal orientations of B 15 000 atoms

  7. Trap geometry dependence of the measured frequency shift BEC always stretches along B Shift of the quadrupole mode frequency (%) Shift of the aspect ratio (%) While the sign of the quadrupole shift depends on the trap geometry Theory PRL 92, 250401 (2004) This exp: Bismut et al., PRL 105, 040404 (2010) Trap anisotropy Good agreement with Thomas-Fermi predictions

  8. e 2nd PART RAMAN-BRAGG SPECTROSCOPY OF A DIPOLAR BEC Sound velocity anisotropy induced by dipolar interactions A more pronounced dipolar effect

  9. Excitation spectrum of a BEC with pure contact interactions c is the sound velocity c is also the critical velocity for superfluidity in the Landau model Rev. Mod. Phys. 77, 187 (2005) where gc= 4pħ2a /m Bogoliubov spectrum when healing length Quasi-particles, phonons free particles

  10. Excitation spectrum of the BEC withDDIs becomes: if , and if , A 20% effect expected on the speed of sound ! Much larger than the (~3%) effects for striction and collective excitations

  11. absorption image after TOF of 5 ms or B┴ frequencies W and W+w Angular separation a Profile at resonance ie when ħ w = e(q)

  12. Raman-Bragg spectroscopy of a BEC Moving lattice upon the BEC a=14° Phonon regime W W+w a From the excitation spectrum we infer the speed of sound Lattice beams crossing with an angle and a detuned causes resonant momentum exchange when a

  13. Anisotropic speed of sound   c┴ ≠ c Width of resonance curve: finite size effects (inhomogeneous broadening) Speed of sound depends on the relative angle between spin and excitation wavevector

  14. Anisotropic speed of sound Bismut el al, arXiv :1205.6305 Good agreement between theory and experiment:

  15. 3 2 1 0 -1 -2 -3 AtultralowT ≈ 300nK and B ≈ 40 nTthe chemicalpotentialbecomesgreaterthan the Zeeman splitting-> NEW PHYSICS Magnetism and thermodynamics - spinor physics TOF + Stern-Gerlach Above and Below threshold Spontaneous demagnetization Spin 3 phase diagram Pasquiou et al, PRL 106, 255303 and 108, 045307

  16. Conclusion Dipolar interactions induce anisotropy in the BEC excitation spectra - collective modes; - phonon dispersion law : sound velocity Magnetism and thermodynamics of a spin 3 BEC with free magnetisation Perspectives Extension of our work on magnetism to strongly correlated states in 3D optical latttices Einstein-de-Haas effect: spin-rotation coupling Improved spatial resolution for in situ imaging of magnetization domains Extension to a Fermi sea of 53Cr atoms

  17. The chromium BEC crew www-lpl.univ-paris13.fr:8082 E.Maréchal, OG, P. Pedri, Q. Beaufils (PhD), B. Laburthe, L. Vernac, B. Pasquiou (PhD), G. Bismut (PhD)

  18. Thank you for your attention … PhD students welcomed in our group…

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