B. Pasquiou (PhD), G. Bismut (PhD) B. Laburthe, E. Maréchal, L. Vernac ,

1. Bragg Spectroscopy of a dipolar BEC B. Pasquiou (PhD), G. Bismut (PhD) B. Laburthe, E. Maréchal, L. Vernac, P. Pedri, O. Gorceix (Group leader) We study the effects of Dipole-Dipole Interactions (DDIs) in a 52 Cr BEC

2. Bragg Spectroscopy of a dipolar BEC I- Dipole – dipole interactions (DDIs) how have they been evidenced in ground state BECs how to see a larger effect ? (with the excitations) II- Excitations of a BEC dispersion of an excited BEC modifications induced by the DDIs III- Experimental realization difficulties… IV- Experimental results excitation spectra modified by DDIs

3. R Different interactions in a BEC polarized BEC GPE : Van-der-Waals interactions Dipole-dipole interactions (DDIs) Isotropic Short Range Anisotropic Long Range Relative strength of dipole-dipole and Van-der-Waals interactions the BEC is unstable for polar molecules alkaline chromium dysprosium for 87Rb

4. Some effects of DDIs on BECs for Cr BECs Striction of the BEC (non local effect) fdd adds a non local anisotropic mean-field DDIs Eberlein, PRL 92, 250401 (2004) Modification of the BEC expansion The effects of DDIs are experimentally evidenced by differential measurements, for two orthogonal orientations of the B field Pfau,PRL 95, 150406 (2005) Collective excitations of a dipolar BEC Bismut et al., PRL 105, 040404 (2010) DDIs change in the few % range the physics of a ground state BEC Aspect ratio DDIs induce changes smaller than edd ! t (ms)

5. A new and larger effect of DDIs: modification of the excitation spectrum of a BEC In the ground state BEC the effects of DDIs are averaged due to its anisotropic nature the dipolar mean field depends on trap geometry no modifications of the frequency of the collective modes due to DDIs for a spherical BEC Example: New idea: probe the effect of DDIs on the excitations of the BEC the excitation spectrum is given by the Fourier Transform of the interactions Experiment: probe dispersion law all dipoles contribute in the same way Quasi-particles, phonons c is sound velocity measure the modification of c due to DDIs ; 15% ?

6. Bragg Spectroscopy of a dipolar BEC I- Dipole – dipole interactions (DDIs) how have they been evidenced in ground state BECs how to see a larger effect ? (with the excitations) II- Excitations of a BEC dispersion of an excited BEC modifications induced by the DDIs III- Experimental realization difficulties… IV- Experimental results excitation spectra modified by DDIs

7. Excitation spectrum of a BEC with pure contact interactions c is the sound velocity c is also the critical velocity Landau criterium for superfluidity Rev. Mod. Phys. 77, 187 (2005) Bogoliubov spectrum healing length Phys. Rev. Lett. 99, 070402 (2007) Quasi-particles, phonons free particles

8. Excitation spectrum of a BEC in presence of DDIs An effect of the momentum-sensitivity of DDI becomes: if , and if , A 20% effect expected on the speed of sound ! Much larger than the (~3%) effects measured on the ground state

9. Excitation spectrum of a BEC: the local density approximation (LDA) = the theory giving predictions that you can compare with * the BEC is trapped, the density is not uniform LDA = consider the gas locally uniform with validity of LDA: LDA not valid at small k * the BEC has a momentum distribution two sources of broadening: the excitation spectrum of the BEC has a non zero width and the effect of DDIs is going to be less than naively expected…

10. Bragg Spectroscopy of a dipolar BEC I- Dipole – dipole interactions (DDIs) how have they been evidenced in ground state BECs how to see a larger effect ? (with the excitations) II- Excitations of a BEC dispersion of an excited BEC modifications induced by the DDIs III- Experimental realization difficulties… IV- Experimental results excitation spectra modified by DDIs

11. Excitation of a BEC with Raman beams: principle Lattice beams detuned and with an angle: Momentum and energy transfer Raman beams very far detuned q w + d w For a givenq, tune d to find a good excitation, and register the excitation spectrum nm d = 100 Hz to 100 kHz

12. Excitation of a BEC with Raman beams: experimentalrealization Two lasers "in phase" are required We use two AOMs driven by a digital double RF source providing two RF signals in phase q w + d w For given (accessible) values of q, we register excitation spectra optical access 6 to 14°, 28°, 83° we measure the excited fraction for a given d excited and non-excited parts spatially separated by momentum transfer

13. Excitation of a BEC with Raman beams: experimental difficulties * poor spatial separation of the excited fraction at low k to have a good spatial separation after expansion k non excited becomes hard to reach in our case excited (we don't work with an elongated BEC) * choice of Dt = the excitation duration (of the raman pulse) Dt >> 1 / Df if Dt is too small, we add a Fourier broadening if Dt is too large, the mechanical effect of the trap comes into play Dt << Ttrap / 4 excited not quite possible at low k…

14. Excitation of a BEC with Raman beams: experimental difficulties q = 6° no excitation poor separation of the excited fraction at low k ! q = 14° data analysis complicated, noisy data q = 83°

15. Bragg Spectroscopy of a dipolar BEC I- Dipole – dipole interactions (DDIs) how have they been evidenced in ground state BECs how to see a larger effect ? (with the excitations) II- Excitations of a BEC dispersion of an excited BEC modifications induced by the DDIs III- Experimental realization difficulties… IV- Experimental results excitation spectra modified by DDIs

16. Bragg Spectroscopy of a dipolar BEC: experimental results Excitation spectra at q =14° From the different spectra, registered for a given q, we deduce the value of: = shift of the excitation spectrum due to DDIs = d Width of resonance curve: finite size effects (inhomogeneous broadening) The excitation spectra depends on the relative angle between spins and excitation

17. Bragg Spectroscopy of a dipolar BEC: experimentalresults 0.2 1 0.1 0 1 1 2 3 4

18. Conclusion: a "non-standard superfluid"… Expansion Striction Stuttgart Collective excitations Villetaneuse Anisotropic speed of sound

19. Thank you for your attention … PhD student welcome in our group…