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T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori, A. Griesmaier, S. Giovanazzi and T. Pfau

Strong dipolar effects in a Chromium BEC A quantum ferrofluid. T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori, A. Griesmaier, S. Giovanazzi and T. Pfau 5. Physikalisches Institut, Universität Stuttgart Assisi – June 6th 2007. Interacting quantum systems in AMO physics.

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T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori, A. Griesmaier, S. Giovanazzi and T. Pfau

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  1. Strong dipolar effects in a Chromium BEC A quantum ferrofluid T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori,A. Griesmaier, S. Giovanazzi and T. Pfau 5. Physikalisches Institut, Universität Stuttgart Assisi – June 6th 2007

  2. Interacting quantum systems in AMO physics Contact interaction Dipole-dipole interaction Coulomb interaction Short range Isotropic Long range - Anisotropic Long range Isotropic MIT Innsbruck

  3. New physics in dipolar quantum gases Dipole-dipole interactions are: - anisotropic - instability - modified dispersion relation (roton) - new equilibrium shapes (biconcave BEC) - long range - new quantum phases in optical lattices - supersolid phase pancake

  4. Outline – BEC with MDDI Chromium How to get a Chromium BEC? Dipolar expansion Demagnetization cooling Strong dipolar effects in a Cr BEC Outlook

  5. Yb I. Chromium Ground state 7S3Magnetic dipole moment m = 6mB.

  6. m = 3 m = 2 m = 1 +E +2E Way to BEC • Continously loaded Ioffe Pritchard trap (CLIP-trap)J. Stuhler et al. PRA 64, 031405 (2001); P. O. Schmidt et al. J. Opt. B5, S170 (2003) • Doppler cooling in compressed IP-trap • P. O. Schmidt, et al., J. Opt. Soc. Am. B 20, 5 (2003) • >108 atoms in the ground state phase space density ~10-7 • Rf-evaporation • Stop by dipolar relaxation! • No cold & dense cloud (no BEC) in MT! • S. Hensler et al., Appl. Phys. B 77, 765 (2003)

  7. s- 7P3 7S3 mj= +3 mj= -3 Transfer to optical dipole trap optical pumping in mj=-3 • Advantages: • all magnetic substates are trapped (no dip. relaxation) • operation at arbitrary magnetic offset field (Feshbach resonance)

  8. Forced evaporation in ODT horizontal beam vertical beam BEC with up to 100.000 atoms

  9. PRL 95, 150406 (2005).PRA 74, 013621 (2006). Dipolar expansion of a BEC Density Mean-field potential due to MDDI First Observation of mechanical effect of a homogenous magnetic field on a gas Elongation along magnetization direction!

  10. II. Demagnetization cooling Why another cooling scheme ????? • doppler cooling techniqueslimited by reabsorption • evaporative coolingthrow away 99 % of your atoms • demagnetization cooling Kastler, Journal de physique et le radium 11, 255 (1950). Cirac, Lewenstein, Phys Rey A 52, 6 (1995).

  11. 1. Initialization 2. Lowering B-field 3. Optical pumping m = -1 m = -2 m = -3 -E s- 7P3 7S3 mj= +3 mj= -3 basic idea • Needed: • Suitable level scheme • Strong enough coupling

  12. gas kB kB kB spinsphonons kB kB kB kB T0? Solid vs.gas solid spinsphonons decrease of B-field kB But we can pump back !

  13. 1G 50mG Results: Single step M. Fattori et.al. Nature Physics 2 , 765 (2006)

  14. Experimental challenges • bad polarization due to • badly polarized light • transverse magnetic fields •  • polarization quality 1/1000 • transverse fields below 5mG

  15. Results: Optimized ramps

  16. Heteronuclear molecules(electric dipole moment d ) Large d (~1 Debye): No BEC yet III. Strong dipolar effects in a BEC Strength of the dipole-dipole interaction: Atoms with large magnetic dipole moment m. Chromium: 6mB. Small edd… but a tunableBEC !!! Griesmaier et.al. PRL 97, 250402 (2006) Griesmaier et.al. PRL 94, 160401 (2005)

  17. Vc Tuning a with a Feshbach resonance collision with molecular potential V(R): V’(R) with Ms’≠ Ms + B-field + coupling: V’(R) V(R) Ec  a ! describes scattering @ low T  a is modified ! scattering length a can be tuned with B-field !

  18. Tuning a with a Feshbach resonance Broadest resonance at 589.1 G (D = 1.7 G)Field stability better than 10-4 required! [J. Werner et al., PRL 94, 183201, (2005)]

  19. Tuning the scattering length Without MDDI: measure a through the released energy a ~ R5 / N Correct for the MDDI effects (hydrodynamic theory, TF regime).

  20. Aspect ratio vs. edd Theory without any adjustable parameter !!!

  21. Dipolar expansion with tunable εdd „εdd=0“ „εdd=0“ „εdd=0“ εdd=0.5 εdd=0.16 εdd=0.16 εdd=0.75 Stuhler et.al. PRL 95 , 150406 (2005) Lahaye et.al. Nature in press

  22. Use of a Feshbach resonance Limits: inelastic losses • 1 / e lifetime of the condensate:

  23. Summary and Outlook I. Dipole-dipole interaction & ultracold Cr atoms II. Demagnetization cooling III. New regime of strong dipolar interactions  New physics 1D lattice: A stack of pancakes

  24. T. Lahaye B. Fröhlich M. Fattori T. Koch T. Pfau A. Griesmaier J. MetzTheory: S. Giovanazzi Thanks for your attention! The Cr team: http://www.pi5.uni-stuttgart.de/ SFB/TR 21 SPP1116

  25. Summary and Outlook • By tuning a we enter a new regime • stabilize the BEC with respect to dipolar collapse? • study spectrum of excitations? • (more) stable molecules? • One-dimensional optical lattice: a stack of pancake traps. • stabilize the BEC with respect to dipolar collapse? • study spectrum of excitations? • (more) stable molecules?

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