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 Strong dipolar effects in a Chromium BEC A quantum ferrofluid

Interacting quantum systems in AMO physics Long range Isotropic Short range Isotropic Coulomb interactionDipole-dipole interactionContact interaction MITInnsbruck Long range - Anisotropic

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

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

I. Chromium Yb Ground state 7 S 3 Magnetic dipole moment  = 6  B.

Way to BEC Continously loaded Ioffe Pritchard trap (CLIP-trap) J. Stuhler et al. PRA 64, (2001); P. O. Schmidt et al. J. Opt. B 5, S170 (2003) Doppler cooling in compressed IP-trap P. O. Schmidt, et al., J. Opt. Soc. Am. B 20, 5 (2003) >10 8 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) +E+E+2  E m = 3 m = 2 m = 1

Transfer to optical dipole trap Advantages: all magnetic substates are trapped (no dip. relaxation) operation at arbitrary magnetic offset field (Feshbach resonance) optical pumping in m j =-3  m j = -3 m j = + 3 7S37S3 7P37P3

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

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

II. Demagnetization cooling Why another cooling scheme ????? ► doppler cooling techniques limited by reabsorption ► evaporative cooling throw 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).

basic idea 1. Initialization 3. Optical pumping 2. Lowering B-field Needed: 1.Suitable level scheme 2.Strong enough coupling  m j = -3 m j = + 3 7S37S3 7P37P3 -E-E m = -1 m = -2 m = -3

T 0 ? Solid vs.gas decrease of B-field solid kBkB spins phonons gas kBkB kBkB kBkB kBkB kBkB kBkB kBkB spins phonons But we can pump back !

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

Experimental challenges bad polarization due to (a) badly polarized light (b) transverse magnetic fields  (a) polarization quality 1/1000 (b) transverse fields below 5mG

Results: Optimized ramps

Atoms with large magnetic dipole moment . Chromium: 6  B. Small  dd … but a tunable BEC !!! III. Strong dipolar effects in a BEC Strength of the dipole-dipole interaction: Heteronuclear molecules (electric dipole moment d ) Large d (~1 Debye): No BEC yet Griesmaier et.al. PRL 97, (2006) Griesmaier et.al. PRL 94, (2005)

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

[J. Werner et al., PRL 94, , (2005)] Broadest resonance at G (  = 1.7 G) Field stability better than required! Tuning a with a Feshbach resonance

Tuning the scattering length Without MDDI: measure a through the released energy a ~ R 5 / N Correct for the MDDI effects (hydrodynamic theory, TF regime).

Aspect ratio vs. B Dipole-dipole interactions: elongation along a / a bg z y

Aspect ratio vs.  dd Theory without any adjustable parameter !!!

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

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

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

Thanks for your attention! T. Lahaye B. Fröhlich M. Fattori T. Koch T. Pfau A. Griesmaier J. Metz Theory: S. Giovanazzi SFB/TR 21SPP1116 The Cr team:

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

Chromium BEC i.Continuous loading of a Ioffe-Pritchard trap. ii.RF evaporation. iii.Transfer to crossed ODT ( nm), optical pumping, and forced evaporation. iv.10 5 atoms in BEC! A. Griesmaier et al., PRL 94, (2005). PRL 95, (2005). PRA 74, (2006). Magnetic dipole-dipole interactions: Cloud more elongated along magnetic field

Use of a Feshbach resonance One can tune the scattering length with an external magnetic field: Feshbach resonances in Chromium [J. Werner et al., PRL 94, , (2005)] Broadest resonance at G (  = 1.7 G) Field stability better than required!

Modified experimental setup Offset coils Crossed ODT z y x - Uniform field ~ 600 G - offset (400 A) + pinch (15 A) - curvature compensation - actively stabilized at level - Absorption imaging in high field - Experimental sequence: Pinch coils time Magnetic field P horiz. beam B evap 5 ms Forced evap. BEC Shape trap (50 ms) B0B0 Ramp to B (5 ms) 5 ms tof Hold (2 ms)

Tuning the scattering length Without MDDI: measure a through the released energy a ~ R 5 / N Correct for the MDDI effects (hydrodynamic theory, TF regime) B-B 0 [G]

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