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 rangeIsotropic

Coulomb interactionDipole-dipole interactionContact interaction

MIT Innsbruck

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

• ChromiumHow to get a Chromium BEC?Dipolar expansion

• Demagnetization cooling

• Strong dipolar effects in a Cr BEC • Outlook

Outline – BEC with MDDI

I. Chromium

Yb

5 1[ ]3 4Cr Ar d s

Ground state 7S3

Magnetic dipole moment = 6B.

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. B 5, S170 (2003)

• Doppler cooling in compressed IP-trapP. 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)

+E +2Em = 3m = 2m = 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 mj=-3

mj= -3 mj= +37S3

7P3

Forced evaporation in ODT

BEC with up to 100.000 atoms

horizontal beam

verticalbeam

Dipolar expansion of a BEC

Elongation along magnetization direction!

Density

Mean-field potentialdue to MDDI

PRL 95, 150406 (2005).PRA 74, 013621 (2006).

First Observation of mechanical effect of a homogenous magnetic field on a gas

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).

basic idea

1. Initialization 3. Optical pumping2. Lowering B-field

Needed:1. Suitable level scheme2. Strong enough coupling

31 S

mj= -3 mj= +37S3

7P3

-Em = -1m = -2m = -3

T0? Solid vs.gas

1f i

i f

T cT c

decr

ease

of B

-fie

ld

solid

kB

spins phonons

gas

kB

kBkB

kB

kBkB

kB

34

f i

i f

T cT c

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

ln 11ln

dd N

Atoms with large magnetic dipole moment .

Chromium: 6B.

Small dd… but a tunableBEC !!!

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, 250402 (2006) Griesmaier et.al. PRL 94, 160401 (2005)

Tuning a with a Feshbach resonance

scattering length a can be tuned with B-field !

V(R)

collision with molecular potential V(R):

Ec

a ! describes scattering @ low T

V’(R)

V’(R) with Ms’ ≠ Ms + B-field

Vc

a is modified !

+ coupling:

[J. Werner et al., PRL 94, 183201, (2005)]

Broadest resonance at 589.1 G ( = 1.7 G)Field stability better than 10-4 required!

Tuning a with a Feshbach resonance

Tuning the scattering lengthWithout MDDI: measure a through the released energy a ~ R5 / N

Correct for the MDDI effects (hydrodynamic theory, TF regime).

Aspect ratio vs. dd

Theory without any adjustable parameter !!!

Dipolar expansion with tunable εdd

εdd=0.16

„εdd=0“

εdd=0.75

εdd=0.5

εdd=0.16

„εdd=0“„εdd=0“

Stuhler et.al. PRL 95 , 150406 (2005)

Lahaye et.al. Nature in press

1 / e lifetime of the condensate:

Limits: inelastic losses

-15 -10 -5 0 5 10 1510

100

1000

Li

fetim

e [m

s]

Magnetic field B-B0 [G]

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

http://www.pi5.uni-stuttgart.de/

SFB/TR 21 SPP1116

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?

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