t. koch, t. lahaye, b. fröhlich, j. metz, m. fattori, a. griesmaier, s. giovanazzi and t. pfau 5....
TRANSCRIPT
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
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 c
T c
decr
ease
of
B-f
ield
solid
kB
spins phonons
gas
kB
kBkB
kB
kBkB
kB
3
4f i
i f
T c
T 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
ln11
ln
d
d 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 length
Without 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
Life
time
[ms]
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?
title