Bose-Einstein condensation of
chromium
Ashok Mohapatra
NISER, Bhubaneswar
1
SFB/TRR21
SFB/TRR 21
The
team
Cr BEC team
Axel
Tilman
Jonas
Yong Wan (diploma student)
Stefan
Ashok
2
Chromium
BEC, 2004
by Pfau’s group
3
Chromium
Large magnetic moment 6μB
36 times more dipolar interaction
compared to alkali atoms
4
Relevant energy levels of chromium
7P
4
7P
3
MOT
425 nm
Repumper
663 nm
5D
4
Optical Pumping
427 nm
7S
3
Ti:Sa 851 nm 2 W
doubled (LBO) (600 mW)
5
Experimental set up
6
Steps to get the BEC
Step 1: CLIP load
Step 2: RF evapolarion to 15 μK and
load to a dipole trap
Step 3: Switch off the MT and optically pump the atoms to the energetically lowest
Zeeman state to prevent the dipolar relaxation (large μB helps to flip the atoms
to wrong Zeeman states)
7
Steps to get the BEC
Step 4: Forced evaporation in a crossed dipole trap
A. Griesmaier,
Phys. Rev. Lett. 94, 160401 (2005)
Condensate of
50,000 atoms
8
Feshbach resonances in
Chromium
J. Werner et al., Phys. Rev. Lett. 94, 183201 (2005)
Braodest
resonance
@ 590 G,
Δ ~ 1.4 G
Control of magnetic field to 10-5 is needed to have a noise of ±1a0 at 0 scattering length
Active stabilization of the current in the Feshbach coils was used
9
Feshbach resonances in Chromium
B0 ~ 590 G
ΔB ~ 1.4 G
10
Broad feshabch resonance

B 
Fit: a  abg  1 

B

B
0 

abg  a0
B0 ~ 590 G
ΔB ~ 1.4 G
11
Strong dipolar effects
T. Lahaye, T. Koch, B. Fröhlich, M. Fattori, J. Metz,
A. Griesmaier, S. Giovanazzi, T. Pfau; Nature 448, 672 (2007)
12
Roton like excitation spectrum in a
dipolar quantum gas
L. Santos et al., PRL 90 250403 (2003)
D-W. Wang et al., arXiv:0812.1838
13
Dipolar coupling in classical ferrofluids
Rosensweig instability
M. D. Cowley and R. E. Rosensweig, J. Fluid Mech. 30, 671 (1967)
14
Roton like excitation spectrum in
classical ferrofluid
Dispersion relation of surface wave in ferrofluid
J. Browaeys et al., Braz. J. Phys. 31, 447 (2001)
R. E. Rosensweig, Ferrohydrodynamics,
Dover Publications, Inc (1997), page 191
15
Roton like excitation spectrum in
classical ferrofluid
Spontaneous static deformation around critical magnetic field
J. Browaeys et al., Braz. J. Phys. 31, 447 (2001)
The rotonization mechanism in quantum dipolar gas a quantum could be a
classical phenomenon…………
Do we really need a superfluidity to have roton in quantum dipolar gas?????
16
Roton and Supersolid phase in
Rydberg dressed BEC
Henkel et al., Phys. Rev.
Lett. 104, 195302 (2010)
Collective interaction using
coherent Rydberg dressed BEC
Honer et al, PRL 105, 160404 (2010)
Thank you
19
Rosensweig instability in
classical ferrofluid
Competition between
the magnetic interaction
with Gravity and surface
free energy
Surface wave
C. Gollwitzer et al.,, J. Fluid Mech. (2006)
Similar pattern in a quantum gas would lead to a supersolid state
20
dipolar interaction in a BEC
Field in y-direction
Field in Z-direction
J. Stuhler et al., Phys. Rev. Lett. 95 , 150406 (2005)
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