1 Bose-Einstein condensation of chromium Ashok Mohapatra NISER, Bhubaneswar

2 Cr BEC team Ashok Axel Stefan Jonas The team SFB/TRR 21 Tilman Yong Wan (diploma student) Yong Wan (diploma student)

3 Chromium BEC, 2004 by Pfau’s group

4 Chromium Large magnetic moment 6μ B 36 times more dipolar interaction compared to alkali atoms

5 Relevant energy levels of chromium 7S37S3 7P47P4 7P37P3 5D45D4 MOT 425 nm Optical Pumping 427 nm Repumper 663 nm Ti:Sa 851 nm 2 W doubled (LBO) (600 mW)

6 Experimental set up

7 Steps to get the BEC Step 1: CLIP load Step 2: RF evapolarion to 15 μK and load to a dipole trap 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 μ B helps to flip the atoms Zeeman state to prevent the dipolar relaxation (large μ B helps to flip the atoms to wrong Zeeman states) to wrong Zeeman states)

8 Steps to get the BEC Step 4: Forced evaporation in a crossed dipole trap Condensate of 50,000 atoms A. Griesmaier, Phys. Rev. Lett. 94, (2005)

9 Feshbach resonances in Chromium J. Werner et al., Phys. Rev. Lett. 94, (2005) 590 G, Δ ~ 1.4 G Control of magnetic field to is needed to have a noise of ±1a 0 at 0 scattering length Active stabilization of the current in the Feshbach coils was used

Feshbach resonances in Chromium 10 B 0 ~ 590 G ΔB ~ 1.4 G

11 Broad feshabch resonance Fit: B 0 ~ 590 G ΔB ~ 1.4 G

12 T. Lahaye, T. Koch, B. Fröhlich, M. Fattori, J. Metz, A. Griesmaier, S. Giovanazzi, T. Pfau; Nature 448, 672 (2007) Strong dipolar effects

13 Roton like excitation spectrum in a dipolar quantum gas L. Santos et al., PRL (2003) D-W. Wang et al., arXiv:

14 Rosensweig instability M. D. Cowley and R. E. Rosensweig, J. Fluid Mech. 30, 671 (1967) Dipolar coupling in classical ferrofluids

15 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

16 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?????

Roton and Supersolid phase in Rydberg dressed BEC Henkel et al., Phys. Rev. Lett. 104, (2010)

Collective interaction using coherent Rydberg dressed BEC Honer et al, PRL 105, (2010)

19 Thank you

20 Rosensweig instability in classical ferrofluid Competition between the magnetic interaction with Gravity and surface 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

21 dipolar interaction in a BEC Field in y-direction Field in Z-direction J. Stuhler et al., Phys. Rev. Lett. 95, (2005)