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Ultracold Chromium a dipolar quantum gas

Ultracold Chromium a dipolar quantum gas. Tilman Pfau University of Stuttgart. Quantum Optyks VI, Krynica 16.6.05. Interacting quantum systems in atomic physics. this talk. d. Dipolar systems. 70 Yb. Cr in periodic table of elements. Cr - element properties. isotopic distribution

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Ultracold Chromium a dipolar quantum gas

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  1. Ultracold Chromium a dipolar quantum gas Tilman Pfau University of Stuttgart Quantum Optyks VI, Krynica 16.6.05

  2. Interacting quantum systems in atomic physics this talk

  3. d Dipolar systems

  4. 70Yb Cr in periodic table of elements

  5. Cr - element properties • isotopic distribution • 3 Bosons (I=0): 52Cr (83.8%), 50Cr (4.3%), 54Cr (2.4%) • 1 Fermion (I=3/2): 53Cr (9.5%) • versatile level scheme • electronic configuration • [Ar]3d54s1 S=3 • large magnetic moment 6 µB! dipole-dipole interaction!

  6. Cr BEC phase transition tof = 5 msec Decreasing T

  7. S. Hensler A.Griesmaier T. Koch M. Fattori J. Stuhler The Dragontamers Former members: J. Werner P.O.Schmidt A. Görlitz • Theory: • K. Rzazewski • S. Giovanazzi • Simoni • E. Tiesinga • P. Pedri • L. Santos

  8. Was it worth all the trouble?

  9. Dipole dipole scattering Exactly solvable in Born approximation S. Hensler, J. Werner, A. Griesmaier, P.O. Schmidt, A. Görlitz, T. Pfau, S. Giovanazzi, K. RzazewskiAppl. Phys. B 77, 765 (2003) elastic scattering spin relaxation collisions spin changing collisions

  10. Dipolar relaxation dipolar relaxation + spin changing collisions atom number • Very good agreement between theory and experiment • no BEC in magnetic trap time [sec]

  11. 1915: Einstein - de Haas for a quantum gas? Demagnetization is a pain!BUTcould it be useful?

  12. Adiabatic demagnetization cooling of a solid e.g. S=3 Einstein‘s model of a solid << kBT << Phonon reservoir Spin reservoir (large S) Cv~NkB Cv<<NkB coupling

  13. Adiabatic demagnetization cooling of a trapped gas? e.g. S=3 >> kBT >> Phonon reservoir Spin reservoir (large S) Cv~NkB Demagnetising collisions Cv<<NkB

  14. Single demagnetization step (kBT>>hntrap) Continuous recycling

  15. Would it work in ODT? S. Hensler, A. Greiner, J. Stuhler, T. Pfau, submitted Example: ntrap = 500 Hz Includes: 3 body losses heat rate due to pump photons BUT no reabsorption …

  16. Elastic dipole dipole interaction? elastic scattering only! 7S3 mJ=+3 mJ=-3 „high field seeker“ optical dipole trap

  17. tunable! d m Feshbach resonance spinning polarization Strength of elastic dipole-dipole interaction dipole interaction compare to contact interaction: contact interaction tuning atoms het.-nucl. molecules Rydberg atoms Rb edd=0.007 Na edd=0.003 Cr edd=0.15 e.g.: Rb (n=40)* edd~108 e.g.: CaH, NH3, CrRb edd~100 J. Werner et al. PRL 94, 183201 (2005) S. Giovanazzi, A. Görlitz, T.P. PRL 89, 130401 (2002) electric dipoles magnetic dipoles

  18. Evaporative cooling – phase space density gain

  19. expansion T>Tc T<Tc T<Tc

  20. lifetime of the condensate t~400msec n0i= 2×1015 cm-3 t~6 sec n0i= 2×1014 cm-3 Cr S=3, mS=-3 L3 10−29 cm6 s−1 (upper limit) Rb F=2, mF=2 L3= 1.8 ×10−29 cm6 s−1 J. Söding et al. APB, 69, 257 (1999)

  21. Condensate fraction nx=581 Hz ny=406 Hz nz=138 Hz ideal gas T=1.1μK corr. for finite size and weak interaction* Tc~700 nK T=625nK exp. • Griesmaier, • et al. • PRL 94, • 160401 (2005) * S. Giorgini, L. P. Pitaevskii, and S. Stringari, Phys. Rev. A 54, R4633 (1996)

  22. time of flight Expansion driven by interaction Cr edd=0.15 T<Tc

  23. Dipolar interaction as perturbation BEC without dipoles in an isotropic harmonic trap in Thomas –Fermi limit 2 RTF nTF(r) perturbation by dipole interaction:  parabolic density profile B Fdd(r)

  24. B B z smaller aspect ratio ! larger aspect ratio ! dipole-dipole interaction & aspect ratio y z ry/rz<1 aspect ratio time of flight ry/rz>1 Magnetostriction!

  25. dipolar expansion - magnetostriction 31 measurements nx=942 Hz ny=712 Hz nz=128 Hz J. Stuhler, A. Griemaier, T. Koch, M. Fattori, S. Giovanazzi, P. Pedri L. Santos, T. Pfau submitted Theory – No free parameters! S. Giovanazzi, A. Görlitz, and T. Pfau, J. Opt. B: Quantum Semiclass. Opt. 5, S208 (2003).

  26. dipolar coupling in fluids Ferrofluids ~ 2-20 nm

  27. tunable! m Feshbach resonance spinning polarization Strength of elastic dipole-dipole interaction dipole interaction compare to contact interaction: contact interaction tuning atoms Rb edd=0.007 Na edd=0.003 Cr edd=0.15 magnetic dipoles

  28. centrifugal potentials (e.g. ) FRs: centrifugal terms & notation Skip Feshbach Quantum numbers - notation S=2 S=4 S=6 Cr2 ab initio potentials: Z. Pavlovic et al., PRA 69, 030701 (2004)

  29. FRs: couplings & selection rules Possible couplings: 2nd order Spin-Orbit Spin - Spin Selection rules: first order not allowed! second order angular momentum conservation:

  30. } 11 expected FRs in Cr S=6 S=4 S=2 MS MS MS 0 -1 -2 -2 -3 -4 -5 -6 -2 -3 -4 g - wave -2 -3 -4 d - wave -4 -5 -6 first order 3 resonances X X second order 8 resonances s - wave -4 -5 -6 initial state (open channel)

  31. How to see the FR resonance B

  32. S=6 S=4 S=2 not yet ass. increasing B-field Feshbach resonances in 52Cr • expectation for collisions of ultracold, fully polarized Cr: 11 resonances (3 first order, 8 second order) • results (assignment by A. Simoni, E. Tiesinga):

  33. exp. vs. theory D/Br~210-4 D/Br≈0.003 av. agreement of resonance positions better than 0.6 G incl. only spin-spin interaction! • despite 6 valence e-, Cr is not too complicated • no hyperfine interaction (I=0) • dipole-dipole interaction is dominant coupling mechanism J. Werner et al. PRL 94, 183201 (2005)

  34. S=2 S=4 S=6 What do we know about Chromium now? a6= 112(14) a0 a4= 58(6) a0 a2= -7(20) a0 C6=733(70) a.u. C8=75(+90/-75) a.u.

  35. summary • Cr-BEC with 105 atoms • new twist: dipole-dipole interaction • dipolar effects visible in Cr • Feshbach resonances • all 11 FR up to 2nd order dipole- dipole interactions detected (+3) • theo. & exp. fit  a6,a4,a2,C6,C8

  36. unstable stable pancake Supersolid Checkerboard outlook • experimental set up • tuning of contact interaction • tuning of dipole-dipole interaction • field compensation • play the dipolar game ! • excitations • stability and ground state of condensate • Roton-Maxon dispersion relation • new quantum phase transitions ...

  37. Further outlook • Demagnetization cooling • Trap fermion (see Poster Villetaneuse) • Lithography: • controlled single atom deposition? • Cr3+:Al203 Cr3+:LiSAF Cr3+:LiNb03 • Cr:GaN • cw atom laser ?

  38. More projects…. postdocs & phd students welcome !!!

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