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Cold Atoms Experiments

Cold Atoms Experiments. D. Jin JILA, NIST and the University of Colorado. $ NSF, NIST. Investigate many-body quantum physics with a model system. Why study atomic gases?. - low density, low temperature - well understood microscopics unique experimental tools

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Cold Atoms Experiments

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  1. Cold Atoms Experiments D. Jin JILA, NIST and the University of Colorado $ NSF, NIST

  2. Investigate many-body quantum physics with a model system Why study atomic gases? • - low density, low temperature • - well understood microscopics • unique experimental tools • - controllable interactions n ~ 1013 cm-3,T ~ 100 nK

  3. Rapidly rotating BECs Strongly interacting BECs Atoms in optical lattices + + _ _ 1d and 2d gases Cold molecules Lots of exciting experiments: * Fermi gas and BCS-BEC crossover

  4. Outline: • Introduction • Controlling Interactions • BCS-BEC Crossover • Recent Experiments at JILA

  5. Some Numbers Numbers: atomselectrons n = 1013 1023 cm-3 d = 10-7 10-10 m N = 105 ∞ m = 7x10-26 9x10-31 kg TF=10-6 105 K vF = 10-2 106 m/s T/TF=0.05 <0.001 Trap inhomogeneous density n x

  6. spin  kT spin  Important properties Ultracold (100 nK!) gas : • Meta-stable. • True ground state is a solid. • Spin degree of freedom is frozen out. • Contact interactions (collisions) are s-wave.

  7. s-wave centrifugal barrier V(R) V(R) kT R Contact Interactions Collisions/interactions are only s-wave. non-s-wave R Spin-polarized fermions stop colliding.

  8. Laser cooling and trapping • Magnetic trapping & evaporative cooling • Optical trapping & evaporative cooling Techniques spin 2 spin 1 • can confine any spin-state • can apply arbitrary B-field

  9. Time-of-flight absorption imaging • Probing the atoms Probing the ultracold gas

  10. Fermi gas of atoms 1999: 40K JILA many experimental groups: 40K, 6Li, 173Yb,3He* EF = kBTF EF = hftrap(6N)1/3 Fermi sea of atoms T ~ 0.05 TF

  11. Quantum degeneracy Get T from the measured velocity distribution. reach T/TFermi~ 0.05 N = 4 ·105 ,T = 16 nK T/TFermi = 0.05

  12. Apparatus

  13. II. Controlling Interactions

  14. V(R) R a Interactions s-wave scattering length, a a > 0 repulsive, a < 0 attractive Large |a| → strong interactions

  15. Controlling interactions a > 0 repulsive, a < 0 attractive Large |a| → strong interactions 40K  0 scattering length, a

  16. Magnetic-field Feshbach resonance A magnetic-field tunable atomic scattering resonance Channels are coupled by the hyperfine interaction. molecule state in channel 2 → ← colliding atoms in channel 1

  17. → ← repulsive DB > attractive molecules Magnetic-field Feshbach resonance repulsive free atoms Ebinding

  18. → ← Magnetic-field Feshbach resonance s-wave scattering length, a repulsive free atoms DB > Ebinding attractive molecules

  19. Magnetic-field Feshbach resonance • spectroscopic measurement of the mean-field energy shift repulsive attractive C. A. Regal and D. S. Jin, PRL 90, 230404 (2003)

  20. → ← The atoms reappear if we sweep back to high B. Turning atoms into molecules energy B Ramp across Feshbach resonance from high to low B

  21. Molecule binding energy C. Regal et al. Nature 424, 47 (2003) • extremely weakly bound ! • long lifetime

  22. III. BCS-BEC crossover

  23. kF spin  spin  Making condensates with fermions generalized Cooper pairs molecules Cooper pairs BCS BEC BCS – BEC crossover BCS-BEC crossover theory(partial list):Eagles, Leggett, Nozieres and Schmitt-Rink, Randeria, Strinati, Haussman, Holland, Timmermans, Griffin, Levin …

  24. alkali atom BEC superfluid 4He high Tc superconductors superfluid 3He superconductors BCS-BEC landscape M. Holland et al., PRL 87, 120406 (2001) BEC transition temperature BCS energy to break fermion pair

  25. BCS-BEC Crossover Gap Chemical potential  BEC BCS 1/kFa characterizes interactions in BCS-BEC crossover M. Marini, F. Pistolesi, and G.C. Strinati, Europhys. J. B 1, 151 (1998)

  26. → ← Magnetic-field Feshbach resonance s-wave scattering length, a repulsive free atoms DB > Ebinding attractive molecules

  27. EF Changing the interaction strength in real time : FAST repulsive 2 ms/G DB > attractive molecules

  28. EF Changing the interaction strength in real time: SLOW 40 ms/G DB > attractive molecules

  29. EF Changing the interaction strength in real time: SLOWER 4000 ms/G DB > attractive molecules Cubizolles et al., PRL 91, 240401 (2003); L. Carr et al., PRL 92, 150404 (2004)

  30. Molecular Condensate initial T/TF: 0.19 0.06 Time of flightabsorption image M. Greiner, C.A. Regal, and D.S. Jin, Nature 426, 537 (2003).

  31. ? 4000 ms/G EF ? Observing a Fermi condensate repulsive 40 ms/G DB > attractive

  32. Condensates without a two-body bound state Dissociation of molecules at low density C. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004) DB (gauss) DB = 0.12 G DB = 0.25 GDB=0.55 G T/TF=0.08

  33. Bose-Einstein Condensate Fermi Condensate 2004 Imaging atom pairs stronger attractive interactions C. A. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004)

  34. 4000 ms/G EF Mapping out a phase diagram repulsive a 40 ms/G DB > T/TF attractive molecules

  35. BCS-BEC Crossover condensate fraction 0 0.01 0.05 0.1 0.15 Initial  BEC BCS C.A. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004)

  36. BCS-BEC Crossover condensate fraction 0 Initial a BCS-BEC crossover theory C.A. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004) Q. Chen, C.A. Regal, M. Greiner, D.S. Jin & K. Levin, PRA 73, 041601 (2006).

  37. Vortices Collective excitations Probing the BCS-BEC crossover Condensate fraction Unbalanced spin population Initial Unitarity and Universality Probes of pairing Thermodynamic measurements Correlations in atom shot noise

  38. Probing the BCS-BEC crossover Increasing interactions 1/(kFa) = -80 -1 0 +1 (Ketterle, MIT)

  39. IV. Recent experiments at JILA • Photoemission spectroscopy for ultracold atoms • Exploring a p-wave Feshbach resonance

  40. Photoemission spectroscopy for ultracold atoms Like ARPES Look at the BCS-BEC crossover (s-wave)

  41. The gap and the pseudogap 2D is the energy to break a pair BCS superconductivity BCS-BEC crossover

  42. rf spectroscopy: A brief history C. Regal et al. Nature 424, 47 (2003)

  43. rf spectroscopy: A brief history 2D Molecule dissociation 40K Energy C.A. Regal, C. Ticknor, J. L. Bohn, & D.S. Jin, Nature 424, 48 (2003)

  44. Measuring the gap? 6Li BEC side On resonance 2D D? Transfer 0 0 Coexistence rf offset Coexistence EF J. Kinnunen, M. Rodrıguez,& P. Torma,Science 305, 1131 (2004) C. Chin et al., Science 305, 1128 (2004)

  45. Controversy… 6Li C. H. Schunck et al., Science 316, 867 (2007) Fermi sea T=0

  46. n x Issues Theory papers Trap inhomogeneity: • Torma, Science 2004 • Levin, PRA 2005 • Griffin, PRA 2005 • Stoof, PRA 2008 • Levin, PRA 2008 • Mueller, preprint 2007 • …

  47. Issues Theory papers 6Li Final-state effects: • Chin & Julienne, PRA 2005 • Yu & Baym, PRA 2006 • Baym, PRL 2007 • Perali & Strinati, PRL 2008 • Punk & Zwerger, PRL 2007 • Basu & Mueller, preprint 2007 • Veillette et al., preprint 2008 • Levin, preprint 2008 • …

  48. C. H. Schunck et al., arXiv:0802.0341v1 RF spectroscopy without final-state effects EF 6Li our 40K data EF

  49. Final-state effects 6Li C. H. Schunck et al., arXiv:0802.0341v2 RF offset

  50. Momentum-resolved rf spectroscopy PES for atoms Conservation of energy

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