Condensed matter physics in dilute atomic gases S. K. Yip Academia Sinica

1. Condensed matter physics in dilute atomic gasesS. K. YipAcademia Sinica

2. possible to cool and trap dilute atomic gases Atoms used: 7Li 23Na 87Rb 1H He* Yb 6Li 40K Some typical numbers: number of atoms 104 106 108 (final) (mostly) peak density n < 10 14 cm-3 distance between particles ~ 104 A (dilute gas) size of cloud ~ mm temperatures: down to nK

3. f = I s = I + 1/2 hyperfine spin nuclear spin electron spin mf = - f, - f + 1, …. , f -1, + f (integers for bosons, half-integers for fermions) _ alkalis nucleus + [ closed electronic shell ] + e _ nucleon N odd atom = boson N even fermion 7Li 23Na 87Rb 1H : Bosons 6Li 40K : Fermions

4. 6Li s = 1/2, i = 1; f = 1/2 , 3/2

5. Magnetic Trap (most experiments): magnetic moment - m. B ( r ) | B ( r ) | increasing from the center U trapped r not trapped typically can trap only one species ( else loss due to collisions ) effectively scalar ( spinless ) particles

6. Optical Trap: 1 U ( r ) = - a ( w ) E2 ( r , w ) 2 ex a ( w ) > 0ifreddetuned ( w < wres ) g atoms attracted to strong field region laser . . . . . . .. . .. . . . . . . . . spin degree of freedom remains (c.f. driven harmonic oscillator)

8. Identical particles fermions bosons many particle wavefunctions: antisymmetric symmetric

9. can occupy the same single particle states at sufficiently low T macroscopic occupation Bose-Einstein Condensation BOSONS:

10. Macroscopic wavefunction (common to all condensed particles) (r, t) (c.f Schrodinger wavefunction) Supercurrent: Phase gradient  supercurrent

11. Quantized vortices: well defined at any position r  If ||  then  unique up to 2n  0  0 2

12. Rotating superfluid: if  = constant, then not rotating (no current) rotating    constant but circulation quantized  quantized vortices  0  2 

13. [MIT, Science, 292, 476 (2001)]

14. FERMIONS:

15. FERMIONS Exclusion principle: Single species (can be done in magnetic traps): T=0 Particles filled up to Fermi energy Normal Fermi gas (liquid) Generally NOT superfluid Momentum space: Fermi sphere

16. FERMIONS Two species: need optical trap T=0 still not much interesting unless interacting Momentum space: Fermi sphere

17. 6Li s = 1/2, i = 1; f = 1/2 , 3/2 }   (need optical trap)

18. Can be superfluid if attractive interaction : Cooper pairing (Bardeen, Cooper, Schrieffer; BCS) k  -k  all k’s near Fermi surface  Underlying mechanism for superconductivity (in perhaps all superconductors)

19.  B How to get strong enough attractive interaction in dilute Fermi gases Feshbach Resonances: (others) (1 2) Bres

20. Hydridization  level repulsion

21. Hydridization  level repulsion Lowering of energy  attractive interaction

22.  B Two particles no coupling:  continuum continuum closed channel molecule

23. with coupling: Bound state effective attractive interaction between   fermions

24. Bound state eventually BEC of molecules effective attractive interaction between   fermions BCS pairing

25. Smooth crossover from BCS pairing to BEC (Leggett 80)

26. Experimental evidence: (resonance) [MIT, Nature, 435, 1047 (2005)]

27. New possibilities: unequal population [c.f. superconductor in external Zeeman field: pair-breaking ] Smooth crossover is destroyed ! ( Pao, Wu, Yip; 2006) N uniform superfluid state unstable in shaded region BF homogenous mixture

28. Many potential ground states for the shaded region experiments suggest phase separation near resonance another likely candidate state: Larkin-Ovchinnikov/ domain-walls (c.f. -junctions in SFS) not yet found Finite T phase diagram open question Interesting interacting system even when it is not superfluid (non-Fermi liquid behaviours?)

29. Many other topics not covered: atoms in periodic lattice (c.f. solid!) “random” potential multicomponent (spin) Bosonic superfluids low dimensional systems (e.g 1D) rapidly rotating Bose gas (maximum number of vortices ?) tunable parameters, often in real time and many more opportunities!!