Center for Quantum Physics Innsbruck Center for Quantum Physics Innsbruck Austrian Academy of Sciences Austrian Academy of Sciences University strongly interacting fermions: … to mixed species strongly interacting fermions: … to mixed species Rudolf Grimm
Devang Naik Devang Naik The team Frederik Spiegelhalder Frederik Spiegelhalder Eric Wille Eric Wille Gerhard Hendl Gerhard Hendl Andreas Trenkwalder Andreas Trenkwalder (Gabriel Kerner) (Gabriel Kerner) Rudi Grimm Rudi Grimm Florian Schreck Florian Schreck Devang Naik Devang Naik
Ultracold alkali fermions Li 6 K 40 last five years: tremendous success in strongly interacting Fermi gases with spin mixtures of 6 Li and 40 K molecular BEC, pair condensates, superfluidity,…
Ultracold alkali fermions Li 6 K 40 & let‘s go a big step further pairing and superfluidity in a Fermi-Fermi mixture
control knobs (single species) interaction strength Innsbruck, JILA, MIT, Duke, ENS, Rice BEC-BCS crossover physics a spin imbalance physics of polarized Fermi gases MIT Rice P trap parameters: anisotropy, ellipticity etc. (very flexible!)
+ Cooper pairs dipolar molecules complex molecules new possibilities in FF mixtures control of mass ratio 87/40 = /6 = /6 = 14.5 fermion pairing with unequal masses, stable heteronuclear molecules, novel quantum phases … m 1 /m 2 Li, K, Sr m Sr /m K = 2.2 m K /m Li = 6.6 m Sr /m Li = 14.5 independent control of optical potentials species #1 species #2 pairing with unequal Fermi surfaces e.g., small trap of 40 K in a large trap of 6 Li or optical lattice for 87 Sr in a bath of 6 Li …
The machine
Lithium-Potassium-Strontium Machine
6 Li MOT: N ~ 10 9 T ~ 300µK 3 mm MOT beams MOT beams dipole trap (100W 1075nm laser): U ~ k B 1 mK w ~ 60 µm dipole trap: N > mm λ/4 all-optical way
6 Li 2 molecular Bose condensation molecules in dipole trap After 10ms time of flight: 4.3 sec evap4.7 sec evap PURE BEC! 5.1 sec evap4.8 sec evap
heteronuclear FF mixture ! absorption images of 6 Li and 40 K atoms after 3 s of forced evaporative cooling at 750G 26 µK trap depths 55 µK temperature ~ 4µK numbers ~ Li 40 K heteronuclear Fermi-Fermi mixture
Cs Feshbach resonances
Feshbach resonance r U(r) incident channel bound state magnetic moment of closed channel differs from the magnetic moment of the incident channel swave scattering length a as a function of magnetic field B B a a bg B0B0 coupling closed channel
coupling to molecular state r U(r) bound state coupling Feshbach resonance as entrance door into the molecular world enhanced three-body recombination near Feshbach resonance: trap loss as a signature of a resonance production of molecules via Feshbach resonances T. Köhler, K. Goral, P. Julienne, RMP 78, 1312 (2006)
Energy [MHz] B [T] LiK spin states m=+1 m= -1
Energy [MHz] B [T] LiK spin relaxation m=+1 m= -1 stable mixtures can be created if one of the species is in the lowest state ! stable mixtures can be created if one of the species is in the lowest state !
Energy [MHz] B [T] |1> |2> |3> |1> |9> LiK spin states nomenclature
Energy [MHz] B [T] |1> |2> |3> |1> |9> LiK initial spin state preparation optical pumping to K|1> resonant laser removes Li|1> RF transfer to K|2> Optical pumping to lower states
ramp to B field prepare mixture of K|1> and Li |1,2> in IR trap evaporative cooling at 760 G wait 10 s losses occur recapture to MOT and observe remaining fluorescence Prepare stable mixture state change with RF resonant laser B [G]760 B Remove unwanted states RF sweep Feshbach spectroscopy Li|1> + K|1> Li|1> + K|2> Li|1> + K|3>. Li|2> + K|1>Li|3> + K|1>
Li|1> K|2> scan K|2> onlyinterspecies
interspecies Feshbach resonances B [G] channelposition [G]width [G] Li|2> + K|1> Li|1> + K|1> Li|1> + K|1> Li|1> + K|1>24911 Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|3> Li|1> + K|3> Li|1> + K|3> Li|1> + K|3>27114
B [G] channelposition [G]width [G] Li|2> + K|1> Li|1> + K|1> Li|1> + K|1> Li|1> + K|1>24911 Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|2> Li|1> + K|3> Li|1> + K|3> Li|1> + K|3> Li|1> + K|3>27114 S. Kokkelmans, T. Tiecke and J. Walraven, E. Tiesinga, P. Julienne did interpret data Only two free parameters fit model to data: position of last bound state in singlet and triplet potential (= a S and a T ) interspecies Feshbach resonances asymptotic bound state moldel coupled channels calculation
p-wave molecules s-wave molecules individual atoms what do we learn? r U(r) incident channel bound state coupling closed channel
Li|1> K|2> scattering length singlet sc. length a s = 52.1 a 0 triplet sc. length a t = 63.5 a 0 interaction tuning ! coupled channels calculation by Eite Tiesinga and Paul Julienne but, s-wave res. all rather narrow !
present experiments optimum strategie for degeneracy three-body physics (3 non-id. atoms)
improved set-up superb optical access (later applications like lattices)
preliminary evaporative cooling 6 Li spin mixture would be a superb cooling agent 760 G molecules mBEC how well does sympathetic cooling of 40 K work ? loss of 40 K atoms
early stage of cooling: three-component atomic gas three atoms with two identical ones three-body recombination Pauli suppressed
early stage of cooling: three-component atomic gas three atoms with two identical ones three-body recombination Pauli suppressed
early stage of cooling: three-component atomic gas three nonidentical atoms (one pair with large a) three-body recombination without Pauli suppression
atom-dimer interaction (involving three atoms) inelastic decay without Pauli suppression final stage of cooling: atom-dimer mixture
three-body physics three-body decay (near-resonant pair of atoms) atom-dimer decay (weakly bound halo dimer) ? ? our educated guess: moderate loss our educated guess: strong loss … but three-body physics is full of surprises, e.g. „Efimov-type“ loss resonances, loss minima… this slide made in May 2008
next experimental step variable B-field explore three-body decay and sympathetic cooling of 40 K immersed into a near-resonant 6 Li spin mixture an important question awaiting experimental answer: does a realistic scenario for a three-component Fermi gas exist where at least two components are strongly interacting ? or will future experiments on strongly interacting Fermi gases be restricted to two-component gases ? this slide made in May 2008
strong loss in a 3-comp. 6 Li spin mixture three-body recombination with lowest three spin states of 6 Li Ottenstein et al., arXiv: S. Jochim‘s group in Heidelberg extremely fast losses !
two spin states of 6 Li Feshbach resonance non-resonant + one spin state of 40 K stable or not ? how about a 6 Li – 40 K mixture?
a > 0 a < 0 resonance high stability on top of the resonance and its a<0-side very good news ! strongly interacting, degenerate spin mixture of 6 Li + 40 K after 1s storage
Devang Naik Devang Naik The team Fermi-Fermi mixture available in the lab! several interspecies resonances observed! scattering properties understood! “friendly” three-body properties