1. Prospects for ultracold metastable helium research: phase separation and BEC of fermionic molecules R. van Rooij, R.A. Rozendaal, I. Barmes & W. Vassen Laser Centre Vrije Universiteit, Amsterdam, the Netherlands I=1/2I=0 The Experiment: Ultracold Helium Helium-3 / Helium-4 mixture is excited to the 2 3 S 1 state using a dc discharge Metastable helium beam is laser cooled and trapped in a MOT Trapped helium is spin polarised The 1 mK atoms are transferred to a cloverleaf magnetic trap 1D Doppler cooling to 0.1 mK is performed within trap Evaporative cooling is performed down to ~1K 3 He* is sympathetically cooled with 4 He* +1 0 +3/2 -1/2 -3/2 3 He* 4 He* 8.4 MHz +1/2 5.6 MHz 2 3 S 1 state (He*): = 8000 s, 20 eV internal energy: single He* atom detection Penning ionization: He + ( He* + He* → He + He + + e ─ ) 3 He* fermion and 4 He* boson Scattering lengths large and positive: a BB =+7.5105(25) nm (exp. ENS Paris) a FB =+27.1 (5) nm (theory Warsaw) 4 He* 3 He* + 4 He* 3 He* Time-of-flight pictures of a BEC (upper figure), a DFG (middle figure) and a mixture (lower figure). The upper figure shows a thermal cloud above the BEC temperature, a mixture of BEC and thermal cloud, and a pure BEC with the typical inverted parabola shape. In the middle figure a fit to a Fermi-Dirac distribution is shown from which we extract a temperature T=0.45 T F. In the lower figure the dashed- dotted line shows the BEC contribution to the signal and the dashed line the DFG contribution. ground state: 1s 2 1 S 0 metastable state: 1s2s 3 S 1 ( 4 He*) F= 1/2, 3/2 ( 3 He*) Magnetic sublevels for both isotopes as a function of position in the cloverleaf trap after compression N c >10 7 N=2×10 6 T/T F =0.45 The setup Boson – Fermion Quantum Phase Separation Optical Dipole Trap Feshbach Resonances in situ spatial separation of fermions and bosons depends on fermion-boson and boson-boson scattering length a 44 and especially a 34 are large: good for the Helium case (B=0) proposed detection: in situ imaging in magnetic trap Very low quantum efficiency of Si at 1083 nm disturbances under investigation Laser: Er-fiber 2 W 1557 nm Trap depth 24 K Never implemented for Helium Access to Feshbach resonances Allows fast trap switching: better absorption imaging Starting point for optical lattice experiments DC discharge Collimation / Deflection Tuning of scattering length by changing the magnetic field Predictions are (being) made for boson-fermion resonances in Helium and fermion-fermion resonances (different m F states) No resonances have (yet) been confirmed experimentally for Helium Scattering length tuning changes phase-separation behaviour Fermion – Fermion interactions Not possible in the s-wave regime due to large PI losses when a two-state mixture is used For spin-polarized gas possible when |M F | 0 and, when a<0, p-wave pairing. Only accessible in optical trap BEC BCS a > 0 rotating molecules a → ±∞ interacting pairs a < 0 cooper pairs B (G) E (GHz) 3 He density profile for N F = N B = 10 6 Calculated by T. Tiecke (UvA) and S. Kokkelmans (TUe) Calculated by finding the self-consistent solution to With, the boson-boson and fermion-boson interaction strength, respectively. Feshbach Resonance at 10G 4 He: (J,M J )=(1,-1) 3 He: (F,M F )=(3/2,-1/2) n F / 10 12 cm - 3 z / μm r / μm