The Lithium group: studies of fermionic and bosonic degenerate lithium gasses

(J. Cubizolles, T. Bourdel, M. Teichmann, L. Khaykovich, L. Tarruel, J. Zhang, F. Chevy, S. Kokkelmans, C. Salomon)

History, Motivation

Since their first production 1995, the study of degenerate bosonic atomic gases (Bose Einstein Condensation, BEC) has become an active and expanding field of research. The studies of their fundamental excitations, coherence properties, and interaction with light has given rise to sometimes surprising results. (See also the Vortex experiment of the Rubidium group.) New methods to produce and manipulate BECs are constantly being developed.

Recent results

The bright soliton

We have produced bright matter-wave solitons made from a ultra-cold gas of 7Li atoms. The effective interaction between the atomes in the BEC was tuned with a Feshbach resonance from repulsive to attractive. Then the condensate was released into a one-dimensional optical waveguide. We observed the propagation of the solition over a macroscopic distance of 1.1mm. [4]

Example of a Soliton

An Example of a Soliton

Production of long-lived ultracold 6Li2 molecules

We create weakly bound Li2 molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach 85% and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings. [2]

production of molecules

The production of molecules: we start at point 1, where there can be no molecules due to the negative scattering length. Then we sweep to point two. The atoms are lost. But when we sweep to point 3, the atoms reappear. This means that they have been hidden as molecules in point 2.

molecule production efficiency

Efficiency of production of molecules as a function of the trap depth.

BEC of Lithium-6 molecules

We studied the expansion of the cloud in the BEC-BCS crossover region.

We have produced a Bose-Einstein condensate of weakly bound 6Li2 molecules in a crossed optical trap near a Feshbach resonance. We measure a molecule-molecule scattering length of 170nm at 770 G, in good agreement with theory. [1]

boson and fermion in the same trap

A Bose-Einstein condensate of 6Li2 molecules (a), and a condensate of 7Li in the same trap (b). The large difference in condensate size after expansion reveals that the molecule-molecule scattering length in this figure is much larger than the Li-7 scattering length.

Study of the BEC-BCS crossover region

We have studied time-of-flight expansion images of condensates across the Feshbach resonance. In this region, the coupling between particles becomes very strong, meaning that the gas parameter na3 becomes around 1. This is a region where the normal mean field theory is no longer applicable. The region of the Feshbach resonance is interesting because it is a crossover region between a BEC of molecules and a predicted state of condensed cooper pairs.

the condensate for various magnetic fields

Change of the condensate for various magnetic fields. The two points indicate the Feshbach resonance. The left picture shows the expansion in the direction of weak confinement, while the right picture shows the epansion along the strong confinement.

change of various parameters with magnetic field

(a): scattering length between the |1/2, 1/2> and |1/2, -1/2> 6Li states. The Feshbach resonance peak is located at 820G (dotted line). (b): anisotropy of the cloud, (c): release energy across the BEC-BCS crossover region. In (c), the dot-dashed line corresponds to a T=0 ideal Fermi gas. The dashed curve is the release energy from a pure condensate in the Thomas-Fermi limit. The solid curve corresponds to a finite temperature mean field model with T=0.5TC.

Description of the experiment

A beam of Lithium atoms is slowed down by a counter-propagating laser beam in a spin-flipped Zeeman slower. Atoms of both Lithium isotops are cooled and accumulated in a magneto-optical trap (MOT). We can capture 1010 bosonic 7Li atoms at the same time with 5 108 fermionic 6Li atoms. The mixture is then transfered to a magnetic trap for evaporative cooling.

artist's view of the experiment

The vacuum chamber consists of a glass cell with a small appendage for the final magnetic trap, an Ioffe trap. We use a magnetic elevator to transfer the atoms from one region to the other (see figure on left). The MOT atoms are first captured (with 30% efficiency) in a quadrupolar magnetic trap (see photo on right). Afterwards, a second quadrupole field is gradually added. This field has its center at the position of the final trap. Afterwards the first field is ramped off. Thus the center of the resulting quadrupole field moves from the MOT region to the Ioffe trap region and takes the atoms with it. After having reached the final position, the atoms are captured in the Ioffe trap. The second pair of quadrupole coils, initially used during the transfer, is now used in a Helmholtz configuration to compensate the large magnetic field bias produced by the pinch coils.

the glass cell

At the end of this process we are left with 6 108 bosonic Lithium atoms at a temperature of 3 mK. This temperature is too high for us to start evaporative cooling, because the scattering cross section for 7Li-7Li collisions is energy-dependent and has a zero point at energies of 4 mK . Therefore, evaporative cooling at 3mK cannot work. We first cool down the atoms to below 1 mK using Doppler cooling inside the Ioffe trap. To prevent atoms from beeing pumped to untrapped states during this process, we work with a bias magnetic field of 450 Gauss and use red-detuned, circularly-polarized light parallel to the magnetic field. This gives a one-dimensional cooling in the axial direction of the trap. Due to anharmonicities in the trapping potential, ergodic mixing cools the other dimensions.

the coils

Sympathetic Cooling between Li-7 bosons and Li-6 fermions leads to simultaneous quantum degeneracy in both gases.

Recent Publications

[1] Experimental Study of the BEC-BCS Crossover Region in Lithium 6, T. Bourdel, L. Khaykovich, J. Cubizolles, J. Zhang, F. Chevy, M. Teichmann, L. Tarruell, S. Kokkelmans, C. Salomon, cond-mat/0403091 (2004)

[2] Weakly bound dimers of fermionic atoms, D.S Petrov, C. Salomon, G.V Shlyapnikov cond-mat/0309010 (2003)

[3] Degenerate atom-molecule mixture in a cold Fermi gas, S.J.J.M.F. Kokkelmans, G.V. Shlyapnikov, C. Salomon, Phys. Rev. A 69 031602(R) (2004)

[4] Weakly bound dimers of fermionic atoms, D. Petrov, C. Salomon and G. Shlyapnikov, cond-mat/0309010 (2003)

[5] Production of Long-Lived Ultracold Li2 Molecules from a Fermi Gas, J. Cubizolles, T. Bourdel, S. Kokkelmans, G. Shlyapnikov, and C. Salomon, Phys. Rev. Lett. 91, 240401 (2003).

[6] Formation of a Matter-Wave Bright Soliton, L. Khaykovich, F. Schreck, G. Ferrari, T. Bourdel, J. Cubizolles, L. D. Carr, Y. Castin, C. Salomon, Science 296, 1290-1293 (2002)

[7] Quasipure Bose-Einstein Condensate Immersed in a Fermi Sea F. Schreck, L. Khaykovich, K. L. Corwin, G. Ferrari, T. Bourdel, J. Cubizolles, and C. Salomon, Phys. Rev. Lett. 87, 080403 (2001)


Christophe Salomon
Laboratoire Kastler Brossel
24, rue Lhomond
75231 Paris, France

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