Dr. Smair Bali
, Duke University
[Host: Olivier Pfister]
A quantum degenerate sample of cold dilute fermions is expected to yield
new exciting physics since Pauli's exclusion principle forbids
congregation of fermions in the same quantum state. A fascinating
possibility is the formation of Cooper pairs, analogous to the BCS phase
transition responsible for superconductivity and for superfluidity in
liquid He-3. Lithium-6, a stable and naturally abundant fermionic isotope,
is an excellent candidate because it has large and attractive interatomic
interactions, a necessary requirement for the superfluid transition.
However, the lowest lying spin states of lithium-6 are not
magnetically trappable. This precludes the use of a magnetic trap, the
only kind of trap in which BEC has been achieved for bosonic atoms.
The possibility of achieving quantum degeneracy, whether boson or
fermion, in an optical trap has been a long sought goal and is of intense
current interest. This is because, unlike their magnetic counterparts,
optical traps can trap all spin states and offer the possibility of
arbitrary control of interatomic interactions via external magnetic
fields. However, owing to the presence of unexplained heating
rates, optical traps have failed to be stable. We have identified
some of the important heating mechanisms. By minimizing their effect
we have constructed an ultrastable optical trap consisting of a focused
far-detuned CO2 laser beam, in which we confine lithium-6 atoms with a
life-time of 300 secs. This is nearly a two order of magnitude improvement
in stability over all previous optical traps, rivalling that of magnetic
We simultaneously confine the two lowest lying spin states of lithium-6
thus enabling efficient evaporative cooling toward Fermi degeneracy and,
possibly, the superfluid transition. Measurements of anomalously large
elastic collision cross-sections and observation of evaporative cooling
of lithium-6 will be presented.
Atomic Physics Seminar
Monday, January 17, 2000
Physics Building, Room 204
Note special time.
Note special room.
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