Thermodynamic Evidence for Fermi Superfluidity
Building a case for a superfluid state in a strongly-attractive
Fermi gas has been complicated, and has been the result of several ground-breaking
experiments. These include observations by groups at JILA,
MIT, Duke,
Rice, and the University
of Innsbruck groups. Recently, Wolfgang Ketterle's group at MIT
has observed vortices in Fermi gases, which appears to clinch the case
for superfluidity.
However, determining the transition temperature (at which
the Fermi gas transforms into a superfluid) requires some good old-fashioned
evidence for a thermodynamic phase transition, as well as calibrating
(determining) the temperatures they are seeing. Researchers from Duke
University report they have delivered the goods in two recent papers.
In the first thermodynamic study of a strongly interacting Fermi gas
(Kinast
et al., Science, 25 February 2005), Duke researchers recorded an
abrupt jump in heat capacity at an estimated temperature of about 0.27
TF (TF is the Fermi temperature scale, which corresponds
to the energy of the uppermost rung of the energy ladder if the atoms
filled all of the lowest available energy states).
In a second paper (Kinast
et al., Physical Review Letters, 6 May 2005), the Duke researchers
noticed an abrupt change in the behavior of the damping time (the time
it takes for the vibrating gas to settle down) below a temperature of
about 0.35 TF. The temperatures are calibrated (determined)
using a a new theory of the spatial profiles of a trapped strongly-interacting
Fermi gas introduced by theorist Kathy Levin and her collaborators at
the University of Chicago.
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