Number 651 #2, August 28, 2003 by Phil Schewe, James Riordon, and Ben Stein
Ultracold Molecular Bose Gases
Ultracold molecular Bose gases, where the gas consists of diatomic
molecules of fermionic atoms (atoms with an overall half-integral spin
value), provide two important opportunities---the chance to do high-precision
spectroscopy of molecules and the chance to study the process by which
fermions (normally unable to form into coherent quantum condensates)
amalgamate into pairs. The pairs are bosons (entities with a whole-number
valued spin) and can form condensates. Randy Hulet and his colleagues
at Rice University, the first to engineer a Bose Einstein condensation
(BEC) in lithium-7 atoms (see PNU
#237), have gotten a batch of Li-6 atoms to pair up (at least 50%
of them at a time) at micro-kelvin temperatures by manipulating an external
magnetic field. Although the group does not yet have evidence that the
pairs, or molecules, have taken the final plunge by forming a BEC, the
atoms have held together (in an optical trap) in their paired state
for as long as 1 second, compared to millisecond times for previous
experiments of this type. Hulet hopes that as the molecular gas hangs
together long enough, it will cool off sufficiently through the evaporative
process to form a BEC. Having a true BEC of molecules would give researchers
the chance to study the Cooper pairing mechanism at work in superconductivity
and in superfluidity of liquid helium-3. In ordinary molecules (joined
by chemical forces) the constituents (atoms) are very close together.
In the Cooper pairs characterizing superconductivity, the constituents
(electrons) are only weakly coupled and are far apart from each other.
Hulet and his group hope to dissociate the molecular condensate in order
to produce Cooper pairs that fall in between these two cases, both as
to the size and in the strength of the force holding the pairs together.
One might even be able to simulate high-temperature superconductivity
by loading ultracold fermion gases into an "optical lattice"
consisting of crossed laser beams. (Strecker
et al., Physical Review Letters, 22 August 2003; see
figure and lab website)