COOL things happen when atoms get close to absolute zero. But getting them
cold enough can be tricky. Now researchers have come a step closer to making one
type of atoms so cold that they behave like the electrons in a
superconductor鈥攁 quantum phenomenon never before seen in atoms.
Atoms come in two varieties: bosons have an even sum total of electrons,
neutrons and protons, while fermions have an odd total. That distinction makes
all the difference when a gas of atoms is cooled to near absolute zero.
Bosons like to clump together in the same quantum state, called a
Bose-Einstein condensate鈥攁 feat first achieved five years ago (New
快猫短视频, 22 July 1995, p 16). Fermions can鈥檛 form such a condensate, because
quantum mechanics forbids more than one fermion from occupying each quantum
state. Fermions effectively avoid each other, and if atoms don鈥檛 collide it鈥檚
difficult to draw heat energy out of them. 鈥淚f you don鈥檛 have collisions, you
don鈥檛 have thermal equilibrium and temperature is not well defined,鈥� says
Deborah Jin of the National Institute of Standards and Technology in Boulder,
Colorado.
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However, the collisions don鈥檛 cease if bosons are mixed in with the fermions,
Randall Hulet and his colleagues at Rice University in Houston, Texas, will
report in a forthcoming issue of Science. The researchers cooled a mixture of
lithium-6 and lithium-7 atoms to within a fraction of a degree of absolute zero.
The lithium-7 bosons fell into the condensed state, and collisions between the
two types of atoms cooled the lithium-6 fermions enough to make them fill up the
lowest energy states of the container one by one, as if occupying the rungs of a
ladder. Previously, only Jin had observed this phenomenon, called 鈥渄egeneracy鈥�,
which is thought to keep neutron stars from collapsing under gravity.
The new technique could be used to reach a long-sought goal: getting fermions
to team up into 鈥淐ooper pairs鈥�, just as electrons do in a superconductor. At
temperatures very near absolute zero, the fermions near the top of the energy
ladder should form Cooper pairs. But making the unsociable fermions cooperate
won鈥檛 be easy. 鈥淐ooper pairing isn鈥檛 going to happen this week or next,鈥� Hulet
says. 鈥淏ut it could happen this year.鈥�