The above figure
shows false-color images of ultracold lithium-atom clouds, produced
by physicists at Rice University. Lithium has two stable isotopes, one
of which is a boson (lithium-7), the other of which is a fermion (lithium-6).
Bosons and fermions are the two fundamental types of quantum particles
found in nature. The lithium species are very similar in many respects;
for example, they contain the same configuration of electrons, making
them chemically identical. But in a low-temperature environment, dramatic
differences appear between the lithium because of the fact that they
comprise two fundamentally different kinds of particles.
The Rice researchers
trapped the two lithium species together in the same cloud and photographed
them one after the other. As it turned out, the bosons (lithium-7) are
used to help cool the fermions (lithium-6) down to a low temperature.
The atom clouds
are shown at three different temperatures: 810, 510 and 240 nano-Kelvin.
One nano-Kelvin is an extremely cold temperature--it's a billionth of
a degree above absolute zero, which is -460 degrees Fahrenheit. As the
temperature gets colder, one can see that the boson gas, shown on the
left, coalesces into a compact cloud, while the size of the fermion
gas stabilizes at a specific size. This illustrates the principle of
"Fermi degeneracy," in which the fermions cannot condense
further, due to a law of quantum mechanics--the Pauli exclusion principle--that
keeps identical fermions from occupying the same space at the same time.
The same effect stabilizes white dwarf stars against collapse under
their own gravitational attraction.
The somewhat anti-social
nature of fermions has made it difficult to cool them to low temperatures.
But the Rice researchers used the ingenious idea of "sympathetic"
cooling, in which one species acts as a refrigerant for another. In
this case, the bosons act as the refrigerator, while the fermions are
cooled by contact with the bosons.
Here's how it works.
The researchers first trap the two types of particles in a magnetic
field. Through a process known as "evaporative cooling," the
hottest, most energetic particles escape from the trap. Only the colder,
more sluggish particles remain, and this brings about a cooler overall
temperature for the gas. The evaporative cooling process is exactly
what happens when you blow on a cup of coffee to cool it off--you are
causing the hotter steam molecules to escape, leaving colder liquid
molecules which lowers the overall temperature of your drink.
Evaporative cooling
works great for bosons, but not as well for fermions. Evaporative cooling
only works effectively when particles are constantly bombarding each
other. When particles collide with one another, they redistribute their
energy. With collisions, even the most sluggish particles could eventually
gain enough energy to escape the trap. Another way to look at it is
that the collisions help energy to escape the trap. But fermions tend
to resist collisions. Bosons, on the other hand, have no such aversion
to collisions. The bosons in the trap collide with fermions and help
them to redistribute their energy, producing hotter, high-energy fermions
which leave the trap, so that only colder fermions remain. This allows
the researchers to achieve low temperatures with fermions.
Although the size
of the fermion cloud cannot be shrunk below a certain point, the researchers
are striving to cool the gas further. Each fermion can be imagined to
contain a tiny bar magnet; the strength of the magnet and its other
characteristics are specified by a property called "spin."
By creating a fermion gas with two different values of spin, and cooling
the gas to temperatures below those already achieved, the researchers
hope to pair together fermions with the two different spin values to
create something known as a "Cooper pair." The Cooper pair
is the basic ingredient of a superconductor, a material in which electricity
can travel without resistance. Studying these Cooper pairs in the simple
setting of a gas will bring new insights into the nature of superconductivity.
Superconductors
are potentially very important for society, as researchers hope that
they will someday lead to a more efficient way of distributing electricity.
Physicists envision this application with solid superconductors. But
superconductivity is not completely understood. Moreover, solid superconductors
are hard to analyze, because they often contain impurity atoms or defects
in their crystal structure.
Studied in a gas,
superconducting Cooper pairs would be much simpler to analyze theoretically.
In addition, the atom trap setup enables researchers to modify the strength
of interactions between the particles, making the fermions anywhere
from weakly attractive to strongly attractive. This can help provide
insights into BCS theory, the theory of low-temperature metal superconductors.
Most superconductors are believed to contain weakly interacting Cooper
pairs, but the recently discovered magnesium boride superconductors,
which are superconducting at record-high temperatures for a metal, seem
to contain Cooper pairs more strongly interacting than other metal superconductors.
(Thanks to Randy
Hulet and colleagues at Rice University for providing the figure and
parts of the caption.)
Reported by:
Andrew G.
Truscott, Kevin E. Strecker, William I. McAlexander, Guthrie Partridge,
and Randall G. Hulet, "Observation of Fermi Pressure in a Gas of
Trapped Atoms," Science, 2 March 2001
Rice
News Release on this work
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