A super-cold collection
of molecules behaving in perfect unison has been created
for the first time from a sea of “fermion” atoms by researchers at JILA,
a joint institute of the Department of Commerce’s National Institute
of Standards and Technology (NIST) and the University of
Colorado at Boulder (CU-Boulder).
Fermions
are a class of particles that are inherently difficult to
coax into a
uniform quantum state. The ability to meld fermions
into this state—a soup of particles that acts like one giant,
super molecule—may
lead to better understanding of superconductivity, in which
electricity flows through certain metals with no resistance. The work was described in a paper posted November 7 on the informal physics archival
Web site at http://arxiv.org and will be published online by the journal Nature on November 26. Researchers Deborah S. Jin of NIST and Markus Greiner and Cindy
A. Regal of CU-Boulder reported that they created a Bose-Einstein condensate
(BEC) of weakly bound molecules starting with a gas of fermionic potassium atoms
cooled to 150 nanoKelvin above absolute zero (about minus 273 degrees Celsius
or minus 459 degrees Fahrenheit).
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False
color images of the molecular Bose-Einstein condensate
forming.
Left—A cloud of gaseous
fermionic potassium cooled to 250 nanoKelvin
and paired into bosonic molecules.
Right—The same experiment
starting at 90 nanoKelvin where the molecules
collapse into a Bose-Einstein condensate.
In both images higher areas indicate a greater density of atoms.
For
a high resolution version of this image, contact Gail
Porter. |
Jin
describes her team’s work as the “first
molecular condensate” and says it is closely related
to “fermionic superfluidity,” a hotly sought
after state in gases that is analogous to superconductivity
in metals. “Fermionic superfluidity is superconductivity
in another form,” says Jin. Quantum physicists are
in a worldwide race to produce fermionic superfluidity because
gases would be much easier to study than solid superconductors
and such work could lead to more useful superconducting materials. While fermionic superfluidity was not demonstrated in the
current experiments, the NIST/CU-Boulder authors note that
their molecular condensate was produced by passing through
the appropriate conditions for fermionic superfluidity.
A separate
research group at the University of Innsbruck in Austria
reported
on November 13 in the online version
of the journal Science that they had created a similar Bose-Einstein “super
molecule” from lithium, fermion
atoms.
Bose-Einstein condensates are a new form of matter first
created by JILA scientists Eric Cornell of NIST and Carl
Wieman of CU-Boulder in 1995 with rubidium atoms. The pair
received the physics Nobel Prize in 2001 for the achievement.
First predicted by Albert Einstein, BECs are an unusual physical
state in which thousands of atoms behave as though they were
a single entity with identical energies and wave forms. Consequently,
BECs have been described as a magnifying glass for quantum
physics, the basic laws that govern the behavior of all matter.
In the world of quantum physics, atomic particles are classified
as either fermions (e.g., electrons, protons and neutrons)
or bosons (e.g., photons) depending on their spin. Fermions
have half-integer spins (1/2, 3/2, 5/2, etc.) and bosons
have integer spins (0, 1, 2, 3, etc.). In addition, whereas
no fermion can be in exactly the same state as another fermion,
bosons have no such restrictions. Light waves or photons
are the most commonly known bosons, and laser light is an
example of how bosons can behave in unison.
Since
1995, dozens of research groups worldwide have created
BECs and several
thousand scientific papers have been published
on the subject. Recently, a number of groups have been working
to produce a condensate from fermions. Superconductivity
occurs when electrons (a type of fermion) combine into pairs.
By producing pairing of ultracold fermionic atoms in a reproducible
fashion, researchers hope to explore the physics underlying
the “super” phenomena in unprecedented detail.
In their
experiment, the JILA scientists paired up individual fermion
atoms
(with half-integer spins) into molecules (with
integer spins) and in doing so formed a Bose-Einstein condensate.
The researchers cooled a gas of potassium atoms (potassium
isotope 40) with lasers and confined them in an optical trap.
They then slowly varied the strength of a magnetic field
applied across the trap to increase the attraction between
pairs of atoms and eventually converted most of the fermionic
atoms into bosonic molecules. “Strikingly,” they
said, the molecular condensate was not formed by further
cooling of the molecules but solely by the increased attractive
forces created with the magnetic field. When the initial
temperature of the fermion atoms was sufficiently low, the
gas collapsed into the BEC as soon as the loosely bound bosonic
molecules formed.
Funding for the research was provided by NIST, the National
Science Foundation and the Hertz Foundation.
In October
2003, Jin received a $500,000 John D. and Catherine T.
MacArthur
Fellowship, often referred to as a “genius
grant.”
As a
non-regulatory agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
The University of Colorado at Boulder is a comprehensive
research institution located in the foothills of the Rocky
Mountains and has an enrollment of 29,151 students. CU-Boulder
was founded in 1876 and is known for its strong programs
in the space sciences, environmental sciences, natural sciences,
education, music and law. It received a record $250 million
in sponsored research funding last fiscal year.
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Created:
11/20/03
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