Opening
Statement from Press Conference
Scientists
at JILA, a joint laboratory of the Department of Commerce’s
National Institute of Standards and Technology (NIST) and
the University of Colorado at Boulder (CU-Boulder) report
the first observation of a “fermionic condensate”
formed from pairs of atoms in a gas, a long-sought, novel
form of matter. Physicists hope that further research with
such condensates eventually will help unlock the mysteries
of high-temperature superconductivity, a phenomenon with the
potential to improve energy efficiency dramatically across
a broad range of applications.
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Left
to right Deborah Jin, Markus Greiner, Cindy Regal
©Geoffrey Wheeler
To obtain a high-resolution copy of this image contact
Gail Porter. |
The research
is described in a paper to be published in the Jan. 24-30
online edition of Physical Review Letters by JILA
authors Deborah S. Jin, a physicist at NIST and an adjoint
associate professor at CU-Boulder, and Markus Greiner and
Cindy Regal, a post-doctoral researcher and graduate student
at CU-Boulder.
“The
strength of pairing in our fermionic condensate, adjusted
for mass and density,” Jin explains, “would correspond
to a room temperature superconductor. This makes me optimistic
that the fundamental physics we learn through fermionic condensates
will eventually help others design more practical superconducting
materials.”
The new
work complements a previous major achievement, creation of
a “Bose-Einstein” condensate, which earned JILA
scientists Eric Cornell and Carl Wieman, the Nobel Prize in
Physics in 2001. Bose-Einstein condensates are collections
of thousands of ultracold particles occupying a single quantum
state, that is, all the atoms are behaving identically like
a single, huge superatom. Bose-Einstein condensates are made
with bosons, a class of particles that are inherently gregarious;
they’d rather adopt their neighbor’s motion than
go it alone.
Unlike
bosons, fermions—the other half of the particle family
tree and the basic building blocks of matter—are inherently
loners. By definition, no fermion can be in exactly the same
state as another fermion. Consequently, to a physicist even
the term—fermionic condensate—is almost an oxymoron.
For many
decades, physicists have proposed that superconductivity (which
involves fermions) and Bose-Einstein condensates (BEC) are
closely linked. Theorists have hypothesized that superconductivity
and BEC are two extremes of superfluid behavior, an unusual
state where matter shows no resistance to flow. Superfluid
liquid helium, for example, when poured into the center of
an open container, will spontaneously flow up and over the
sides of the container.
In the
current experiment, a gas of 500,000 potassium atoms was cooled
to temperatures below 50 billionths of a degree Celsius above
absolute zero (minus 459 degrees Fahrenheit) and then a magnetic
field was applied near a special “resonance” strength.
This magnetic field coaxed the fermion atoms to match up into
pairs, akin to the pairs of electrons that produce superconductivity,
the phenomenon in which electricity flows with no resistance.
The Jin group detected this pairing and the formation of a
fermionic condensate for the first time on Dec. 16, 2003.
The temperature
at which metals or alloys become superconductors depends on
the strength of the “pairing” interaction between
their electrons. The highest known temperature at which superconductivity
occurs in any material is about minus 135 degrees Celsius
(minus 216 degrees Fahrenheit).
Funding
for the research was provided by NIST, the National Science
Foundation, and the Hertz Foundation of Livermore, Calif.
In October
2003, Jin, 35, 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 natural sciences,
space sciences, environmental sciences, education, music and
law. It received a record $250 million in sponsored research
funding last fiscal year.
Background:
History and Research Details
In 2001
JILA researcher Murray Holland and co-workers predicted that
fermionic atom condensates would turn out to be the link between
superconductivity and BECs. Holland’s group suggested
that magnetic fields could be used to “tune” a
gas of atoms to create a “resonance condensate”
between superconductivity and BEC behaviors.
The experiments
conducted by Jin’s team appear to confirm these predictions.
“We expect that the fermionic condensates that we observed,”
notes Jin, “will exhibit superfluid behavior. They represent
a novel phase that lies in the crossover between superconductors
and BEC.”
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image credit: NIST/University of Colorado
Figure
1
False color images of a condensate formed from pairs
of fermion potassium atoms. Higher areas indicate a
greater density of atoms.
Images from left to right correspond to the increasing strength of attraction between the atoms that form fermion pairs as the magnetic field strength is varied.
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In November
2003, Jin’s team (as well as a separate research group
in Innsbruck, Austria) reported producing a Bose-Einstein
condensate of molecules. In those experiments, a time-varying
magnetic field was applied to fermionic atoms that forced
them to combine into bosonic molecules. Fermions have half-integer
“spins” (1/2, 3/2, 5/2, etc.), while bosons have
integer “spins” (1, 2, 3, etc.). Spins are additive,
so that a molecule containing two fermionic atoms is a boson.
However, even if two fermions are not bound into one molecule,
but merely move together in a correlated fashion, then as
a pair they can act like a boson, and undergo condensation.
It is this second, more subtle form of condensation that has
been observed in the current experiments.
The current
work was performed by applying a particular magnetic field
at values where individual fermionic atoms cannot bind together
to form bosonic molecules. Instead, pairing of fermions is
caused by the collective behavior of many atoms, similar to
what causes “Cooper pairs” of electrons to form
in a superconductor.
Paradoxically,
in order to detect that the experiment produced a condensate
from paired fermions (and not molecules), the researchers
had to first convert the pairs into molecules. A magnetic
field at the right strength for molecular bonding was rapidly
applied to the fermionic condensate and simultaneously the
optical “trap” holding the gas was opened. This
magnetic field change can create molecules, but was too fast
to create a molecular BEC, as previously shown. Nonetheless,
a “picture” of the molecules’ motion showed
the characteristic shape of a condensate cloud. (See figure
1.)
“It
happens too fast for anything to move around,” says
Jin. “The condensate that appears in our ‘snapshot’
of the gas has to have existed before the molecules were formed.”
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Figure 2
Illustration by Markus Greiner
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Figure
3
Illustration by Markus Greiner |
In
simple terms, the fermion pairs are like high-schoolers at a
dance. When the band plays fast music, many dancers pair up
and move together in a coordinated way (Figure 2). If the band
suddenly switches to a slow dance, the dancers in each pair
move closer and “bond” (Figure 3). If a flash photograph
is then taken immediately, the ‘snapshot’ will show
“bound” dancers (molecules), but the arrangement
of those dancers was determined earlier when the pairs first
matched up.
“Even
in this first observation we were able to see the fermionic
atom condensates in a much more direct way than anyone had
anticipated,” says Jin. “This opens up the very
exciting potential to study superconductivity and superfluid
phenomena under extreme conditions that have never existed
before.”
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