Supercold, Wiggling 'Jelly' Presents Evidence of New Kind of Superfluidity
If confirmed, finding would provide a new way to study superconductivity
Tuesday, April 13,
2004 | DURHAM,
N.C. - Duke University researchers may have reached a milestone in
physics by cooling and confining a gas of lithium-6 atoms into a kind
of oscillating "jelly" exhibiting group behavior uncharacteristic of
this antisocial "fermion" atom class. The observed behavior, the Duke
group's leader said, would conform to what some theorists predict for
the fermion form of superfluidity, a rare state in which matter can
flow without resistance.
If fermion atoms can indeed form a superfluid state, as the
"friendlier" class of boson atoms are already known to, this would
provide scientists with new insights for studying such phenomena as
very high temperature superconductivity, said John Thomas,
the physics professor who heads the Duke team. Because electrons are
also fermions, neutral lithium atoms in a superfluid state could serve
as a stand in for a high temperature version of a superconducting
system where electricity flows without resistance.
"The really long-term interest would be applications to
superconductivity," said Michael Gehm, a Duke postdoctoral researcher
who along with Thomas and others wrote a new paper announcing these
observations. "The near term interest is to try to understand the
physics of what is going on in these systems," Gehm said.
As a result of their special electronic properties, fermions such as
lithium-6 cannot interact as closely as the bosonic class of atoms.
That has made it more difficult to induce superfluidity in fermions
than in socially gregarious bosons, which can co-mingle as closely as
atoms possibly can.
But in their paper posted April 13 in the online issue of the
journal Physical Review Letters, the Duke team announced promising
observations of certain "hydrodynamic" traits of fluids under pressure
as the scientists manipulated the temperature of a supercooled
"Fermi gas" of fermionic atoms in a tiny trap formed using a single laser beam.
The cigar-shaped gas congealed into a jelly-like state that
oscillated as the researchers cooled it significantly below 50
billionths of a degree above absolute zero, the team wrote. Absolute
zero, about 273 degrees Centigrade below the freezing point of water,
is the temperature at which theoretically all atomic motion stops.
This quivering of this jelly did not degrade in response to these
temperature manipulations as an ordinary gas would, said Thomas, who is
Duke's Fritz London Distinguished Professor of Physics. Instead it
became what Thomas called "a more perfect jelly" that continued
oscillating, and did so at a rate that some theoreticians have recently
predicted would be characteristic of a fermionic superfluid.
"These observations provide the first evidence for superfluid
hydrodynamics in a resonantly interacting Fermi gas," the Duke
physicists wrote in their paper, the first author of which was Joseph
Kinast, one of Thomas's graduate students. "It is…difficult to see how
the observations can be explained without invoking superfluidity," the
Thomas, however, declined to categorically claim his group has
observed fermionic superconductivity, saying theories that would
explain all their experimental observations are still incomplete. "We
have no definitive way of being absolutely sure until everything is
corroborated by theory," he said. "Until you get me a theory that
predicts what I observe completely and says 'this is a superfluid' I'm
not going to claim it is completely impossible that there's another
Besides Kinast, Thomas and Gehm, other authors of the Physical
Review Letters paper include graduate student Staci Hemmer and
postdoctoral researcher Andrey Turlapov. The research was supported by
the U.S. Department of Energy, the Army Research Office, the National
Science Foundation and NASA.
The Duke group confined a few hundred thousand atoms of gaseous
lithium-6 atoms in an "optical bowl" formed by a laser beam, cooling
down the atoms to temperatures so low that their normal atomic motion
Under those extreme conditions fermion atoms become what is called a
"degenerate" gas, swelling in size and approaching as close to each
other as the rules of Nature permit. Fermionic atoms are more
stand-offish than bosonic atoms. While members of the boson class of
atoms can occupy the same energy states, fermions cannot.
This antisocial trait means that in a degenerate state fermionic
atoms are too cold and closely confined to collide. "They're basically
told 'the place you want to go is occupied; therefore this collision
cannot happen,'" Thomas said. It was in this "collision-less" regime
that his group found signs of superfluidity.
While degenerate fermion gases cannot collide, theory says they can
be magnetically adjusted to interact by forming molecule-like pairs of
atoms. Because these "atomic pairs" act like bosons instead of
fermions, they are permitted to vibrate together in a coordinated
fashion that Thomas's group observed, he said.
In January, 2004 scientists at a joint laboratory of the National
Institute of Standards and Technology and the University of Colorado at
Boulder announced the first observations of a "fermionic condensate"
composed of pairs of fermions.
"That was a good experiment," Thomas said, "but it doesn't establish
superfluidity. To have superfluidity you've got to observe something
like hydrodynamics, like what we observed."
Thomas, Hemmer and Gehm were also among the authors of an earlier
Nov. 7, 2002, paper in the journal Science's online Science Express
that found less-obvious hints of superconductivity. In that research
the authors also created a "degenerate" fermion gas under similar
conditions. But instead of confining and manipulating the gas they
On release, that gas swelled in free space in a startlingly lopsided
way, expanding in one direction but not in the other. While
theoreticians have predicted such an "anisotropic expansion" as another
possible sign of superfluidity, Thomas and his colleagues realized
there was a second possible explanation. There was a chance that the
gas could have exhibited that behavior if it lost its collision-less
properties on release from confinement.
The new set of experiments was designed to rule out the possibility
of the gas's becoming "collisional" by keeping it confined in its trap
while slightly adjusting its temperature, Thomas said.
Available theory said that if the gas remained truly collision-less
at the temperatures the Duke group observed, the jiggling of jelly it
formed would not degrade. If it continued oscillating at a certain
characteristic rate, theory postulated that the gas was in the
superfluid state. The oscillation rate the Duke group observed is
"exactly what it's supposed to be," he said.
Thomas said some critics might challenge the Duke group's
interpretations because they did not observe "some really abrupt
transition" connected with a switch to the superfluid state. However
some theoreticians have proposed this system might be "a gapless
superfluid," he said. "That doesn't have to have such an abrupt change."
Superfluidity, which is the flow of a fluid without resistance, has
similarities to superconductivity, the flow of electric current without
resistance. Both involve the special interaction of pairs of particles,
Thomas added. "But in a superconductor those particles are electrons
that carry charge; in a system with neutral atoms there is no charge."
Because this superfluid forms at a relatively high temperature for
such gaseous atoms, it would behave like an extremely high temperature
superconductor, Thomas said. The Duke group's system would thus allow
scientists to model what would happen if metals like copper could act
like high temperature superconductors with transition temperatures
above the melting point.
And because such systems are also "strongly interacting" - meaning
their constituent particles affect each other at much greater than
normal distances - they could also be used to model matter under
especially extreme conditions, Thomas added. Examples are processes
within neutron stars, and the natures of quark-gluon plasmas postulated
to have formed microseconds after the universe began in a colossal "Big
For more information, contact:
Monte Basgall | phone: (919) 681-8057 | email: firstname.lastname@example.org.