MIT scientists have brought a supercool end to a heated race among physicists: They have become the first to create a new type of matter, a gas of atoms that shows high-temperature superfluidity.

MIT scientists have brought a supercool end to a
heated race among physicists: They have become the first to create a new
type of matter, a gas of atoms that shows high-temperature superfluidity.

The rotating superfluid gas of fermions is
pierced with the vortices, which are like mini-tornadoes. (Image Credit: Andre
Schirotzek, MIT)

Their work, to be reported in the June 23 issue of Nature,
is closely related to the superconductivity of electrons in metals. Observations
of superfluids may help solve lingering questions about high-temperature superconductivity,
which has widespread applications for magnets, sensors and energy-efficient
transport of electricity, said Wolfgang Ketterle, a Nobel laureate who heads
the MIT group and who is the John D. MacArthur Professor of Physics as well
as a principal investigator in MIT’s Research Laboratory of Electronics.

Seeing the superfluid gas so clearly is such a dramatic
step that Dan Kleppner, director of the MIT-Harvard Center for Ultracold Atoms,
said, ‘This is not a smoking gun for superfluidity. This is a cannon.’

For several years, research groups around the world have
been studying cold gases of so-called fermionic atoms with the ultimate goal
of finding new forms of superfluidity. A superfluid gas can flow without
resistance. It can be clearly distinguished from a normal gas when it is
rotated. A normal gas rotates like an ordinary object, but a superfluid can
only rotate when it forms vortices similar to mini-tornadoes. This gives
a rotating superfluid the appearance of Swiss cheese, where the holes are
the cores of the mini-tornadoes. ‘When we saw the first picture of the vortices
appear on the computer screen, it was simply breathtaking,’ said graduate
student Martin Zwierlein in recalling the evening of April 13, when the team
first saw the superfluid gas. For almost a year, the team had been working
on making magnetic fields and laser beams very round so the gas could be set
in rotation. ‘It was like sanding the bumps off of a wheel to make it perfectly
round,’ Zwierlein explained.

‘In superfluids, as well as in superconductors, particles
move in lockstep. They form one big quantum-mechanical wave,’ explained Ketterle.
Such a movement allows superconductors to carry electrical currents without

The MIT team was able to view these superfluid vortices
at extremely cold temperatures, when the fermionic gas was cooled to about
50 billionths of a degree Kelvin, very close to absolute zero (-273 degrees
C or -459 degrees F). ‘It may sound strange to call superfluidity at 50 nanokelvin
high-temperature superfluidity, but what matters is the temperature normalized
by the density of the particles,’ Ketterle said. ‘We have now achieved by
far the highest temperature ever.’ Scaled up to the density of electrons
in a metal, the superfluid transition temperature in atomic gases would be
higher than room temperature.

Ketterle’s team members were MIT graduate students Zwierlein,
Andre Schirotzek, and Christian Schunck, all of whom are members of the Center
for Ultracold Atoms, as well as former graduate student Jamil Abo-Shaeer.

The team observed fermionic superfluidity in the lithium-6
isotope comprising three protons, three neutrons and three electrons. Since
the total number of constituents is odd, lithium-6 is a fermion. Using laser
and evaporative cooling techniques, they cooled the gas close to absolute
zero. They then trapped the gas in the focus of an infrared laser beam; the
electric and magnetic fields of the infrared light held the atoms in place.
The last step was to spin a green laser beam around the gas to set it into
rotation. A shadow picture of the cloud showed its superfluid behavior: The
cloud was pierced by a regular array of vortices, each about the same size.

The work is based on the MIT group’s earlier creation of
Bose-Einstein condensates, a form of matter in which particles condense and
act as one big wave. Albert Einstein predicted this phenomenon in 1925. Scientists
later realized that Bose-Einstein condensation and superfluidity are intimately

Bose-Einstein condensation of pairs of fermions that were
bound together loosely as molecules was observed in November 2003 by independent
teams at the University of Colorado at Boulder, the University of Innsbruck
in Austria and at MIT. However, observing Bose-Einstein condensation is not
the same as observing superfluidity. Further studies were done by these groups
and at the Ecole Normale Superieure in Paris, Duke University and Rice University,
but evidence for superfluidity was ambiguous or indirect.

The superfluid Fermi gas created at MIT can also serve
as an easily controllable model system to study properties of much denser
forms of fermionic matter such as solid superconductors, neutron stars or
the quark-gluon plasma that existed in the early universe.

The MIT research was supported by the National Science
Foundation, the Office of Naval Research, NASA and the Army Research Office.

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