time you boil water in a kettle, you witness a phenomenon known as a phase
transition—water transforms from a liquid to a gas, as you can see from the
bubbling water and hissing steam. Massachusetts Institute of Technology (MIT)
physicists have now observed a much more elusive phase transition: that from a
gas into a superfluid, a state where particles flow without any friction.
MIT work, published in an online edition of Science, also sheds light on
the superconductivity of electrons in metals, including high-temperature
superconductors that have the potential to revolutionize energy efficiency.
researchers, led by MIT assistant professor of physics Martin Zwierlein, carried
out their experiment with an isotope of lithium that has an odd number of
electrons, protons, and neutrons. Such particles are called fermions. In order
to become superfluid and flow without friction, fermions need to team up in
pairs. This is what happens in superconductors, where electrons form so-called
Cooper pairs, which can flow without any resistance.
to the transition from water to steam, the transition from the superfluid
(pairs) to the normal gas (single unpaired atoms) should be accompanied by a
dramatic change in the gas’s pressure, density, and energy. To directly observe
such a transition in a gas, the MIT team had to first trap the lithium gas in
an atom trap (in which atoms are held in place by electromagnetic fields) and
cool it to ultralow temperatures—less than a hundred billionths of a degree
above absolute zero.
this point, a superfluid comprising atom pairs was expected to form in the
center of the atom trap, surrounded by a normal region of unpaired atoms. A
light was then used to cast this atom cloud’s shadow on a camera.
the shadow images, Zwierlein and MIT graduate students Mark Ku, Ariel Sommer,
and Lawrence Cheuk set out to precisely measure the relationship between the
pressure, density, and temperature of the gas. The relation between these three
variables is known as an “equation of state” for the system. An equation of
state completely determines the thermodynamic properties of a system, including
its phase transitions.
A new ‘thermometer’
An obstacle in previous experiments on the thermodynamics of ultracold gases
was the absence of a reliable thermometer that can measure the temperature of a
puff of gas more than 10 million times colder than interstellar space. The
researchers solved this problem by carefully characterizing the properties of
their atom trap.
geometers who measure the height lines of a landscape, we determined the exact
shape of our trap,” explains graduate student Mark Ku. “These height lines then
served as our thermometer.”
of the trap as a valley filled with fog: In the upper regions, one would
encounter less dense regions of fog, while down in the valley the fog gets
denser. By measuring three quantities—the gas density at a given height line,
its change from one line to the next and the total amount of gas encountered on
the way down to that height—the researchers could determine the equation of state
of their gas of fermions.
atoms in these gases interact very strongly, not unlike the electrons in
high-temperature superconductors. The exact mechanism for superconductivity is
not yet understood, and so far, physicists have not been able to predict
materials that would become superconducting at room temperature. The MIT team
has now measured the critical temperature for superfluidity in their atomic
Fermi gas and shown that scaled to the density of electrons in a metal,
superfluidity would occur far above room temperature.