hunt for superfluidity because quantum
materials, when placed in a spinning container, don’t spin along. Imagine rotating a bucket of water. If the water were
a superfluid, it wouldn’t slosh around
with the rotation but would instead sit
unmoving, decoupled from the bucket’s
moving sides. Supersolids, if they exist,
would do the same thing.
Thus, as a solid transitions
to a supersolid state at low
temperatures, the period of
time it takes for the oscillator to rotate back and forth
would drop — because less
mass would be sloshing
around.
Chan and his colleague
Eun-Seong Kim didn’t think they had
much chance of finding supersolidity.
“I remember telling Eun-Seong that
our chances of seeing something were
close to zero—like buying a lottery
ticket,” Chan says.
Yet when they filled their torsional
oscillator with solid helium and spun
the machine, they saw its period get
shorter — presumably because some of
the solid helium was becoming decou-
pled from the system instead of rotating
with it. And when they put solid helium
in a spinning device the shape of a
doughnut, then blocked a portion of the
doughnut before oscillating it, the effect
mostly went away — suggesting that flow
was indeed occurring within the helium
and could be cut off at will.
That discovery, reported in 2004,
touched off a rush of excitement among
physicists, who thought they had finally
seen the long-predicted
supersolidity (SN: 1/17/04,
p. 35). But the observations
that followed made it far
from crystal clear.
“The rumor of
the demise
of the subject
of supersolidity
is highly
exaggerated.”
MOSES CHAN
Super or not so super
For one thing, researchers
struggled with understand-
ing what role defects played
in supersolidity. In 2007, a team led by
physicist John Reppy of Cornell Uni-
versity reported heating helium crystals
long enough to heal many of the defects
within. These cleaner crystals, when put
into a torsional oscillator, changed the
rotational period of the device far less
than would be expected if supersolidity
were happening. Scientists were puzzled.
Crystal purity Researchers in Paris have grown solid helium crystals in a transparent torsional oscillator. in the top series, a single pure crystal
grows slowly at a temperature of about 25 millikelvins. Below, a highly disordered polycrystal grows quickly, at 600 millikelvins. By watching these crystals
develop, scientists hope to tease out whether supersolidity occurs in pure helium or whether defects must be present for the solid to start flowing.
Pure crystal
