A material that oozes through itself presents hop from one place to another, so that
different apartments become empty at
a super physics puzzle By Alexandra Witze
different times. The moving vacancies
could start to display Bose-Einstein–
like behavior, and part of the solid could
Solids are supposed to be the because of what’s at stake. Not only is begin to move without friction.
reliable state of matter. supersolidity an entirely new form of If supersolidity exists, scientists
Gases are flighty and flitting material behavior, but it’s also Nobel thought, they should look for it in solid
andexpandtofillanyavailable Prize territory. The study of superflu- helium. With just two protons in its
space. Liquids will also mold themselves idity, supersolidity’s older cousin, has nucleus, a helium atom is relatively light.
to whatever shape they occupy, from netted a series of Nobels (see Page 25). It is also not strongly attracted to other
soda bottle to swimming pool. Solids, helium atoms. So if vacancies could shift
though, are steadfast and unyielding, From one super state to another around within a crystal lattice, they
stable and dependable, like the rise and Superfluidity was discovered in 1937, would do so most easily in helium.
fall of tides, the guidance of the North when researchers in England and the Yet it took more than three decades
Star or the love of a dog. Soviet Union independently found that, for experimentalists to discover super-
But that truism may turn out not to be below temperatures of 2 degrees above solidity in helium- 4, which has two neu-
so true. In the past few years, physicists absolute zero, liquid helium could flow trons along with the two protons in its
have learned of a solid that doesn’t adhere without any friction. Intrigued, theorists nucleus. The breakthrough came after
to fixed rules. Within this solid — helium suggested that the atoms had morphed John Goodkind of the University of Cal-
at very low temperatures — some atoms into a new type of quantum state — a ifornia, San Diego reported that sound
appear to leave their rigidly defined “Bose-Einstein condensate,” in which waves traveled in an unexpected manner
positions and begin moving through the atoms lose their individual identities and through solid helium. Intrigued, Moses
rest of the material without friction. In begin to flow as a collective mass. Chan of Pennsylvania State University
essence, the atoms are simultaneously Superfluidityhasananaloginelectric- in University Park decided to launch
solid and fluid, and the material slides ity. Asuperconductingmaterialisonein a new study of solid helium using a
through itself. which electrons carry electricity without device called a torsional oscillator. This
Star Trek captains might be familiar any resistance. Superconductivity machine oscillates a sample back and
with such shape-shifting, but research- also occurs at low temperatures, and forth around a central axis, like a merry-
ers on Earth aren’t. Ever since the alsobecausetheelectronsstopbehaving go-round spinning first one way and
phenomenon, dubbed “supersolidity,” normally and instead pair up in a then the other at 1,000 times a second.
was described, physicists have struggled Bose-Einstein condensate of their own. Such oscillators had been used to
to explain what might be going on within By the middle of the last
the mutable material. century, then, scientists Super helium Under normal conditions, helium- 4 is a
Pressure (bar)
“Most people probably thought this knew that both fluids and
gas, but at temperatures close to absolute zero it can exist
in other phases. a superfluid phase has long been studied;
would be crystal clear in 18 months,” says electrons could flow with-
scientists are now exploring its probable supersolid phase.
John Beamish, a physicist at the Univer- out resistance thanks to
70
sity of Alberta in Edmonton, Canada. quantum effects. Why not
“It’s a surprise that five years later we’re solids, too?
60
still not sure — which is telling you it’s On the face of it, super-more complicated than we’d thought.” solidity seems counter-
Some studies suggest that observations intuitive, since atoms in a
50 Supersolid Normal
solid
once attributed to supersolidity might solid are arranged in a rigid
instead result from materials behaving crystal lattice. But almost all
40
bizarrely for other reasons. Recent work, solids have empty spaces —
for supersolidity. While theorists move atoms are missing, like
forward with explanations for how flaws an apartment complex in
20
within crystals could give rise to super- which some rooms don’t
solid behavior, experimentalists are have tenants. In 1969, Rus-
Super uid
10
building new laboratory devices to try sian theorists proposed
to pin down what is really going on with that quantum effects at low
this superpuzzle. temperatures could cause
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