My question is, would this experiment or other experiments that have
been done be able to tell if a the helium 4 was reverting from solid
back to superfluid with a drop in temperature, instead of changing from
a solid to a supersolid. The experiment reminded me a little of what
happens when a superconductor is put in a magnetic field and becomes
nonsuper conducting then subsequently becomes superconducting again
with a further drop in temperature. Though theory might have predicted
that we would get a supersolid instead of a superfluid under these
conditions, the empirical results seem not able to distinguish between
the two conditions of superfluidinty and supersolidity. I have no real
problem with the idea of supersolids, but wouldn't Occam's razor
require us to assume that if an already observed state of matter can
explain a phenomena that we use that as our assumption until we can
empirically test for the difference between the two.
Supersolid is seen in the lab
14 January 2004
A new "supersolid" phase of matter has been created by physicists
in the US by cooling helium-4 to ultracold temperatures. Eun-Seong Kim
and Moses Chan of Pennsylvania State University say their supersolid
behaves like a superfluid - a liquid that flows without resistance -
but has all the characteristics of a crystalline solid (E Kim and M H W
Chan 2004 Nature 427 225).
All atoms are either bosons or fermions depending on whether their
intrinsic angular momentum or "spin" is an integer or half-integer
in quantum units. When bosonic atoms like helium-4 are cooled to near
absolute zero, they can all collapse into the same quantum ground state
to form a Bose-Einstein condensate. Fermionic atoms, on the other hand,
obey the Pauli exclusion principle and cannot form such a condensate.
When liquid helium-4 is cooled to below about 2 kelvin, it undergoes
Bose-Einstein condensation to become a superfluid. Although theory
predicts that superfluidity should also exist in solid helium-4, no
such supersolid phase has ever been observed in an experiment.
Chan and Kim first compressed helium-4 gas into a small glass disk made
of fused silica that contains atomic-sized pores. Next, they placed
this disk in a "torsional oscillator" capsule (see figure) and applied
a pressure of over 60 atmospheres. Then they rotated the capsule and
measured its oscillation rate as the temperature was lowered to almost
absolute zero.
The pair observed that the oscillation rate of the capsule increased
suddenly when the temperature fell below about 175 millikelvin.
According to Kim and Chan, this meant that the helium-4 had
"decoupled" from the pores in the glass disk and had entered a
supersolid phase. "In this supersolid, the individual helium-4 atoms
are continually flowing - without any friction - but because all the
particles are in an identical quantum state, it remains a solid,"
Chan told PhysicsWeb.
To test their findings, the duo repeated their experiment with helium-3
atoms - which are fermions. As expected, they found no change in the
oscillation period of the capsule. "If our results are confirmed,
this means that researchers have now been able to observe Bose-Einstein
condensation in gases, liquids and solid phases," write Kim and Chan.
About the author
.
|
