Ref: http://physicsweb.org/article/news/8/1/6
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.
See: http://physicsweb.org/article/news/8/1/6
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