Supersolidity

Evidence for “Supersolidity” Becomes More Solid. ‘In recent years, no topic in condensed matter physics has been hotter than the study of ultracold solid helium. Subtle experiments suggest that as temperatures dip toward absolute zero, crystalline helium can bizarrely flow like a liquid with no viscosity, a phenomenon known as supersolidity. Now, a new experiment lends credence to that controversial claim by revealing a possible second sign of the transition.’

Penn State Live: High-quality helium crystals show supersolid behavior. ‘High-quality, single-crystal, ultra-cold solid helium exhibits supersolid behavior, suggesting that this frictionless solid flow is not a consequence of defects and grain boundaries in poor-quality, polycrystalline, solid helium, according to a team of Penn State researchers.

In 2004, Penn state physicists — Eunseong Kim, then-graduate student and Moses Chan, the Evan Pugh professor of physics– announced the observance of frictionless superflow in solid helium at nearly absolute zero. This new phenomenon is a cousin of Bose-Einstein condensate observed in gases in 1995 and in liquid helium in 1938.

Since then, their results have been replicated at the University of Tokyo, Keio University, Japan, and Cornell University. While the experiment was duplicated at Cornell, one experiment there found that if the solid helium was annealed –- cooled slowly from the melting point –- the supersolid behavior disappeared. This suggested that the theoretical idea of supersolidity is possible only in poor-quality solid helium and that the superflow is due to defects in the poorly grown crystals.

To create solid helium, the gaseous helium must be cooled very close to absolute zero and put under at least 25 atmospheres. Unlike other gases, helium remains a liquid at ambient pressure all the way down to absolute zero. Determining that the solid helium acts as a supersolid or Bose Einstein condensate is tricky. In a Bose-Einstein condensate all the atoms are at the lowest possible energy state, and they all behave in unison. The supersolid portion of the crystalized helium appears to flow without friction. For liquids and gases, this idea is less difficult because the atoms of both move around more and can easily slide past each other. But, in a solid, especially a very cold one, atoms do not usually flow easily or without friction.’

PhysicsWorld: The quantum solid that defies expectation. ‘In 2004 researchers reported the first clear evidence for superfluidity in solid helium-4. However, Matthew Chalmers describes recent experimental and theoretical work that has brought the very existence of such a “supersolid” into question.’

Physical Review Focus: Supersolid, with a Twist. ‘The strange quantum phenomenon known as supersolidity occurs when atoms flow without friction through a solid block of helium, possibly along a network of defects extending through an otherwise perfect crystal. In the 20 July Physical Review Letters, physicists find such a flow in computer simulations even when the atoms that make up the defects form a regular pattern, unlike the disorderly arrangement used in previous calculations. The results open a new way of understanding this extraordinary state of matter, which has properties of both solids and liquids.’

Kavli Institute Rapid Response to Supersolidity: ‘Is It There or Is It Not?’ May Be the Answer as Well as the Question (PDF).



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