Condensed-matter physics: Supersolid helium

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Author: John Beamish
Date: Jan. 15, 2004
From: Nature(Vol. 427, Issue 6971)
Publisher: Nature Publishing Group
Document Type: Article
Length: 1,292 words
Lexile Measure: 1440L

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Author(s): John Beamish [1]

At temperatures below 2.176 K, helium-4 enters a superfluid state and flows without friction. This 'perpetual motion' makes superfluidity -- perhaps even more than its electronic counterpart, superconductivity -- the most dramatic manifestation of quantum mechanics on a macroscopic scale. Despite its appeal, and despite many searches for superfluidity in other systems, it remains an uncommon phenomenon. From 1938, when superfluidity was discovered [1, 2], helium-4 was the only known example until 1972 when the phenomenon was seen [3], at much lower temperatures, in helium-3. That temperature difference between the two isotopes' behaviour reflects the intimate connection of superfluidity to Bose-Einstein condensation -- the transition that occurs when 'bosons' collect in a single quantum mechanical state. Atoms of helium-4 are bosons, but helium-3 atoms are 'fermions' and must pair up before they can condense into a single state.

In 1995, advances in laser-cooling and magnetic-trapping techniques led to the achievement [4] of Bose-Einstein condensation in rubidium vapour, adding to the list of superfluid systems. That list now includes other gases, such as spin-polarized hydrogen gas [5], and, most recently, molecular gases of paired fermions [6, 7]. On page 225 of this issue, Kim and Chan [8] claim the first observation of superfluid behaviour in a solid. A sample of solid helium-4, confined in the nano-scale pores of Vycor glass and rotated in a torsional oscillator, underwent a transition below about 175 mK that indicated the onset of 'supersolid' behaviour (Fig. 1). If it can be confirmed that superflow is occurring in the solid helium, this is a remarkable result indeed.

Despite their rarity, superfluids are fundamental to, for example, statistical mechanics and fluid dynamics, and they are a valuable test bed in fields as diverse as turbulence and cosmology. So it is not surprising that superfluidity has been sought in new systems. Solids, with their atoms localized on a periodic lattice, are certainly the most unexpected phase of matter...

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Gale Document Number: GALE|A186371740