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Low-temperature physics

Condensate forced out of hiding

Nature volume 391, pages 25–26 (1998)Cite this article

Below a temperature of 2.17 K, liquid 4He undergoes a phase transition to a new state of matter with superfluid properties. It has long been accepted that a Bose-Einstein condensate is responsible, namely that a macroscopic number of atoms occupy the same quantum state. Direct evidence for this condensate has eluded our best efforts for many decades, but finally, as reported on page 56 of this issue by Wyatt1, a macroscopic fraction of atoms with zero momentum has been observed in liquid 4He. A Bose condensate of Cooper pairs also underlies superfluidity in superconductors and liquid 3He.

Recalling a little history should make the significance of these results clear. Within weeks of the discovery of the superfluid properties of liquid 4He in 1938, Fritz London3 suggested that they were a consequence of the fact that 4He atoms have zero spin, and hence obey Bose statistics. (Bosons are particles with integer spin 0, 1, 2, ⃛; fermions have spin 1/2, 3/2, ⃛.) In particular, London argued that superfluidity was related to the occurrence of Bose-Einstein condensation (BEC) in a liquid, similar to that predicted in an ideal gas a decade earlier by Einstein4 in a paper that had fallen into disfavour. London's colleague Tisza5 quickly developed a two-fluid model, in which the superfluid component had its origin in the macroscopic occupation of atoms of a single quantum state. The superfluid component is a new, coherent, collective degree of freedom which moves without resistance.

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Figure 1: Quantum evaporation with liquid helium.

References

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  1. the Department of Physics, University of Toronto, Toronto, M5S 1A7, Ontario, Canada

    Allan Griffin

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Griffin, A. Condensate forced out of hiding. Nature 391, 25–26 (1998). https://doi.org/10.1038/34058

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