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Materials science: Superdiffusion in solid helium

Nature volume 400, pages 512–513 (1999)Cite this article

Low-temperature physics is a field that can appear sealed off from much of the rest of science, and metallurgy in particular. Its practitioners think in terms of millikelvins rather than, like metallurgists, in thousands of kelvins, and observe strange quantum-linked phenomena such as superfluidity and superconductivity. But when a group of low-temperature physicists — in this case, Emil Polturak and colleagues at the Technion in Israel — focus, exceptionally, on the mechanical behaviour of solid helium their results hold particular interest for metallurgists1,3. Following on from their discovery of unusually rapid diffusion in solid helium4, Polturak and co-workers have shown that this ‘superdiffusion’ occurs at the exact temperature at which solid helium changes crystal structure1 and is linked to a reduction in resistance to stress at this temperature2. In a computer simulation3, they relate this anomalous behaviour to a new theory of melting. This work covers a whole range of phenomena normally studied by metallurgists, but all within one or two kelvin of absolute zero.

Helium cannot be solidified at ambient pressure, but at pressures exceeding 25 atmospheres and below about 2.5 K it can be turned into a hexagonal-close-packed (h.c.p.) crystal. Over a narrow range of pressures and temperatures near 30 atmospheres and 1.8 K, this structure changes allotropically to a body-centred cubic (b.c.c) form5. Natural helium consists almost exclusively of the 4He isotope, but there is another stable isotope, 3He, a by-product of the production of tritium in nuclear reactors, which has quite different quantum characteristics. This rare isotope has also been extensively studied, both alone and in the form of 4He-3He mixtures (the two isotopes are partially soluble in each other5).

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Figure 1: Superdiffusion in solid helium.

References

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  1. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB3 0DT, UK

    Robert W. Cahn

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  1. Robert W. Cahn
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Cahn, R. Materials science: Superdiffusion in solid helium. Nature 400, 512–513 (1999). https://doi.org/10.1038/22894

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