How is He-3 different to He-4?
At low temperatures, that is, below a few Kelvin, the two isotopes begin to behave very differently. Around 2.17 K (at vapour pressure), helium-4 enters the superfluid (He-II) phase. Helium-3, on the other hand, remains in a normal (viscous) fluid phase until a much lower temperature is reached. The superfluid transition in helium-3 only occurs below 0.001 K.
It is hardly suprising then that much of the older literature on the subject of low temperature physics reports that the superfluid transition does not exist for the lighter helium-3 isotope. The superfluid transition in helium-3 is a relatively recent discovery, a feat for which David M. Lee, Douglas D. Osheroff and Robert C. Richardson shared the 1996 Nobel Prize in Physics. However, the earliest recognition of their pioneering work with superfluid helium-3 came from the UK's Institute of Physics in the form of the 1976 Simon Memorial Prize. George Pickett and Tony Guénault, founders of the Lancaster ULT group, were awarded the Simon Memorial Prize in 1998 in recognition of their outstanding contributions in the field of low temperature physics.
The incredibly low transition temperature of helium-3 combined with the rarity and hence expense of significant quantities of helium-3 has ensured that experimental access to this superfluid requires a significant dedication of time, effort, expertise and funds.
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