Pomeranchuk cooling (named after Isaak Pomeranchuk) is the phenomenon in which liquid helium-3 will cool if it is compressed isentropically when it is below 0.3 K. This occurs because helium-3 has the unusual property that its solid state can have a higher entropy than its liquid state.
The effect was first observed by Yuri Anufriev in 1965.
[1] This can be used to construct a cryogenic cooler.
[2] In 2021 an analog effect has been observed on twisted bilayer graphene[3][4][5] and in TMDs[6]
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