The Pomeranchuk effect*

A central part of the story of the discovery of superfluid 3He is the cooling technique used for the experiments, the Pomeranchuk effect. Although it is not an especially useful technique for obtaining low temperatures today, it contains my favorite example of the use of the Clausius-Clapeyron equation. The cooling technique is fun to describe in undergraduate physics classes on thermodynamics. In 1950, I. Pomeranchuk, a well-known particle theorist, suggested that melting 3He could be cooled by squeezing it (Pomeranchuk, 1950). At the time of his suggestion 3He was quite rare and had not yet even been liquefied. He observed that at low enough temperatures the thermal phenomena in condensed 3He would be dominated by spin properties instead of phonon properties. The liquid of 3He would obey Fermi statistics with an entropy proportional to the temperature, much like the free electrons in a good metal. On the other hand, the entropy of solid 3He would be that of the disordered collection of weakly interacting spin1/2 nuclei. At temperatures greater than the (then) expected nuclear-magnetic ordering temperatures less than 1 mK, the entropy per mole of solid 3He would be S5R ln 2, independent of temperature until the hightemperature phonon modes of the solid become important. (The Debye temperature of solid 3He is approximately 30 K.) The idea of the method is represented in Fig. 1. The entropy of solid 3He exceeds that of liquid 3He at temperatures less than 0.3 K. If the mixture is compressed without heat input it will cool as liquid is converted into solid.