Pomeranchuk Effect

the anomalous character of the melting, or solidification, of the light helium isotope³ He: at a temperature below 0.3°K, the entropy of liquid³ He is less than that of the solid, and heat absorption occurs when the solid phase is formed. According to the Clausius-Clapeyron equation, the dependence of the melting point T_melt on pressure is in this case also anomalous—that is, as the pressure rises, T_melt decreases. The effect was theoretically predicted by I. Ia. Pomeranchuk in 1950 and experimentally detected by the American physicists W. M. Fairbank and G. K. Walters in 1957.

The Pomeranchuk effect is made use of to obtain temperatures as low as 1–1.5 millikelvins. Upon adiabatic compression of³He along the melting curve the formation of the solid phase and a corresponding temperature drop occur. Below 1–1.5 millikelvins, the ordering of the nuclear spins of³ He in the solid phase leads to a sharp decrease in the entropy of solid³ He; as a result, the Pomeranchuk effect is precluded at these temperatures.