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 Tmelt on pressure is in this case also anomalous—that is, as the pressure rises, Tmelt 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.
L. G. ASLAMAZOV