On condensed helium at absolute zero

Abstract

The properties of helium at very low temperatures have recently been the subject of many investigations. Helium is the only substance which cannot be transformed into the solid state merely by cooling. It has, it is true, at 2·19° abs a thermal anomaly, the so-called λ-point. At this temperature the specific heat has a discontinuity, indicating that liquid helium undergoes at this point that characteristic type of phase transition which, in Ehrenfest's well-known classification, corresponds to a phase equilibrium of the "second kind." But even below this temperature helium remains liquid; moreover, it has been found that its viscosity decreases suddenly to about a tenth when passing through the λ-point. It is only under pressure amounts to about 25 atmospheres. The entropy difference between the liquid and the solid phase tends towards zero with falling temperature, which means that the liquid phase goes into a peculiar ordered state. This change towards an ordered state of energy which occurs continuously in a relatively small temperature range.