Kinetic energy of liquid and solid ⁴He

We report new path-integral calculations and measurements of the kinetic energy of condensed helium and construct an overall dependence of kinetic energy on temperature for densities less than 70 atoms nm^-3. In the solid phase we find the kinetic energy is almost temperature independent and, surprisingly, has a smaller kinetic energy than the fluid near freezing at the same density. In the high temperature fluid phase, the excess kinetic energy decreases to zero very slowly because of pair scattering from the repulsive interatomic potential.