Kinetic energy of 4He along the T=6.1 K isotherm

We report the results of a neutron Compton scattering experiment on He-4, on the T=6.1 K isotherm, in a wide density range extending from the low-density gas phase to the compressed solid. These experiments give access to the atomic momentum distribution and we have derived the average kinetic energy. This is a function of the density, due to the increasing effect of interactions in the quantum fluid, and equals the classical value 3/2k(B)T only in the zero-density limit. The data allow us to give a coherent picture of the evolution of the kinetic energy of 4He, on the T=6.1 K isotherm, up to the compressed solid phase (p(max)=539 bars) where the measured value of the kinetic energy is similar to 5 times larger than the classical value. The data in the fluid phase are well described by a power law in the density with exponent 2.46. We confirm that the kinetic energy data display a change of behavior across the freezing transition, with the solid data lower than the extrapolation of the fluid data. [S0163-1829(98)02922-1].