Thermodynamic evaluation of the cubic term in the phonon spectrum of superfluid helium

When the approximation of overlapping momenta, in calculating the phonon and roton contributions to the thermodynamic properties of liquid helium II, is removed and the resulting expression for the specific heat of the system is fitted to experimental data at low temperatures, seemingly improved values of the phonon dispersion coefficient γ are obtained. At saturated vapor pressure, γ turns out to be --- 5.6 ×{} 10³⁷ cgs units, which is in excellent agreement with the value resulting from x-ray-scattering measurements. Comparison with neutron-scattering data suggests that γ may be strongly temperature dependent.