Phonon spectrum of the ferromagnetic superconductor UGe$_2$ : Consequences for the specific heat

We report inelastic neutron scattering measurements of the phonon spectrum of the pressure-induced ferromagnetic superconductor UGe$_{2}$. No changes of the spectrum were found on cooling down to low temperature. The phonon contribution to the specific heat was estimated from a fit to our data. The excess specific heat previously noted at around $T_{x} approx$ 30 K is not due to phonons but is well described by the temperature dependence of the magnetic order parameter at the molecular field level.