Figure 12

Fig. 12: Excitation energies, (Q), derived from the fit of the IXS data using a damped harmonic oscillator model, for the new high resolution data ( = 1.4 meV) and from previous IXS experiments ( = 3.2 meV, = 5.0 meV). The open symbols refer to the dispersing excitation and the full diamond to the weakly dispersing ones. The blue line, with a slope of 3200 m/s results from a fit for Q > 4 nm^-1. The inset shows an enlargement of the low Q region, where the transition from fast toward normal sound takes place, as emphasised by the two lines corresponding to the fast and normal sound branches.