Fig. 6

(a) and (b): Comparison of the spectra measured in the transverse configuration at values \(q=0.2\), \(q=0.25\) and \(q=0.3\) at \(T<T_{co}\) (14K, upper panel) and \(T>T_{co}\) (180K, lower panel, with spectra shifted by 25 counts for clarity). This comparison shows that, at \(T<T_{co}\), the TA’ branch exhibits a maximum intensity at \(q=0.25\) and that the LA branch acquires a large intensity in the transverse configuration for \(q\ge 0.25\). (c) and (d): Comparison of raw data obtained in longitudinal configuration at \(q=0.35\), for \(T<T_{co}\) (14K left panel) and \(T>T_{co}\) (180K, right panel), showing the occurrence of the LA’ mode at \(T>T_{co}\).