The extra brillouin doublets and central peak of KTaO3: Second sound vs. two-phonon difference scattering

Low-T Brillouin spectra of the incipient ferroelectric KTaO3 exhibit a broa d central peak (CP), and additional Brillouin doublets (BD), that can both be related to phonon-density fluctuations. On the basis of new high-resolut ion neutron data obtained of low-lying phonon branches, we analysed the pho non-kinetics mechanisms that are possibly the origin of these unusual featu res. Firstly, transverse acoustic (TA) phonons whose normal damping is fast er than the ED frequency can produce hydrodynamic second sound. Secondly, t wo-phonon difference scattering from low damping thermal transverse phonons contribute to the spectra with either a sharp or a broader doublet, depend ing on the phonon group velocity and anisotropy of dispersion surfaces. The position of the observed sharp doublet is consistent with both mechanisms, but a comparison of the computed and experimental anisotropies favours the second process.