The Extra Brillouin Doublets and Central Peak of KTaO_3: Second Sound vs. Two-Phonon Difference Scattering

摘要

Low-T Brillouin spectra of the incipient ferroelectric KTaO_3 exhibit a broad central peak (CP), and additional Brillouin doublets (BD), that can both be related to phonon-density fluc-tuations. On the basis of new high-resolution neutron data obtained of low-lying phonon branches, we analysed the phonon-kinetics mechanisms that are possibly the origin of these unusual features. Firstly, transverse acoustic (TA) phonons whose normal damping is faster than the BD frequency can produce hydrodynamic second sound. Secondly, two-phonon dif-ference scattering from low damping thermal transverse phonons contribute to the spectra with either a sharp or a broader doublet, depending on the phonon group velocity and anisot-ropy 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.