Investigation of the relaxation dynamics of phonons in NaNO2

The study of phonon relaxation and dephasing mechanisms in NaNO2 has been carried out experimentally with a high-resolutioninterferometric Raman technique. For the linewidth measurements a piezoelectrically scanned Fabry-Perot interferometer has been coupled to a double monochromator. This allowed the accurate determination of the linewidth of phonons in a wide range of energies and free spectral ranges with minor modifications of the setup. External and internal (vibron)LO, TO and oblique phonon linewidths of NaNO2 have been measured as a function of temperature, from 10 to 200 K. The temperature behaviour of lineshapes has been rationalized in terms of artharmonic coupling between phonons and energy conservation requirements. The external phonons, either LO or TO, show a linear temperature dependence at the classical limit, i.e. kT≫, h{stroke}ω, until about room temperature. The linewidth is controlled by three-phonon relaxation mechanisms of the phonon energy into lower energy phonons of the bath (down conversion) or higher energy phonons (up conversion). The v2 vibron linewidth showed instead a complicated nonlinear temperature behaviour which was fitted with a four-phonon dephasing mechanism. No efficient relaxation energy decay channels of lower order are allowed for this mode thus explaining the nonlinear temperature dependence of the linewidth. v3 TO exhibits a similar behaviour. The tuning of the coupling between the v1 and v3 vibrons can be accomplished in the bc scattering plane by changing the angle between the phonon k vector and the crystallographic axes. A larger linewidth has been observed for these vibrons at the angie of maximum coupling.