The Central Peak in SbSI

Abstract

Abstract New measurements of light scattering in SbSI as a function of temperature are reported which extend the previous results to lower frequencies by improving the spectral purity of the spectrometer. The light is found to couple to the central peak in SbSI only indirectly via the soft mode. Using this fact as a condition, from a two‐coupled‐oscillator analysis of the soft‐mode/central‐mode system all the oscillator parameters are determined. Within the first 10 deg below the phase transition the central mode turns out to be a strongly overdamped oscillator of width Γω /2γ 1 of 0.4 to 4 cm −1 . This width compares very well with the value obtained from the hydrodynamic theory of light scattering; using measured thermal conductivity and specific heat data of SbSI one obtains a width of 0.3 to 1 cm −1 for a Δ T of 2 to 10 deg. Thus, the central peak in SbSI has been directly spectrally resolved and it seems justified to assign this feature to light scattering by phonon density fluctuations.