Consideration of a Frequency-Independent Damping Coefficient from Raman Measurements

The possibility of a frequency-independent or frequency-dependent damping constant, $\gamma$, is experimentally investigated, using the right-angle Raman results for the heavily damped soft-$E$ mode in the ferroelectric PbTi${\mathrm{O}}_3$ at 491°C (just below the transition temperature). The fits, over a wide frequency range, using a frequency-independent $\gamma$ are so good that there is little room for any appreciable frequency dependence. This is in disagreement with several claims made in a recent paper by Heiman, Ushioda, and Remeika.