Spectrum of fast dynamics in glass forming liquids: Does the “knee” exist?

A knee-shaped feature observed earlier in light scattering spectra of ${\mathrm{Ca}}_{\mathrm{0.4}}{\mathrm{K}}_{\mathrm{0.3}}{\mathrm{(NO}}_{\mathrm{3}}{\mathrm{)}}_{\mathrm{1.4}}$ (CKN) below $T_c$ is used as a strong argument in favor of mode-coupling theory of the glass transition (MCT). Our careful measurements reveal no “knee” in the spectra of two glass forming liquids, CKN and ortho-terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither expected in the asymptotic MCT predictions nor in any other model.