Anomaly of the non-ergodicity parameter and crossover to white noise in the fast relaxation spectrum of a simple glass former

Abstract

We present quasi-elastic light scattering (LS) and dielectric (DS) spectra of the glass former alpha-picoline (13K < T < 320K). At high temperatures the evolution of the susceptibility minimum is well described by mode coupling theory (MCT) yielding a critical temperature T_c = 162K +- 5K. At T_c > T > T_g the excess wing of the alpha-process identified in the DS spectra and characterized by a power-law exponent c is rediscovered in the LS spectra. Introducing a universal evolution for c = c[lg(tau_{alpha})] as suggested by DS data of several glass formers, the fast dynamics spectrum is singled out, allowing to determine the non-ergodicity parameter f(T). The latter shows the predicted cusp-like anomaly identifying T_c as well. Another discontinuous change of f(T) is observed at T_g. The fast dynamics spectra exhibit a crossover to ``white noise'' (T < T_c). Concerning the fast dynamics, we conclude that idealized MCT predictions hold even below T_c.