Origin of the excess wing and slow beta relaxation of glass formers: a unified picture of local orientational fluctuations.

Here we consider the relation between the excess wing and the slow beta relaxation, focusing on the degree of the coupling between cooperative rotational motion associated with creation and annihilation of metastable islands (alpha mode) and local rotational jump motion in a cage (slow beta mode). For a strongly coupled case the slow beta mode appears as the excess wing having the character of the alpha mode, while for a fully decoupled case as an independent local mode having the Arrhenius temperature dependence. We argue that the degree of the coupling between the two modes is controlled by the following two factors: (i) the relation between the characteristic size of metastable islands and the cage size and (ii) the disparity between the high-temperature Arrhenius behavior of the alpha mode and the Arrhenius behavior of the slow beta mode.