Quasielastic neutron scattering evidence of coupling of caged molecule dynamics to JG β-relaxation

In several publications we studied the fast caged dynamics of many glass-formers observed in the glassy state at high frequencies above circa 1 GHz by various techniques, including neutron scattering, Brillouin scattering, H-2 NMR spin-lattice relaxation, and THz dielectric spectroscopy. On increasing temperature, the intensity of the caged dynamics exhibit two step increases in intensity at two temperatures, first at T-HF below the glass transition temperature T-g alpha and subsequently at T-g alpha. T-HF is nearly coincident with the Johari-Goldstein (JG)beta-glass transition temperature T-g beta. The phenomenon is general and found in polyalcohols, pharmaceuticals, many amorphous polymers, and two small molecular van der Waals glass-formers. It is remarkable since T-HF is determined from measurements of fast caged dynamics at short time scales typically in the ns to ps range, while T-g beta characterizes the JG beta-glass transition at which the JG beta-relaxation time tau(beta) reaches a long time similar to 10(3) s. The phenomenon was explained by coupling of the caged dynamics to the JG beta-relaxation. In this paper we analyze exceptionally high quality quasielastic neutron scattering data of two molecular van der Waals glass-formers, cumene (isopropylbenzene) and cis-decalin (cis-decahydro-naphthalene) to demonstrate the generality of the phenomenon and validity of its theoretical explanation. (C) 2017 Elsevier B.V. All rights reserved.