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, 2H 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 THF below the glass transition temperature Tgα and subsequently at Tgα. THF is nearly coincident with the Johari-Goldstein (JG) β-glass transition temperature Tgβ. 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 THF is determined from measurements of fast caged dynamics at short time scales typically in the ns to ps range, while Tgβ characterizes the JG β-glass transition at which the JG β-relaxation time τβ reaches a long time ~ 103 s. The phenomenon was explained by coupling of the caged dynamics to the JG β-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.