Microscopic investigation of the Johari-Goldstein relaxation in cumene Insights on the mosaic structure in a van der Waals liquid

The Johari-Goldstein (β_JG) relaxation anticipates in time, and is closely connected to, the structural relaxation in deeply supercooled liquids. Probing its microscopic properties is a crucial step for a complete understanding of the glass-transition. We here report the investigation of the van der Waals glass-former cumene using time-domain interferometry, a technique able to probe microscopic density fluctuations at the spatial and temporal scales relevant for the β_JG-relaxation. We find that the molecules participating in it undergo a restricted motion, though sufficient to induce local, cage-breaking events at the characteristic time-scale for molecular re-orientations. A detailed characterization of the relaxation strength, i.e. the fraction of molecules involved in the relaxation process, shows that such molecules are connected in a percolating cluster which, above the glass-transition temperature, T_g, is weakly dependent on temperature. Our results confirm thus previous observations of a mosaic structure associated to the β_JG-relaxation in the supercooled state, and provide additional information on its temperature evolution above the glass-transition temperature. We conclude that the observed microscopic properties of the β_JG-relaxation, and thus of the associated mosaic structure, are generic and independent of the molecular interaction potential. In addition, we show that, while the dynamics within the percolating cluster becomes progressively slower on approaching T_g, the fraction of the molecules involved in cage-breaking events within the β_JG-relaxation is not affected by temperature.