Picosecond Dynamic Heterogeneity, Hopping and Johari-Goldstein Relaxation in Glassforming Liquids

We show that incoherent quasi-elastic neutron scattering (QENS) from molecular liquids reveals a two-state dynamic heterogeneity on a 1 ps timescale, where molecules are either highly confined or are free to undergo relatively large excursions. Data ranging from deep in the glassy state to well above the melting point allows us to observe temperature-dependent population levels and exchange between these two states. A simple physical picture emerges from this data, combined with published work, that provides a mechanism for "hopping" and for the Johari-Goldstein ($\beta_{JG}$) relaxation, and allows us to accurately calculate the diffusion coefficient, $D_T$, and characteristic times for $\alpha$, and $\beta_{JG}$ relaxations from ps timescale neutron data.