Dynamic correlations around the glass transition in systems with different degrees of fragility

We have investigated two ionic salt systems with different degrees of fragility, Ca0.4K0.6(NO3)1.4 (CKN) and ZnCl2, by cold-neutron spectroscopy. The spectra were studied in detail around the glass-transition temperatures. Harmonic vibrations dominate the dynamics in both systems below the glass transition. With increasing temperature a strong anharmonic additional intensity was observed in the picosecond range in both systems. The results of our study show correlations between the low-energy excitations and the degree of fragility: harmonic vibrations are better pronounced in less fragile systems, while the anharmonic picosecond dynamics are stronger in fragile glass-forming liquids.