Density scaling and isochronal superposition in liquids -- combining neutrons and dielectric spectroscopy

By using a high pressure cell designed for simultaneous dielectric spectroscopy and neutron scattering, we have studied the pressure and temperature dependent dynamics in a handful of different glass-forming liquids including van der Waals liquids, hydrogen bonding liquids and a room temperature ionic liquid. Combining dielectric spectroscopy and quasielastic neutron scattering gives access to time scales from pico- to kilosecond and make it possible to follow the relaxation dynamics in the liquid from above the melting point and all the way down to the glass transition. Moreover, the neutron scattering data give information on the boson peak and fast relaxation in the deeply supercooled liquid. The data is analysed in view of the so-called isomorph theory which predicts the existence of lines in the temperature-pressure phase diagram where dynamics is invariant both in terms of time scale and spectral shape. Put in other words: density scaling and isochronal superposition are predicted by isomorph theory. Isomorph theory is only expected to hold in liquids without directional bonds, nevertheless the predictions hold to a very large extend in all the studied liquids, particularly in the high temperature / low pressure region.