Influence of pressure on fast picosecond relaxation in glass-forming materials

Understanding the microscopic mechanism of the fast dynamics (gigahertz-terahertz frequency range) in amorphous materials remains a challenge. Disordered systems usually exhibit two additional contributions in this frequency range in comparison to their crystalline counterparts: a low-frequency excess vibrations, the so-called boson peak, and the fast picosecond relaxation that appears as a quasielastic scattering (QES) in the light- and neutron-scattering spectra. The nature of both contributions remains a subject of active discussions. In particular, QES intensity varies significantly with temperature. These variations might be caused by pure thermal effect and/or by change in density. To separate these two contributions we performed detailed light (Raman and Brillouin) scattering studies of the fast dynamics at different experimental conditions: isothermal, isobaric, isokinetic, and isochoric. The analysis demonstrates that the volume contribution dominates the fast relaxation behavior in a liquid state while the thermal energy becomes more important in the glassy state. Moreover, the presented analysis of the light-scattering data reveals significant difference in sensitivity of the fast dynamics to pressure among seven glass-forming materials (van der Waals-bonding and hydrogen-bonding molecular systems and polymers) studied in this work. It appears that the fast dynamics in orthoterphenyl and glycerol depends on pressure (density) significantly weaker than in other materials. However, the earlier observed correlations between pressure-induced variations in the QES and boson peak intensities and between the boson peak frequency and intensity are confirmed for all the studied materials. The obtained experimental results are compared to predictions of different models.