Influence of Pressure on the Boson Peak: Stronger than Elastic Medium Transformation

We study the changes in the low-frequency vibrational dynamics of poly(isobutylene) under pressure up to 1.4 GPa, corresponding to a density change of 20%. Combining inelastic neutron, x-ray, and Brillouin light scattering, we analyze the variations in the boson peak, transverse and longitudinal sound velocities, and the Debye level under pressure. We find that the boson peak variation under pressure cannot be explained by the elastic continuum transformation only. Surprisingly, the shape of the boson peak remains unchanged even at such high compression.

Figure 1

The reduced vibrational density of states [$g(E)/E^2$] obtained by INS, of PIB at different pressures: atmospheric pressure (●), 0.4 (▾), 0.8 (■), and 1.4 GPa (◆).

Figure 2

The dispersion of longitudinal sound modes measured by IXS at $P_{\mathrm{atm}}$ (▪) and 0.3 GPa (●). Solid lines: Linear dispersion corresponding to the low $Q$ sound speed measured by BLS. Horizontal dashed lines: Position of the boson peak at the same pressures.

Figure 3

(a) The relative shift in energy as a function of pressure. Boson peak (◆), BLS longitudinal modes (◂), BLS transverse modes (●), IXS longitudinal modes (▪). The solid line represents a $P^{1/3}$ fit of the higher pressure values of the boson peak energy and the dashed line the relative changes with $P$ of the $E_{\mathrm{Debye}}$. (b) The Debye level (●) and the boson peak intensity (▪) as a function of pressure, and the product of the boson peak intensity and boson peak energy $I_{\mathrm{BP}}{E}_{\mathrm{BP}}$ (◆). Inset: Pressure dependence of the amplitude $1/g(E,P)$ at a fixed frequency 2.2 meV on the left side of the boson peak.

Figure 4

The boson peak at different pressures: atmospheric pressure (●), 0.4 (▾), 0.8 (■), and 1.4 GPa (◆). The data from Fig. 1 scaled with the boson peak intensity and the boson peak position.