Pressure dependence of the boson peak in glasses: Correlated and uncorrelated perturbations

The pressure dependence of the boson peak in glasses within the framework of the extended soft potential model is reconsidered, taking effects at small pressures into account. One of these is the pressure dependence of the elastic constants, changing the interaction between the soft localized modes and thus changing the quasilocalized vibrations (QLVs) of the boson peak. This and other effects require the introduction of additional parameters to describe all the different influences of the pressure in detail. As in the first treatment of the problem, the dominating high-pressure influence remains the creation of pressure forces, which have to be added to the random forces responsible for the boson peak formation. The pressure forces consist of a correlated and an uncorrelated part (correlated with respect to the already existing random forces). Both lead to the P-1/3 dependence observed in high-pressure experiments, but the uncorrelated part vanishes at small pressure P. The comparison to experiment is divided into a small pressure part, accessible through low-temperature heat capacity and thermal expansion measurements, and the high-pressure part, mostly Raman scattering measurements of the boson peak under pressure. The results suggest that the latter are dominated by the uncorrelated part of the forces, probably due to pressure-induced relaxations.