Low-frequency Raman scattering under high pressure in diamond anvil cell: Experimental protocol and application to GeO2 and SiO2 boson peaks

Low frequency vibrations, a universal feature of amorphous solids which is responsible for thermodynamical anomalies at low temperature, are complicated to record in high pressure device, notably because of the closeness of the elastic line. We first present an experimental protocol allowing to record the in situ low-frequency Raman scattering of samples under high pressure in the diamond anvil cell apparatus with a high quality. This protocol is particularly adapted to study the evolution of the boson peak of glassy materials. The second part is dedicated to the study of the boson peak of a-GeO2, a typical strong glass. The results, which clearly show the non-Debye behaviour of this material at the beginning of the compression where an anomalous compression takes place, are compared to previous measurements on a-SiO2. Interpretation in terms of local structural transitions and discussions about the elastic nanoheterogeneities of the amorphous state are addressed.