Surface investigation of phonon-state desities in glasses

The non-crystalline states of matter, namely the amorphous and glassy state, have attracted considerable interest over the last few decades from both the experimental and theoretical point of view. The interest in these materials has arisen owing to: (i) the rich new physics underlying the glass transition and its accompanying effects, and (ii) the important and continuously growing number of applications that amorphous materials meet in everyday life and high-tech processes. An excess phonon density of states compared to the Debye model (the standard description for the low-energy phonon state densities in crystals) is an intriguing and widely discussed universal property of disordered media and glasses. This phenomenon - commonly termed the boson peak - has been observed in bulk measurements. Despite the intensive efforts which have been undertaken in the past in order to resolve the origin of these excess phonon density of states, its nature is not fully understood. Recently, this fundamental phenomenon in the vibrational spectra of amorphous substances was discovered as a surface effect by the applicant. The discovery was made using helium atom scattering, a technique well established for surface investigations of crystals, but not previously implemented in the study of amorphous materials. Helium scattering is particularly well suited for the investigation of surface dynamics due to the low energy of the probing He atoms. Their energy is in the range of the lowest energy phonons (~18 meV), thus especially sensitive to the typical boson peak range. With this first time observation of the boson peak at the surface of vitreous silica, we have opened a door to new interesting physics which we propose to explore in detail for this research. At present, efforts in understanding the origin of the boson peak have resorted to numerical simulations but are lacking new experimental results. An investigation of the phonon state densities at reduced dimensionality, i. e., by studying the low-energy vibrations at the surface, offers the potential of significantly contributing to the ongoing discussions. Surface sensitive experiments by helium atom scattering are expected to unveil a different behavior of the boson peak compared to the existing bulk measurements. The phonons that constitute the surface boson peak are subject to more restrictions than in case of the bulk boson peak due to the reduced symmetry of the surface (the translational symmetry in the direction normal to the surface is broken) compared to that of the bulk. Thus, any new result from studying the surface boson peak will act as a test case for the existing theoretical models. Planned experiments cover a detailed study of the temperature dependence, the relation between the boson peak intensity and the parallel momentum transfer, investigations at varying beam energy, and an experimental investigation of the disorder-order phase transition.