12 research outputs found
An atomic mechanism for the boson peak in metallic glasses
The boson peak in metallic glasses is modeled in terms of local structural
shear rearrangements. Using Eshelby's solution of the corresponding elasticity
theory problem (J. D. Eshelby, Proc. Roy. Soc. A241, 376 (1957)), one can
calculate the saddle point energy of such a structural rearrangement. The
neighbourhood of the saddle point gives rise to soft resonant vibrational
modes. One can calculate their density, their kinetic energy, their fourth
order potential term and their coupling to longitudinal and transverse sound
waves.Comment: 9 pages, 7 figures, 31 references, contribution to 11th International
Workshop on Complex Systems, Andalo (Italy), March 200
The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica
Spectroscopic results on low frequency excitations of densified silica are
presented and related to characteristic thermal properties of glasses. The end
of the longitudinal acoustic branch is marked by a rapid increase of the
Brillouin linewidth with the scattering vector. This rapid growth saturates at
a crossover frequency Omega_co which nearly coincides with the center of the
boson peak. The latter is clearly due to additional optic-like excitations
related to nearly rigid SiO_4 librations as indicated by hyper-Raman
scattering. Whether the onset of strong scattering is best described by
hybridization of acoustic modes with these librations, by their elastic
scattering (Rayleigh scattering) on the local excitations, or by soft
potentials remains to be settled.Comment: 14 pages, 6 figures, to be published in a special issue of J. Phys.
Condens. Matte