Vibrations and relaxations in a soft sphere glass: boson peak and structure factors

• Published in: Journal of physics. Condensed matter Vol. 16; no. 27; pp. S2659 - 2670
• Main Author: Schober, H R
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 14.07.2004; Institute of Physics
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Lattice dynamics; Physics; Vibrational states in disordered systems; Dynamic structure factor; Structure factors; Soft sphere model; Phonon mode; Molecular dynamics method; Glass; Theoretical study; Boson peak; Phonon density of states; Modelling; Soft modes
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/16/27/005

The dynamics of a soft sphere model glass, studied by molecular dynamics, is investigated. The vibrational density of states divided by *w2 shows a pronounced boson peak. Its shape is in agreement with the universal form derived for soft oscillators interacting with sound waves. The excess vibrations forming the boson peak have mainly transverse character. From the dynamic structure factor in the Brillouin regime pseudo dispersion curves are calculated. Whereas the longitudinal phonons are well defined up to the pseudo zone boundary the transverse ones rapidly get over-damped and go through the Ioffe-Regel limit near the boson peak frequency. In the high q regime constant-*w scans of the dynamic structure factor for frequencies around the boson peak are clearly distinct from those for zone boundary frequencies. Above the Brillouin regime, the scans for the low frequency modes follow closely the static structure factor. This still holds after a deconvolution of the exact harmonic eigenmodes into local and extended modes. Also the structure factor for local relaxations at finite temperatures resembles the static one. This semblance between the structure factors mirrors the collective motion of chain-like structures in both low frequency vibrations and atomic hopping processes, observed in the earlier investigations. The aim of the present paper is to use a simple model glass, representative for densely packed metallic glasses, to see what information can be gained from the dynamic structure both in the Brillouin regime and for higher q-values.