Quasilocal vibrations and the Boson peak in glasses

Abstract

The problem of the low-frequency harmonic excitations and of the Boson peak in glasses is reviewed in the scope of recent theoretical developments. It is shown that the Boson peak inevitably appears in the reduced density of states g(ω)/ω2 of quasilocal vibrations in glasses which are additional to phonons harmonic excitations. We show that the same physical mechanism is fundamental for such seemingly different phenomenon as formation of the two-level systems in glasses. The fundamental reason for the Boson peak and two-level system formation is an instability of the spectrum of quasilocal harmonic modes weakly interacting with the high frequency surrounding and with each other. The instability controlled by the anharmonicity creates a new stable universal spectrum of harmonic vibrations with a Boson peak feature. We show that under pressure the Boson peak in glasses is always shifted to higher frequencies. For high enough pressures P the Boson peak frequency ωb ∝ P1/3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)