Origin of the High-Frequency Doublet in the Vibrational Spectrum of Vitreous SiO2
The vibrational properties of amorphous SiO2 were studied within first-principles density functional theory. The calculated spectrum is in good agreement with neutron data, showing, in particular, a double peak in the high-frequency region. This doublet results from different local modes of the tetrahedral subunits and cannot be ascribed to a longitudinal-optic-transverse-optic (LO-TO) effect. This solves a long-standing controversy about the origin of the doublet in neutron spectra. A LO-TO splitting is recovered only when the long-wavelength limit is probed, as in optical experiments. These findings should be a general feature of tetrahedral AX2 amorphous networks.
Sarnthein
J
J. Sarnthein, Institut Romand de Recherche Numerique en Physique des Materiaux (IRRMA), IN-Ecublens, CH-1015 Lausanne, Switzerland. A. Pasquarello and R. Car, IRRMA, IN-Ecublens, CH-1015 Lausanne, Switzerland, and Department of Condensed Matter Physics, University of Geneva, CH-1211 Geneva, Switzerland.
Pasquarello
A
Car
R