Origin of the High-Frequency Doublet in the Vibrational Spectrum of Vitreous SiO2
Johannes Sarnthein,
Alfredo Pasquarello,
*
Roberto Car
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.
J. Sarnthein, Institut Romand de Recherche Numérique en
Physique des Matériaux (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.
*
To whom correspondence should be addressed.
