Correlation of low-energy excitations with photodarkening in
Kirchhoff-Institut für Physik der Universität Heidelberg
Albert-Ueberle-Strasse 3-5, D-69120 Heidelberg, Germany
Accepted: 2 November 2001
Low-temperature sound velocity and absorption of differently treated amorphous films have been measured around using surface acoustic waves. The data show the typical behaviour of glasses governed by low-energy excitations. Illumination of the films at using photon energies equal to and higher than the band gap increases the absorption and temperature dependence of the sound velocity by creation of defects. Between and the change does not depend on the wavelength but on the number of absorbed photons. The initial state is only reset after annealing at the glass transition temperature. The photoinduced change of the sound velocity data suggests that upon illumination quasiharmonic oscillators are transformed into two-level systems on average. The strong correlation of the photoinduced changes with photodarkening favours the twisting of adjacent pyramids as microscopic model for the low-energy excitations.
PACS: 62.65.+k – Acoustical properties of solids / 63.50.+x – Vibrational states in disordered systems / 78.66.Jg – Amorphous semiconductors; glasses
© EDP Sciences, 2002