Far-infrared study of low-frequency vibrational states in As2S3 glass.

Abstract

Far-infrared absorption and reflection spectra of ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ glass have been measured in the frequency range 10--60 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at room temperature. The absorption coefficient \ensuremath{\alpha}(\ensuremath{\nu}) has been determined as a function of frequency \ensuremath{\nu} and found to deviate from the quadratic frequency dependence. A broad absorption peak corresponding to a boson peak is clearly found at about 27 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Infrared coupling coefficient ${\mathit{C}}_{\mathrm{ir}}$(\ensuremath{\nu}) is determined experimentally from \ensuremath{\alpha}(\ensuremath{\nu}) and the density of vibrational states previously reported. It is found that ${\mathit{C}}_{\mathrm{ir}}$(\ensuremath{\nu}) shows the frequency dependence similar to that of the Raman coupling coefficient ${\mathit{C}}_{\mathit{r}}$(\ensuremath{\nu}) obtained in the same manner for ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ glass and is not described by the Martin-Brenig model. Both ${\mathit{C}}_{\mathrm{ir}}$(\ensuremath{\nu}) and ${\mathit{C}}_{\mathit{r}}$(\ensuremath{\nu}) show no peak and the frequency dependence close to that predicted by a fractal model. The far-infrared absorption peak may be interpreted as a result of the phonon-fracton crossover.