Low-frequency vibrational states in As₂S₃glasses

Abstract

The heat capacity C(T) of quenched and annealed samples of bulk As2S3 glasses has been measured in the temperature range 1.5 to 20K, and a far-infrared laser has been used to determine the room-temperature absorption alpha ( nu ) as a function of frequency nu in the range 0.6 to 3 THz (20 to 100 cm-1). Both C(T)/T3 and alpha ( nu )/ nu 2 show broad peaks, centred at about 5K and 0.75 THz respectively. Derived values of the density of vibrational states g( nu ) show a peak in g( nu )/ nu 2 that decreases on annealing. Calculation of the Raman coupling constant C( nu ) from published Raman data using g( nu ) shows that observed changes in the Raman spectra of As2S3 can result from changes in g( nu ) and do not necessarily imply changes in C( nu ). Moreover the form of C( nu ) throws doubt on the usual interpretation of Raman scattering in glasses, and suggests that scattering is directly controlled by a correlation between vibrational displacements. The product of C( nu ) and g( nu ) predicts a peak in alpha ( nu )/ nu 2 in agreement with experiment. Both infrared and Raman experiments can therefore be explained in terms of a single vibrational correlation function at frequencies below 1.5 THz.