Correlated excitations and Raman scattering in glasses

The dominant Raman line in $A{X}_2$ tetrahedral glasses is demonstrated to be caused by correlated symmetric stretch motions of the bridging $X$ atoms. Using a bond-polarizability model and the central-force model of Sen and Thorpe, we derive simple analytic forms for Raman spectra. We show that correlations may cause qualitative changes in the polarized spectrum leading to a peak at the edge of a band of vibrational states, whereas the depolarized spectrum is less affected and mimics the density of states. Both features are clearly observed in Ge${\mathrm{O}}_2$ and explain features in many other glasses. The same reasoning leads to a prediction of unusual spectral features in electronic spectra of disordered solids which may be observable in angular-dependent photoemission.