NEUTRON BRILLOUIN AND UMKLAPP SCATTERING FROM GLASSY SELENIUM

Scattering from high-frequency acoustic modes is measured in glassy Se using two neutron spectrometries. Both Brillouin scattering under kin ematic conditions and the boson peak are observed. Owing to an absolut e calibration, the strengths of the various contributions can be compa red to each other and to theory. Umklapp scattering via the static str ucture factor provides unique information on transverse acoustic modes . It shows that all acoustic excitations cease to propagate beyond sim ilar to 5 meV.