Neutron Brillouin and Umklapp Scattering from Glassy Selenium

Abstract

Scattering from high-frequency acoustic modes is measured in glassy Se using two neutron spectrometries. Both Brillouin scattering under kinematic conditions and the boson peak are observed. Owing to an absolute calibration, the strengths of the various contributions can be compared to each other and to theory. Umklapp scattering via the static structure factor provides unique information on transverse acoustic modes. It shows that all acoustic excitations cease to propagate beyond $\sim 5\mathrm{meV}$.