Anharmonic interference effects in potassium

The contributions to the dynamic form factor S(Q, omega ) arising from interference between its one-phonon and multiphonon parts have been observed in the scattering of neutrons from potassium. Interference makes two qualitatively distinct contributions to S(Q, omega ). One is approximately proportional to the one-phonon part, S1(Q, omega ), and contributes scattering intensity largely within the one-phonon peak region. The other is asymmetric around the one-phonon frequency and for narrowly peaked phonon groups contributes intensity of opposite sign in the multiphonon region directly on each side of the one-phonon peak. Both contributions, and their dependence on wavevector Q and frequency omega , can be described by an anharmonic theory (Ambegaokar et al. (1965)) which includes interference between the one- and two-phonon parts of S(Q, omega ) via the cubic anharmonic term.