Anharmonic interference effects in potassium

The contributions to the dynamic form factor S(Q, U) 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, U). One is approximately proportional to the one-phonon part, S,(Q, U), and as a result contributes scattering intensity largely within the one-phonon peak region which is symmetric about the one-phonon frequency. 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 have been observed here, the second contribution apparently for the first time. Both contributions, and their dependence on wavevector Q and frequency o, can be described by an anharmonic theory (Ambegaokar et al) which includes interference between the oneand two-phonon parts of S(Q, o) via the cubic anharmonic term.