Study of two-phonon excitations in superfluid 4He

Abstract

We explain the second branch of excitations in superfluid 4 He observed by Cowley and Woods, by accounting for two-phonon contributions to the dynamic structure function, S( k , ω). Our theory gives a good fit with the experimental data in the high energy region for several values of momentum transfer. It is observed that the contribution to S( k , ω) due to two-phonon excitations is of the order of k 2 as against its k 4 dependence reported in earlier theories.