Excitation spectrum of a He3 atom moving in He II at zero temperature

The recoil spectrum S(q,ω) for a single He3 moving in He II at zero temperature is calculated within a mode-coupling approximation. The effective mass of the He3 particle is found to be m*=2.35m3. The dispersion curve for elementary excitations is shown to deviate considerably from a parabolic one yielding a higher density of states for energies near the He II roton gap. The undamped quasiparticle excitations cease to exist for a momentum about 2-1; their excitation strength drops to zero. The dispersion curve continues above 2-1 as a resonance in the multiple-excitation background.