ROTON WAVE-PACKETS IN 3-DIMENSIONAL AND 2-DIMENSIONAL HE-4

We have obtained new information on the elementary excitations in supe rfluid He-4 by studying local observables relative to roton wave-packe ts built from accurate variational representations of the excited stat es. The same technique has been applied also to study a pure 2-Dimensi onal system, where the spectrum, computed through a variational Monte Carlo simulation, still shows a modified phonon-maxon-roton dispersion relation. In the roton region, collective quantum effects are present , but when the average momentum of the wave-packet matches with the wa ve-vector of the roton minimum the interference effects between neighb oring atoms disappear and the local density and momentum essentially c oincide with those of a single particle excitation. A roton, as well a s a single particle excitation, has a velocity vector field which is v ery similar to that of a smoke-ring scaled down to atomic size.