Ph.D Thesis


Ph.D StudentMenachem Shay
SubjectRaman Scattering from Superfluid and Solid Helium
DepartmentDepartment of Physics
Supervisors Professor Emeritus Lipson Stephen
Professor Emeritus Polturak Emil
Full Thesis textFull thesis text - English Version


Abstract

This work presents an investigation of the interaction between rotons in superfluid 4He using Raman scattering. Raman spectra taken at several pressures reveal an asymmetric peak at about twice the energy of a roton, consistent with previous works. According to a well established interpretation, this asymmetric peak is proportional to the two rotons joint density of states. A model for this density of states, which includes roton-roton interaction is described and developed. The measured spectra, as well as previous measurements by Ohbayashi et al, are analyzed according to that model and according to other, simpler, models. It is shown that the models without roton-roton interaction do not fit the experimental data. From the fits to the interacting rotons model we conclude that an l=2 bound state exists at pressures lower than 10 bar but not above. The pressure dependence of the interaction energy and the coupling strength are determined by the analysis. It is found that the coupling constant is negative at low pressure, it reaches zero at about 10 bar and becomes positive, in broad agreement with a theoretical prediction by Bedell, Pines and Zawadowski (BPZ). Furthermore, the temperature dependence of the roton lifetime is extracted from the analysis. It is found to be in agreement with the theoretical prediction of the BPZ model. Finally, the influence of roton-roton interaction on the helium dispersion curve is discussed. In addition, the Raman spectrum of a bcc solid 3He-4He mixture was measured. Preliminary results show that the spectrum is consistent with inelastic neutron scattering measurements done on bcc 4He. The interpretation of the spectrum is discussed.