טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMarkovich Tuvy
SubjectInelastic Neutron Scattering from bcc 4He
DepartmentDepartment of Physics
Supervisor Professor Emeritus Emil Polturak


Abstract

The aim of this work is a better understanding of the physics of quantum solids. These solids are defined as having large zero point amplitude of vibration in comparison with the atomic separation. Solid helium fits this definition rather well, having a zero point motion amplitude of about 30% of the lattice spacing. In addition, He atoms in the solid reside inside a highly anharmonic potential well. This results in lattice dynamics unique to solid helium. The self consistent phonon (SCP) theory which emerged over the years, treats the short range correlations in addition to the inclusion of anharmonic terms in the potential. Recently, a new description was proposed for the ground state of bcc He. In this description, the anisotropic crystalline environment is reflected in the zero point motion being directional and highly correlated. The model corrects previous predictions of the SCP theory for one of the transverse phonons. In addition, it predicts a new dispersionless mode that should appear along the [110] direction. As of now, such mode was not reported in the literature. We carried out inelastic neutron scattering experiments on bcc 4He. Indeed, we found a new mode in the [110] direction that has exactly the predicted energy at the origin of the Brillouin zone. However, this mode is not dispersionless, and in addition it is strongly coupled to the [110] phonons. A possible interpretation of our data is in terms of point-defects. The dispersion of the mode can be thus inherent, or a consequence of mode-coupling, depending on the model.

Other aspects of solid He that we have explored concerns the possibility of a soft-mode instability at the origin of both the melting transition and the bcc-hcp structural transformation.