|Ph.D Student||Pelleg Oshri|
|Subject||Investigation of Excitations in BCC 4He Solid Using|
|Department||Department of Physics||Supervisor||Professor Emeritus Emil Polturak|
|Full Thesis text|
The research presented in this thesis is an experimental study of the excitation spectrum of a quantum crystal, in particular body centered cubic (bcc) crystal made of helium. The main experimental method employed in this research is inelastic neutron scattering using a triple axis spectrometer (TAS). The phrase "quantum solid" stands for a solid in which the atoms vibrate with an amplitude which is a large fraction of the atomic separation. In this solid, an atom cannot be considered localized at a lattice point. Nevertheless, taking an average over long time scales, the atoms are arranged in a perfect solid order. One of the best candidates for a quantum solid is solid helium. We studied the phonon branches along all 3 major crystalline directions, and through the whole Brillouin zone. A comparison of our phonon data with previous work, a Self-Consistent-Phonon theory and with numerical calculation is presented. We also studied the recently discovered optic-like excitation branch along the main crystalline directions. By analyzing this mode and the phonons using a mode coupling approach, we conclude that this mode interacts with phonons and has a small intrinsic dispersion. At zero momentum transfer, this mode has energy of 1.2meV. It is anisotropic, in the sense that it exists long some, but not all directions. To our surprise, we discovered another optic-like mode with a lower energy (0.95meV). This mode has no dispersion, its energy line-width is very small, and it does not interact with phonons. A classical bcc crystal supports only acoustic modes which represent all the degrees of freedom of the system. No optic modes are allowed, since there is only one atom per one cubic unit cell. We present some suggestions regarding possible interpretation of our findings. In the appendices, we describe two additional experiments about the unusual properties of the bcc
solid phase. Appendix A describes a Laue camera investigation of the structural dynamics of the bcc solid. Appendix B is devoted to our search of "supersolidity" in the bcc phase.