Inelastic Neutron Scattering of Nanoconfined Superfluid Helium

Abstract

The dynamics of liquid $^4$He con fined in a mesoporous powder FSM-16 are reported in this dissertation, including the roton linewidth, excitation spectrum, and Compton pro file. With an ordered triangular lattice structure, FSM-16 is a high surface area porous glass with hexagonal pores a few nanometers in diameter. Neutron backscattering results examined the roton linewidth as a function of temperature. Observed linewidths in confinement are consistent with the theoretical and experimental results of the bulk liquid. The temperature-filling phase diagram was explored at intermediate fillings and low temperatures. The maxon and roton excitations are used as indicators of density for a thin-film that transitions into a three dimensional con fined fluid. The resulting excitation spectrum at low fillings does not correspond to the bulk liquid at any pressure. The deep inelastic neutron scattering results found an enhanced single particle kinetic energy, with full pore and thin film liquid deviating from the bulk momentum distribution in shape.