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|Title:||On the Microscopic Theory of Liquid Helium-4 (Superfluid, Quantum, Variational Calculations, Correlated Basis Function)|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||A microscopic description of the ground state and elementary excitations of liquid ('4)He is presented. The variational theory is used in conjunction with a perturbative scheme called correlated basis perturbation theory. The hypernetted-chain technique is used to calculate the various matrix elements.
First, the ground state calculations are reviewed with emphasis on the most recent variational calculations. The best available variational wave functions are used to calculate the momentum distribution of the atoms in the ground state and the condensate fraction.
We, then, study in detail the energy spectrum of an elementary excitation traveling with momentum (')k in the liquid. A perturbation theory in a correlated basis generated by Feynman-Cohen (FC) excitations is developed. The expansion in this basis appears to have good convergence. We calculate, up to second order, the effects of the coupling of the one FC excitation to two FC excita- tions. These corrections to the FC excitations bring the theory in close agreement with the neutron scattering measurements.
This perturbation expansion is also used to microscopically calculate the dynamic liquid structure function S(k,(omega)), known from neutron inelastic scattering experiments. The calculated strength Z(k) of the one quasiparticle excitation and the contribution of the two quasiparticle states to S(k,(omega)) are in semiquantitative agreement with those inferred from the data.
Finally, the structure of the excitations is studied by evaluating the change (delta)n(,(,k))((')p) in the momentum distribution of the particles due to the creation of one quasiparticle excitation traveling with momen- tum (')k in the liquid. This study provides an insight in the nature of the elementary excitations; it brings out the collective and "quasi-free particle" character of the excitations in the long and short wave length limits respectively, and the interplay between these two behaviours at intermediate momenta. The (delta)n(,(,k))((')p) is used to deter- mine the momentum distribution and condensate fraction at low temperatures.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.
|Date Available in IDEALS:||2015-05-13|