X-ray diffraction study of the high-density phases of solid 4He
Venkataraman, Chitra T.
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https://hdl.handle.net/2142/18834
Description
Title
X-ray diffraction study of the high-density phases of solid 4He
Author(s)
Venkataraman, Chitra T.
Issue Date
1996-05
Doctoral Committee Chair(s)
Simmons, R.O.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
lattice dynamics
x-ray diffraction
helium
high-density phases (solids)
Language
en
Abstract
At low densities the helium solids are highly quantum in nature, with the small atomic mass and weak interactive forces giving rise to large vibrational motions of the atoms
about their mean lattice sites. While these distances can be as much as 25 % of the
nearest-neighbor distance at the lowest densities, the atoms are significantly more confined ·as the density of the solid is increased. Since 4He is a highly compressible solid, it is possible to examine the density dependence of the an harmonicity of the interatomic potential within accessible experimental conditions. One can then study the decreasing
quantum nature of the solid.
The research presented in this thesis involves x-ray diffraction studies of the high density phases of 4He to 60 atoms/nm3
• This includes in situ single crystal growth and characterization; measurement of the De bye-Waller factor through the dependence of the scattered intensities on the wave-vector transfer,q; study of the inelastic diffuse scattering to relate it to crystal elastic constants and other dynamical information; and lattice
parameter measurements to study thermal defect properties.
Analysis of the data indicate that the average displacement amplitudes are reduced to 10 % of the nearest -neighbour distance at 60 atoms/nm3 Related De bye temperatures are
within the range of those inferred from neutron dispersion curves and from PIMC
calculations. The dynamic diffuse scattering data can be qualitatively understood in terms
of the 4He elastic constants. At low q, the 4He diffuse scattering is intense enough to
warrant detailed analysis. It appears that both defects and dynamic processes contribute
to the diffuse intensity. Estimates of vacancy concentrations in the fcc phase, as measured from lattice parameter shifts, greatly extend the density range over which these
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