5,399 research outputs found
Freezing and pressure-driven flow of solid helium in Vycor
The recent torsional oscillator results of Kim and Chan suggest a supersolid
phase transition in solid He-4 confined in Vycor. We have used a capacitive
technique to directly monitor density changes for helium confined in Vycor at
low temperature and have used a piezoelectrically driven diaphragm to study the
pressure-induced flow of solid helium into the Vycor pores. Our measurements
showed no indication of a mass redistribution in the Vycor that could mimic
supersolid decoupling and put an upper limit of about 0.003 um/s on any
pressure-induced supersolid flow in the pores of Vycor.Comment: 5 pages, 4 figure
Intrinsic and dislocation induced elastic behavior of solid helium
Recent experiments showed that the shear modulus of solid 4He stiffens in the
same temperature range (below 200 mK) where mass decoupling and supersolidity
have been inferred from torsional oscillator measurements. The two phenomena
are clearly related and crystal defects, particularly dislocations, appear to
be involved in both. We have studied the effects of annealing and the effects
of applying large stresses on the elastic properties of solid 4He, using both
acoustic resonances and direct low-frequency and low-amplitude measurements of
the shear modulus. Both annealing and stressing affect the shear modulus, as
expected if dislocations are responsible. However, it is the high temperature
modulus which is affected; the low temperature behavior is unchanged and
appears to reflect the intrinsic modulus of solid helium. We interpret this
behavior in terms of dislocations which are pinned by isotopic 3He impurities
at low temperatures and so have no effect on the shear modulus. At higher
temperatures they become mobile and weaken the solid. Stressing the crystal at
low temperatures appears to introduce new defects or additional pinning sites
for the dislocation network but these effects can be reversed by heating the
crystal above 500 mK. This is in contrast to dislocations produced during
crystal growth, which are only annealed at temperatures close to melting
Finite-element modeling of liquid-crystal hydrodynamics with a variable degree of order
A finite-element model of liquid-crystal hydrodynamics based on the Qian and Sheng formulation has been developed. This formulation is a generalization of the Ericksen-Leslie theory to include variations in the order parameter, allowing for a proper description of disclinations. The present implementation is well suited to treat properly the various length scales necessary to model large regions yet resolve the rapid variations in the order parameter in proximity to disclinations
Modeling of weak anisotropic anchoring of nematic liquid crystals in the Landau-de Gennes theory
The anisotropic anchoring effect of a treated solid surface on a nematic liquid crystal is described in the Landau-de Gennes theory using a power expansion on the tensor-order parameter and two mutually orthogonal unit vectors. The expression has three degrees of freedom, allowing for independent assignment of polar and azimuthal anchoring strengths and a preferred value of the surface-order parameter. It is shown that in the limit for a uniaxial constant-order parameter, the expression simplifies to the anisotropic generalization of the Rapini-Papoular anchoring energy density proposed by Zhao et al. Experimentally measurable values with a physical meaning in the Oseen-Frank theory can be scaled and assigned to the scalar coefficients of the tensor-order-parameter expansion. Results of numerical experiments comparing the anchoring according to the study of Zhao et al. in the Oseen-Frank theory and the power expansion in the Landau-de Gennes theory are presented and shown to agree well
Non-linear Elastic Response in Solid Helium: critical velocity or strain
Torsional oscillator experiments show evidence of mass decoupling in solid
4He. This decoupling is amplitude dependent, suggesting a critical velocity for
supersolidity. We observe similar behavior in the elastic shear modulus. By
measuring the shear modulus over a wide frequency range, we can distinguish
between an amplitude dependence which depends on velocity and one which depends
on some other parameter like displacement. In contrast to the torsional
oscillator behavior, the modulus depends on the magnitude of stress, not
velocity. We interpret our results in terms of the motion of dislocations which
are weakly pinned by 3He impurities but which break away when large stresses
are applied
Some Factors Related to Student Achievement of Entering Freshmen at the Utah State Agricultural College
The need for study in regard to factors related to student achievement and their possible application as guidance devices for entering freshmen in any college is very evident in our modern life. We have only to look at the increasing college enrollments and to the scarcity of positions for college graduates to realize this. In an effort to meet this need, educators should more and more base their practices on the findings of statistical and experimental studies
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