900 research outputs found
Dissipative superfluid mass flux through solid 4He
The thermo-mechanical effect in superfluid helium is used to create an
initial chemical potential difference, , across a solid He
sample. This causes a flow of helium atoms from one reservoir
filled with superfluid helium, through a sample cell filled with solid helium,
to another superfluid-filled reservoir until chemical potential equilibrium is
restored. The solid helium sample is separated from each of the reservoirs by
Vycor rods that allow only the superfluid component to flow. With an improved
technique, measurements of the flow, , at several fixed solid helium
temperatures, , have been made as function of in the pressure
range 25.5 - 26.1 bar. And, measurements of have been made as a function of
temperature in the range ~mK for several fixed values of . The temperature dependence of the flow above ~mK shows a reduction
of the flux with an increase in temperature that is well described by . The non-linear functional dependence , with independent of temperature but dependent on pressure,
documents in some detail the dissipative nature of the flow and suggests that
this system demonstrates Luttinger liquid-like one-dimensional behavior. The
mechanism that causes this flow behavior is not certain, but is consistent with
superflow on the cores of edge dislocations.Comment: 11 pages, 14 figure
Mass flux characteristics in solid 4He for T> 100 mK: Evidence for Bosonic Luttinger Liquid behavior
At pressure 25.7 bar the flux, , carried by solid \4he for
100 mK depends on the net chemical potential difference between two reservoirs
in series with the solid, , and obeys ,
where is independent of temperature. At fixed the
temperature dependence of the flux, , can be adequately represented by , K, for K. A
single function fits all of the available
data sets in the range 25.6 - 25.8 bar reasonably well. We suggest that the
mass flux in solid \4he for mK may have a Luttinger liquid-like
behavior in this bosonic system.Comment: 4 pages, 5 figure
Observation of thermo-mechanical equilibration in the presence of a solid 4He conduit
We observe a thermo-mechanical effect when a chemical potential difference is
created by a temperature difference imposed between two liquid reservoirs
connected to each other through Vycor rods in series with solid hcp 4He. By
creating a temperature difference, , between the two reservoirs, we
induce a rate-limited growth of a pressure difference between the two
reservoirs, . In equilibrium is in
quantitative agreement with the thermo-mechanical effect in superfluid helium.
These observations confirm that below 600 mK a flux-limited flow exists
through the solid helium.Comment: 4 pages, 4 figure
Job Loss and Effects on Firms and Workers
This paper serves as an introduction and (incomplete) survey of the wide-ranging literature on job loss. We begin with a discussion of job stability in the US and the commitment between firms and workers, and how this has changed in recent years. We then focus on the short and long-term consequences to workers (i.e. wages, health outcomes) following a layoff, and the effect which mass layoffs have on future firm performance. The changing nature of these relationships over the past several decades is a central theme of this paper. We review the common data sources used to examine these questions, and identify many influential papers on each topic. Additionally, we discuss alternative policies to the typical mass layoff, such as worksharing
Growth of solid hcp \^4He off the melting curve
We report studies of the growth of solid hcp \4he at pressures higher than
the bulk freezing pressure using a cell design that allows us to inject atoms
into the solid. Near the melting curve during injection we observe random
events during which the pressure recorded in the cell drops abruptly. These
events are accompanied by transient increases in the temperature of the cell.
We discuss these transients and conclude that they represent the solidification
of meta-stable liquid regions and the associated relief of strain in the local
solid. We also observe that further from the melting curve the transients are
no longer recorded, but that we can continue to add atoms to the solid,
increasing its density at fixed volume. We document these changes in density
with respect to changes in the chemical potential as a function of temperature
and discuss these in the context of recent theoretical work.Comment: 7 pages, 8 figure
Mass flow through solid 4He induced by the fountain effect
Using an apparatus that allows superfluid liquid 4He to be in contact with
hcp solid \4he at pressures greater than the bulk melting pressure of the
solid, we have performed experiments that show evidence for 4He mass flux
through the solid and the likely presence of superfluid inside the solid. We
present results that show that a thermomechanical equilibrium in quantitative
agreement with the fountain effect exists between two liquid reservoirs
connected to each other through two superfluid-filled Vycor rods in series with
a chamber filled with solid 4He. We use the thermomechanical effect to induce
flow through the solid and measure the flow rate. On cooling, mass flux appears
near T = 600 mK and rises smoothly as the temperature is lowered. Near T = 75
mK a sharp drop in the flux is present. The flux increases as the temperature
is reduced below 75 mK. We comment on possible causes of this flux minimum.Comment: 20 pages, 22 figures, 7 table
The optical field angle distortion calibration feasibility study for the Hubble Space Telescope fine guidance sensors
The results of an analytical study to investigate the feasibility of calibrating the Hubble Space Telescope's (HST's) fine guidance sensors (FGSs) within HST mission accuracy limits are presented. The study has two purposes: (1) to determine the mathematical feasibility of the optical field angle distortion (OFAD) calibration algorithm and (2) to confirm that the OFAD, plate scale, and FGS-to-FGS alignment calibration algorithms produced a calibration of the FGSs that satisfied mission requirements. The study concluded that the mathematical specification of the OFAD algorithm is adequate and permits a determination of the FGS calibration parameters (accurate to better than 0.003 arc-second) sufficient to meet the mission requirements. The algorithms implemented, the characteristics of the simulated data and procedures for data analysis, and the study's results are discussed. In addition, several useful techniques for improving the stability and accuracy of the OFAD solution are outlined
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