19,503 research outputs found
Excitations of amorphous solid helium
We present neutron scattering measurements of the dynamic structure factor,
, of amorphous solid helium confined in 47 pore diameter
MCM-41 at pressure 48.6 bar. At low temperature, = 0.05 K, we observe
of the confined quantum amorphous solid plus the bulk
polycrystalline solid between the MCM-41 powder grains. No liquid-like
phonon-roton modes, other sharply defined modes at low energy ( 1.0
meV) or modes unique to a quantum amorphous solid that might suggest superflow
are observed. Rather the of confined amorphous and bulk
polycrystalline solid appear to be very similar. At higher temperature ( 1
K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad
peaked near 0 characteristic of normal liquid
He under pressure. Expressions for the of amorphous and
polycrystalline solid helium are presented and compared. In previous
measurements of liquid He confined in MCM-41 at lower pressure the
intensity in the liquid roton mode decreases with increasing pressure until the
roton vanishes at the solidification pressure (38 bars), consistent with no
roton in the solid observed here
Federal mandates by popular demand
This paper proposes a new framework for studying federal mandates regarding public policies in areas such as environmental quality, public health, highway safety, and the provision of local public goods. Voters have single-peaked preferences along a single policy dimension. There are two levels of government, federal and local. The federal level can constrain local policy by mandating a minimum (or maximum) policy. Localities are free to adopt any policy satisfying the constraint imposed by the federal mandate. We show that voters choose federal mandates that are too strict, which leads to excessively severe mandates. We show that similar results can obtain when federal provision of the public-provided good is more efficient than local provision
Transverse Contraction Criteria for Existence, Stability, and Robustness of a Limit Cycle
This paper derives a differential contraction condition for the existence of
an orbitally-stable limit cycle in an autonomous system. This transverse
contraction condition can be represented as a pointwise linear matrix
inequality (LMI), thus allowing convex optimization tools such as
sum-of-squares programming to be used to search for certificates of the
existence of a stable limit cycle. Many desirable properties of contracting
dynamics are extended to this context, including preservation of contraction
under a broad class of interconnections. In addition, by introducing the
concepts of differential dissipativity and transverse differential
dissipativity, contraction and transverse contraction can be established for
large scale systems via LMI conditions on component subsystems.Comment: 6 pages, 1 figure. Conference submissio
Output-Feedback Control of Nonlinear Systems using Control Contraction Metrics and Convex Optimization
Control contraction metrics (CCMs) are a new approach to nonlinear control
design based on contraction theory. The resulting design problems are expressed
as pointwise linear matrix inequalities and are and well-suited to solution via
convex optimization. In this paper, we extend the theory on CCMs by showing
that a pair of "dual" observer and controller problems can be solved using
pointwise linear matrix inequalities, and that when a solution exists a
separation principle holds. That is, a stabilizing output-feedback controller
can be found. The procedure is demonstrated using a benchmark problem of
nonlinear control: the Moore-Greitzer jet engine compressor model.Comment: Conference submissio
Fixed and Flexible Rates: A Renewal of The Debate
The paper reviews the extent to which a decade of analysis and experience has altered thinking about the choice of an exchange rate system. The advantages of flexible rates are viewed to have been exaggerated. They do not permit governments to have permanently higher rates of economic activity at the expense of higher inflation as some thought. Further, the slow speed of adjustment to relative price changes limits the contribution of flexible rates to external adjustment in the short-run, and the degree of insulation from external influences that they provide. Finally, flexible rates tend to be fluctuating rates, and, although there is little empirical evidence so far showing that the fluctuations have had adverse effects on trade and capital flows, the exchange rate instability more than any other factor has led to a certain disillusionment with the floating rate system. Notwithstanding the drawbacks of flexible rates, there will be a continuing need for exchange rate flexibility over the next few years, and some analysis is given of the problems of achieving greater stability under flexible rates or the requisite amount of flexibility under pegged rates.
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