8,066 research outputs found
Failure of classical elasticity in auxetic foams
A recent derivation [P.H. Mott and C.M. Roland, Phys. Rev. B 80, 132104
(2009).] of the bounds on Poisson's ratio, v, for linearly elastic materials
showed that the conventional lower limit, -1, is wrong, and that v cannot be
less than 0.2 for classical elasticity to be valid. This is a significant
result, since it is precisely for materials having small values of v that
direct measurements are not feasible, so that v must be calculated from other
elastic constants. Herein we measure directly Poisson's ratio for four
materials, two for which the more restrictive bounds on v apply, and two having
values below this limit of 0.2. We find that while the measured v for the
former are equivalent to values calculated from the shear and tensile moduli,
for two auxetic materials (v < 0), the equations of classical elasticity give
inaccurate values of v. This is experimental corroboration that the correct
lower limit on Poisson's ratio is 0.2 in order for classical elasticity to
apply.Comment: 9 pages, 2 figure
Quasi-Local Energy Flux of Spacetime Perturbation
A general expression for quasi-local energy flux for spacetime perturbation
is derived from covariant Hamiltonian formulation using functional
differentiability and symplectic structure invariance, which is independent of
the choice of the canonical variables and the possible boundary terms one
initially puts into the Lagrangian in the diffeomorphism invariant theories.
The energy flux expression depends on a displacement vector field and the
2-surface under consideration. We apply and test the expression in Vaidya
spacetime. At null infinity the expression leads to the Bondi type energy flux
obtained by Lindquist, Schwartz and Misner. On dynamical horizons with a
particular choice of the displacement vector, it gives the area balance law
obtained by Ashtekar and Krishnan.Comment: 8 pages, added appendix, version to appear in Phys. Rev.
Interface behavior of a multi-layer fluid configuration subject to acceleration in a microgravity environment, supplement 1
With the increasing opportunities for research in a microgravity environment, there arises a need for understanding fluid mechanics under such conditions. In particular, a number of material processing configurations involve fluid-fluid interfaces which may experience instabilities in the presence of external forcing. In a microgravity environment, these accelerations may be periodic or impulse-type in nature. This research investigates the behavior of a multi-layer idealized fluid configuration which is infinite in extent. The analysis is linear, and each fluid region is considered inviscid, incompressible, and immiscible. An initial parametric study of confiquration stability in the presence of a constant acceleration field is performed. The zero mean gravity limit case serves as the base state for the subsequent time-dependent forcing cases. A stability analysis of the multi-layer fluid system in the presence of periodic forcing is investigated. Floquet theory is utilized. A parameter study is performed, and regions of stability are identified. For the impulse-type forcing case, asymptotic stability is established for the configuration. Using numerical integration, the time response of the interfaces is determined
Properties of the symplectic structure of General Relativity for spatially bounded spacetime regions
We continue a previous analysis of the covariant Hamiltonian symplectic
structure of General Relativity for spatially bounded regions of spacetime. To
allow for near complete generality, the Hamiltonian is formulated using any
fixed hypersurface, with a boundary given by a closed spacelike 2-surface. A
main result is that we obtain Hamiltonians associated to Dirichlet and Neumann
boundary conditions on the gravitational field coupled to matter sources, in
particular a Klein-Gordon field, an electromagnetic field, and a set of
Yang-Mills-Higgs fields. The Hamiltonians are given by a covariant form of the
Arnowitt-Deser-Misner Hamiltonian modified by a surface integral term that
depends on the particular boundary conditions. The general form of this surface
integral involves an underlying ``energy-momentum'' vector in the spacetime
tangent space at the spatial boundary 2-surface. We give examples of the
resulting Dirichlet and Neumann vectors for topologically spherical 2-surfaces
in Minkowski spacetime, spherically symmetric spacetimes, and stationary
axisymmetric spacetimes. Moreover, we show the relation between these vectors
and the ADM energy-momentum vector for a 2-surface taken in a limit to be
spatial infinity in asymptotically flat spacetimes. We also discuss the
geometrical properties of the Dirichlet and Neumann vectors and obtain several
striking results relating these vectors to the mean curvature and normal
curvature connection of the 2-surface. Most significantly, the part of the
Dirichlet vector normal to the 2-surface depends only the spacetime metric at
this surface and thereby defines a geometrical normal vector field on the
2-surface. Properties and examples of this normal vector are discussed.Comment: 46 pages; minor errata corrected in Eqs. (3.15), (3.24), (4.37) and
in discussion of examples in sections IV B,
Electronic Structure of Electron-doped Sm1.86Ce0.14CuO4: Strong `Pseudo-Gap' Effects, Nodeless Gap and Signatures of Short Range Order
Angle resolved photoemission (ARPES) data from the electron doped cuprate
superconductor SmCeCuO shows a much stronger pseudo-gap
or "hot-spot" effect than that observed in other optimally doped -type
cuprates. Importantly, these effects are strong enough to drive the
zone-diagonal states below the chemical potential, implying that d-wave
superconductivity in this compound would be of a novel "nodeless" gap variety.
The gross features of the Fermi surface topology and low energy electronic
structure are found to be well described by reconstruction of bands by a
order. Comparison of the ARPES and optical data from
the sample shows that the pseudo-gap energy observed in optical data is
consistent with the inter-band transition energy of the model, allowing us to
have a unified picture of pseudo-gap effects. However, the high energy
electronic structure is found to be inconsistent with such a scenario. We show
that a number of these model inconsistencies can be resolved by considering a
short range ordering or inhomogeneous state.Comment: 5 pages, 4 figure
Analogue-to-digital interface technique for digital controllers in DC-DC converters
A digital controller for a DC-DC converter is implemented with an efficient analogue-to-digital interface scheme. Conventional digital controllers require flash-type ADCs, which need large analogue circuits and thus significantly diminish the advantage of digital implementation. The presented delta-sigma modulated single-bit interface effectively reduces the analogue circuit area, while it achieves good voltage regulations. A silicon chip is implemented and measured to prove the successful operation of the controller
The effects of peripheral and central high insulin on brain insulin signaling and amyloid-β in young and old APP/PS1 mice
Hyperinsulinemia is a risk factor for late-onset Alzheimer's disease (AD). In vitro experiments describe potential connections between insulin, insulin signaling, and amyloid-β (Aβ), but in vivo experiments are needed to validate these relationships under physiological conditions. First, we performed hyperinsulinemic-euglycemic clamps with concurrent hippocampal microdialysis in young, awake, behaving APP(swe)/PS1(dE9) transgenic mice. Both a postprandial and supraphysiological insulin clamp significantly increased interstitial fluid (ISF) and plasma Aβ compared with controls. We could detect no increase in brain, ISF, or CSF insulin or brain insulin signaling in response to peripheral hyperinsulinemia, despite detecting increased signaling in the muscle. Next, we delivered insulin directly into the hippocampus of young APP/PS1 mice via reverse microdialysis. Brain tissue insulin and insulin signaling was dose-dependently increased, but ISF Aβ was unchanged by central insulin administration. Finally, to determine whether peripheral and central high insulin has differential effects in the presence of significant amyloid pathology, we repeated these experiments in older APP/PS1 mice with significant amyloid plaque burden. Postprandial insulin clamps increased ISF and plasma Aβ, whereas direct delivery of insulin to the hippocampus significantly increased tissue insulin and insulin signaling, with no effect on Aβ in old mice. These results suggest that the brain is still responsive to insulin in the presence of amyloid pathology but increased insulin signaling does not acutely modulate Aβ in vivo before or after the onset of amyloid pathology. Peripheral hyperinsulinemia modestly increases ISF and plasma Aβ in young and old mice, independent of neuronal insulin signaling. SIGNIFICANCE STATEMENT The transportation of insulin from blood to brain is a saturable process relevant to understanding the link between hyperinsulinemia and AD. In vitro experiments have found direct connections between high insulin and extracellular Aβ, but these mechanisms presume that peripheral high insulin elevates brain insulin significantly. We found that physiological hyperinsulinemia in awake, behaving mice does not increase CNS insulin to an appreciable level yet modestly increases extracellular Aβ. We also found that the brain of aged APP/PS1 mice was not insulin resistant, contrary to the current state of the literature. These results further elucidate the relationship between insulin, the brain, and AD and its conflicting roles as both a risk factor and potential treatment
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