5,179 research outputs found
Displacement Detection with a Vibrating RF SQUID: Beating the Standard Linear Limit
We study a novel configuration for displacement detection consisting of a
nanomechanical resonator coupled to both, a radio frequency superconducting
interference device (RF SQUID) and to a superconducting stripline resonator. We
employ an adiabatic approximation and rotating wave approximation and calculate
the displacement sensitivity. We study the performance of such a displacement
detector when the stripline resonator is driven into a region of nonlinear
oscillations. In this region the system exhibits noise squeezing in the output
signal when homodyne detection is employed for readout. We show that
displacement sensitivity of the device in this region may exceed the upper
bound imposed upon the sensitivity when operating in the linear region. On the
other hand, we find that the high displacement sensitivity is accompanied by a
slowing down of the response of the system, resulting in a limited bandwidth
From solid solution to cluster formation of Fe and Cr in -Zr
To understand the mechanisms by which Fe and Cr additions increase the
corrosion rate of irradiated Zr alloys, a combination of experimental (atom
probe tomography, x-ray diffraction and thermoelectric power measurements) and
modelling (density functional theory) techniques are employed to investigate
the non-equilibrium solubility and clustering of Fe and Cr in binary Zr alloys.
Cr occupies both interstitial and substitutional sites in the {\alpha}-Zr
lattice, Fe favours interstitial sites, and a low-symmetry site that was not
previously modelled is found to be the most favourable for Fe. Lattice
expansion as a function of alloying concentration (in the dilute regime) is
strongly anisotropic for Fe additions, expanding the -axis while contracting
the -axis. Defect clusters are observed at higher solution concentrations,
which induce a smaller amount of lattice strain compared to the dilute defects.
In the presence of a Zr vacancy, all two-atom clusters are more soluble than
individual point defects and as many as four Fe or three Cr atoms could be
accommodated in a single Zr vacancy. The Zr vacancy is critical for the
increased solubility of defect clusters, the implications for irradiation
induced microstructure changes in Zr alloys are discussed.Comment: 15 pages including figure, 9 figures, 2 tables. Submitted for
publication in Acta Mater, Journal of Nuclear Materials (2015
How model sets can be determined by their two-point and three-point correlations
We show that real model sets with real internal spaces are determined, up to
translation and changes of density zero by their two- and three-point
correlations. We also show that there exist pairs of real (even one
dimensional) aperiodic model sets with internal spaces that are products of
real spaces and finite cyclic groups whose two- and three-point correlations
are identical but which are not related by either translation or inversion of
their windows. All these examples are pure point diffractive.
Placed in the context of ergodic uniformly discrete point processes, the
result is that real point processes of model sets based on real internal
windows are determined by their second and third moments.Comment: 19 page
Icosahedral multi-component model sets
A quasiperiodic packing Q of interpenetrating copies of C, most of them only
partially occupied, can be defined in terms of the strip projection method for
any icosahedral cluster C. We show that in the case when the coordinates of the
vectors of C belong to the quadratic field Q[\sqrt{5}] the dimension of the
superspace can be reduced, namely, Q can be re-defined as a multi-component
model set by using a 6-dimensional superspace.Comment: 7 pages, LaTeX2e in IOP styl
On the effect of Ti on Oxidation Behaviour of a Polycrystalline Nickel-based Superalloy
Titanium is commonly added to nickel superalloys but has a well-documented
detrimental effect on oxidation resistance. The present work constitutes the
first atomistic-scale quantitative measurements of grain boundary and bulk
compositions in the oxide scale of a current generation polycrystalline nickel
superalloy performed through atom probe tomography. Titanium was found to be
particularly detrimental to oxide scale growth through grain boundary
diffusion
Random fields on model sets with localized dependency and their diffraction
For a random field on a general discrete set, we introduce a condition that
the range of the correlation from each site is within a predefined compact set
D. For such a random field omega defined on the model set Lambda that satisfies
a natural geometric condition, we develop a method to calculate the diffraction
measure of the random field. The method partitions the random field into a
finite number of random fields, each being independent and admitting the law of
large numbers. The diffraction measure of omega consists almost surely of a
pure-point component and an absolutely continuous component. The former is the
diffraction measure of the expectation E[omega], while the inverse Fourier
transform of the absolutely continuous component of omega turns out to be a
weighted Dirac comb which satisfies a simple formula. Moreover, the pure-point
component will be understood quantitatively in a simple exact formula if the
weights are continuous over the internal space of Lambda Then we provide a
sufficient condition that the diffraction measure of a random field on a model
set is still pure-point.Comment: 21 page
Limit-(quasi)periodic point sets as quasicrystals with p-adic internal spaces
Model sets (or cut and project sets) provide a familiar and commonly used
method of constructing and studying nonperiodic point sets. Here we extend this
method to situations where the internal spaces are no longer Euclidean, but
instead spaces with p-adic topologies or even with mixed Euclidean/p-adic
topologies.
We show that a number of well known tilings precisely fit this form,
including the chair tiling and the Robinson square tilings. Thus the scope of
the cut and project formalism is considerably larger than is usually supposed.
Applying the powerful consequences of model sets we derive the diffractive
nature of these tilings.Comment: 11 pages, 2 figures; dedicated to Peter Kramer on the occasion of his
65th birthda
Dark-Matter Decays and Self-Gravitating Halos
We consider models in which a dark-matter particle decays to a slightly less
massive daughter particle and a noninteracting massless particle. The decay
gives the daughter particle a small velocity kick. Self-gravitating dark-matter
halos that have a virial velocity smaller than this velocity kick may be
disrupted by these particle decays, while those with larger virial velocities
will be heated. We use numerical simulations to follow the detailed evolution
of the total mass and density profile of self-gravitating systems composed of
particles that undergo such velocity kicks as a function of the kick speed
(relative to the virial velocity) and the decay time (relative to the dynamical
time). We show how these decays will affect the halo mass-concentration
relation and mass function. Using measurements of the halo mass-concentration
relation and galaxy-cluster mass function to constrain the
lifetime--kick-velocity parameter space for decaying dark matter, we find
roughly that the observations rule out the combination of kick velocities
greater than 100 km/s and decay times less than a few times the age of the
Universe.Comment: 17 pages, 10 figures, replaced with published versio
- …