5,657 research outputs found
Dispersive force between dissimilar materials: geometrical effects
We calculate the Casimir force or dispersive van der Waals force between a
spherical nanoparticle and a planar substrate, both with arbitrary dielectric
properties. We show that the force between a sphere and a plane can be
calculated through the interacting surface plasmons of the bodies. Using a
Spectral Representation formalism, we show that the force of a sphere made of a
material A and a plane made of a material B, differ from the case when the
sphere is made of B, and the plane is made of A. We found that the difference
depends on the plasma frequency of the materials, the geometry, and the
distance of separation between sphere and plane. The differences show the
importance of the geometry, and make evident the necessity of realistic
descriptions of the sphere-plane system beyond the Derjaguin Approximation or
Proximity Theorem Approximation
Retardation turns the van der Waals attraction into Casimir repulsion already at 3 nm
Casimir forces between surfaces immersed in bromobenzene have recently been
measured by Munday et al. Attractive Casimir forces were found between gold
surfaces. The forces were repulsive between gold and silica surfaces. We show
the repulsion is due to retardation effects. The van der Waals interaction is
attractive at all separations. The retardation driven repulsion sets in already
at around 3 nm. To our knowledge retardation effects have never been found at
such a small distance before. Retardation effects are usually associated with
large distances
Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces
There is an attractive Casimir-Lifshitz force between two silica surfaces in
a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin
(5-50{\AA}) metallic nanocoating to one of the surfaces results in repulsive
Casimir-Lifshitz forces above a critical separation. The onset of such quantum
levitation comes at decreasing separations as the film thickness decreases.
Remarkably the effect of retardation can turn attraction into repulsion. From
that we explain how an ultrathin metallic coating may prevent
nanoelectromechanical systems from crashing together.Comment: 4 pages, 5 figure
Body-assisted van der Waals interaction between two atoms
Using fourth-order perturbation theory, a general formula for the van der
Waals potential of two neutral, unpolarized, ground-state atoms in the presence
of an arbitrary arrangement of dispersing and absorbing magnetodielectric
bodies is derived. The theory is applied to two atoms in bulk material and in
front of a planar multilayer system, with special emphasis on the cases of a
perfectly reflecting plate and a semi-infinite half space. It is demonstrated
that the enhancement and reduction of the two-atom interaction due to the
presence of a perfectly reflecting plate can be understood, at least in the
nonretarded limit, by using the method of image charges. For the semi-infinite
half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure
The two-fluid model with superfluid entropy
The two-fluid model of liquid helium is generalized to the case that the
superfluid fraction has a small entropy content. We present theoretical
arguments in favour of such a small superfluid entropy. In the generalized
two-fluid model various sound modes of HeII are investigated. In a
superleak carrying a persistent current the superfluid entropy leads to a new
sound mode which we call sixth sound. The relation between the sixth sound and
the superfluid entropy is discussed in detail.Comment: 22 pages, latex, published in Nuovo Cimento 16 D (1994) 37
Rotating Superconductors and the London Moment: Thermodynamics versus Microscopics
Comparing various microscopic theories of rotating superconductors to the
conclusions of thermodynamic considerations, we traced their marked difference
to the question of how some thermodynamic quantities (the electrostatic and
chemical potentials) are related to more microscopic ones: The electron's the
work function, mean-field potential and Fermi energy -- certainly a question of
general import.
After the correct identification is established, the relativistic correction
for the London Moment is shown to vanish, with the obvious contribution from
the Fermi velocity being compensated by other contributions such as
electrostatics and interactions.Comment: 23 pages 4 fi
Bounds on New Physics from B -> V1 V2 Decays
We consider the possibility that physics beyond the standard model
contributes to the decays B -> V1 V2, where V1 and V2 are vector mesons. We
show that a time-dependent angular analysis of B -> V1 V2 decays provides many
tests for this new physics (NP). Furthermore, although one cannot solve for the
NP parameters, we show that this angular analysis allows one to put bounds on
these parameters. This can be useful in estimating the scale of NP, and can
tell us whether any NP found directly at future high-energy colliders can be
responsible for effects seen in B -> V1 V2 decays.Comment: 23 pages, plain LaTeX, 5 figures (included
Observables in the Decays of B to Two Vector Mesons
In general there are nine observables in the decay of a B meson to two vector
mesons defined in terms of polarization correlations of these mesons. Only six
of these can be detected via the subsequent decay angular distributions because
of parity conservation in those decays. The remaining three require the
measurement of the spin polarization of one of the decay products.Comment: 12 pages, no figur
Widespread dysregulation of MiRNAs by MYCN amplification and chromosomal imbalances in neuroblastoma: association of miRNA expression with survival
MiRNAs regulate gene expression at a post-transcriptional level and their dysregulation can play major roles in the pathogenesis of many different forms of cancer, including neuroblastoma, an often fatal paediatric cancer originating from precursor cells of the sympathetic nervous system. We have analyzed a set of neuroblastoma (n = 145) that is broadly representative of the genetic subtypes of this disease for miRNA expression (430 loci by stem-loop RT qPCR) and for DNA copy number alterations (array CGH) to assess miRNA involvement in disease pathogenesis. The tumors were stratified and then randomly split into a training set (n = 96) and a validation set (n = 49) for data analysis. Thirty-seven miRNAs were significantly over-or under-expressed in MYCN amplified tumors relative to MYCN single copy tumors, indicating a potential role for the MYCN transcription factor in either the direct or indirect dysregulation of these loci. In addition, we also determined that there was a highly significant correlation between miRNA expression levels and DNA copy number, indicating a role for large-scale genomic imbalances in the dysregulation of miRNA expression. In order to directly assess whether miRNA expression was predictive of clinical outcome, we used the Random Forest classifier to identify miRNAs that were most significantly associated with poor overall patient survival and developed a 15 miRNA signature that was predictive of overall survival with 72.7% sensitivity and 86.5% specificity in the validation set of tumors. We conclude that there is widespread dysregulation of miRNA expression in neuroblastoma tumors caused by both over-expression of the MYCN transcription factor and by large-scale chromosomal imbalances. MiRNA expression patterns are also predicative of clinical outcome, highlighting the potential for miRNA mediated diagnostics and therapeutics
Risk, commercialism and social purpose: Repositioning the English housing association sector
Originally seen as the ‘third arm’ of UK housing policy, the independent, not-for-profit housing association sector had long been seen as effective in ‘filling the gap’ where the state or market were unable to provide for households in need. Since the 1980s in particular, successive governments had viewed housing associations in favourable terms as efficient, semi-autonomous social businesses, capable of leveraging significant private funding. By 2015, in contrast, central government had come to perceive the sector as inefficient, bureaucratic and wasteful of public subsidy. Making use of institutional theory, this paper considers this paradigm shift and examines the organisational responses to an increasingly challenging operating environment. By focusing, in particular, on large London housing associations, the paper analyses their strategic decision-making to address the opportunities and threats presented. The paper argues that in facing an era of minimal subsidy, low security and high risk, the 2015 reforms represent a critical juncture for the sector. Housing organisations face a stark dilemma about whether to continue a strategy of ‘profit for purpose’ or to embrace an unambiguously commercial ethos. The article contends that the trajectory of decision-making (although not unidirectional) leads ultimately towards an increased exposure to risk and vulnerability to changes in the housing market. More fundamentally, the attempt to reconcile social and commercial logics is likely to have wider consequences for the legitimacy of the sector
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