22,504 research outputs found
Phase diagram of a polydisperse soft-spheres model for liquids and colloids
The phase diagram of soft spheres with size dispersion has been studied by
means of an optimized Monte Carlo algorithm which allows to equilibrate below
the kinetic glass transition for all sizes distribution. The system
ubiquitously undergoes a first order freezing transition. While for small size
dispersion the frozen phase has a crystalline structure, large density
inhomogeneities appear in the highly disperse systems. Studying the interplay
between the equilibrium phase diagram and the kinetic glass transition, we
argue that the experimentally found terminal polydispersity of colloids is a
purely kinetic phenomenon.Comment: Version to be published in Physical Review Letter
Separation and fractionation of order and disorder in highly polydisperse systems
Microcanonical Monte Carlo simulations of a polydisperse soft-spheres model
for liquids and colloids have been performed for very large polydispersity, in
the region where a phase-separation is known to occur when the system (or part
of it) solidifies. By studying samples of different sizes, from N=256 to N=864,
we focus on the nature of the two distinct coexisting phases. Measurements of
crystalline order in particles of different size reveal that the solid phase
segregates between a crystalline solid with cubic symmetry and a disordered
phase. This phenomenon is termed fractionation.Comment: 8 pages, 5 figure
The confined hydrogen atom with a moving nucleus
We study the hydrogen atom confined to a spherical box with impenetrable
walls but, unlike earlier pedagogical articles on the subject, we assume that
the nucleus also moves. We obtain the ground-state energy approximately by
means of first--order perturbation theory and by a more accurate variational
approach. We show that it is greater than the one for the case in which the
nucleus is clamped at the center of the box. Present approach resembles the
well-known treatment of the helium atom with clamped nucleus
Further analysis of the connected moments expansion
We apply the connected moments expansion to simple quantum--mechanical
examples and show that under some conditions the main equations of the approach
are no longer valid. In particular we consider two--level systems, the harmonic
oscillator and the pure quartic oscillator.Comment: 19 pages; 2 tables; 4 figure
Accuracy of MUAC in the detection of severe wasting with the new WHO growth standards.
OBJECTIVES: The objectives of this study were to estimate the accuracy of using mid-upper-arm circumference (MUAC) measurements to diagnose severe wasting by comparing the new standards from the World Health Organization (WHO) with those from the US National Center for Health Statistics (NCHS) and to analyze the age independence of the MUAC cutoff values for both curves. METHODS: We used cross-sectional anthropometric data for 34,937 children between the ages of 6 and 59 months, from 39 nutritional surveys conducted by Doctors Without Borders. Receiver operating characteristic curves were used to examine the accuracy of MUAC diagnoses. MUAC age independence was analyzed with logistic regression models. RESULTS: With the new WHO curve, the performance of MUAC measurements, in terms of sensitivity and specificity, deteriorated. With different cutoff values, however, the WHO standards significantly improved the predictive value of MUAC measurements over the NCHS standards. The sensitivity and specificity of MUAC measurements were the most age independent when the WHO curve, rather than the NCHS curve, was used. CONCLUSIONS: This study confirms the need to change the MUAC cutoff value from <110 mm to <115 mm. This increase of 5 mm produces a large change in sensitivity (from 16% to 25%) with little loss in specificity, improves the probability of diagnosing severe wasting, and reduces false-negative results by 12%. This change is needed to maintain the same diagnostic accuracy as the old curve and to identify the children at greatest risk of death resulting from severe wasting
Ricci flow, quantum mechanics and gravity
It has been argued that, underlying any given quantum-mechanical model, there
exists at least one deterministic system that reproduces, after
prequantisation, the given quantum dynamics. For a quantum mechanics with a
complex d-dimensional Hilbert space, the Lie group SU(d) represents classical
canonical transformations on the projective space CP^{d-1} of quantum states.
Let R stand for the Ricci flow of the manifold SU(d-1) down to one point, and
let P denote the projection from the Hopf bundle onto its base CP^{d-1}. Then
the underlying deterministic model we propose here is the Lie group SU(d),
acted on by the operation PR. Finally we comment on some possible consequences
that our model may have on a quantum theory of gravity.Comment: 8 page
Roper Excitation in Alpha-Proton Scattering
We study the Roper excitation in the reaction. We consider
all processes which may be relevant in the Roper excitation region, namely,
Roper excitation in the target, Roper excitation in the projectile, and double
excitation processes. The theoretical investigation shows that the
Roper excitation in the proton target mediated by an isoscalar exchange is the
dominant mechanism in the process. We determine an effective isoscalar
interaction by means of which the experimental cross section is well
reproduced. This should be useful to make predictions in related reactions and
is a first step to construct eventually a microscopic
transition potential, for which the present reaction does not offer enough
information.Comment: Latex 17 pages; figures available by request; Phys. Rev. C in prin
Inversion of perturbation series
We investigate the inversion of perturbation series and its resummation, and
prove that it is related to a recently developed parametric perturbation
theory. Results for some illustrative examples show that in some cases series
reversion may improve the accuracy of the results
Majorana Zero Modes in Graphene
A clear demonstration of topological superconductivity (TS) and Majorana zero
modes remains one of the major pending goal in the field of topological
materials. One common strategy to generate TS is through the coupling of an
s-wave superconductor to a helical half-metallic system. Numerous proposals for
the latter have been put forward in the literature, most of them based on
semiconductors or topological insulators with strong spin-orbit coupling. Here
we demonstrate an alternative approach for the creation of TS in
graphene/superconductor junctions without the need of spin-orbit coupling. Our
prediction stems from the helicity of graphene's zero Landau level edge states
in the presence of interactions, and on the possibility, experimentally
demonstrated, to tune their magnetic properties with in-plane magnetic fields.
We show how canted antiferromagnetic ordering in the graphene bulk close to
neutrality induces TS along the junction, and gives rise to isolated,
topologically protected Majorana bound states at either end. We also discuss
possible strategies to detect their presence in graphene Josephson junctions
through Fraunhofer pattern anomalies and Andreev spectroscopy. The latter in
particular exhibits strong unambiguous signatures of the presence of the
Majorana states in the form of universal zero bias anomalies. Remarkable
progress has recently been reported in the fabrication of the proposed type of
junctions, which offers a promising outlook for Majorana physics in graphene
systems.Comment: 14 pages, 8 figures. Included simulations of Andreev spectroscopy and
mor
Large rescaling of the Higgs condensate: theoretical motivations and lattice results
In the Standard Model the Fermi constant is associated with the vacuum
expectation value of the Higgs field, `the condensate', usually believed to be
a cutoff-independent quantity. General arguments related to the `triviality' of
theory in 4 space-time dimensions suggest, however, a dramatic
renormalization effect in the continuum limit that is clearly visible on the
relatively large lattices available today. The result can be crucial for the
Higgs phenomenology and in any context where spontaneous symmetry breaking is
induced through scalar fields.Comment: LATTICE99(Higgs) 3 pages, 3 figure
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