14,910 research outputs found
Calculable inverse-seesaw neutrino masses in supersymmetry
We provide a scenario where naturally small and calculable neutrino masses
arise from a supersymmetry breaking renormalization-group-induced vacuum
expectation value. We adopt a minimal supergravity scenario without ad hoc
supersymmetric mass parameters. The lightest supersymmetric particle can be an
isosinglet scalar neutrino state, potentially viable as WIMP dark matter
through its Higgs new boson coupling. The scenario leads to a plethora of new
phenomenological implications at accelerators including the Large Hadron
Collider.Comment: LaTeX, 5 pages, 4 figures. Comments and references added. Final
version to appear in PR
Disordered graphene Josephson junctions
A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method
is used to describe disordered single-layer graphene Josephson junctions.
Scattering by vacancies, ripples or charged impurities is included. We compute
the Josephson current and investigate the nature of multiple Andreev
reflections, which induce bound states appearing as peaks in the density of
states for energies below the superconducting gap. In the presence of single
atom vacancies, we observe a strong suppression of the supercurrent that is a
consequence of strong inter-valley scattering. Although lattice deformations
should not induce inter-valley scattering, we find that the supercurrent is
still suppressed, which is due to the presence of pseudo-magnetic barriers. For
charged impurities, we consider two cases depending on whether the average
doping is zero, i.e. existence of electron-hole puddles, or finite. In both
cases, short range impurities strongly affect the supercurrent, similar to the
vacancies scenario
Tight-binding description of intrinsic superconducting correlations in multilayer graphene
Using highly efficient GPU-based simulations of the tight-binding
Bogoliubov-de Gennes equations we solve self-consistently for the pair
correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by
considering a finite intrinsic s-wave pairing potential. We find that the two
different stacking configurations have opposite bulk/surface behavior for the
order parameter. Surface superconductivity is robust for ABC stacked multilayer
graphene even at very low pairing potentials for which the bulk order parameter
vanishes, in agreement with a recent analytical approach. In contrast, for
Bernal stacked multilayer graphene, we find that the order parameter is always
suppressed at the surface and that there exists a critical value for the
pairing potential below which no superconducting order is achieved. We
considered different doping scenarios and find that homogeneous doping strongly
suppresses surface superconductivity while non-homogeneous field-induced doping
has a much weaker effect on the superconducting order parameter. For multilayer
structures with hybrid stacking (ABC and ABA) we find that when the thickness
of each region is small (few layers), high-temperature surface
superconductivity survives throughout the bulk due to the proximity effect
between ABC/ABA interfaces where the order parameter is enhanced.Comment: 7 page
Critical behavior of a bounded Kardar-Parisi-Zhang equation
A host of spatially extended systems, both in physics and in other
disciplines, are well described at a coarse-grained scale by a Langevin
equation with multiplicative-noise. Such systems may exhibit non-equilibrium
phase transitions, which can be classified into universality classes. Here we
study in detail one of such classes that can be mapped into a
Kardar-Parisi-Zhang (KPZ) interface equation with a positive (negative)
non-linearity in the presence of a bounding lower (upper) wall. The wall limits
the possible values taken by the height variable, introducing a lower (upper)
cut-off, and induce a phase transition between a pinned (active) and a depinned
(absorbing) phase. This transition is studied here using mean field and field
theoretical arguments, as well as from a numerical point of view. Its main
properties and critical features, as well as some challenging theoretical
difficulties, are reported. The differences with other multiplicative noise and
bounded-KPZ universality classes are stressed, and the effects caused by the
introduction of ``attractive'' walls, relevant in some physical contexts, are
also analyzed.Comment: Invited paper to a special issue of the Brazilian J. of Physics. 5
eps Figures. 9 pagres. Revtex
Development of polymer network of phenolic and epoxies resins mixed with linseed oil: pilot study
Epoxy resin was mixed with phenolic resins in different percentages by weight. Composite 40/60 means the proportion by weight of epoxy resin is 40 percent. It was found that only composites 50/50 and 40/60 could be cured in ambient conditions. Dynamic mechanical analysis showed that only these two composites form interpenetrating polymer network. The addition of linseed oil to the two resins results also in the formation of interpenetrating network irrespective of proportion by weight of the resins; the mechanical properties will only be better when the percentage by weight of epoxy resin is higher; the aim of reducing cost and at the same time maintaining the mechanical properties cannot be fully achieved because epoxy resin is much more expensive than its counterpart
Dynamical phase coexistence: A simple solution to the "savanna problem"
We introduce the concept of 'dynamical phase coexistence' to provide a simple
solution for a long-standing problem in theoretical ecology, the so-called
"savanna problem". The challenge is to understand why in savanna ecosystems
trees and grasses coexist in a robust way with large spatio-temporal
variability. We propose a simple model, a variant of the Contact Process (CP),
which includes two key extra features: varying external
(environmental/rainfall) conditions and tree age. The system fluctuates locally
between a woodland and a grassland phase, corresponding to the active and
absorbing phases of the underlying pure contact process. This leads to a highly
variable stable phase characterized by patches of the woodland and grassland
phases coexisting dynamically. We show that the mean time to tree extinction
under this model increases as a power-law of system size and can be of the
order of 10,000,000 years in even moderately sized savannas. Finally, we
demonstrate that while local interactions among trees may influence tree
spatial distribution and the order of the transition between woodland and
grassland phases, they do not affect dynamical coexistence. We expect dynamical
coexistence to be relevant in other contexts in physics, biology or the social
sciences.Comment: 8 pages, 7 figures. Accepted for publication in Journal of
Theoretical Biolog
Constraining differential renormalization in abelian gauge theories
We present a procedure of differential renormalization at the one loop level
which avoids introducing unnecessary renormalization constants and
automatically preserves abelian gauge invariance. The amplitudes are expressed
in terms of a basis of singular functions. The local terms appearing in the
renormalization of these functions are determined by requiring consistency with
the propagator equation. Previous results in abelian theories, with and without
supersymmetry, are discussed in this context.Comment: 13 pages, LaTeX. Some equations corrected and a reference added.
Complete ps paper also available at http://www-ftae.ugr.es/papiros.html or
ftp://ftae3.ugr.es/pub/rmt/ugft73.p
Performance limits of graphene-ribbon-based field effect transistors
The performance of field effect transistors based on an single graphene
ribbon with a constriction and a single back gate are studied with the help of
atomistic models. It is shown how this scheme, unlike that of traditional
carbon-nanotube-based transistors, reduces the importance of the specifics of
the chemical bonding to the metallic electrodes in favor of the carbon-based
part of device. The ultimate performance limits are here studied for various
constriction and metal-ribbon contact models. In particular we show that, even
for poorly contacting metals, properly taylored constrictions can give
promising values for both the on-conductance and the subthreshold swing.Comment: 5 pages, 4 figure
Trust Central Eases Funding Decisions with Data
The Children’s Trust is a local government taxing authority that uses property tax dollars to fund programming for Miami-Dade County children and families. To become more efficient, Trust Central was created and now automates our full business cycle. Data flow is solicitation → contracting → program metrics → solicitation. Agencies apply for funding, contract to provide services, report their progress. Their progress is used to determine future solicitation criteria. Data automatically flows from one module to another.
Trust Central allowed us to move from using 5 data points to make funding decisions to 24 data points. We were able to look across our various initiatives to ensure that our funding decisions were equitable. Funding decisions were backed by data and easy to share with applicants. We created context and communicated funding decisions in a way that reduced emotional conflicts and appeals. As a reference point, we had 96 appeal meetings last funding cycle - this funding cycle no appeal meetings and only 19 review meetings; a cost savings of 68M in funding decisions without any negative feedback from the community. Our relationship with the community pivoted from negative to positive. This is a first for us! We are now positioned to be a mentor for both governmental and non-governmental funders
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