55,843 research outputs found
Intrinsic Curie temperature bistability in ferromagnetic semiconductor resonant tunneling diodes
We predict bistability in the Curie temperature-voltage characteristic of
double barrier resonant-tunneling structures with dilute ferromagnetic
semiconductor quantum wells. Our conclusions are based on simulations of
electrostatics and ballistic quantum transport combined with a mean-field
theory description of ferromagnetism in dilute magnetic semiconductors.Comment: 10 pages, 3 figures, submitted to Phys. Rev. B; typo removed in
revised version - spurious eq.12 immediately after eq.1
A Model for Dark Matter Halos
A halo model is presented which possesses a constant phase space density (Q)
core followed by a radial CDM-like power law decrease in Q. The motivation for
the core is the allowance for a possible primordial phase space density limit
such as the Tremaine-Gunn upper bound. The space density profile derived from
this model has a constant density core and falls off rapidly beyond. The new
model is shown to improve the fits to the observations of LSB galaxy rotation
curves, naturally provides a model which has been shown to result in a
lengthened dynamical friction time scale for the Fornax dwarf spheroidal galaxy
and predicts a flattening of the density profile within the Einstein radius of
galaxy clusters. A constant gas entropy floor is predicted whose adiabatic
constant provides a lower limit in accord with observed galaxy cluster values.
While `observable-sized' cores are not seen in standard cold dark matter (CDM)
simulations, phase space considerations suggest that they could appear in warm
dark matter (WDM) cosmological simulations and in certain hierarchically
consistent SuperWIMP scenarios.Comment: 14 pages, 3 figures, accepted for publication in Ap
Constitutive Models for Tumour Classification
The aim of this paper is to formulate new mathematical models that will be able to differentiate not only between normal and abnormal tissues, but, more importantly, between benign and malignant tumours. We present preliminary results of a tri-phasic model and numerical simulations of the effect of cellular adhesion forces on the mechanical properties of biological tissues.
We pursued the following three approaches:
(i) the simulation of the time-harmonic linear elastic models to examine coarse scale effects and adhesion properties,
(ii) the investigation of a tri-phasic model, with the intent of upscaling this model to determine effects of electro-mechanical coupling between cells,
and (iii) the upscaling of a simple cell model as a framework for studying interface conditions at malignant cells.
Each of these approaches has opened exciting new directions of research that we plan to study in the future
Classical resolution of black hole singularities via wormholes
In certain extensions of General Relativity, wormholes generated by
spherically symmetric electric fields can resolve black hole singularities
without necessarily removing curvature divergences. This is shown by studying
geodesic completeness, the behavior of time-like congruences going through the
divergent region, and by means of scattering of waves off the wormhole. This
provides an example of the logical independence between curvature divergences
and space-time singularities, concepts very often identified with each other in
the literature.ns of curvature divergences in the context of space-time
singularities.Comment: 6 pages, 4 figures; several improvements in main body and abstract;
final version to appear in Eur. Phys. J.
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
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