14,705 research outputs found
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
A Proposal of a Renormalization Group Transformation for Lattice Field Theories
We propose a new Real Space Renormalization Group transformation useful for
Monte Carlo calculations in theories with global or local symmetries. From
relaxation arguments we define the block-spin transformation with two tunable
free parameters, adapted to the system's action. Varying them it is possible to
place the fixed point very close to the simulation point. We show how the
method works in a simple model with global symmetry: the three dimensional XY
model.Comment: 26 pages, uuencoded compressed postscript single file, 8 figures
include
Two-point one-dimensional - interactions: non-abelian addition law and decoupling limit
In this contribution to the study of one dimensional point potentials, we
prove that if we take the limit on a potential of the type
, we
obtain a new point potential of the type , when and are related to , , and
by a law having the structure of a group. This is the Borel subgroup of
. We also obtain the non-abelian addition law from the
scattering data. The spectra of the Hamiltonian in the exceptional cases
emerging in the study are also described in full detail. It is shown that for
the , values of the couplings the
singular Kurasov matrices become equivalent to Dirichlet at one side of the
point interaction and Robin boundary conditions at the other side
The influence of microlensing on the shape of the AGN Fe K-alpha line
We study the influence of gravitational microlensing on the AGN Fe K-alpha
line confirming that unexpected enhancements recently detected in the iron line
of some AGNs can be produced by this effect. We use a ray tracing method to
study the influence of microlensing in the emission coming from a compact
accretion disc considering both geometries, Schwarzschild and Kerr.
Thanks to the small dimensions of the region producing the AGN Fe K-alpha
line, the Einstein Ring Radii associated to even very small compact objects
have size comparable to the accretion disc hence producing noticeable changes
in the line profiles. Asymmetrical enhancements contributing differently to the
peaks or to the core of the line are produced by a microlens, off-centered with
respect to the accretion disc.
In the standard configuration of microlensing by a compact object in an
intervening galaxy, we found that the effects on the iron line are two orders
of magnitude larger than those expected in the optical or UV emission lines. In
particular, microlensing can satisfactorily explain the excess in the iron line
emission found very recently in two gravitational lens systems, H 1413+117 and
MG J0414+0534.
Exploring other physical {scenario} for microlensing, we found that compact
objects (of the order of one Solar mass) which belong to {the bulge or the
halo} of the host galaxy can also produce significant changes in the Fe
K line profile of an AGN. However, the optical depth estimated for
this type of microlensing is {very small, , even in a favorable
case.Comment: Astron. Astrophys. accepte
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