16,826 research outputs found
Leptoproduction of charm revisited
We calculate the energy--momentum distribution of the charmed quarks produced
in neutrino reactions on protons, quantifying the importance of mass and
current non--conservation effects. We study the strange and charm distributions
probed in neutrino interactions in the presently accessible kinematical region.
Some ambiguities inherent to the extraction of the parton densities from dimuon
data are pointed out.Comment: 9 pages, DFTT 72/9
Charged branes interactions via Kalb-Ramond field
Because of its versatility, the 2-form field has been employed to describe a
multitude of scenarios that range from high energy to condensed matter physics.
Pushing forward in this endeavor we study the interaction energy, intermediated
by this kind of field, between branes in a variety of configurations. Also, the
so-called Cremmer-Scherk-Kalb-Ramond model, which consists of the
electromagnetic field coupled to the Kalb-Ramond gauge potential, is
considered. It turns out that these models exhibit a much richer class of
sources than usually thought, able to intermediate novel forms of interactions
in different scenarios.Comment: 12 latex pages, more general result
Unitarization of Structure Functions at Large
We discuss the effects of the -channel unitarization on the and
dependence of structure functions. The unitarization is implemented at
the level of photoabsorption cross sections by resorting to the light--cone
wave functions of virtual photons and to the diagonalization property of the
scattering matrix in a basis of Fock states of the photon with fixed transverse
size. Triple pomeron effects are also explicitly taken into account. We find
large unitarity corrections to the structure functions at . The
results are in very good agreement with the existing NMC and the preliminary
HERA data.Comment: 12 page
Enhanced Half-Metallicity in Edge-Oxidized Zigzag Graphene Nanoribbons
We present a novel comprehensive first-principles theoretical study of the
electronic properties and relative stabilities of edge-oxidized zigzag graphene
nanoribbons. The oxidation schemes considered include hydroxyl, carboxyl,
ether, and ketone groups. Using screened exchange density functional theory, we
show that these oxidized ribbons are more stable than hydrogen-terminated
nanoribbons except for the case of the etheric groups. The stable oxidized
configurations maintain a spin-polarized ground state with antiferromagnetic
ordering localized at the edges, similar to the fully hydrogenated
counterparts. More important, edge oxidation is found to lower the onset
electric field required to induce half-metallic behavior and extend the overall
field range at which the systems remain half-metallic. Once the half-metallic
state is reached, further increase of the external electric field intensity
produces a rapid decrease in the spin magnetization up to a point where the
magnetization is quenched completely. Finally, we find that oxygen containing
edge groups have a minor effect on the energy difference between the
antiferromagnetic ground state and the above-lying ferromagnetic state.Comment: 5 pages,5 figures, 1 tabl
Magnetic Exchange Couplings from Noncollinear Spin Density Functional Perturbation Theory
We propose a method for the evaluation of magnetic exchange couplings based
on noncollinear spin-density functional calculations. The method employs the
second derivative of the total Kohn-Sham energy of a single reference state, in
contrast to approximations based on Kohn-Sham total energy differences. The
advantage of our approach is twofold: It provides a physically motivated
picture of the transition from a low-spin to a high-spin state, and it utilizes
a perturbation scheme for the evaluation of magnetic exchange couplings. The
latter simplifies the way these parameters are predicted using
first-principles: It avoids the non-trivial search for different spin-states
that needs to be carried out in energy difference methods and it opens the
possibility of "black-boxifying" the extraction of exchange couplings from
density functional theory calculations. We present proof of concept
calculations of magnetic exchange couplings in the H--He--H model system and in
an oxovanadium bimetallic complex where the results can be intuitively
rationalized.Comment: J.Chem. Phys. (accepted
Field theoretic description of electromagnetic boundaries: The Casimir effect between dissimilar mirrors from external potentials
In a previous work we formulated a model of semitransparent dielectric
surfaces, coupled to the electromagnetic field by means of an effective
potential. Here we consider a setup with two dissimilar mirrors, and compute
exactly the correction undergone by the photon propagator due to the presence
of both plates. It turns out that this new propagator is continuous all over
the space and, in the appropriate limit, coincides with the one used to
describe the Casimir effect between perfect conductors. The amended Green
function is then used to calculate the Casimir energy between the uniaxial
dielectric surfaces described by the model, and a numerical analysis is carried
out to highlight the peculiar behavior of the interaction between the mirrors.Comment: 7 pages, 3 figure
Identification and molecular characterization of Pear blister canker viroid isolates in Campania (Southern Italy)
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