7,505 research outputs found
Antiparticle Contribution in the Cross Ladder Diagram for Bethe-Salpater Equation in the Light-Front
We construct the homogeneous integral equation for the vertex of the bound
state in the light front with the kernel approximated to order g^4. We will
truncate the hierarchical equations from Green functions to construct dynamical
equations for the two boson bound state exchanging interacting intermediate
bosons and including pair creation process contributing to the crossed ladder
diagram.Comment: 12 pages, 2 figure
Singularity-softening prescription for the Bethe-Salpeter equation
The reduction of the two fermion Bethe-Salpeter equation in the framework of
light-front dynamics is studied for one gauge A+=0. The arising effective
interaction can be perturbatively expanded in powers of the coupling constant
g, allowing a defined number of gauge boson exchanges. The singularity of the
kernel of the integral equation at vanishs plus momentum of the gauge is
canceled exactly in on approuch. We studied the problem using a
singularity-softening prescription for the light-front gauge.Comment: 6 pages, Prepared for 25th Brazilian Meeting of Particles Physics and
Fields, Caxambu, Brazil, 24/27 aug. 2004, Caxambu, Minas Gerais, Brazi
Gauge transformations are not canonical transformations
In classical mechanics, we can describe the dynamics of a given system using
either the Lagrangian formalism or the Hamiltonian formalism, the choice of
either one being determined by whether one wants to deal with a second degree
differential equation or a pair of first degree ones. For the former approach,
we know that the Euler-Lagrange equation of motion remains invariant under
additive total derivative with respect to time of any function of coordinates
and time in the Lagrangian function, whereas the latter one is invariant under
canonical transformations. In this short paper we address the question whether
the transformation that leaves the Euler-Lagrange equation of motion invariant
is also a canonical transformation and show that it is not.Comment: 4 page
Antiparticle Contribution in the Cross Ladder Diagram for Two Boson Propagation in the Light-front
In the light-front milieu, there is an implicit assumption that the vacuum is
trivial. By this " triviality " is meant that the Fock space of solutions for
equations of motion is sectorized in two, one of positive energy k- and the
other of negative one corresponding respectively to positive and negative
momentum k+. It is assumed that only one of the Fock space sector is enough to
give a complete description of the solutions, but in this work we consider an
example where we demonstrate that both sectors are necessary.Comment: 10 pages, 5 figure
The Light Front Gauge Propagator: The Status Quo
At the classical level, the inverse differential operator for the quadratic
term in the gauge field Lagrangian density fixed in the light front through the
multiplier (nA)^2 yields the standard two term propagator with single
unphysical pole of the type (kn)^-1. Upon canonical quantization on the
light-front, there emerges a third term of the form (kn^(mu)n^(nu))(kn)^-2.
This third term in the propagator has traditionally been dropped on the grounds
that is exactly cancelled by the "instantaneous" term in the interaction
Hamiltonian in the light-front. Our aim in this work is not to discuss which of
the propagators is the correct one, but rather to present at the classical
level, the gauge fixing conditions that can lead to the three-term propagator.Comment: 5 pages. Talk given in Light-Cone Workshop: Hadrons and Beyond, LC03,
Grey College, University of Durham, Durham, 5-9 August, 200
Angular Tunneling Effect
We investigate the tunneling of an electron with momentum p in the direction
of V potential and under an angle {\theta} to the normal potential. Using the
boundary conditions, the conditions of continuity and Snell's law, we obtain
tunneling for various angles.Comment: 2 figure
A Model of the Normal State Susceptibility and Transport Properties of Ba (Fe1-xCox)2As2: An Explanation of the Increase of Magnetic Susceptibility with Temperature
A simple two-band model is used to describe the magnitude and temperature
dependence of the magnetic susceptibility, Hall coefficient and Seebeck data
from undoped and Co doped BaFe2As2. Overlapping rigid parabolic electron and
hole bands are considered as a model of the electronic structure of the
FeAs-based semimetals. The model has only three parameters: the electron and
hole effective masses and the position of the valence band maximum with respect
to the conduction band minimum. The model is able to reproduce in a
semiquantitative fashion the magnitude and temperature dependence of many of
the normal state magnetic and transport data from the FeAs-type materials,
including the ubiquitous increase in the magnetic susceptibility with
increasing temperature.Comment: 4 figs 5 pages submitted to PRB - replaces 0906.213
Effect of Eu magnetism on the electronic properties of the candidate Dirac material EuMnBi2
The crystal structure and physical properties of the layered material EuMnBi2
have been characterized by measurements on single crystals. EuMnBi2 is
isostructural with the Dirac material SrMnBi2 based on single crystal x-ray
diffraction, crystallizing in the I4/mmm space group (No. 139). Magnetic
susceptibility measurements suggest antiferromagnetic (AFM) ordering of moments
on divalent Eu ions near T_N=22K. For low fields, the ordered Eu moments are
aligned along the c-axis, and a spin-flop is observed near 5.4T at 5K. The
moment is not saturated in an applied field of 13T at 5K, which is uncommon for
compounds containing Eu^{2+}. The magnetic behavior suggests an anisotropy
enhancement via interaction between Eu and the Mn moments that appear to be
order antiferromagnetically below approximately 310K. A large increase in the
magnetoresistance is observed across the spin-flop, with absolute
magnetoresistance reaching approximately 650% at 5K and 12T. Hall effect
measurements reveal a decrease in the carrier density below T_N, which implies
a manipulation of the Fermi surface by magnetism on the sites surrounding the
Bi square nets that lead to Dirac cones in this family of materials.Comment: Accepted for publication in Phys. Rev.
Classes of complex networks defined by role-to-role connectivity profiles
Interactions between units in phyical, biological, technological, and social
systems usually give rise to intrincate networks with non-trivial structure,
which critically affects the dynamics and properties of the system. The focus
of most current research on complex networks is on global network properties. A
caveat of this approach is that the relevance of global properties hinges on
the premise that networks are homogeneous, whereas most real-world networks
have a markedly modular structure. Here, we report that networks with different
functions, including the Internet, metabolic, air transportation, and protein
interaction networks, have distinct patterns of connections among nodes with
different roles, and that, as a consequence, complex networks can be classified
into two distinct functional classes based on their link type frequency.
Importantly, we demonstrate that the above structural features cannot be
captured by means of often studied global properties
Is the Bohr's quantization hypothesis necessary ?
We deduce the quantization of Bohr's hydrogen's atomic orbit without using
his hypothesis of angular momentum quantization. We show that his hypothesis is
nothing more than a consequence of the Planck's energy quantization.Comment: 5 page
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