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