Search results for "Field Theory"
showing 10 items of 1188 documents
μ-e conversion in nuclei versus μ→e γ: an effective field theory point of view
1998
Using an effective lagrangian description we analyze possible new physics contributions to the most relevant muon number violating processes: $\mu \to e \gamma$ and $\mu$--$e$ conversion in nuclei. We identify a general class of models in which those processes are generated at one loop level and in which $\mu$--$e$ conversion is enhanced with respect to $\mu \to e \gamma$ by a large $\ln(m^2_\mu/\Lambda^2),$ where $\Lambda$ is the scale responsible for the new physics. For this wide class of models bounds on $\mu$--$e$ conversion constrain the scale of new physics more stringently than $\mu \to e \gamma$ already at present and, with the expected improvements in $\mu$--$e$ conversion experim…
Chaotic Scattering in the Gaussian Potential
1995
It is well known that general classical Hamiltonian dynamical systems have as a rule chaotic behaviour. By such a term one usually understands a sensitive dependence on initial conditions which manifests itself in the topology of phase space. For the most studied case of bounded motions this behaviour is detected, for example, by analysing the Poincare surfaces of section and by calculating Lyapunov characteristic exponents. The question then naturally arises of what are the effects of this complexity on the unbounded motions, i.e., on scattering phenomena. The signature of chaotic dynamics in these scattering regions of phase space has been the object of several papers appeared mainly in t…
M-theory, graphene-branes and superconducting wormholes
2017
Exploiting an M-brane system whose structure and symmetries are inspired by those of graphene (what we call a graphene-brane), we propose here a similitude between two layers of graphene joined by a nanotube and wormholes scenarios in the brane world. By using the symmetries and mathematical properties of the M-brane system, we show here how to possibly increase its conductivity, to the point of making it as a superconductor. The questions of whether and under which condition this might point to the corresponding real graphene structures becoming superconducting are briefly outlined.
Mean-field theory for superconductivity in twisted bilayer graphene
2018
Recent experiments show how a bilayer graphene twisted around a certain magic angle becomes superconducting as it is doped into a region with approximate flat bands. We investigate the mean-field $s$-wave superconducting state in such a system and show how the state evolves as the twist angle is tuned, and as a function of the doping level. We argue that part of the experimental findings could well be understood to result from an attractive electron--electron interaction mediated by electron--phonon coupling, but the flat-band nature of the excitation spectrum makes also superconductivity quite unusual. For example, as the flat-band states are highly localized around certain spots in the st…
Surface-induced disordering at first-order transitions in body-centered cubic binary alloys: A Monte-Carlo simulation
1990
Surface effects on the phase transition from theDO3 phase to the disordered phase are studied for a bcc Ising antiferromagnet with nearest and next-nearest neighbor exchange interactions in a magnetic field. This model can also be considered to represent binary alloys such as the FeAl-system; missing interactions near the surface translate then into surface magnetic fields. The change of the local magnetization near the surface then corresponds to “surface enrichment” of one component. For a plausible choice of parameters surface-induced disordering is found and the associated critical behavior is studied. Varying the bulk fieldH near the transition fieldHc, we find that the thickness of th…
Description of light nuclei in pionless effective field theory using the stochastic variational method
2016
We construct a coordinate-space potential based on pionless effective field theory with a Gaussian regulator. Charge-symmetry breaking is included through the Coulomb potential and through two- and three-body contact interactions. Starting with the effective field theory potential, we apply the stochastic variational method to determine the ground states of nuclei with mass number $A\leq 4$. At next-to-next-to-leading order, two out of three independent three-body parameters can be fitted to the three-body binding energies. To fix the remaining one, we look for a simultaneous description of the binding energy of $^4$He and the charge radii of $^3$He and $^4$He. We show that at the order con…
A New Iterative Estimation Procedure for the Localization of Passive Stationary Objects from Received RF Signals in Indoor Environments
2019
This paper deals with the localization of passive stationary objects from the received radio- frequency (RF) signals in 3-dimensional (3D) indoor environments. Each object located in the 3D indoor environment is modelled by a single point scatterer. The propagation space is equipped with a multiple-input multiple-output (MIMO) wireless communication system. The employed channel model is flexible and allows to have a line-of-sight (LOS) component as well as single- and double- bounce scattering components. Here, we present a new accurate iterative estimation technique for computing the optimal coordinates as well as the number of the main stationary objects (scatterers) in indoor areas. The …
RANDOM SEQUENTIAL ADSORPTION ON A LINEAR LATTICE: EFFECT OF DIFFUSIONAL RELAXATION
1992
In this paper, the authors offer phenomenological arguments, supported by numerical Monte Carlo data, suggesting that the asymptotic large-time behavior of the coverage in the 1D lattice deposition of k-mers with k {gt} 3, accompanied by k-mer diffusion, is governed by the same mean-field dynamics as the lattice chemical reaction kA {yields} inert. The latter reaction is considered to occur with partial probability. The coverage in the deposition process approaches full saturation for any nonzero diffusion rate, and the void fraction decreases according to the power-law t{sup {minus}1/(k{minus}1)}.
Electronic Shell Structure and the Crystal Field Splitting in Simple Metals Clusters
1991
An upper limit for the number of atoms in metal clusters capable of exhibiting electronic shell structure has been estimated by comparing the energy difference between the highest occupied and the lowest unoccupied state with the crystal field splitting. The former is obtained by solving the Schrodinger equation for a spherical potential well with hard walls while the latter is obtained from the band structure of the solid. The results indicate that shell structures may persist in clusters containing as many as a million atoms.
The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles
2008
We consider the influence of interparticle interaction on the magnetization reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we solve a kinetic equation for the relaxation of the overall ensemble magnetization to its equilibrium value in some effective mean field. Latter field consists of external magnetic field and interaction mean field proportional to the instantaneous value of above magnetization. We show that the interparticle interaction influences the temperature dependence of a coercive field. This influence manifests itself in the noticeable coercivity at $T>T_{b}$ ($T_{b}$ is so-called blocking temperature). The above interaction can also lead to a forma…