Search results for " symmetry"
showing 10 items of 701 documents
Stability of a Tensioned Axially Moving Plate Subjected to Cross-Direction Potential Flow
2015
We analyze the stability of an axially moving Kirchhoff plate, subjected to an axial potential flow perpendicular to the direction of motion. The dimensionality of the problem is reduced by considering a cross-directional cross-section of the plate, approximating the axial response with the solution of the corresponding problem of a moving plate in vacuum. The flow component is handled via a Green’s function solution. The stability of the cross-section is investigated via the classical Euler type static linear stability analysis method. The resulting eigenvalue problem is solved numerically using Hermite type finite elements. As a result, the critical velocity and the corresponding eigenfun…
Fractal square zone plates
2013
[EN] In this paper we present a novel family of zone plates with a fractal distribution of square zones. The focusing properties of these fractal diffractive lenses coined fractal square zone plates are analytically studied and the influence of the fractality is investigated. It is shown that under monochromatic illumination a fractal square zone plate gives rise a focal volume containing a delimited sequence of two-arms-cross pattern that are axially distributed according to the self-similarity of the lens.
Quantum dots in magnetic fields: Unrestricted symmetries in the current spin-density functional formalism
1999
We apply the current spin-density functional formalism (CSDFT) of Vignale and Rasolt to two-dimensional quantum dots in magnetic fields. Avoiding any spatial symmetry restrictions of the solutions, we find that a broken rotational symmetry of the electronic charge density can occur in high magnetic fields.
A kinematic method to obtain conformal factors
2000
Radial conformal motions are considered in conformally flat space-times and their properties are used to obtain conformal factors. The geodesic case leads directly to the conformal factor of Robertson-Walker universes. General cases admitting homogeneous expansion or orthogonal hypersurfaces of constant curvature are analyzed separately. When the two conditions above are considered together a subfamily of the Stephani perfect fluid solutions, with acceleration Fermi-Walker propagated along the flow of the fluid, follows. The corresponding conformal factors are calculated and contrasted with those associated with Robertson-Walker space-times.
Muon physics — Survey
1992
The empirical basis of the minimal standard model has been consolidated in an impressive way, over the last seventeen years, by precision experiments at the meson factories. I illustrate this by means of selected examples of muonic weak interaction processes. I then describe an extension of Yang-Mills theory, inspired by noncommutative geometry, that yields precisely the standard model but fixes and explains some of its empirical input. In particular, this new approach yields a simple geometrical interpretation of spontaneous symmetry breaking. The algebraic framework of this approach offers a natural place for the lepton and quark matter fields and for inter-family mixing.
Modulating the magnetosphere of magnetars by internal magneto-elastic oscillations
2014
We couple internal torsional, magneto-elastic oscillations of highly magnetized neutron stars (magnetars) to their magnetospheres. The corresponding axisymmetric perturbations of the external magnetic field configuration evolve as a sequence of linear, force-free equilibria that are completely determined by the background magnetic field configuration and by the perturbations of the magnetic field at the surface. The perturbations are obtained from simulations of magneto-elastic oscillations in the interior of the magnetar. While such oscillations can excite travelling Alfv\'en waves in the exterior of the star only in a very limited region close to the poles, they still modulate the near ma…
Radio source evolution and the interplay with the host galaxy
2019
There is compelling evidence showing that extragalactic jets are a crucial ingredient in the evolution of host galaxies and their environments. Extragalactic jets are well collimated and relativistic, both in terms of thermodynamics and kinematics at sub-parsec and parsec scales. They generate strong shocks in the ambient medium, associated with observed hotspots in FRII radio galaxies, and carve cavities that are filled with the shocked jet flow, dragging a large fraction of the interstellar gas along, in the form of slow, massive outflows within the host galaxies. In this paper, I discuss relevant processes associated to jet evolution in the frame of FRI-FRII dichotomy. In particular, I f…
Explosion and Final State of an Unstable Reissner-Nordström Black Hole
2016
A Reissner-Nordstr\"om black hole (BH) is superradiantly unstable against spherical perturbations of a charged scalar field, enclosed in a cavity, with frequency lower than a critical value. We use numerical relativity techniques to follow the development of this unstable system -- dubbed a charged BH bomb -- into the non-linear regime, solving the full Einstein--Maxwell--Klein-Gordon equations, in spherical symmetry. We show that: $i)$ the process stops before all the charge is extracted from the BH; $ii)$ the system settles down into a hairy BH: a charged horizon in equilibrium with a scalar field condensate, whose phase is oscillating at the (final) critical frequency. For low scalar fie…
Clockwork Goldstone Bosons
2017
The clockwork mechanism has recently been proposed as a natural way to generate hierarchies among parameters in quantum field theories. The mechanism is characterized by a very specific pattern of spontaneous and explicit symmetry breaking, and the presence of new light states referred to as `gears'. In this paper we begin by investigating the self-interactions of these gears in a scalar clockwork model and find a parity-like selection rule at all orders in the fields. We then proceed to investigate how the clockwork mechanism can be realized in 5D linear dilaton models from the spontaneous symmetry breaking of a complex bulk scalar field. We also discuss how the clockwork mechanism is mani…
Hawking Radiation by Kerr Black Holes and Conformal Symmetry
2010
The exponential blueshift associated with the event horizon of a black hole makes conformal symmetry play a fundamental role in accounting for its thermal properties. Using a derivation based on two-point functions, we show that the full spectrum of thermal radiation of scalar particles by Kerr black holes can be explicitly derived on the basis of a conformal symmetry arising in the wave equation near the horizon. The simplicity of our approach emphasizes the depth of the connection between conformal symmetry and black hole radiance.