Order reductions of Lorentz-Dirac-like equations

We discuss the phenomenon of preacceleration in the light of a method of successive approximations used to construct the physical order reduction of a large class of singular equations. A simple but illustrative physical example is analyzed to get more insight into the convergence properties of the method.Comment: 6 pages, LaTeX, IOP macros, no figure

On the model of a classical relativistic particle of constant and universal mass and spin

The deformation of the classical action for a free relativistic particle recently suggested by A. Staruszkiewicz gives rise to a spin structure which constrains the values of the invariant mass and the invariant spin to be the same for any solution of the equations of motion. We prove that both these Casimir invariants, the square of the four-momentum vector and the square of the Pauli-Luba\'{n}ski pseudo-vector, preserve the same fixed values even in the presence of an arbitrary external electromagnetic field. In the "free" case, in the centre of mass reference frame, the particle moves along a circle of fixed radius. In a homogeneous magnetic field, a number of rotational "states" is possible with frequencies slightly different from the cyclotron frequency, and "phase-like" transitions with spin flops occure at some critical value of the particle's three-momentum. In the last section, the article of Kuzenko, Lyakhovich and Segal (1994) in which, in fact, an equivalent model had been proposed and elaborated, is briefly reviewed and compared with Staruszkiewicz's approach and our results.Comment: 19 pages, 4 figures; some comments, important references and afterword added. Accepted to J. Phys. A: Math. Theo

Characterization of quantum angular-momentum fluctuations via principal components

We elaborate an approach to quantum fluctuations of angular momentum based on the diagonalization of the covariance matrix in two versions: real symmetric and complex Hermitian. At difference with previous approaches this is SU(2) invariant and avoids any difficulty caused by nontrivial commutators. Meaningful uncertainty relations are derived which are nontrivial even for vanishing mean angular momentum. We apply this approach to some relevant states.Comment: 10 pages, Two column. New section II and some clarifying comment

Shaping the fluorescent emission by lattice resonances in plasmonic crystals of nanoantennas

We demonstrate that the emission of light by fluorescent molecules in the proximity of periodic arrays of nanoantennas or plasmonic crystals can be strongly modified when the arrays are covered by a dielectric film. The coupling between localized surface plasmon resonances and photonic states leads to surface modes which increase the density of optical states and improve light extraction. Excited dye molecules preferentially decay radiatively into these modes, exhibiting an enhanced and directional emission.Comment: 5 figure

Homopolar bond formation in ZnV2_2O4_4 close to a metal-insulator transition

Electronic structure calculations for spinel vanadate ZnV$_2$O$_4$ show that partial electronic delocalization in this system leads to structural instabilities. These are a consequence of the proximity to the itinerant-electron boundary, not being related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover. For the case of ZnV$_2$O$_4$, this leads to the formation of V-V dimers along the [011] and [101] directions that readily accounts for the intriguing magnetic structure of ZnV$_2$O$_4$.Comment: 5 pages, 3 figures, 1 tabl

Estudio geológico del sector de Puerto-López (Granada, zona subbética)

La geología del sector de Puerto-López, situado al NW de Granada, ofrece varios aspectos de interés. Hemos podido establecer la sucesión  estratigráfica, con términos comprendidos entre el Lías y el Senonense, con dataciones muchas veces precisas gracias a las faunas de Ammonites. La estructura es relativamente violenta y son de destacar los pliegues vergentes al S y las fallas inversas del mismo sentido. Por la naturaleza de la serie y por su posición en el Subbético medio, se ha llegado a la correlación de la serie establecida con otras series subbéticas, de la transversal de Granada

Long-range surface polaritons in ultra-thin films of silicon

We present an experimental and theoretical study of the optical excitation of long-range surface polaritons supported by thin layers of amorphous silicon (a-Si). The large imaginary part of the dielectric constant of a-Si at visible and ultraviolet (UV) frequencies allows the excitation of surface polariton modes similar to long-range surface plasmon polaritons on metals. Propagation of these modes along considerable distances is possible because the electric field is largely excluded from the absorbing thin film. We show that by decreasing the thickness of the Si layer these excitations can be extended up to UV frequencies, opening the possibility to surface polariton UV optics compatible with standard Si technology

Hyperbolic Scar Patterns in Phase Space

We develop a semiclassical approximation for the spectral Wigner and Husimi functions in the neighbourhood of a classically unstable periodic orbit of chaotic two dimensional maps. The prediction of hyperbolic fringes for the Wigner function, asymptotic to the stable and unstable manifolds, is verified computationally for a (linear) cat map, after the theory is adapted to a discrete phase space appropriate to a quantized torus. The characteristic fringe patterns can be distinguished even for quasi-energies where the fixed point is not Bohr-quantized. The corresponding Husimi function dampens these fringes with a Gaussian envelope centered on the periodic point. Even though the hyperbolic structure is then barely perceptible, more periodic points stand out due to the weakened interference.Comment: 12 pages, 10 figures, Submited to Phys. Rev.

Hipoparatiroidismo primario en un perro

Una perra Terrier de 8 años se presentó con convulsiones generalizadas agudas. El animal sufría hipocalcemia severa persistente, fosfatemia en el rango superior o elevada y niveles de PTH compatibles con hipoparatirodismo primario. Tras 21 meses de tratamiento el animal sólo requiere la administración de calcitriol a días alternos.