Stability properties of periodically driven overdamped pendula and their implications to physics of semiconductor superlattices and Josephson junctions

We consider the first order differential equation with a sinusoidal nonlinearity and periodic time dependence, that is, the periodically driven overdamped pendulum. The problem is studied in the case that the explicit time-dependence has symmetries common to pure ac-driven systems. The only bifurcation that exists in the system is a degenerate pitchfork bifurcation, which describes an exchange of stability between two symmetric nonlinear modes. Using a type of Prufer transform to a pair of linear differential equations, we derive an approximate condition of the bifurcation. This approximation is in very good agreement with our numerical data. In particular, it works well in the limit of large drive amplitudes and low external frequencies. We demonstrate the usefulness of the theory applying it to the models of pure ac-driven semiconductor superlattices and Josephson junctions. We show how the knowledge of bifurcations in the overdamped pendulum model can be utilized to describe effects of rectification and amplification of electric fields in these microstructures.Comment: 15 pages, 7 figures, Revtex 4.1. Revised and expanded following referee's report. Submitted to journal Chaos

A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses

Multipath propagation in the interstellar medium distorts radio pulses, an effect predominant for distant pulsars observed at low frequencies. Typically, broadened pulses are analyzed to determine the amount of propagation-induced pulse broadening, but with little interest in determining the undistorted pulse shapes. In this paper we develop and apply a method that recovers both the intrinsic pulse shape and the pulse broadening function that describes the scattering of an impulse. The method resembles the CLEAN algorithm used in synthesis imaging applications, although we search for the best pulse broadening function, and perform a true deconvolution to recover intrinsic pulse structre. As figures of merit to optimize the deconvolution, we use the positivity and symmetry of the deconvolved result along with the mean square residual and the number of points below a given threshold. Our method makes no prior assumptions about the intrinsic pulse shape and can be used for a range of scattering functions for the interstellar medium. It can therefore be applied to a wider variety of measured pulse shapes and degrees of scattering than the previous approaches. We apply the technique to both simulated data and data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical Journa

Resistive state of superconducting structures with fractal clusters of a normal phase

The effect of morphologic factors on magnetic flux dynamics and critical currents in percolative superconducting structures is considered. The superconductor contains the fractal clusters of a normal phase, which act as pinning centers. The properties of these clusters are analyzed in the general case of gamma-distribution of their areas. The statistical characteristics of the normal phase clusters are studied, the critical current distribution is derived, and the dependencies of the main statistical parameters on the fractal dimension are found. The effect of fractal clusters of a normal phase on the electric field induced by the motion of the magnetic flux after the vortices have been broken away from pinning centers is considered. The voltage-current characteristics of fractal superconducting structures in a resistive state for an arbitrary fractal dimension are obtained. It is found that the fractality of the boundaries of normal phase clusters intensifies magnetic flux trapping and thereby increases the current-carrying capability of the superconductor.Comment: 15 pages with 8 figures, revtex3, alternative e-mail of author is [email protected]

Electronic structure, magnetic and optical properties of intermetallic compounds R2Fe17 (R=Pr,Gd)

In this paper we report comprehensive experimental and theoretical investigation of magnetic and electronic properties of the intermetallic compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these two systems was probed by optical measurements in the spectral range of 0.22-15 micrometers. On top of that charge carriers parameters (plasma frequency and relaxation frequency) and optical conductivity s(w) were determined. Self-consistent spin-resolved bandstructure calculations within the conventional LSDA+U method were performed. Theoretical interpetation of the experimental s(w) dispersions indicates transitions between 3d and 4p states of Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical optical conductivity coincides well with our experimental data. Calculated by LSDA+U method magnetic moments per formula unit are found to be in good agreement with observed experimental values of saturation magnetization.Comment: 16 pages, 5 figures, 1 tabl

Magnetothermopower and Magnetoresistivity of RuSr2Gd1-xLaxCu2O8 (x=0, 0.1)

We report measurements of magnetothermopower and magnetoresistivity as a function of temperature on RuSr2Gd1-xLaxCu2O8 (x = 0, 0.1). The normal-state thermopower shows a dramatic decrease after applying a magnetic field of 5 T, whereas the resistivity shows only a small change after applying the same field. Our results suggest that RuO2 layers are conducting and the magnetic field induced decrease of the overall thermopower is caused by the decrease of partial thermopower decrease associated with the spin entropy decrease of the carriers in the RuO2 layers.Comment: 21 pages, 6 figure

Disorder Induced Ferromagnetism in Restricted Geometries

We study the influence of on-site disorder on the magnetic properties of the ground state of the infinite $U$ Hubbard model. We find that for one dimensional systems disorder has no influence, while for two dimensional systems disorder enhances the spin polarization of the system. The tendency of disorder to enhance magnetism in the ground state may be relevant to recent experimental observations of spin polarized ground states in quantum dots and small metallic grains.Comment: 4 pages, 4 figure

Measurement of the Pion Form Factor in the Energy Range 1.04-1.38 GeV with the CMD-2 Detector

The cross section for the process $e^+e^-\to\pi^+\pi^-$ is measured in the c.m. energy range 1.04-1.38 GeV from 995 000 selected collinear events including 860000 $e^+e^-$ events, 82000 $\mu^+\mu^-$ events, and 33000 $\pi^+\pi^-$ events. The systematic and statistical errors of measuring the pion form factor are equal to 1.2-4.2 and 5-13%, respectively.Comment: 5 pages, 2 figure

Constraints for hypothetical interactions from a recent demonstration of the Casimir force and some possible improvements

The Casimir force is calculated in the configuration of a spherical lens and a disc of finite radius covered by $Cu$ and $Au$ thin layers which was used in a recent experiment. The correction to the Casimir force due to finiteness of the disc radius is shown to be negligible. Also the corrections are discussed due to the finite conductivity, large-scale and short-scale deviations from the perfect shape of the bounding surfaces and the temperature correction. They were found to be essential when confronting the theoretical results with experimental data. Both Yukawa-type and power-law hypothetical forces are computed which may act in the configuration under consideration due to the exchange of light and/or massless elementary particles between the atoms of the lens and the disc. New constraints on the constants of these forces are determined which follow from the fact that they were not observed within the limits of experimental errors. For Yukawa-type forces the new constraints are up to 30 times stronger than the best ones known up today. A possible improvement of experimental parameters is proposed which gives the possibility to strengthen constraints on Yukawa-type interactions up to $10^4$ times and on power-law interactions up to several hundred times.Comment: 15 pages, 3 figures, subm. to Phys. Rev.

Induced pseudoscalar coupling of the proton weak interaction

The induced pseudoscalar coupling $g_p$ is the least well known of the weak coupling constants of the proton's charged--current interaction. Its size is dictated by chiral symmetry arguments, and its measurement represents an important test of quantum chromodynamics at low energies. During the past decade a large body of new data relevant to the coupling $g_p$ has been accumulated. This data includes measurements of radiative and non radiative muon capture on targets ranging from hydrogen and few--nucleon systems to complex nuclei. Herein the authors review the theoretical underpinnings of $g_p$, the experimental studies of $g_p$, and the procedures and uncertainties in extracting the coupling from data. Current puzzles are highlighted and future opportunities are discussed.Comment: 58 pages, Latex, Revtex4, prepared for Reviews of Modern Physic

Arecibo 430 MHz Pulsar Polarimetry: Faraday Rotation Measures and Morphological Classifications

We have measured Faraday Rotation Measures (RMs) at Arecibo Observatory for 36 pulsars, 17 of them new. We combine these and earlier measurements to study the galactic magnetic field and its possible temporal variations. Many RM values have changed significantly on several-year timescales, but these variations probably do not reflect interstellar magnetic field changes. By studying the distribution of pulsar RMs near the plane in conjunction with the new NE2001 electron density model, we note the following structures in the first galactic longitude quadrant: (1) The local field reversal can be traced as a null in RM in a 0.5-kpc wide strip interior to the Solar Circle, extending \~7 kpc around the Galaxy. (2) Steadily increasing RMs in a 1-kpc wide strip interior to the local field reversal, and also in the wedge bounded by 42<l<52 deg, indicate that the large-scale field is approximately steady from the local reversal in to the Sagittarius arm. (3) The RMs in the 1-kpc wide strip interior to the Sagittarius arm indicate another field reversal in this strip. (4) The RMs in a final 1-kpc wide interior strip, straddling the Scutum arm, also support a second field reversal interior to the Sun,between the Sagittarius and Scutum arms. (5) Exterior to the nearby reversal, RMs from 60<l<78 deg show evidence for two reversals, on the near and far side of the Perseus arm. (6) In general, the maxima in the large-scale fields tend to lie along the spiral arms, while the field minima tend to be found between them. We have also determined polarized profiles of 48 pulsars at 430 MHz. We present morphological pulse profile classifications of the pulsars, based on our new measurements and previously published data.Comment: Accepted by Astrophysical Journal Supplement, 56 pages, 22 fig