Cherenkov Radiation from Fluxon in a Stack of Coupled Long Josephson Junctions

We present a systematic study of the Cherenkov radiation of Josephson plasma waves by fast moving fluxon in a stack of coupled long Josephson junctions for different fluxon modes. It is found that at some values of parameters current-voltage characteristic may exhibit a region of the back-bending on the fluxon step. In the opposite limit the emission of the Cherenkov radiation takes place. In the annular junctions of moderate length the interaction of the emitted waves with fluxon results in the novel resonances which emerge on the top of the fluxon step. We present more exact formulas which describe the position of such resonances taking into account difference between junction and non-linear corrections. The possibility of direct detection of the Cherenkov radiation in junctions of linear geometry is discussed.     

Numerical Simulations of the Fluxon Dynamics of a Two-Dimensional Josephson Junction System

A direct perturbation method applied to a long Josephson junction molded by one-dimensional sine-Gordon equation is presented and the modulation of perturbations on fluxon velocity and stable state is studied. Simulations of a two-dimensional Josephson junction with dc bias current in an external magnetic field provide an I–V characteristic curve for the system and determine the various dynamic behaviors, including the periodic, quasiperiodic, and chaotic motions. Physical explanations for the behaviors are presented. The numerical results are in good agreement with the energetic analysis.     

Observation of high voltage resonances in one-dimensional arrays

Dynamic states in parallel one-dimensional arrays of small Josephson tunnel junctions are experimentally investigated in the presence of an external magnetic field H. In addition to conventional Fiske modes and flux-flow branches, the current-voltage characteristics exhibit a new type of resonances which appear in higher voltage regions as additional groups of resonant steps. These resonances can be explained in terms of a simple kinematic model, in the limit where the fluxon frequency is higher than the maximum linear wave frequency. The model is in good agreement with reported experiments.     

Annular Long Josephson Junctions in a Magnetic Field: Engineering and Probing the Fluxon Interaction Potential

The interaction of a Josephson fluxon with an externalmagnetic field-induced potential in a long Josephson junctionis investigated experimentally. The thermal activation of thefluxon from a potential well is observed and experimentsprobing its predicted quantum properties are discussed. Amethod for engineering a magnetic double-well potential for afluxon is proposed and the use of the coupled fluxon statesfor quantum computation is suggested.     

Fluxons and superfluxons in Josephson junctions with periodical structure of the magnetic inclusions

The motion of fluxons in Josephson junctions with periodical systems of structure defects with a magnetic moment is investigated. A new type of fluxon pinning, namely the pinning outside the defects, is predicted. The dependence of the critical current on the external magnetic field and distance between defects has been obtained. It is shown that in the case of a fast fluxon its interaction with the magnetic defect is only magnetic. The existence of supersolitons in the fluxon chains is predicted. The results of the theory is used to explain the dependence of critical current on the magnetic field in high temperature superconductors.     

Confocal Annular Josephson Tunnel Junctions with Large Eccentricity

Confocal annular Josephson tunnel junctions (CAJTJs), which are the natural generalization of the circular annular Josephson tunnel junctions, have a rich nonlinear phenomenology due to the intrinsic non-uniformity of their planar tunnel barrier delimited by two closely spaced confocal ellipses. In the presence of a uniform magnetic field in the barrier plane, the periodically changing width of the elliptical annulus generates a asymmetric double-well for a Josephson vortex trapped in a long and narrow CAJTJ. The preparation and readout of the vortex pinned in one of the two potential minima, which are important for the possible realization of a vortex qubit, have been numerically and experimentally investigated for CAJTJs with the moderate aspect ratio 2:1. In this work, we focus on the impact of the annulus eccentricity on the properties of the vortex potential profile and study the depinning mechanism of a fluxon in more eccentric samples with aspect ratio 4:1. We also discuss the effects of the temperature-dependent losses as well as the influence of the current and magnetic noise.     

Stochastic Resonance in Superconducting Nb Film with Periodic Array of Holes

Stochastic fluxon dynamics and spatial ordering are studied in a superconducting Nb film patterned with a periodic (3×3 micron) square array of holes. First, imaging of the film is performed using Scanning Hall Probe Microscope following application of the random noise magnetic drive. We observe formation of the fluxon stripe patterns oriented preferentially along the principal axes of the array. Next, ac magnetic field is applied as superposition of harmonic and random noise components and the local magnetic response is probed with the micron-sized Hall sensor at a single array site. We find that hopping of the fluxons becomes synchronized with the harmonic driving at certain amplitude of the random noise; the latter is found to be dependent upon the harmonic drive frequency. We attribute our observations to the phenomenon of stochastic resonance that is ubiquitous in non-linear multi-level systems with dynamic threshold and discuss a possibility of flux manipulation using stochastically-resonant periodic driving.     

Interactions between spin waves and elastic waves in one-domain ferromagnetic insulators

The interaction between spin waves and the elastic waves is theoretically investigated in the ferromagnetic insulators which are made to behave like one-domain bodies by means of an applied magnetic field. The free energy of the material, that is, the potential of stresses, is derived by continuum approximations of a microscopic model, and the physical meaning of various contributions in the free energy is discussed. The dipole-dipole interaction which may be considered to be a long-range interaction is approximated by a nearest neighbor interaction. As an application of the theory, interactions of spin waves and elastic waves are studied. A correspondence between the microscopic and the continuum theories of ferromagnetic resonance is presented.     

On the deficiency of the fluxon mass derived from the lattice elastic energy

Inadequacies of a result for the fluxon mass per unit length m derived from the nonlocal elasticity of the flux line lattice are discussed. Unusual properties of m are pointed out, including a divergent temperature dependence, showing the need to replace this quantity and the associated viscosity in dynamical expressions. An Appendix illustrates one of many alternative fluxon masses and viscosities, the electromagnetic contribution of AJ fluxons.     

Scattering of waves by a planar junction between resistive half-screen and perfectly electric conducting wedge

The diffraction of plane waves by a planar junction between a resistive half-plane and perfectly conducting wedge is investigated. First of all, the relation between the scattered geometrical optics waves and diffracted fields by a wedge is outlined. Then the scattered GO fields of the planar junction are obtained by subtracting the initial fields from the total geometrical optics waves. The diffracted waves by the junction is derived with the aid of the scattered GO waves’ structure. The behaviour of the fields is analysed numerically.     

Far-field optical superlens

Far-field optical lens resolution is fundamentally limited by diffraction, which typically is about half of the wavelength. This is due to the evanescent waves carrying small scale information from an object that fades away in the far field. A recently proposed superlens theory offers a new approach by surface excitation at the negative index medium. We introduce a far-field optical superlens (FSL) that is capable of imaging beyond the diffraction limit. The FSL significantly enhances the evanescent waves of an object and converts them into propagating waves that are measured in the far field. We show that a FSL can image a subwavelength object consisting of two 50 nm wide lines separated by 70 nm working at 377 nm wavelength. The optical FSL promises new potential for nanoscale imaging and lithography.     

Polarization of almost-plane waves

The general polarization behavior of almost-plane waves, in which the electric field varies slowly over a circular pupil, is considered, on the basis of an axial Hertz potential treatment and expansion in Zernike polynomials. The resultant modes of a circular aperture are compared with the well-known waveguide (or optical fiber) modes and Gaussian beam modes. The wave can be decomposed into partial waves of electric and magnetic types. The modes for a square pupil are also considered. The particular application of the effect on polarization of focusing the waves is discussed. Another application discussed is the Fresnel reflection from a dielectric interface, it being shown that the Fresnel reflection alters the relative strength of the electric and magnetic components.     

The theory of the boundary diffraction wave for wedge diffraction

A new potential function, line integration which gives the edge diffracted fields, is constructed for wedge diffraction by using the method of modified theory of physical optics. The surface integrals are transformed into line integrals by the technique of asymptotic reduction. As an application of the novel potential function, the diffracted field is obtained for the geometry of a wedge for arbitrary incidence of plane waves.     

The scattering of electro-elastic waves by a spherical piezoelectric particle in a polymer matrix

This paper is devoted to the study of scattering of plane harmonic waves by a piezoelectric sphere with spherical isotropy embedded in an unbounded isotropic polymer matrix. The scattered displacement field and the electric potential in the matrix are expressed in terms of spherical vector wave functions and spherical harmonic functions, respectively. For the field points inside the inhomogeneity, new displacement functions are introduced. Expansion of the new displacement functions and the electric potential in terms of spherical harmonic functions, the equations of motion and electrostatic lead to four second order ordinary differential equations (odes), where three of them are coupled. The coupled system of odes is solved by the generalized Frobenius series. This approach is readily used to handle low and high frequencies. Three different types of piezoelectric inhomogeneities, PZT-4, PZT-5H, and BaTiO₃ are considered and the associated piezoelectric effects on the electro-mechanical fields, differential and total scattering cross-sections are addressed.     

Modulational instability of electromagnetic waves in long Josephson junctions

This paper examines the modulational instability of electromagnetic waves in long Josephson junctions under conditions where the electrodynamics of the junction are nonlocal. A region is found containing a modulational instability of nonlinear plane electromagnetic waves in the junction. Pis’ma Zh. Tekh. Fiz. 23, 8–11 (January 26, 1997)     

Transmission of dipole radiation through interfaces and the phenomenon of anti-critical angles

Radiation emitted by an electric dipole consists of traveling and evanescent plane waves. Usually, only the traveling waves are observable by a measurement in the far field, since the evanescent waves die out over a length of approximately a wavelength from the source. We show that when the radiation is passed through an interface with a medium with an index of refraction larger than the index of refraction of the embedding medium of the dipole, a portion of the evanescent waves are converted into traveling waves, and they become observable in the far field. The same conclusion holds when the waves pass through a layer of finite thickness. Waves that are transmitted under an angle larger than the so-called anti-critical angle theta (1) ac are shown to originate in evanescent dipole waves. In this fashion, part of the evanescent spectrum of the radiation becomes amenable to observation in the far field. We also show that in many situations the power in the far field coming from evanescent waves greatly exceeds the power originating in traveling waves.     

Second and Third Peaks in the Non-resonant Microwave Absorption Spectra of Superconducting Bi2212 Crystals

Non-resonant microwave absorption (NMA) measurements at liquid nitrogen temperature with systematic microwave power variation showed a two-peak structure in the Bi-2212 textured crystals, similar to that observed in the Bi-2212 single crystals (Srinivasu et al., in J. Supercond. Incorp. Nov. Magn. 14:41, 2001). NMA signals from the aged Bi-2212 single crystals show an emergence of a ‘third peak’ as a function of microwave power. We qualitatively interpret these second and third peaks as due to the microwave power induced phase locking of several number of junctions into coherent groups and then the destruction of the phase locking by the applied DC field leading to decoupling or fluxon motion, which gives the loss in individual junctions belonging to these otherwise coherent groups.     

Large-scale electron vortex structure formation in a plasma lens

The first experimental data on the observation of electron vortices in an electrostatic plasma lens at a considerable radial gradient of electron density are reported. It is established that anharmonic potential waves of large amplitude appear and propagate in the azimuthal direction. The results of measurements of the electric field distribution show that electron bunches formed as a result of instability development contain trapped particles, which rotate at a large velocity around the centers of bunches, thus creating vortices. The main factor leading to limitation of the maximum amplitude of vortices in a strong magnetic field has been experimentally established.     

Influence of grain properties on the Abrikosov vortices interaction with Josephson junction

The problem of the displacement of an Abrikosov vortex is investigated when the magnetic field decreases to H _cl ^G in a grain of a type II superconductor. It is shown that near grain boundaries the vortex line generates an intergranular current which depends on the normalized grain size τ and the anisotropy ratio ν. These parameters strongly influence the conditions of the Josephson vortex generation as a result of the interaction of the Abrikosov vortex with the Josephson junction. We support our theory with calculations of the intergrain critical current taking into account two types of the vortex configuration, triangular and square, for different grain characteristics. The results are of interest for the charge transport in granular superconductors as well as the relaxation process in devices that contain Josephson junctions in micro- and nanoelectronics in magnetic field.