An investigation into density-wave propagation within high-temperature superconducting plasmas for use in creating traveling-wave devices

This paper theoretically investigates a novel application of high-temperature superconductors where the superconductor serves as the active component in a microwave or millimeter traveling-wave amplifier. A guided electromagnetic wave interacts with a dc superconducting electron current to set up charge-density gradients within the superconducting electron "plasma." The electromagnetic wave gradually extracts energy from the superconducting electrons by traveling in phase synchronism with these charge gradients. The interaction mechanism is similar to that of a conventional traveling-wave tube amplifier or oscillator. We have modeled the wave behavior of superconducting electrons using the London equations and a two-fluid approach. Our model includes dissipation within the superconductor, and it shows that traveling-wave devices may be possible using high-quality thin-film superconductors in which dissipation is kept low.     

Tunneling spectroscopy: A probe for high-Tc superconductivity

After the pioneering work of Giaever [Phys. Rev. Lett. 5 (1960) 147. [1]] and Shapiro in the 1960s, tunneling spectroscopy has become an important method for studying different properties of superconductors, such as the superconducting gap and the density of states. At the interface with a superconductor there is a variety of tunneling processes depending on barrier properties and the type of materials in both sides of the junction. Among other phenomena, we have single electron (Giaever) tunneling, cooper pairs (Josephson) tunneling [B.D. Josephson, Phys. Rev. Lett. 1 (1962) 251. [17]] and mixed (Andreev-Saint-James reflections [A.F. Andreev, Zh. Eksp. Teor. Fiz. 46 (1964) 1823, Saint-James D., J. Phys 25 (1964) 899]) tunneling. In high-Tc superconductors these phenomena are enriched by the anisotropy of the energy gap, which adds new features, like the so-called ‘Andreev-Saint-James bound states’, which originate the zero bias conductance peak. These peculiarities add to this technique the capability of studying the symmetry of the superconducting gap and the normal state pseudogap. In this report we present a brief review of tunneling spectroscopy applied to high-Tc superconductors and some results about the symmetry of the superconducting gap, supporting the hypothesis of a quantum critical point, which may be at the origin of the superconductive transition.     

Modeling and analysis of high-temperature superconductor terahertz photomixers

Photomixing phenomenon in superconductors is studied. For a photo-excited superconductor film the fundamental equations for the carriers motion inside the superconductor material have been used in connection with the two-temperature model to find an analytic expression for the generated terahertz (THz) photocurrent inside the film. The dependency of the resulting photocurrent on the material and laser parameters, temperature, and photomixer configuration for a photomixer made of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// high-temperature superconductor is studied.     

在微型计算机上建立的一个超导体模型系统

本文叙述了新近在微型机IBM-PC上研制的超导体模型系统。它收集了至今所发现的超导材料和相关的化合物,显示和绘制了三维的晶胞结构和配位多面体模型,寻找了超导结构的特征规律,并实现了屏幕网格模块设计。本系统特别适合于高温氧化物超导材料的构效关系的研究,并为寻找新型高温超导材料开辟了可能的途径。     

Soliton shape stabilization in a superlattice with next-to-nearest neighbor spectrum in a field of a nonlinear wave

The effect of an electric field of a nonlinear (cnoidal) electromagnetic wave (pumping field) upon the shape of the solitary electromagnetic wave (soliton) in the quantum semiconductor superlattice with two harmonics of the electron spectrum is studied. Propagation of electromagnetic waves is shown in this case to be described by the modified double sine-Gordon equation. The possibility of the amplification of the pulse and its transformation into the dissipative soliton is noticed. The speed and width of a soliton depend on the presence of the second harmonic in the superlattice electron energy spectrum. The dependence of the dissipative soliton parameters on temperature and pumping field amplitude is also noticed. The possibility of propagation of electromagnetic waves described by solutions of the modified sine-Gordon equation in the superlattice with the spectrum under study is found.     

Analysis and simulation of the effects of distributed nonlinearities in microwave superconducting devices

This paper presents a comprehensive study of microwave nonlinearities in superconductors, with an emphasis on intermodulation distortion and third-harmonic generation. It contains the analysis of various resonant and nonresonant test devices and its validation using numerical simulations based on harmonic balance (HB). The HB simulations made on test devices show that the closed-form equations for intermodulation and third-harmonic generation are only valid at low power levels. The paper also contains examples of application of HB to illustrate that this technique is useful to simulate superconductive devices other than simple test devices, and that the validity of the simulations is not restricted to low drive power levels. Most of the analyses and simulations of this paper are based on electrical parameters that describe the nonlinearities in the superconducting material. These parameters are compatible with many existing models of microwave nonlinearities in superconductors. We discuss the particulars on how to relate these electrical parameters with one of the existing models that postulates that the nonlinear effects are due to a dependence of the penetration depth on the current density in the superconductor.     

相对论电子在高斯激光场中的加速

为了研究激光电磁场对真空中电子的作用,从洛伦兹方程出发,得出了电子运动轨迹,实现了激光场对电子的加速。继而由运动电子产生的流密度,得到了电子的辐射能量谱,分析了相对论电子的辐射特点。结果表明,高斯激光场对真空中电子有很好的加速效果,最大轴向速度可达0.9c。     

Phenomenological and electron-theoretical study of the electrodynamics of rotating systems

A phenomenological and electron-theoretical study of the electrodynamics of rotating systems is developed in the three-dimensional form. In the former part of the present paper, the electromagnetic field equations in an isotropic and homogeneous medium performing arbitrary accelerated motion relative to an inertial reference frame are derived from the phenomenological viewpoint, under the assumptions that the observer is at rest in the inertial or rotating reference frame and that the macroscopic properties of the material medium are not changed by the acceleration acting upon it. With the aid of the classical electron theory, the latter part of the present paper discusses the electromagnetic field equations in rotating media as viewed from the inertial or rotating frame, presenting an electron-theoretical basis for the electrodynamics of rotating media. In addition, the electromagnetic effects caused by the inertial forces accompanying the rotational motion, i.e., the centrifugal and Coriolis forces, are investigated in detail.     

Superconductor electronic device applications

Superconductors are becoming important in many applications where high sensitivity or speed is required. A brief introduction to superconductive device physics is presented, and the role of high-temperature superconductors is considered. The basic principles of a number of applications in metrology, electromagnetic sensing, and analog and digital circuits are presented. The prospects for hybrid semiconductor-superconductor devices, circuits, and systems are discussed     

A theoretical study of ion plasma oscillations

A theoretical study is made of oscillations in an ion plasma, which is in an electron beam. The effect of ion motion on the electrons is neglected, and the ions and the electrons are assumed to have constant and equal density in the equilibrium position. Symmetric and transverse oscillations are studied, both in planar and cylindrical geometry. For planar geometry, the frequency of oscillations for both symmetric and transverse modes is independent of amplitude, while the frequency increases with amplitude for cylindrical symmetric oscillations. For both cylindrical and planar geometry, the presence of the anode boundaries reduces the frequency for transverse ion oscillations, but does not affect the frequency for symmetric-type oscillations.     

Influence of diffusion and nonequilibrium populations on noble-gas plasmas in electric arcs

Measurements and calculations of temperatures, densities, and field-strength-current characteristics of cascade arcs burning in noble gases under atmospheric pressure are reported. The evaluation of measured arc data assuming Saha equilibrium [complete local thermal equilibrium (LTE)] is not in agreement with the detailed solution of the balance equations. The temperatures of electrons and heavy particles and the density of electrons and neutrals have to be determined from the set of rate equations, from the equation of state, connection with the electron energy balance and the equation of state, the energy balance of the electron gas, and of the total plasma. Solutions of these equations are compared with results following from measured line intensities only solving the rate equations in connection with the electron energy balance and the equation of state. For helium, both methods give results which agree within a few percent. The deviations from Saha equilibrium are caused by diffusion and the overpopulation of ground state atoms. The excited atoms, however, are nearly in equilibrium with free electrons in the range of electron densities reached in our experiment (partial LTE). Measurements of E-I characteristics agree with calculated data, if diffusion is taken into account. A simple criterion for the limit between diffusion-dominated plasma and a plasma in thermal equilibrium is derived.     

Self-stabilization of dense soliton trains in a passivelymode-locked ring laser

The generation of stable dense soliton trains in passively mode locked ring fiber lasers was explored theoretically. We found equilibrium states of even and odd numbers of interacting solitons in a ring configuration by employing soliton perturbation theory. In a lossless ring, these equilibrium states were unstable due to the lack of a damping mechanism both for the oscillations in the solitons parameters as well as for modulation instability. In a passively mode-locked ring fiber laser, these dense soliton trains were self-stabilized to form ordered trains. The stabilization process was initiated by the local repulsive soliton-soliton interactions and preceeded by a global soliton phase locking stemming from multimode self-injection locking. We describe in detail the evolution from a disordered to ordered dense soliton train     

Radiation from a uniformly moving distribution of electric charge in an anisotropic, compressible plasma

The radiation characteristics of a linear distribution of electric charge moving with a uniform velocity in a homogeneous electron plasma of infinite extent are investigated for the case in which a uniform static magnetic field is impressed externally throughout the medium. The linear distribution of charge and its direction of motion are assumed to be parallel and perpendicular, respectively, to the direction of the external magnetic field. Of the two possible modes of waves of small amplitude, namely, the modified electromagnetic mode and the modified electron plasma mode, the uniformly moving charge distribution excites the modified electron plasma mode. The emitted radiation has no frequencies less than the plasma frequency. For a particular value of the ratio of the gyrotropic to the plasma frequency of the electrons, the frequency and the angular spectrum of the emitted radiation are determined for two values of the velocity of the charge.     

Mode competition in the orotron

A nonlinear, self-consistent and multimode analysis of the orotron is presented. The field in the cavity is expanded into the Hermite-Gaussian modes with time-dependent amplitudes, for which a set of ordinary differential equations is obtained from Maxwell's equations. The equations for the amplitudes are coupled to the equations of motion for the electrons. To yield a self-consistent solution, this set of coupled equations is solved simultaneously. The calculations yield transient and steady state behaviour, saturated efficiency, mode competition and multi-frequency behaviour.     

Charge separation in organic photovoltaic cells

We consider a simple model for the geminate electron–hole separation process in organic photovoltaic cells, in order to illustrate the influence of dimensionality of conducting channels on the efficiency of the process. The Miller–Abrahams expression for the transition rates between nearest neighbor sites was used for simulating random walks of the electron in the Coulomb field of the hole. The non-equilibrium kinetic Monte Carlo simulation results qualitatively confirm the equilibrium estimations, although quantitatively the efficiency of the higher dimensional systems is less pronounced. The lifetime of the electron prior to recombination is approximately equal to the lifetime prior to dissociation. Their values indicate that electrons perform long stochastic walks before they are captured by the collector or recombined. The non-equilibrium free energy considerably differs from the equilibrium one. The efficiency of the separation process decreases with increasing the distance to the collector, and this decrease is considerably less pronounced for the three dimensional system. The simulation results are in good agreement with the extension of the continuum Onsager theory that accounts for the finite recombination rate at nonzero reaction radius and non-exponential kinetics of the charge separation process.     

Influence of flux creep on dynamic behavior of magnetic levitation systems with a high-T/sub c/ superconductor

Significant flux creep may be generated in some high-T/sub c/ superconductors with weak pinning, which could yield an influence on the dynamic behavior of a high-T/sub c/ superconductor-magnet levitation system. To investigate this influence, this article presents a numerical analysis of dynamic features of the levitation generated by an interaction between a high-T/sub c/ superconductor (HTSC) and a permanent magnet (PM) after the flux creep in the superconductor is taken into account in a macro-model of superconductivity. The influence is comprehensively displayed by comparing the predictions of dynamic responses of such systems in which the flux creep in the superconductor is and is not taken into account. The obtained results show that whether or not the flux creep results in a noticeable influence to the levitation of superconductor-magnet systems is mainly dependent upon properties of superconductivity and applied excitation, e.g., critical current density of superconductors, and amplitude and frequency of external excitations. When the critical current density is less than 4.5/spl times/10/sup 8/ A/m/sup 2/, and the system is subjected to a periodic excitation, the influence of flux creep should be taken into account in the theoretical analysis.