We examine energy relaxation of non-equilibrium quasiparticles in different vortex configurations in ``dirty'' $s$-wave superconductors. The heat flow from the electronic subsystem to phonons in a mesoscopic superconducting disk with a radius of the order of several coherence lengths is calculated both in the Meissner and giant vortex states using the Usadel approach. The recombination process is shown to be strongly affected by interplay of the subgap states, located in the vortex core and in the region at the sample edge where the spectral gap $E_{\rm g}$ is reduced by the Meissner currents. In order to uncover physical origin of the results, we develop a semiquantitative analytical approximation based on the combination of homogeneous solutions of Usadel equations in Meissner and vortex states of a mesoscopic superconducting disc and analytically calculate the corresponding spatially resolved electron-phonon heat rates. Our approach provides an important information about non-equilibrium quasiparticles cooling by the magnetic-field induced traps in various mesoscopic superconducting devices.

The dynamics of large superwavelength open systems which are free-standing multi-wire lines with a large number of Josephson junctions, DC bias batteries and other lumped elements is analyzed using direct numerical simulation. Such systems represent the simplest version of the active Josephson antennas proposed in our earlier studies and show promise as terahertz and subterahertz radiation sources. We have studied dependences of the radiation characteristics on the antenna geometry, number of junctions, DC bias current and the lumped elements position and parameters. Detailed simulation results reveal a multitude of phase transitions between the dynamical states, which differ in the number of Josephson junctions synchronized by the excited waves of current and, hence, in the radiation power emitted into open space also as in radiation patterns. It is shown that, in the absence of noise, the Josephson antennas can emit completely coherent radiation.

The previously proposed technique of microwave volt–impedance spectroscopy of semiconductors has been experimentally verified. The technique allows one to determine the local values of electrical parameters of a semiconductor. The investigations have been carried out on a homogeneous single-crystal GaAs wafer with a system of concentric antennas formed on the top of it. The resolution is determined by the antenna central disk diameter, which has been 12, 27, and 57 μm. A dc bias voltage of 0 V ≤ U ≤ 5 V has been applied between antenna contact pads. The complex Z(f, U) impedance spectrum of each antenna has been measured on a Cascade Microtech probe station in the frequency range of f = 0.1–10 GHz. The electrical characteristics of the semiconductor have been determined from the Z(f, U) spectra by solving the inverse problem. The n-type of the semiconductor has been established, and the contact potential difference at the interface with a metal has been determined. The local values of electron density, mobility and electrical conductivity have been found have been found. The four-point probe Hall effect measurements of the surface-averaged values of the same parameters have shown good agreement of the results for the investigated homogeneous wafer.

We have tested experimentally the proposed method of microwave volt-impedance spectroscopy of semiconductors. The method allows to determine the local values of the semiconductor electrophysical parameters. The studies were performed on a homogeneous single-crystal GaAs wafer with a concentric antenna system formed on its surface. The resolution is determined by the diameter of the antenna central disk, which was amounted a = 12, 27, 57 μm. A constant bias voltage of 0 ≤ U ≤ 5 V was applied between the contact pads of the antennas. The complex impedance spectrum Z (f, U) of each antenna was measured using a Cascade Microtech probe station in the frequency range f = 0.1 - 10 GHz. The electrophysical characteristics of the semiconductor were determined from Z(f, U) spectra by the inverse problem solving. We have established the n-type for our semiconductor and determined the electrical potential difference on the metal-semiconductor interface. We have found as well the electron concentration, mobility and conductivity. Measurements of the same parameters by Hall four-probe method (giving the surface averaging) showed good mutual agreement of the results for the homogeneous sample under study.

The electronic structure of the giant vortex states in a mesoscopic superconducting disk is studied in a dirty limit using the Usadel approach. The local density of states profiles are shown to be strongly affected by the effect of quasiparticle (QP) tunneling between the states localized in the vortex core and the ones bound to the sample edge. Decreasing temperature leads to a crossover between the edge-dominated and core-dominated regimes in the magnetic field dependence of the tunneling conductance. This crossover is discussed in the context of the efficiency of quasiparticle cooling by the magnetic field induced QP traps in various mesoscopic superconducting devices.

We propose a method for determining electrophysical characteristics (free charge carrier concentration, mobility, and conductivity) of semiconductors from the results of measurements of the microwave spectrum of the impedance of a coaxial probe as a function of applied constant voltage U . The sought parameters have been determined by solving the corresponding inverse problem using the theory of a near-field antenna that was developed earlier. We have developed a computer program that seeks the solution by minimization of the multiparametric residual function in accordance with the Nelder–Mead algorithm. The precision of the method has been analyzed from the results of simulation in which the impedance was calculated preliminarily considering resultant concentration profile n ( x , U ) of the depleted layer in the vicinity of the metal–semiconductor contact. The possibility of diagnostics with a micrometer lateral resolution has been demonstrated.

We propose a method for determining semiconductors electrophysical characteristics (concentration and mobility of free charge carriers, conductivity) using measurements of the microwave impedance spectrum of a coaxial probe as a function of the applied constant voltage U. The parameters under study are found by solving the corresponding inverse problem using the developed theory of the near-field antenna. A computer program was created that searches for a solution by minimizing the multiparameter discrepancy function using the Nelder–Mead algorithm. The accuracy of the method is analyzed from simulation results in which the impedance is calculated via the obtained charge concentration profile n(x, U) of the depleted layer in the vicinity of the metal-semiconductor contact. The possibility of diagnostics with micron lateral resolution is demonstrated.

Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

A terahertz superconducting oscillators based on large amount of Josephson junction embedded in open system guiding traveling electromagnetic wave is theoretically considered and computer simulated. It is shown that such active Josephson antennae represent oscillator effectively radiated into open space with power scaled with the system size and number of junctions. Dynamics and directivity patterns of such Josephson antennae depending on bias current is investigated and it is shown that such oscillator with sufficiently large junction amount can be competitive with quantum cascade lasers.

A method for determining the parameters of a layered semiconductor structure, using the data obtained by near-field microwave probing with a micron-size lateral resolution, was developed and tested experimentally. We have measured a frequency spectrum of the impedance of a coaxial antenna formed on a test structure surface. The corresponding inverse problem has been solved based on the quasistatic theory for the impedance of a monopole antenna interacting with a layered medium, which was proposed earlier [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)]. This method was applied to a low-barrier Mott diode structure with a nearly 100 nm thick undoped layer grown on a conducting substrate GaAs. Computer simulation allowed us to establish the optimal frequency intervals and estimate the accuracy of determining the structure parameters. Measurements were taken in the frequency range of 0.1–67 GHz on commercially available equipment. Three antennas with a radius of the central conductor of 5.5, 11, and 25 μm, respectively, were used. The accuracy of the experimental evaluation of the layer thickness d and conductivity σ was ∼1–3%, and for the substrate conductivity, it came to about 15%. As an example, we also present the parameters σ and d in four points of the sample surface image. These data show strong lateral inhomogeneity of the structure under study.

We propose a new approach to the problem of obtaining coherent radiation from systems with a great number of Josephson junctions, which is based on the concept of traveling-wave antennas. The traveling wave in a line ensures identity of the electrodynamic conditions, under which the junctions operate, whereas the energy leakage to radiation in the lateral direction prevents saturation of the nonlinearity of the individual junctions having a small dynamic range. Simple analytical models, which demonstrate feasibility of the traveling-wave regime, are considered. A code for direct numerical simulation of Josephson microchips including microantennas, lumped elements, and power supply circuits have been developed. Using the direct numerical simulation, a version of the Josephson antenna, which is similar to the simplest single-wire antenna, is studied and the possibility to realize the traveling-wave regime is demonstrated.

The problem of determining the center and radius of a substrate in the shape of a circular disc is considered. We propose an original functional having a clear geometric interpretation. Determination of the extremum of this functional reduces to a linear problem. The matrix of the linear system corresponding to the functional is positively defined and well-posed for rather long arcs. Experimental studies show that the proposed method enables the determination of the center of the substrate with an accuracy of 10 μm, based on a small number of measurements of the coordinates of the substrate edge. This accuracy is sufficient for solving a number of applied problems.

We present the mathematical model and algorithm for simulation of active Josephson antenna, which consists of a few lumped Josephson junctions and sources of bias voltage, connected by perfectly conducting wires placed on dielectric substrate. For simple model of such antenna, we present some results of simulation, in particular, current-voltage characteristics of junctions and examples of antenna patterns.

This paper presents a statistical analysis of multipactor initiation on a transmission window with finite spatial length. It is found that the presence of the tangential electron motion along the window (under the action of RF electric field) can significantly alter the multipacting discharge cutoff conditions in comparison with the unbounded emission surface model: the decrease of the window wave size leads to the growth of the threshold of multipactor breakdown. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752457]

A technique for the profiling of free carrier concentration N(z) in semiconductors, based on the near-field measurements in microwave frequency range, is proposed. A high accuracy in retrieving the N(z) function with characteristic spatial scales of 10–100 nm using 2–3 probes with apertures of 3–15 μm is demonstrated.

We present the numerical method for retrieving of the profile of layered media permittivity from the near field microscopy data. The method is based on the minimization of the discrepancy function describing the difference between the experimental measured of the near field antenna impedance and the calculated one for given permittivity profile. Such calculation is based on the numerical algorithm which developed on the base of rigorous solution of corresponding direct problem. We suppose that the profile to be retrieved is described by the function with finite number of unknown parameters and the discrepancy is minimized with respect of these parameters. We present some results of numerical experiments with method suggested.

A statistical theory is constructed to evaluate the multipactor induced breakdown on a dielectric. The calculation employs the integral equation allowing predicting the threshold of multipactor growth. It is found that the action of the rf magnetic field considerably changes the shape of the upper susceptibility curve and, depending on the secondary electron yield, leads either to rising of the slope of the upper boundary or to its complete elimination. The critical value of the secondary yield above which the region of multipactor existence is not restricted from above by any limiting rf electric field strength is estimated.