I. M. Nefedov

Inhomogeneous states and the mechanism of magnetization reversal of a chain of classical dipoles

The inhomogeneous states (solitons) in a single chain of classical dipoles are studied numerically and analytically. An analytical solution to the problem is based on the long-wave approximation for dipole sums which holds for high magnetic fields perpendicular to the dipole chain. The analytical and numerical solutions are in reasonable agreement. The magnetization reversal is investigated by numerical simulation based on the Landau-Lifshitz stochastic equations. It is demonstrated that the magnetization reversal of a dipole chain at a finite temperature has a thermal activation nature and occurs through the formation of a stable phase nucleus (a soliton at the edge of the chain) and its growth (the motion of the soliton along the chain).

Multipactor breakdown in waveguide irises

A detailed analysis is made in order to establish the multipactor breakdown threshold in wave guide irises. The analysis involves an analytical investigation based on the conformal mapping technique and also extensive numerical simulations. Comparison with recent experimental results shows good agreement. The main goal of the work is to obtain multipactor charts for the iris, i.e. the breakdown voltage as a function of the frequency - gap product, for different height to width ratios of the iris.

Microwave Corona Breakdown Around Metal Corners and Wedges

This paper presents an analytical, numerical, and experimental analysis of the breakdown strength of microwave gas-filled RF devices containing sharp corners and wedges. For the idealized case of a wedge-shaped geometry, it is shown that only under certain physical circumstances does the singularity and the concomitant strongly enhanced microwave field determine the breakdown strength. In particular, when diffusion is the dominating loss mechanism for the electron density, breakdown is a volumetric process, and the field singularity does not determine the breakdown threshold. In such situations, excessive accuracy in numerical calculations is not required. Conditions for volumetric and localized breakdown, respectively, are established analytically, and the validity is demonstrated by numerical simulations. Finally, the analysis is extrapolated and compared with experimentally observed breakdown thresholds in commercially available resonators of nonidealized geometry. Good agreement between theoretical predictions and experimental results is demonstrated.

Charge transport in superlattices with low-strength barriers and the problem of a terahertz bloch oscillator

Charge-transport properties of superlattices with low-strength barriers and the possibility of designing a Bloch oscillator based on these superlattices are discussed. A terahertz Bloch oscillator based on n-GaAs-GaAlAs structures with low-strength barriers is suggested. Because of interminiband tunneling, the current is an increasing function of electric-field strength, so that domains cannot be formed. At the same time, tunneling and Bloch oscillations give rise to dynamic negative electrical conductivity in the terahertz region. Monte Carlo simulations show that dynamic negative conductivity exists in the frequency range of 1–7 THz for superlattices with moderate charge-carrier mobility at 77 K. A Bloch oscillator should include a superlattice with 350–700 periods of 150-Å, with this superlattice being sandwiched between contact regions, which are in fact strip-line sections (the oscillator cavity). Presumably, such an oscillator can operate at 77 K in the continuous-wave mode.

Microwave Breakdown in RF Devices Containing Sharp Corners

The present work reports on an analytical, numerical, and experimental analysis of the importance of electric field singularities around sharp corners for the determination of the breakdown strength of microwave RF devices. It is shown that only under certain physical circumstances, does the singularity and the concomitant strongly enhanced microwave field determine the breakdown strength. In particular, in situations where diffusion is the dominating loss mechanism for the electron density, it is shown that breakdown is a volumetric process and that the field singularity does not determine the breakdown threshold. Conditions for volumetric and localized breakdown respectively are established analytically and the validity is demonstrated by numerical simulations. Finally an experimental investigation is made which confirms the predicted behavior and demonstrates the accuracy which is possible to obtain for the determination of the breakdown threshold

Influence of the microcrystalline structure on the magnetic properties of ferromagnetic films and structures on their base

This paper presents the results of investigating the crystal structure of thin films of the Co40Fe60 composition obtained by magnetron sputtering on their magnetic properties and properties of ferromagnetic nanostructures formed from these films using the optical and electron lithography. It has been found experimentally that magnetic properties of the structures depend on the crystalline-grain sizes, which can be controlled by means of deposition of additional buffer layers of various materials ∼1 nm thick on the substrate under the ferromagnetic film. It has been shown that the crystallite size can be the main factor that determines the minimal lateral sizes of the structures with specified magnetic properties, which are formed from thin ferromagnetic films.

Phase transitions in a two-dimensional dipole ferrimagnet

The magnetic configurations of the system of magnetic dipoles that have different values and are arranged in a staggered order on a square lattice are studied. A numerical simulation is used to study the phase transitions in the system when the mismatch between the dipoles changes. The restructuring of the magnetic configuration of the system induced by a change in the mismatch is shown to proceed via sequential second-order phase transitions between collinear and noncollinear phases. The numerical simulation results are supported by analytical calculations performed with trial functions.

The Accuracy of Reconstructing the Semiconductor Doping Profile from Capacitance-Voltage Characteristics Measured during Electrochemical Etching

The accuracy of a number of techniques for the reconstruction of doping profiles on the basis of the capacitance-voltage measurements in an electrochemical cell are numerically analyzed. The two previous simple methods proposed by us are shown not only to allow the direct determination of the doping profile at the surface but also to be potentially more accurate than the conventionally used procedure. However, our calculation techniques require experimental data of a higher accuracy. In particular, the relative error of measurements should be within 5×10−4, which is an order of magnitude smaller than the commonly available values.

Approach to electrochemical C-V profiling in semiconductor with sub-Debye-length resolution

Three methods for the determination of the detailed structure of dopant distribution in semiconductors, based on the data of electrochemical C-V profiling, are proposed. The methods give the possibility of determining a dopant distribution directly from a semiconductor surface and providing a sub-Debye length resolution. The results of numerical simulation confirm the possibility of determination of semiconductor dopant profile with nanometer depth resolution.

Magnetostatic mechanism for control of chirality of magnetization distributions

It has been shown that the magnetostatic interaction in an inhomogeneous medium leads to the removal of the chiral degeneracy of magnetic distributions. Noncollinear states of two magnetic dipoles and a helical cycloid placed over a superconducting half-space have been considered as examples. The influence of a finite penetration depth of the magnetic field on the efficiency of removal of the chiral degeneracy has been studied in the framework of the London approximation.