Nearest-neighbor distribution for singular billiards

The exact computation of the nearest-neighbor spacing distribution P(s) is performed for a rectangular billiard with point-like scatterer inside for periodic and Dirichlet boundary conditions and it is demonstrated that for large s this function decreases exponentially. Together with the results of [Bogomolny et al., Phys. Rev. E 63, 036206 (2001)] it proves that spectral statistics of such systems is of intermediate type characterized by level repulsion at small distances and exponential fall-off of the nearest-neighbor distribution at large distances. The calculation of the n-th nearest-neighbor spacing distribution and its asymptotics is performed as well for any boundary conditions.Comment: 38 pages, 10 figure

Effect of Preparation Conditions in the Pressure Range of Atmospheric Nitrogen (2 ... 50) 10-4 Torr on the Structural and Phase State of the Vacuum-Arc Coatings of Mo-N

Nanocrystalline vacuum-arc nitride coatings possess the totality of unique structural states and properties (high hardness, wear resistance, oxidation stability, etc.). The coatings of the Mo-N system show a high hardness and low solubility of nonferromagnetic materials, thereby attracting great interest in their industrial use. Unfortunately, at present there is an apparent lack of information on the regularities of phase-structural state formation in the Mo-N system. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3535

Spectral Statistics: From Disordered to Chaotic Systems

The relation between disordered and chaotic systems is investigated. It is obtained by identifying the diffusion operator of the disordered systems with the Perron-Frobenius operator in the general case. This association enables us to extend results obtained in the diffusive regime to general chaotic systems. In particular, the two--point level density correlator and the structure factor for general chaotic systems are calculated and characterized. The behavior of the structure factor around the Heisenberg time is quantitatively described in terms of short periodic orbits.Comment: uuencoded file with 1 eps figure, 4 page

Accelerated Algorithm of Least-Sguares Approximation of Signals by Exponentials for Wideband Frequency-Domain Reflectometry

In the paper the possibility of acceleration of procedure of best least-squares approximation of signals by exponentials was considered. For this purpose the analytical expressions for components of gradient vector and Hessian matrix of the objective function were obtained. The algorithms of quasisolution searching were constructed. Method of conjugate gradient and modified Newton method were used. The obtained algorithms were compared with modification of Nelder-Mead method which used information about only values of objective function. The comparison of the novel method and Prony’s method and matrix pencil method was held.В статье рассмотрена возможность ускорения аппроксимации сигналов экспонентами методом наименьших квадратов. Для этого были получены аналитические выражения для компонент вектора градиента и матрицы Гессе оптимизируемой функции. Был сконструирован алгоритм поиска квазирешения. Использовались метод сопряженных градиентов и модифицированный метод Ньютона. Полученные алгоритмы были сравнены с модификацией метода Нелдера-Мида, который использует информацию только о значениях оптимизируемой функции. Было проведено сравнение нового метода с методом Прони и методом пучка матриц.У статті розглянуто можливість прискорення апроксимації сигналів експонентами методом найменших квадратів. Для цього було отримано аналітичні вирази для компонент вектора градієнта та матриці Гессе функції, що оптимізується. Було сконструйовано алгоритм пошуку квазірозв’язку. Використано метод спряжених градієнтів та модифікований метод Ньютона. Отримані алгоритми були порівняні з модифікацією методу Нелдера-Міда, який використовує інформацію тільки про значення функції, що оптимізується. Було проведене порівняння нового методу з методом Проні та методом пучка матриць

Wigner--Dyson statistics for a class of integrable models

We construct an ensemble of second--quantized Hamiltonians with two bosonic degrees of freedom, whose members display with probability one GOE or GUE statistics. Nevertheless, these Hamiltonians have a second integral of motion, namely the boson number, and thus are integrable. To construct this ensemble we use some ``reverse engineering'' starting from the fact that $n$--bosons in a two--level system with random interactions have an integrable classical limit by the old Heisenberg association of boson operators to actions and angles. By choosing an $n$--body random interaction and degenerate levels we end up with GOE or GUE Hamiltonians. Ergodicity of these ensembles completes the example.Comment: 3 pages, 1 figur

A pseudointegrable Andreev billiard

A circular Andreev billiard in a uniform magnetic field is studied. It is demonstrated that the classical dynamics is pseudointegrable in the same sense as for rational polygonal billiards. The relation to a specific polygon, the asymmetric barrier billiard, is discussed. Numerical evidence is presented indicating that the Poincare map is typically weak mixing on the invariant sets. This link between these different classes of dynamical systems throws some light on the proximity effect in chaotic Andreev billiards.Comment: 5 pages, 5 figures, to appear in PR

Modification of the Gelfand-Levitan Method for 1-D Multylaered Structure Inverse Problem

For dielectric slab with step profile of dielectric constant the Gelfand-Levitan method is correct if peaks of time-domain reflected signal are close to δ-pulses. Combination of parametric spectral methods for obtaining time-domain signal from frequency domain data and Gelfand-Levitan method for time-domain signal processing can help to improve the solution of the problem. Results of numerical simulation are presented.Для диэлектрической плиты со ступенчатым профилем диэлектрической постоянной применим метод Гельфанда-Левитана, если пики отраженного сигнала близки к δ -импульсам. Комбинация параметрических спектральных методов для получения сигнала во временной области по данным из частотной области и метод Гельфанда-Левитана для обработки сигнала во временной области позволяют получить усовершенствованный алгоритм решения задачи. Приведены результаты численного моделирования.Для діелектричної плити зі східчастим профілем діелектричної сталої метод Гельфанда-Левітана застосовний, якщо піки відбитого сигналу близькі до δ - імпульсів. Комбінація параметричних спектральних методів для отримання сигналу в часовій області та метод Гельфанда-Левітана для обробки сигналу в часовій області дозволяють отримати удосконалений алгоритм розв’язання задачі. Наведено результати чисельного моделювання

Phase coherence phenomena in superconducting films

Superconducting films subject to an in-plane magnetic field exhibit a gapless superconducting phase. We explore the quasi-particle spectral properties of the gapless phase and comment on the transport properties. Of particular interest is the sensitivity of the quantum interference phenomena in this phase to the nature of the impurity scattering. We find that films subject to columnar defects exhibit a `Berry-Robnik' symmetry which changes the fundamental properties of the system. Furthermore, we explore the integrity of the gapped phase. As in the magnetic impurity system, we show that optimal fluctuations of the random impurity potential conspire with the in-plane magnetic field to induce a band of localized sub-gap states. Finally, we investigate the interplay of the proximity effect and gapless superconductivity in thin normal metal-superconductor bi-layers.Comment: 13 pages, 8 figures include

Current correlations and quantum localization in 2D disordered systems with broken time-reversal invariance

We study long-range correlations of equilibrium current densities in a two-dimensional mesoscopic system with the time reversal invariance broken by a random or homogeneous magnetic field. Our result is universal, i.e. it does not depend on the type (random potential or random magnetic field) or correlation length of disorder. This contradicts recent sigma-model calculations of Taras-Semchuk and Efetov (TS&E) for the current correlation function, as well as for the renormalization of the conductivity. We show explicitly that the new term in the sigma-model derived by TS&E and claimed to lead to delocalization does not exist. The error in the derivation of TS&E is traced to an incorrect ultraviolet regularization procedure violating current conservation and gauge invariance.Comment: 8 pages, 3 figure

Spectral correlations : understanding oscillatory contributions

We give a different derivation of a relation obtained using a supersymmetric nonlinear sigma model by Andreev and Altshuler [Phys. Rev. Lett. 72, 902 (1995)], which connects smooth and oscillatory components of spectral correlation functions. We show that their result is not specific to the random matrix theory. Also, we show that despite an apparent contradiction, the results obtained using their formula are consistent with earlier perspectives on random matrix models