Investigation of vacancy-type complexes in GaN and AlN using positron annihilation

The momentum distribution of electron-positron pairs in GaN and AlN has been investigated for the first time by measuring the one-dimensional angular correlation of the annihilation radiation (1D-ACAR). The characteristic parameter of the electron density r _s ^′ (the radius of the sphere occupied by an electron) estimated from the experimental data differs noticeably from r _s calculated in terms of a standard model of noninteracting particles: r _s ^′ (AlN)≈1.28r _s, and r _s ^′ (GaN)≈1.66r _s, where r _s(AlN)≈1.6091 au, and r _s(GaN)≈1.6568 au. The chemical nature of atoms in the environment of the annihilating positron in AlN and GaN was identified from the electron-positron ionic radius of Al³⁺, Ga³⁺, and N⁵⁺ cores, which were determined from the characteristics of the high-momentum component of 1D-ACAR curves. The analysis was based on a comparison of the electron-positron ionic radii with those considered standard for Al and Ga metals and the related compounds GaP, GaAs, and GaSb. A conclusion is made that positron annihilation dominates in the regions of vacancy-type “nitrogen antisite”-“vacancy” complexes, [N _Ga ⁺ V _Ga] and [N _Al ⁺ V _Al], in GaN and AlN, respectively.     

The Quantum Phase Slip Phenomenon in Superconducting Nanowires with High-Impedance Environment

Quantum phase slip (QPS) is the particular manifestation of quantum fluctuations of the order parameter of a current-biased quasi-1D superconductor. The QPS event(s) can be considered a dynamic equivalent of tunneling through conventional Josephson junction containing static in space and time weak link(s). At low temperatures T<<T _c, the QPS effect leads to finite resistivity of narrow superconducting channels and suppresses persistent currents in tiny nanorings. Here, we demonstrate that the quantum tunneling of phase may result in Coulomb blockade: superconducting nanowire, imbedded in high-Ohmic environment, below a certain bias voltage behaves as an insulator.     

Reduction of additional loads on piles due to loading-friction forces using antifriction coatings

Conclusions 1. The use of low-strength (sand-clay, lime-clay, etc.) grouts for filling the gap between piles and the walls of leader holes having a diameter of no less than the diagonal of the cross section of the pile and a depth equal to the stratum prone to slump-type settlements is one of the most effective measures for reducing additional loads on precast piles due to loading-friction forces in soils classed as type-II in terms of proneness to slumptype settlement.2. Use of jackets formed from polyethylene film on cast-in-place piles ensures significant reduction in additional load due to loading-friction forces. In this case, however, it is necessary to adopt measures to ensure the film's longevity over the entire period during which the building or structure is occupied.3. Considering the extremely high efficiency of anti-friction coatings on piles in soils classed as type-II in terms of proneness to slump-type settlement, it is necessary to continue and expand research in this direction.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 18–20, September–October, 1989.     

Artifacts in Atomic Force Microscopy

The origin of artifacts in atomic force microscopy is discussed. They are classified by basic distinct features. Various solutions to this problem are suggested using the literature data. Metrological issues of the problem are touched upon.     

Charge-coupled devices with compensated charge transfer

It was shown that charge-transfer inefficiency in charge-coupled devices can effectively be compensated by doubling the number of unit cell required for charge transfer. An empty cell should follow each charged one, and charge readout in the output unit is performed via the summation of the charges on every pair of such cells.     

Optimality conditions in abnormal extremal problems

In this paper we study the minimization problem with constraints. The first- and second-order necessary conditions are obtained. These conditions are also informative in the abnormal case in contrast to the classical ones. We introduce the class of 2-normal constraints. It is shown that for 2-normal constraints the gap between the obtained necessary conditions and the sufficient conditions is minimal. It is proved that a generic map is 2-normal.     

A Problem of Optimal Distribution of Resources over a Set of Independent Operations

A problem is considered for the optimal distribution of resources over a set of independent operations for the case where the rate of operations depends linearly on the amount of resources and the state of operations. The necessary optimality conditions in the form of the maximum principle of Pontryagin are obtained.