Controlled bandpass frequency-selective surfaces

Bandpass frequency-selective surfaces (FSSs) controlled by varactors and representing biperiodic lattices formed from slotted squares in a thin metallic screen are considered. Main factors affecting the FSS tuning range with different circuits for connection of control varactors are experimentally evaluated. Experimental results obtained by the waveguide method of measuring the transmission coefficient through samples of both individual FSS elements loaded with varactors of various types and lattices consisting of these elements at microwave frequencies are presented. Based on the measured data, attainable values of the relative tuning of the FSS resonance frequency are evaluated: f _max/f _min = 1.4 for control circuits with two MA46N120 low-capacitance varactors in each FSS element and f _max/f _min = 1.5 for control circuits with one VV857 medium-capacitance varactor.     

Adjusting the linear expansion coefficient Lin Fe-Ni-Co-Ti invars by aging and phase hardening

The N30K10T3 and N40K10T3 invars with the Curie points θ_C ≈ 200°C and θ_C ≈ 310°C and the martensite temperatures M _s ≈ −80°C and M _s < −196°C, respectively, have been studied. The two alloys were hardened by quenching in the range of temperatures from 100 to 750°C. In addition, the first alloy was hardened by a combination treatment including phase-transformation-induced hardening and aging. The method of phase hardening consisted in the use of a forward (γ → α) and a reverse (α → γ_ph) martensitic transformations. It has been shown that the temperature dependences of the linear expansion coefficient and the dependences of the hardness on the temperature and time of aging are considerably different for both alloys upon decomposition of the supersaturated solid solution. Both the ordinary and the double aging have been studied.     

Photoresponse recovery in silicon photodiodes upon VUV irradiation

The spatial homogeneity of the photoresponse is studied for silicon photodiodes based on p-n and n-p junctions a year after their irradiation at a wavelength of 121.6 nm and those based on n-p junctions four years after their irradiation with soft X-rays. It is demonstrated that silicon photodiodes based on p-n junctions exhibit a photoresponse recovery effect on being irradiated at a wavelength of 121.6 nm. No recovery effect is observed for silicon photodiodes based on n-p structures.     

A layer radiowave absorber based on double-period lattices of resistive squares

A radiowave-absorbing composite material based on double-period lattices of resistive squares is considered. Methods of the electric circuit and long line theories are applied to obtain expressions for the estimate of the effective permittivity of such a composite. It is shown that the frequency dependence of the effective permittivity of the realized composite is close to the relaxation value. The reflection characteristics of a radiowave absorber based on a composite with double-period lattices of resistive squares are investigated. It is found that, for such a radiowave absorber, the ratio of the difference of the extreme wavelengths of its operating band (corresponding to the minus-10-dB reflection level) to the thickness of the radiowave absorber is within an interval of 4.2–4.5.     

Martensitic and magnetic transformations in Ni-Mn-In alloys

Dilatometry, magnetometry, and optical metallography were used to study structural and magnetic transformations in Ni-Mn-In alloys of different compositions. Temperatures of structural and magnetic transformations of the Ni_{47 − x} Mn_{42 + x} In₁₁ alloys (with x = 0–2) were determined. It is shown that, as the valence electron concentration decreases, the martensitic transformation temperature decreases. Some of the Ni-Mn-In alloys exhibit a ferromagnetic transformation in both the initial austenite and in the arising martensite.     

Principles for selecting a system of construction machines and evaluating their productivity

Conclusions  

Study of light absorption in n-type and p-type GaInAs and the possibility of making 1.55-μm GaInAs/InP Bragg mirrors

Effects of impurity doping on optical properties of epitaxially grown Ga_0.47In_0.53As semiconductors lattice-matched to InP have been studied. The absorption coefficient decreased and the photoluminescent peak energy increased at very high doping levels, due to a Burstein–Moss shift. Absorption in n-type Ga_0.47In_0.53As was negligible at a wavelength of 1.55 m when the donor (Si) concentration was higher than 7.7 × 10¹⁸cm^–3, but was higher than 4000 cm^–1 at = 1.3 m. Absorption in p-type Ga_0.47In_0.53As was 2600 cm^–1 and 7000 cm^–1 at 1.55 and 1.3 m, respectively, when the acceptor (Be) concentration was 3.4 × 10¹⁹cm^–3. Emission from n-type Ga_0.47In_0.53As experienced a maximum shift of 170 meV towards higher energies relative to emission from undoped Ga_0.47In_0.53As, while emission from p-type Ga_0.47In_0.53As shifted 45 meV. The Burstein–Moss effect was used in growth of 1.55-m distributed Bragg mirrors having 20 pairs of quarter-wave n–Ga_0.47In_0.53As/InP layers. Reflectivity greater than 95% and a stop-band width of 60 nm were demonstrated.     

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T ₁ ^–1) and the echo-decay rate (T ₂ ^–1) of ⁶³Cu were measured for the electron-doped infinite-layer superconductor Sr_0.93La_0.07CuO₂/T _c ^onset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-T _c material. Above T _c the ⁶³Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the ⁶³Cu Knight shift in the electron-doped CuO₂ layer can be understood as a compensation effect between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4p_z) and/or Cu(3d_z ²) orbitals.     

Semi-insulating GaN:C epilayers grown by metalorganic vapor phase epitaxy using propane as a carbon source

The influence of propane present in a reactor at various stages of GaN growth by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates on the character of epitaxial process and the properties of epilayers has been studied. Doped GaN epilayers with carbon concentration 5 × 10¹⁸ cm^–3 characterized by high crystalline perfection, an atomically smooth surface, and electric breakdown voltage above 500 V at a doped layer thickness of 4 μm have been obtained.