Volume 42, Nº 6 (2016)

It has been found that the pulsed laser deposition of a thin tungsten selenide film, followed by thermal treatment at 550°C in an Ar + O₂ mixture of gases, results in the formation of a hybrid structure that is made up of ultrathin WSe₂ and WO_3–y platelets. The structural and size characteristics of the nanoplatelets deposited on microcrystalline graphite provide the effective hydrogen evolution reaction in a 0.5 M H₂SO₄ solution, with the cathode current made about seven times higher at a potential of–100 mV and the slope of the Tafel characteristic reduced from 340 to 90 mV/dec.

Results of an experimental study of the noise temperature (T_n) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (T_b) are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (T_c) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature T_b close to T_c was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at T_b ≪ T_c for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/ metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.

The influence of the solder layer between a semiconductor solar cell and heat-removing ceramics on the nonstationary heat-transfer processes has been investigated by the laser thermal-wave method. A theoretical model taking into account the presence of additional thermal resistance and thermal capacitance at the soldered junction is proposed. Different soldering modes are considered. It is shown that the laser thermal- wave methods within the developed model allow one to correctly estimate the thermophysical properties of multilayer structures.

(Ba_0.5Sr_0.5)Nb₂O₆ films were synthesized on a Pt(111)/Si(001) substrate by RF gas-discharge sputtering in pure oxygen atmosphere. It was found that the films have a dominant crystallographic orientation in the [001] direction and natural unipolarity, which was revealed through analysis of dielectric and piezoelectric parameters. It was demonstrated that the optical parameters of film material in the Ba_0.5Sr_0.5, Nb₂O₆/Pt(111)/Si(001) heterostructure match those typical for a (Ba_0.5Sr_0.5)Nb₂O₆ single crystal.

Using thermal desorption time-of-flight mass spectrometry and thermionic methods, it is shown that oxygen does not intercalate under a graphene monolayer grown correctly on iridium, at least at temperatures of T = 300–400 K and exposures below 12000 L. However, if the graphene film on iridium is preliminary intercalated with samarium atoms (up to coverage of θ_Sm = 0.2–0.45), the penetration of oxygen atoms under the graphene film is observed. The oxygen atoms in the intercalated state are chemically bonded to samarium atoms and remain under graphene up to high temperatures (~2150 K).

We investigate the structural and electrical properties of planar superconducting structures based on the YBa₂Cu₃O_7–d (YBCO) epitaxial films obtained by preliminary modification of the substrate surface. A special master mask was formed on the substrates, so that, at the standard YBCO film deposition onto such a substrate, an insulator layer grew in the modified areas and a superconducting film, in the unmodified ones. Thus, the planar superconducting structure of a desired topology was formed, and the YBCO deposition finished the process. Using this technique, YBCO bridges with widths of 4, 10, and 50 μm on films of different thicknesses and a planar inductive coil were formed. The superconducting transition temperature of the bridges was about 90 K, and the critical current density at a temperature of 77 K was up to 3 MA/cm². The Q factor of the planar inductive coil at a frequency of 85 MHz was 53000 at a temperature of 77 K.

We investigate oxygen-deficient anatase using quantum-chemical simulation within the density functional theory and X-ray photoelectron spectroscopy. It is demonstrated that etching of anatase with argon ions with an energy of 2.4 keV results in the formation of oxygen vacancies and polyvacancies at a concentration of approximately 10²⁰ cm^–3 in the crystal. It was found that the most energetically favorable spatial configuration of an oxygen polyvacancy is a three-dimensional chain in crystallographic direction [100] or [010]. The ability of oxygen polyvacancy in the form of a chain to act as a conductive filament and to participate in the resistive switching is discussed.

Suppression of parasitic electron flows and positive ions formed in the beam tract of a tandem accelerator with vacuum insulation allowed a more than threefold increase (from 1.6 to 5 mA) in the current of accelerated 2-MeV protons. Details of the modification are described. Results of experimental investigation of the suppression of secondary charged particles and data on the characteristics of accelerated proton beam with increased current are presented.

We have studied the process of electron-stimulated desorption (ESD) of Cs atoms from a Cs/CsAu/Au/W system. It is established that ESD takes place from a Cs adlayer and the adjacent CsAu layer of this system. A model of Cs atom desorption from the Cs/CsAu/Au/W system is proposed. The results confirm the semiconductor nature of CsAu compounds.

A miniature bandpass filter of new design with original stripline resonators on suspended substrate has been studied. The proposed filters of third to sixth order are distinguished for their high frequency-selective properties and mush smaller size in comparison to analogs. It is shown that a broad stopband extending above three-fold central bandpass frequency is determined by weak coupling of resonators at resonances of the second and third modes. A prototype sixth-order filter with a central frequency of 1 GHz, manufactured on a ceramic substrate with dielectric permittivity ε = 80, has contour dimensions of 36.6 × 4.8 × 0.5 mm³. Parametric synthesis of the filter, based on electrodynamic 3D model simulations, showed quite good agreement with the results of measurements.

It is shown that a solution featuring the Briggs–Rauscher (BR) oscillating chemical reaction can exhibit the properties of a photonic crystal with alternating bandgap width. Thicknesses and dielectric permittivities of structural elements in the BR reaction solution have been determined by measuring the reflection and transmission spectra of microwave radiation in the range of 5–8 GHz.

Polarization-induced p-type doping of AlGaN layers with high aluminum content during plasmaassisted MBE growth has been studied. It is shown that a gradient of the AlN molar fraction in AlGaN (composition gradient) on a level of 0.005 nm^–1 must be set in order to obtain a hole concentration of ~10¹⁸ cm^–3 (measured by the C–V method) in Al_xGa_1–xN:Mg (x = 0.52–0.32) layers with dopant concentration [Mg] = 1.3 × 10¹⁸ cm^–3. p–i–n photodiodes based on AlGaN heterostructures with such layers as p-emitters showed maximum photoresponsitivity in the solar-blind wavelength range (λ = 281 nm) about 35 and 48 mA/W at reverse bias voltage U = 0 and–5 V, respectively, and exhibited a dark current density of 3.9 × 10^–8 A/cm² at U =–5 V.

We have experimentally studied temporal variation of the temperature of gas–vapor mixture in the trace of water droplets moving in the counterflow of high-temperature combustion products. The initial gas temperature was within 500–950 K. The water droplet radius in the aerosol flow varied from 40 to 400 μm. The motion of water droplets in the counterflow of combustion products in a 1-m-high hollow quartz cylinder with an internal diameter of 20 cm was visualized by optical flow imaging techniques (interferometric particle imaging, shadow photography, particle tracking velocimetry, and particle image velocimetry) with the aid of a cross-correlation complex setup. The scale of temperature decrease in the mixture of combustion products and water droplets was determined for a pulsed (within 1 s) and continuous supply of aerosol with various droplet sizes. Retention times of reduced temperature (relative to the initial level) in trace of water droplets (aerosol temperature trace) are determined. A hypothesis concerning factors responsible for the variation of temperature in the trace of droplets moving in the counterflow of combustion products is experimentally verified.

Thermopower, magnetothermopower, resistivity, magnetoresistance, and magnetization of singlecrystalline samples of the Sm_1–xSr_xMnO₃ system (x = 0.15, 0.2, 0.25, 0.3) have been experimentally studied. These compositions consist of ferromagnetic clusters of the ferron type dispersed in the A-type antiferromagnetic matrix. Colossal thermo- and negative magnetothermopower values reaching 94.5% in a composition with x = 0.3 in the region of Curie point T_C in a magnetic field of 1.323 T. This result implies that thermopower is mostly related to ferron-type clusters, since their breakage under the action of magnetic field or heating above T_C leads to a sharp decrease in thermopower values. These results imply that thermopower in doped manganite semiconductors is determined by the concentration of impurity and volume of a sample.

The electrical properties of dispersions of a powdered ferroelectric nanocomposite based on ilmenite (FeTiO₃) and hollandite (K_1.46Ti_7.2Fe_0.8O₁₆) in dioctyl phthalate have been studied by impedance spectroscopy techniques in a frequency range of 10^–1–10⁶ Hz. The influence of stabilizing additives of cationic and anionic surfactants and iron acetylacetonate on the permittivity, conductivity, and dielectric losses was determined for dispersions containing 40 mass % of the solid composite. The influence of composition on the mechanisms of relaxation processes in the system is discussed.