No 12 (2024)

The paper investigates a new type of terahertz radiation source based on a molecular crystal of rubidium hydrophthalate (RbAP) and a tunable metamaterial that performs the function of a filter. The high Q-factor of the vibrational response of the RbAP crystal lattice in the terahertz frequency range allows the generation of narrowband terahertz radiation simultaneously at several frequencies with high spectral brightness and peak power. The crystal is excited by single femtosecond laser pulses. Switching between the individual generated spectral lines is realized using a planar metamaterial, the absorption lines of which depend on the polarization of the radiation incident on it. The developed source allows for dynamic restructuring of the spectral line of radiation, which makes it more versatile and efficient compared to traditional narrow-band sources, such as, for example, quantum cascade lasers.

The results of a study of nanoclusters at the interface of a Si₃N₄ film on a Si substrate implanted with ⁶⁴Zn⁺ ions with a dose of 5×10¹⁶/cm² and energy of 40 keV are presented. The Si₃N₄ film was deposited on a silicon substrate using the gas-phase method. Then the implanted samples were annealed in air in steps of 100°C for 1 hour at each step in the temperature range of 400–700°C. The surface morphology of the samples was studied using scanning probe microscopy. The profiles of the implanted impurity and film elements, as well as the chemical state of the Zn ion, were studied using X-ray photoelectron and Auger electron spectroscopy. The shock pulse method revealed that after implantation, individual metallic zinc nanoclusters with a size of about 100 nm or less were detected near the surface of the Si₃N₄ film. During the annealing process, they grow with simultaneous transformation into the ZnSiN₂ phase and, possibly, into the phases of zinc oxide and silicide near the surface. After annealing at a temperature of 700°C, Zn-containing nanoclusters with a size of about 100 nm are formed in the Si₃N₄ film.

The surface physicochemical properties of the polylactic acid modified by flows of low-temperature glow discharge plasma were studied. A mixture of argon and air acted as the plasma-forming gas; diethylamine vapor was injected into the plasma as a precursor of amino groups. The elemental composition and chemical state of the surface were studied using X-ray photoelectron spectroscopy. The attachment of nitrogen atoms to the polylactic acid surface and the formation of a bond between the surface carbon and the penetrated nitrogen have been established. It was shown that the hydrophilicity of the plasma modified polylactic acid surface was significantly increased. The obtained polylactic acid-based materials with the argon/air/diethylamine plasma modified surface may have prospects for use in biomedicine due to improved hydrophilicity and the presence of reactive oxygen- and nitrogen-containing functional groups on the surface.

A comparative analysis of radiation defect formation in the gallium and nitrogen lattices of gallium nitride under irradiation with 15 MeV protons and 0.9 MeV electrons was performed. For proton deceleration, numerical simulations were carried out using the SRIM program, and for electrons, analytical calculations were carried out. Under proton irradiation, the total vacancy generation rate in the gallium lattice η_FP(Ga) was shown to be ~560 cm^–1, and in the nitrogen lattice η_FP(N) ~1340 cm^–1. Detailed numerical calculations in the Full Cascade mode showed that in the gallium lattice, the vacancy formation rate due to protons was 110 cm^–1, and due to cascade processes was 450 cm^–1. In the nitrogen lattice, this “disproportion” looked even stronger (60 and 1280 cm^–1, respectively). Under electron irradiation, the vacancy generation rate in the gallium lattice η_FP(Ga) was ~4.7 cm^–1, and in the nitrogen lattice η_FP(N) ~2.0 cm^–1. To experimentally study radiation defects in n-GaN, which create deep levels and compensate for the conductivity of the material, direct current–voltage characteristics of Schottky diodes based on n-GaN were measured. It was shown that the rates of charge carrier removal in n-GaN were 0.47 cm^–1 under electron irradiation and 150 cm^–1 under proton irradiation. Comparison of the calculated and experimental parameters of radiation defect formation allows us to draw a conclusion about the mechanism of the compensation process and the radiation defects responsible for this process.

Using the original magnetocapacitance technique based on simultaneous measurements of magnetocapacitances between a quasi-two-dimensional electron system in a single GaAs quantum well and two gates placed on its opposite sides we have studied magnetic field induced quantum phase transitions between double-layer and single-layer-like states of the system. The measurements have been performed with samples of quantum well width 50 and 60 nm. The double-layer state was composed of layers of two-dimensional electrons confined near the opposite walls of the quantum well. It is characterized by the quantum magneto-oscillations of the compressibility of each of the layers with a frequency determined by the density of electrons in the corresponding layer. In a single-layer-like state, the compressibility minima have been observed only when all electrons occupied one or two spin sublevels of the lowest Landau level (when the total filling factor ν_tot = 1 and 2), and the ratio of the measured capacitances in this state was characteristic of the case when only one electronic layer existed between the gates. It has been found that the first transition from a double-layer to a single-layer-like state took place when the quantum limit was reached, i.e. when ν_tot ≈ 2, independent of either the density of electrons in the system or the quantum well width. A different behavior of electronic systems has been found in wells of different widths in the region 1 < ν_tot < 2. In a 50 nm well, the single-layer-like state existed in the whole studied region of filling factors ν_tot ≤ 2. In a 60 nm well, a double-layer region has been observed within 1 < ν_tot < 2 accompanied by an incompressible state of electrons in the layer with the largest density at filling factor unity in this layer. As a result, three magnetic-filed-induced quantum phase transitions have been observed in samples with 60 nm quantum well width, whereas only one quantum phase transition has been observed in a sample with 50 nm quantum well width. Such a dependence of the character of the quantum phase transition on the quantum well width is supposedly caused by the different tunneling strength between the layers. The formation of magnetic-field-induced compressible single-layer-like state in a nominally double-layer electronic system has been discovered.

Two types of reflectometers have been simulated for the compact neutron source DARIA (Dedicated for Academical Research and Industrial Application), depending on the type of target assembly with thermal or cryogenic moderators. Modeling and optimization of reflectometers were carried out using McStas software package by Monte–Carlo method with given momentum transfer resolution Δq/q ≤ 5% for reflection angles greater than the θ_cr critical angle and horizontal divergence of the neutron beam of Δθ ≤ 0.1° for θ < θ_cr and Δθ ≤ 0.033° for θ > θ_cr. To reduce losses in neutrons, neutron guides with a supermirror coating have been proposed. A system of choppers makes it possible to create a neutron spectrum of the required width on a sample.