The melting front thickness of ice has been estimated from the measured kinetics of the melting of ice balls in air at room temperature (≈22°C) and in cooled water ( ≲ 1^∘C ) taking into account the temperature of the ice ball surface and the bulk temperature inside the balls. In both cases, the input heat is absorbed by ice in the form of the latent heat of fusion within the layer called the melting front. In order to describe the kinetics of the ice ball melting, we have developed a model with allowance for the heat transfer through the entire surface of the ice ball, which decreases during melting. The measured temperatures of the ice surface and the temperature inside the balls are approximately 0.4 and 0°C, respectively. The ice melting kinetics in cold water has been determined by means of the continuous measurement of the weight of the submerged ice ball (containing a lead ball frozen inside) as a function of time. In both cases (melting in air and water), the thickness of the ice melting front estimated from the fit the proposed model of experimental data is approximately 3 mm.

The effect of the Si+ ion implantation on the gas-sensing properties of single-crystal (0001) $\alpha $ -Ga2O3 films grown by halide vapor phase epitaxy (HVPE) has been studied. It is established that irradiation with a dose of $8\times 10^{{12}}$ – $8\times 10^{{15}}$ cm $^{-{2}}$ at an energy of 100 keV followed by postimplantation annealing increases the response of $\alpha $ -Ga2O3 films to 3 vol% of H2 by 43 times at 400 °C, reduces the response time by a factor of 6, and expands the operating temperature range down to 30 °C. In addition, $\alpha $ -Ga2O3 layers irradiated with a Si+ ion dose of $8\times 10^{{13}}$ – $8\times 10^{{15}}$ cm $^{-{2}}$ demonstrate high sensitivity to CO and NH3 gases. The mechanism of Si+ ion irradiation effect on the gas-sensing properties of $\alpha $ -Ga2O3 structures is proposed.Index $\alpha $ -Ga2O3, gas sensitivity, halide vapor phase epitaxy (HVPE), ion implantation.

Variation in the parameters of the positron annihilation lifetime spectra in pure water presubjected to freeze–thaw processes is studied. It is shown that in freshly defrosted water (at 1–2°С), the ortho-positronium lifetime and formation probability tend to the corresponding values in ice (near its melting point), and after about 10 h they reach the values corresponding to ordinary water at the same temperature. It indicates that no complete (at molecular level) defragmentation of the ice lattice occurs immediately after ice melting, and rigid ice-like clusters, in which positronium formation is also possible remain in the liquid phase for several hours.

The electrical conductivity of pseudohexagonal ε(κ)‐Ga 2 O 3 films under different ambient gases (H 2 , NO 2 , O 2 , and CO) is studied in a range of temperatures from 400 to 550 °C. The exposure of ε(κ)‐Ga 2 O 3 to reducing gases such as H 2 and CO results in a reversible increase in current and conductance. The exposure to the oxidizing gases such as NO 2 and O 2 has the opposite effect. The maximum response to reducing gases (H 2 and CO) is observed at 500 °C and to oxidizing gases at 550 and 450 °C for NO 2 and O 2 , respectively. The highest sensitivity to H 2 is achieved at low applied voltages (≤7.9 V). In contrast, the highest sensitivity to NO 2 is observed at high applied voltages. The response and recovery times and temporal drift of ε(κ)‐Ga 2 O 3 characteristics under different ambient are estimated. Polycrystalline ε(κ)‐Ga 2 O 3 exhibits the semiconducting mechanism of electron transport at high temperatures. A qualitative model of the gas‐sensing effect based on the modulation of electron concentration near the surface region of ε(κ)‐Ga 2 O 3 due to the chemisorption of gas molecules is described. Tin doping of ε(κ)‐Ga 2 O 3 increases the response to H 2 at the temperature range from 25 to 550 °C.

We report on growth and electrical properties of α-Ga2O3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr2O3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr2O3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr2O3 bandgap and a sharp MCL line near 700 nm due to the Cr+ intracenter transition. Ohmic contacts to Cr2O3 were made with both Ti/Au or Ni, producing linear current–voltage (I–V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2-μm-thick α-Ga2O3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at EC of 0.3 eV. The I–V and capacitance–voltage (C–V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr2O3 buffer indicated the presence of two face-to-face junctions, one between n-Ga2O3 and p-Cr2O3, the other due to the Ni Schottky diode with n-Ga2O3. The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga2O3 system.

Chemical carcinogens are known to be one of the main causes of cancer. The review considers the biological and physicochemical methods for determining the carcinogenic activity of substances. The physicochemical methods are based on the fact that the majority of carcinogens are effective scavengers of electrons generated by ionizing particles as they pass through a substance that simulates the intracellular medium. It was shown that if a chemical compound completely inhibits the formation of a positronium atom, this can serve as an indication of its carcinogenic properties. The positron method is similar to the radiation-chemical test proposed by G. Bakale (he used nanosecond pulsed radiolysis setup), but is simpler, more rapid, and efficient.

A spin torque nano-oscillator in the form of a three-layer magnetic tunneling junction of small diameter (120 nm), where the magnetizations in both magnetic layers are in vortex state, is considered. The effect of the thickness of a nonmagnetic layer on the coupled dynamics of two magnetic vortices in a spin torque nano-oscillator has been studied. The thick permalloy magnetic layer has a thickness of 15 nm, the middle non-magnetic layer has a thickness in the first case of 12.5 and in the second 15 nm, and the thin permalloy magnetic layer has a thickness of 4 nm. Numerical calculation of the dynamics of magnetostatically coupled vortices was carried out using the software package SpinPM for micromagnetic modeling. The features of the vortex motion dynamic are studied for different thicknesses of the nonmagnetic interlayer. It is shown that in all cases of thickness of the nonmagnetic interlayer, three regimes of vortex dynamics are observed: the oscillations of magnetic vortices damped over time, the mode of stationary coupled oscillations of magnetic vortices, and regime, when vortices “leave” the edge of the disk. It is found that increasing in the thickness of the nonmagnetic layer leads to decreasing in the values of the first, second, and third critical currents.

Heavily Sn-doped films of α -Ga 2 O 3 were grown by halide vapor phase epitaxy (HVPE) on basal plane c-sapphire and on (10-12) r-sapphire substrates with and without α -Cr 2 O 3 thin buffers prepared by magnetron sputtering and annealing in air at 500 °C for 3 h. For both substrate orientations, the use of α -Cr 2 O 3 buffers led to three major effects. The first was a substantial decrease of the half-width of the symmetric and asymmetric x-ray reflections. The second was an order of magnitude decrease of the net donor concentration produced by flowing the same amounts of Sn into the reactor. Third, there was a reduction in the concentration of the major electron trap in the films near E c − 1 eV by more than a factor of two. These results show the major influence of sapphire substrate orientation on the electrical and structural properties of α -Ga 2 O 3 grown by HVPE.

Films of α-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 1015 cm−2 and with 7 MeV C4+ ions with a fluence of 1.3 × 1013 cm−2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at Ec-0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies VGa–VO, and oxygen vacancies VO. Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of VGa and VGa–VO.

We investigate coupled dynamics of two-vortex spin torque nanooscilators using micromagnetic simulation. Dynamic transformation of the vortex structure leading to formation of the C-states in this system has been studied. We demonstrate dynamic formation of the edge vortices, which until now have been observed only in the presence of the Dzyaloshinskii field or in the presence of an inhomogeneous external field.

The growth of cubic and rhombohedral In 2 O 3 and (InGa) 2 O 3 epitaxial films by halide vapor phase epitaxy (HVPE) is reported. The deposition is carried out at 625 °C using indium trichloride (InCl 3 ), gallium monochloride (GaCl), and O 2 precursors on (0001) sapphire substrates, HVPE‐grown Ga 2 O 3 /Al 2 O 3 and metal‐organic chemical vapor deposition (MOCVD)‐grown GaN/Al 2 O 3 templates. The HVPE growth of phase pure In 2 O 3 and (InGa) 2 O 3 with corundum structure is reported for the first time.

Films of Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al2O3 deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga2O3/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al2O3 interlayer was mainly composed of (−201) β-Ga2O3 with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 1015 cm−3, with deep electron traps spectra consisting of the well documented centers for β-Ga2O3 near Ec 0.27, Ec 0.7, and Ec 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga2O3 films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).

The structure and magnetization dynamics are investigated in a vortex spin-transfer nanooscillator, which is a three-layer spin-valve magnetic nanocolumn with a large diameter of 400 nm, under flowing spin-polarized current through it. The dynamic variation in the structure of vortices and their trajectory of motion are studied, using micromagnetic modeling, as a function of the value of the spin-polarized current. It is demonstrated that different modes of motion of vortices may exist: decaying oscillations of vortices, stationary oscillations of vortices, and the mode of switching polarity of one of vortices. The time for which different dynamic modes are settled is determined. The dependence of the oscillation frequency on the value of spin-polarized current is determined for the case of stationary dynamics of coupled vortices. It is shown that, at large values of current, switching of vortex polarity is only possible in a thick layer with the dynamic switching mechanism accompanied by generation of a vortex–antivortex pair. The software package for micromagnetic simulation, SpinPM, is used for numerical calculations of the dynamics of magnetic vortices.

The joint effect of the spin polarized current and an external magnetic field on the dynamics of magnetization in vortex spin-transfer nano-oscillators with a diameter of 400 nm is investigated. For the numerical calculation of the coupled dynamics of magnetic vortices, the SpinPM software package for micromagnetic modeling was used. The dependence of the frequency of stationary coupled oscillations of vortices on the magnitude of the magnetic field, which determines the operating frequency range of a tunable vortex spin-transfer nano-oscillator.

The joint effect of a spin-polarized current and an external magnetic field on the magnetization dynamics in vortex spin-transfer nano-oscillators 400 nm in diameter has been investigated. The coupled dynamics of magnetic vortices has been calculated numerically using the SpinPM software package for micromagnetic simulation. The dependence of the frequency of stationary coupled vortex oscillations on the magnetic field, which determines the operating frequency range of a tunable vortex spin-transfer nano-oscillator, is found.

DOI: 10.17725/rensit.2020.12.107 Dissolved oxygen and positronium atom in liquid media Sergey V. Stepanov, Alexander V. Bokov, OlgaV. Ilyukhina, Vsevolod M. Byakov National Research Centre “Kurchatov Institute'' ITEP, http://www.itep.ru/ Moscow 117218, Russian Federation E-mail: stepanov@itep.ru, av.bokov@yandex.ru, ilyukhina@itep.ru, byakov@itep.ru Received February 06, 2020; peer reviewed February 14, 2020; accepted February 18, 2020 Abstract. The interaction of positronium (Ps) with molecular oxygen dissolved in liquids (isooctane, cyclohexane, isopropanol and water) is experimentally investigated. For variation of the O2 concentration the liquids were bubbled with Ar, air and O2. For interpretation of the lifetime annihilation spectra a simple deconvolution of the spectra into three exponentials decaying in time was used. These exponents correspond to para-positronium, free positron and ortho-positronium annihilations. The total reaction rate constant of the Ps atom with dissolved oxygen (which is the sum of two ortho-para-conversion (Ps → 1⁄4 p-Ps + 3⁄4 o-Ps) and oxidation (Ps + O2 → e + + O2–) reaction rate constants) is obtained for each liquid.

The interaction of positronium (Ps) with molecular oxygen dissolved in liquids is experimentally investigated. Computer software has been developed for fitting the positron annihilation lifetime spectra in liquids using parameters with clear physical meaning: rate constants of the Ps chemical reactions, annihilation rate constants of the different positron states, probability of Ps formation in a quasi-free state, typical formation time of a Ps nanobubble. Such processing of the spectra allowed identification of the dominant interaction of the Ps atom with dissolved oxygen. It turns out to be mainly ortho-para-conversion (Ps → 1/4 p-Ps + 3/4 o-Ps), but not oxidation (Ps + O2 → e+ + O2-). Values of the reaction rate constants are obtained.

The changes of short range ordering and electron density were investigated by means of the nuclear gamma-resonance and the positron annihilation spectroscopies in model alloys containing tungsten, chromium, molybdenum, and vanadium used as dopants. The change of the short-range ordering parameter sign was detected in alloys containing vanadium. Different ordering was also observed in binary and ternary iron alloys. It was shown that dislocations were the main defects in these materials after rolling.