Journal of V. N. Karazin Kharkiv National University. Series Physics

CONDUCTIVITY ANISOTROPY AND METAL-INSULATOR TRANSITION OF Y1-zPrzBa2Cu3O7–δ SINGLE CRYSTALS WITH VARIOUS PRASEODYMIUM CONTENT (review)

Wed, 26 Nov 2025 00:00:00 +0000

The paper analyzes the results of studies of the influence of praseodymium on the anisotropy of the electrical resistance ρ_с(Т)/ρ_ab(Т) of high-temperature superconductors Y_1-zPr_zBa₂Cu₃O_7–δ. The influence of Pr impurities on the anisotropy of conductivity in this system is important both for clarifying the nature of high-temperature superconductivity (HTSC) and for increasing its critical parameters. The temperature dependences ρ_ab(Т) and ρ_с(Т) in the normal state have linear sections at high temperatures. An increase in the praseodymium content, z, leads to a semiconductor shape of the ρ_с(Т) curves, but the ρ_ab(Т) curves retain a linear section longer. A downward deviation from the linear course with decreasing temperature indicates the appearance of excess conductivity. The critical temperatures, T_c, along and across the basal plane are different, which is associated with the Friedel transition in the 1-2-3 system, which consists in the suppression of transverse superconductivity by the growth of ring Josephson vortices in a layered superconductor. This is possible when the conducting subsystem breaks down into several regions with different T_c. The breakup occurs during structural disordering due to doping of HTSC cuprates with an inovalent impurity or changing the oxygen content, and leads to the emergence of electroneutral regions in the system – metallic and dielectric. In this case, the conductivity of the sample is jumpy with a variable jump length, and the resistance anisotropy is well described by the universal "law 1/2". Praseodymium-induced clustering of the sample can initiate a metal-dielectric transition of the "Anderson" type. The curves ρ_ab(Т) and ρ_с(Т) can be approximated over the entire temperature range, taking into account the metallic, semiconducting, and fluctuation conductivity. This allows us to determine the temperature dependences of the anisotropy of the charge carrier scattering mechanisms at different z. An increase in z causes a metal-dielectric transition, which always precedes the superconducting transition, and a decrease in the transition temperature. The difference in the conduction mechanisms along and across the layers emphasizes the difference between HTSC cuprates and Fermi-liquid metals.

SUPERPLASTICITY OF MEDIUM STRENGTH DEFORMABLE ALUMINUM ALLOY 1201

V. P. Poyda, A. V. Poyda — Wed, 26 Nov 2025 00:00:00 +0000

The article presents the results of an experimental study of the superplasticity of medium-strength deformable aluminum alloy 1201 of the Al-Cu-Mn system, which is used for the manufacture of welded containers, cylinders and other constructions intended for operation at low (up to – 269 °C), room and elevated temperatures. The temperature-strain rate conditions under which samples of alloy 1201, deformed in the creep mode at high homologous temperatures, exhibit the effect of structural superplasticity have been determined, and the structural changes that occur in the working parts of samples of this alloy during superplastic deformation have been investigated. The influence of the phase composition on the structural state of the samples and on the stability of their grain structure at high homologous temperatures was analyzed. The average grain size in samples of alloy 1201 prepared for mechanical testing is 20 μm. This indicates that under industrial production conditions, the entire volume of deformed semi-finished products of alloy 1201, from which samples for mechanical testing were made, underwent almost complete recrystallization, the purpose of which was the formation of a fine-grained grain structure. During deformation of samples at high homologous temperatures, there is no increase in the average grain size. It has been established that during superplastic deformation in samples of alloy 1201, intensive grain boundary sliding occurs, and grain boundary cavities also nucleate, grow, and coalesce. Their origin is probably associated with the relaxation of local stresses that arise during grain boundary sliding at triple grain junctions, as well as near clusters of intermetallic particles localized at grain boundaries. During superplastic deformation, grain boundary cavities likely create the opportunity for grains to perform intensive movements in the working part of the sample by grain boundary sliding. The accumulation of grain boundary cavities during superplastic deformation in the working part of the samples and their merging into magistral cracks leads to the failure of the samples, which occurs without neck formation.

THE EFFECT OF PRESSURE ON THE SCATTERING OF NORMAL AND FLUCTUATION CARRIERS IN Y0.66Pr0.34Ba2Cu3O7-δ SINGLE CRYSTALS

G. Ya. Khadzhai, V. Yu. Gres', M. V. Korobkov, V. F. Korshak, R. V. Vovk — Wed, 26 Nov 2025 00:00:00 +0000

The experimentally obtained temperature dependences of the electrical resistance, ρ(T), of the single-crystal high-temperature superconductor (HTSC) Y_0.66Pr_0.34Ba₂Cu₃O_7–δ in the normal state (T* ≤ T ≤ 300 K) at different values of quasi-hydrostatic pressure (0 ≤ P ≤ 1 GPa) are approximated by the Bloch–Grüneisen relation, which takes into account the scattering of charge carriers on phonons and defects. The temperature derivative of the resistance, dρ(Т)/dT, in the normal state passes through a maximum (T_max ≈ θ/3, θ is the Debye temperature), which fully corresponds to the model used. The presence of a high-temperature maximum dρ(Т)/dT indicates the absence of a linear dependence of the electrical resistance in the studied sample at least in the region T ≲ 300 K. The superconducting transition leads to the appearance of a low-temperature maximum at T = T_c. The temperature of the minimum between these maxima is identified with the pseudogap opening temperature T*. The latter increases with increasing hydrostatic pressure. Extrapolation of the normal conductivity (in the Bloch–Grüneisen model) to the temperature range T_c < T ≲ T* allows us to calculate the fluctuation conductivity. The fluctuation conductivity, obtained as the difference between the experimental resistance values and the values extrapolated using the Bloch – Grüneisen relation, is described with good accuracy by the Lorentz – Doniach relation, taking into account the inhomogeneity of the sample. The baric dependences of the parameters of the Lorentz – Doniach model show that hydrostatic pressure contributes to the improvement of the sample structure. The pressure-induced evolution of the fluctuation conductivity in Pr-doped Y_0.66Pr_0.34Ba₂Cu₃O_7–δ single crystals can be determined by two circumstances: an increase in the “three-dimensionality” of the system due to a change in the ratio between the coherence length ξ_c and the interlayer distance d and a shift of the Fermi level relative to the features of the density of electronic states. In contrast to undoped (or lightly Pr-doped) YBCO samples, the application of high pressure leads to a significant increase in the baric derivatives, dT_c/dP and dξ_c/dP.

FLEXIBLE THERMOELECTRIC MATERIALS WITH COPPER IODIDE ON TEXTILE DIELECTRIC SUBSTRATES FOR POWERING WEARABLE ELECTRONICS

Wed, 26 Nov 2025 00:00:00 +0000

The paper analyses the results of research into the thermoelectric properties of copper iodide layers deposited on flexible dielectric substrates made from various fabric materials. The main object of study was film systems based on copper iodide obtained by the SILAR chemical synthesis method. Such deposition occurs from aqueous solutions at atmospheric pressure, which ensures high technological efficiency of the method. Samples deposited by the SILAR method were studied using scanning electron microscopy, characteristic X-ray radiation analysis, UV-VIS spectroscopy, X-ray structural and thermoelectric studies. It has been shown that precipitation from aqueous solutions allows the production of copper γ-iodide films with internal nanostructure elements. In such samples, the band gap width typical for iodide is observed. Its confirming the production of CuI semiconductor layers. Separately, it is shown that the proposed method of sample synthesis provides the creation of a composite consisting of a fabric base, a layer of nanocellulose, and copper iodide itself. The high efficiency of the created structures is demonstrated, which also retain their flexible properties. Issues of mechanical stability of film systems and the development of methods to increase their resistance to bending and abrasion are analysed. Methods for creating stable functional layers on the surface of cotton and polyester fibres are considered, and the possibility of using nanocellulose hydrogel to prevent CuI cracking during actual operation is established. The maximum specific thermoelectric power output of the created nanocomposite samples is 15.6 μW/cm2. This value is one of the best among modern solid-state miniature, flexible, and textile thermoelectric materials.

NEW EXPERIMENTAL APPROACHES TO STUDYING THE COMBUSTION OF WATER-FUEL EMULSION DROPLETS

M. O. Ivanov, O. S. Chernenko — Wed, 26 Nov 2025 00:00:00 +0000

The based on a review of existing methods for studying droplet combustion, it is concluded that the combustion of emulsions is accompanied by the phenomenon of micro-explosions: the division of the primary droplet into secondary droplets. Depending on the intensity of heat input, both a small material ejection and complete droplet destruction can occur. The use of emulsions is important in reducing nitrogen oxide emissions and increasing the combustion efficiency of liquid fuels. The aim of this work is to review the authors' methods for studying the combustion of emulsion droplets, which have several advantages.

A new method for rapid analysis of the combustion rate of a liquid is described in the porous ball method. A ball soaked in a flammable liquid is placed on a special stand on a balance. The mass of the ball is measured over time, and a linear section of the mass vs. time curve defines the combustion rate of the liquid. Compared to existing methods, the advantage of this method is its speed, repeatability, low liquid consumption, and the ability to quickly compare different compositions, including emulsions.

An original method involves the use of high-frequency electrical discharge as an electric trigger for micro-explosion initiation. Even a small thermal power of the streamer is sufficient to observe the pulsations of the droplet diameter over time. The behavior of an emulsion droplet of water-diesel fuel (50%/50%) was studied, where water has a significantly lower boiling point than diesel fuel. The streamer passing near the droplet on a thermocouple does not fully heat the entire droplet initially. As a result, a steam bubble forms near the thermocouple joint, which expands, moves away from the joint, and bursts near the droplet's outer surface, leading to the emission of emulsion material (pulse period of 10–25 ms). As the entire droplet heats up, multiple bubbles expand and burst simultaneously, resulting in more mini-pulsations (2–3 ms).

In the stationary droplet method, continuous emulsion feeding through a needle to a porous metal particle is implemented. The visualization of micro-explosion formation on the droplet surface during combustion is demonstrated. The possibility of using high-frequency electrical discharge in the combustion of emulsions with a high water content (up to 60%) is also indicated.

MANIFESTATIONS OF VORTEX BEHAVIOR OF FLUIDS IN VARIOUS PHYSICAL EXPERIMENTS

A. O. Belova, V. I. Lymar, Ye. D. Makovetskyi, Yu. S. Malyi — Wed, 26 Nov 2025 00:00:00 +0000

The paper presents a comprehensive examination of experimental manifestations of vortex behaviour in fluids, focusing on phenomena generated by physical mechanisms of fundamentally different nature. Through a systematic analysis of diverse hydrodynamic systems, this work demonstrates that nonlinear vortical dynamics represents an inherent and fundamental element of fluid motion across multiple spatial and temporal scales. The study employs a range of illustrative examples, beginning with Albert Einstein's classical "small experiment" and the formation and evolution of meanders in lowland river channels. These seemingly simple phenomena are shown to be governed by the same underlying principles that drive more complex hydrodynamic instabilities. Particular attention is devoted to the development of Rayleigh-Taylor-type vortical instabilities, which are characterized by the mutual "overturning" of heavy and light fluid components in a flowing medium. The paper explores these instabilities through experimental configurations, including thermal convection processes and the formation of "underwater crater" structures in granular materials settling through liquids. A central theoretical framework is established through the identification of common universal features of vortical excitations, all of which are fundamentally linked to the classical Helmholtz vortices in ideal fluid dynamics. The authors demonstrate that despite the diversity of mechanisms generating vortex behavior in real fluids, and the wide range of conditions under which such behavior manifests, there exist universal properties stemming from their connection to Helmholtz vortices. This unifying approach contributes significantly to the formation of a coherent theoretical framework capable of describing a remarkably broad spectrum of physical phenomena observed both in nature and in controlled laboratory experiments, from microscale optical self-defocusing patterns to astronomical structures like the Crab Nebula.