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

STRUCTURAL RELAXATION AND DIFFUSION OF THE LABILE COMPONENT IN NON-STOCHIOMETRICAL CUPRATES ReBa2Cu3O7-Δ (Re = Y, HO) AND ASSOCIATED OXIDES (overview)

L. O. Pashchenko — 2024-05-30

The paper investigates the effect of annealing at room temperature on the electrical resistance in the ab-plane ρab(T) of ReBa₂Cu₃O₇_-δ (Re = Y, Ho) single crystals with oxygen deficiency. It was found that the reduction of oxygen content induces phase segregation, accompanied by the diffusion of a labile component and structural relaxation in the sample volume, confirming the critical role of oxygen deficiency in determining the structural and electrophysical properties of these materials.

Annealing at room temperature with varying oxygen deficiencies results in an expansion of the linear range of ρab(T) and a narrowing of the temperature region where the pseudogap regime occurs, highlighting the importance of oxygen concentration in forming the electronic structure. Excess conductivity follows an exponential temperature dependence over a broad temperature range, indicating the presence of nonlinear effects. The pseudogap temperature dependence is well described within the BKD-BEK crossover theory, confirming a transition between two modes of material behavior.

Replacing yttrium with holmium (Re = Ho) affects the charge distribution in the CuO planes, leading to disorder in the oxygen subsystem, which alters the electronic structure, manifesting as shifts in temperature regions corresponding to metal-insulator transitions and pseudogap anomaly. Thus, the results demonstrate that controlling the oxygen content and chemical composition of ReBa₂Cu₃O₇-δ crystals is an effective method for managing their electrophysical properties. The obtained short-range interatomic potential values can also be used for systematic studies of the complex behavior and defect chemistry of these materials at the atomic level. The use of "atomic scale techniques" allows not only for improving the physicochemical parameters of various rare earth oxides (REO) compounds but also for validating the adequacy of numerous current theoretical studies.

PECULARITIES OF HIRST EXPONENT ESTIMATION FOR NATURAL PHYSICAL PROCESSES

O. V. Lazorenko — 2024-05-30

In bounds of the non-linear and system paradigms, been formulated by L. F. Chernogor in the last 1980th, all processes in open, non-linear, dynamical systems are very complex, non-linear, ultra-wideband or fractal ones.

According to the fractal paradigm put forward in the early 2000s by V. V. Yanovsky, fractality is one of the fundamental properties of the surrounding world. Therefore, the study of fractal characteristics, in particular, of natural physical processes is actual, interesting and useful.

The fractal dimension based on the Hurst exponent is one of the oldest and most famous ones. Based on the study of model fractal signals, it is demonstrated that the dependence between the estimate of the Hurst fractal dimension, obtained by the normalized range method, and its true value is significantly non-linear. To decrease of influence of the errors arising as a result of this, it is proposed to use the method of the corrective function.

The practical effectiveness of the proposed method is demonstrated on the example of the analysis of experimental results obtained in the middle 1960s by H. E. Hurst, which discovered the presence of a somewhat strange grouping of the values of the Hurst fractal dimension around the value of 1.27 for various natural physical processes. A hypothesis about the possibility of explaining this fact precisely by the nonlinearity of the mentioned dependence for R/S-method was proposed.

EVOLUTION OF THE TEMPERATURE DEPENDENCES OF ELECTRICAL RESISTANCE OF Y1-XPrXBa2Cu3O7-δ SINGLE CRYSTALS IN A WIDE RANGE OF APPLIED PRESSURE AND CONCENTRATIONS OF PRASEODIUM IMPURITIES

G. Ya. Hadzhai — 2024-05-30

The effect of high hydrostatic pressure 0 ≤ Р ≤ 17 kbar on the electrical resistance ρ(Т) in the basic ab-plane of praseodymium-doped single crystals of the Y_1–xPr_xBa₂Cu₃O_7–δ system at 0 ≤ х ≤ 0.34 was investigated in the paper. Single crystals of compounds Y_1–xPr_xBa₂Cu₃O_7–δ were grown by solution-melt technology in a gold crucible. Hydrostatic pressure was created in a piston-cylinder multiplier. The pressure was determined with a manganin manometer, and the temperature – with a copper-constantane thermocouple.

It was established that an increase in pressure leads to an increase in the critical temperature of the studied compounds and a decrease in their electrical resistance, which corresponds to literature data for polycrystalline samples. Near the superconducting transition, the appearance of the temperature derivatives of the electrical resistance, dr/dT, of the Y_1–xPr_xBa₂Cu₃O_7–δ compounds indicates a certain heterogeneity of the samples, i.e., the presence of areas in the samples with different temperatures of the superconducting transition, T_с, which transition to the superconducting state as T_с is reached. At intermediate concentrations of praseodymium, such regions pass into the superconducting state sequentially, but at x = 0 and x = 0.34, the form dr/dT indicates the presence of percolation paths for the flow of the transport current. The value of T_c and literature data on the Debye temperature indicate that the McMillan formula can be applied only to compounds Y_1–xPr_xBa₂Cu₃O_7–δ that have T_c < 65 K (that is, for x, d > 0.3), and at the same time gives a very high values of the electron-phonon interaction constant, l ~ 1. It was found that, in contrast to pure YBa₂Cu₃O_7–δ samples with optimal oxygen content, the application of high pressure leads to a multiple increase in the value of the baric derivative dT_c/dP. It was established that within the limits of experimentally achieved pressures, there was no change in the sign of baric derivatives dT_c/dP with increasing pressure, which was observed on polycrystalline samples with close values of praseodymium concentration. The possible mechanisms of the effect of high pressure on the critical temperature of the transition to the superconducting state T_c are discussed, taking into account the features in the electronic spectrum of carriers.

PHOTON IRRADIATION AND “HIGH-TEMPERATURE” ELECTRICAL SUPERCONDUCTIVITY OF POLYCOMPONENT METAL-OXIDE COMPOUNDS

Yu. I. Boyko — 2024-05-30

The paper discusses the possibility of increasing the transition temperature of metal-oxide compounds ("high-temperature" superconductors) to the superconducting state (zero electrical resistance) when irradiating them with a sufficiently powerful photon flow. In this case, in the irradiated substance, as a result of internal photo-ionization, changes occur in both the important parameters of the phonon spectrum and the electrons energy spectrum.

The value of the photon energy must satisfy this condition: hν ≥ W (ν is the photon frequency, W is the energy of the photoinduced chemical reaction. According to the estimates made in the work, the minimum wavelength of photons that can realize the described effect should be characterized by the value l≈10^–4m. Photons of this wavelength correspond to infrared light radiation, a flow of sufficient power of which is easily achieved by laser technology.

A change in the parameters of the phonon spectrum and the energy state of electrons in the discussed compounds when they are irradiated with photons causes the formation of an additional number of clusters of special ion complexes (negative U-centers), which are capable of generating quasiparticles: "Cooper" pairs of electrons (bosons).

The formation of clusters of negative U-centers leads to the formation of a special energy spectrum of the electronic subsystem in metal oxides, which allows pair transitions of electrons. Paired electrons carry an electric charge without losing energy. Irradiation of multicomponent metal oxides with optical photons intensifies the process of formation of the maximum possible total length of U-centers clusters, therefore intensifies the process of metal oxides transition to a superconducting state: the temperature of the complete transition of these compounds to the superconducting state T_c can approach the value of the temperature of the opening of a pseudogap in the energy spectrum of electrons that is, this transition is realized at higher temperatures than it occurs under normal conditions.

EFFECT OF HIGH PRESSURE ON TEMPERATURE DEPENDENCE OF PSEUDOGAP IN Y0.66Pr0.34Ba2Cu3O7-δ SINGLE CRYSTALS

G. Ya. Hadzhai — 2024-05-30

The paper investigates the effect of high hydrostatic pressure on the conductivity, s(T), in the basic ab-plane of the HTSC single crystal Y_0.66Pr_0.34Ba₂Cu₃O_7–_δ. The YBa₂Cu₃O_7–_δ single crystal was grown using the well-known solution-melt technology. Y₂O₃, BaCO₃, CuО and Pr₅O₁₁ compounds were used in the appropriate percentage ratio as initial components for growing Y_0.66Pr_0.34Ba₂Cu₃O_7–_δ single crystal. The modes of growth and oxygen saturation of the Y_1–xPr_xBa₂Cu₃O_7–_δ crystal were the same as for undoped single crystals. Electrical resistance in the ab-plane was measured according to the standard 4-pin method. Hydrostatic pressure was created in a piston-cylinder multiplier. The temperature dependences of the specific electrical resistance, r(Т), of the studied single crystals in the ab-plane in the temperature range T_c–300 K at pressures of 0–10 kbar were obtained experimentally. At all applied pressures, the experimental curves r(T) contain linear sections at temperatures T > T*. When T < T*, the r(T) curves deviate downward from their linear extrapolation, i.e., excess conductivity, Ds(T), appears in the studied samples. It was established that Ds(T) of the samples in a wide temperature range T_f < T < T* are characterized by an exponential temperature dependence Δs ~ (1–Т/Т*)exp(Δ*_ab/T), where T* is the mean-field temperature of the superconducting transition, and can be interpreted in terms of the BCS-BEC crossover theory. Using this ratio, the temperature dependence of the pseudogap, Δ*_ab(T), from T* to the temperatures corresponding to the maximum value of the pseudogap was constructed. An increase in the applied pressure leads to the effect of narrowing the temperature interval of realization of the pseudogap (PG) mode and, as a result, to the expansion of the interval of the linear dependence of the specific electrical resistance in the ab-plane. As the pressure increases, the temperature dependence of the pseudogap shows a crossover from the dependence of the BCS type to the dependence of the BEK type.

PECULIARITIES OF TEACHING PHYSICS AND ASTRONOMY AT THE FACULTY OF PHYSICS IN THE CONDITIONS OF DISTANCE LEARNING

M. O. Makarovsky — 2024-05-30

Development of high quality teaching remotely is currently at the forefront of higher education developments. Particularly challenging is remote delivery of the teaching programs that require deep understanding of complex scientific phenomena, as well as development of practical skills and professional competencies. In this work, we discuss the experience of remote delivery of Physics degree. It is noted that different approaches are needed for different teaching elements. Based on this, we make recommendations for approaches to enhance student experience and to enable successful completion of the degree programs.

Remote defense of the diploma projects in the format of research conference (using Zoom, Skype, GoogleMeet and other video communication) showed that this stage of the educational process is successfully implemented and is almost identical to the face-to-face defense. Consultations (on training courses, research projects, diploma projects) can be conducted remotely.

When conducting written assessments of learning: tests, quizzes and examinations in the distance mode (using standard procedures such as distribution of questions and tasks), it is quite difficult to control the quality of students' performance, since there are opportunities for use various sources of information. A test system can provide a suitable substitute.

The experience of conducting lectures and practical classes in the distance mode has shown that the lecturer does not have the feedback that occurs in classroom teaching. The lecturer is unable to interact with the audience, since students hardly ask questions and during the lecture they can go about their own business rather than listen to the lecturer.