Physics

A semi-empirical universal model describing the temperature dependence of heat capacity is developed using the Laplace distribution as a physical basis and its log-logistic analogue as an analytical form. The proposed formulation unifies the classical Debye, Einstein, and Tarasov approaches and provides an accurate representation of experimental data for solids, liquids, and gases over the entire temperature range of their stability. The model has only two parameters: the characteristic temperature and the rate of change of state, which correlate with the Debye temperature and the structural dimensionality of the substance. Analytical expressions derived from the model allow direct calculation of enthalpy and entropy without numerical integration. The log-logistic form preserves approximation accuracy within the uncertainty of measurements while enabling analytical differentiation and integration of thermodynamic functions. The proposed system of functions offers a unified, physically consistent, and computationally efficient framework for evaluating and predicting the heat capacity and related thermodynamic properties of materials.

The results of experimental studies aimed at the determination of the temperature-strain rate conditions under which samples of cryogenic aluminum alloys 1201 and AMg6, deformed in the creep mode at high homologous temperatures, exhibit the effect of superplasticity, as well as at studying of the structural changes which occur in the working parts of samples of these alloys during superplastic deformation are presented. The probable influence of magnesium, as the main alloying element of the AMg6 alloy, on the partial melting temperature of this alloy and on the mechanism of formation of fibrous structures during superplastic deformation of its samples at high homologous temperatures is considered.

When phonon sheets collide, they can produce a hot line in liquid 4He. The dependence of the energy in the hot line on the angle between two phonon sheets and the energy density in them was measured. It found that the magnitude of the maximum of the low-energy phonon signal of the hot line is directly proportional to the value of this angle at its small values and falls to zero at high angles. The dynamic equilibrium of the hot line was modeled by equating the energy gain from the consumption of the sheets with the energy loss due to the creation of high-energy phonons. At the same time, using theoretical values of the rate of formation of high-energy phonons from low-energy ones, good agreement with experimental results was obtained. We showed that the key factor in this process is the solid angle in the momentum space occupied by low-energy phonons, under the assumption that the magnitude of this solid angle is proportional to the initial energy density in a phonon system.

Significant efforts have been made to elucidate the properties of high-temperature superconductors (HTSCs) using hydrostatic pressure (HP). We studied the changes of resistivity ρ(T), superconduting transition tempetature Tc, fluctuation conductivity σ′(T), and pseudogap Δ*(T) of Y1−xPrxBa2Cu3O7−δ (YPrBCO) (x = 0.23) single crystal under HP up to ∼1.1 GPa. Defects created by magnetic inclusions of PrBa2Cu3O7–δ (PrBCO) were found to play a significant role in the behavior of the sample. The largest decrease in ρ(T) was found depending on HP at a rate of d ln ρ(100 K)/dP = –(29 ± 0.2) %⋅GPa−1. This indicates that the mechanisms of the influence of HP on the ρ(T) and Tc of YPrBCO and YBa2Cu3O7–δ single crystals are different. From the analysis of the pseudogap, it was revealed that the mechanism of the interaction of charge carriers with magnetic PrBCO impurities changes three times with increasing HP. Relatively low HP, up to ∼0.5 GPa, promotes the formation of defects caused by PrBCO magnetic inclusions. Starting from 0.6 GPa, the HP neutralizes the influence of magnetic impurities, and the magnetic maximum on Δ*(T) disappears. Above ∼1.0 GPa, HP aligns the magnetic moments of PrBCO, and the magnetic maximum reappears on Δ*(T), more pronounced than at P = 0. Comparison with the Peters–Bauer theory showed that the local pair density in HTSC does indeed increase with increasing pressure.

The results of experimental studies on the regularities of crystal lattice rotations during plastic deformation of large-grained aluminum samples are presented. The studies were carried out using an original high-resolution technique based on obtaining color orientation maps from the entire surface of the samples. A wide variety of different ways in which the crystal lattice can rotate has been discovered. These include grain rotation and fragmentation, the formation of various types of rotational structures, flare-shaped rotations, complex rotations occurring simultaneously in adjacent grains, rotations that alter the configuration of grain boundaries, and others. For the first time, possible mechanisms of grain rotation during the deformation of large-crystal samples have been identified. They consist of the restructuring of the subgrain structure within the grain. Such restructuring can be relay-like or chaotic in nature. Experiments have shown that with such grain rotation mechanisms, the magnitude can be 10–15 deg, and the magnitude of the relative grain deformation can exceed that for the entire sample.

We investigate the relationship between the low-temperature thermal conductivity plateau and the hump in reduced heat capacity (Cp/T3) in a range of disordered solids, including polymers, glasses, and complex crystalline materials. Using a unified framework that separates particle-like propagation (Peierls term) and wave-like coherence contributions, we analyze experimental data to extract characteristic temperatures associated with each contribution. Our results demonstrate that the temperature of the Cp/T3 hump Tmax correlates systematically with the onset of the thermal-conductivity plateau and the crossover between particle-like and coherence-dominated transport. Normalized scaling of the particle-like contribution reveals a universal behavior across chemically and structurally diverse systems. These findings provide clear evidence of a direct link between the thermal-conductivity plateau and low-temperature heat capacity anomalies, establishing Tmax it as a robust characteristic parameter for predicting thermal transport in disordered solids.

The use of modern methods of consolidation of ceramic materials in combination with approaches of organic and inorganic chemistry, sol-gel technology and mechanochemistry allows controlling the processes of phase synthesis at the molecular level and creating highly effective composite materials. The properties of composites based on refractory compounds become size-dependent when the particle size is reduced to the nanoscale. The paper presents the results of the synthesis and consolidation of nanocomposites based on SiC, ZrO2–Y2O3, and WC. The microstructure, phase composition, and mechanical properties are studied, and the mechanisms for strengthening and increasing crack resistance are substantiated. It has been shown that the introduction of WC nanoparticles into the ZrO2 matrix contributes to the formation of a finely dispersed structure with high density and an increase in hardness, compressive strength, and crack resistance. The results obtained indicate the possibility of using such nanocomposites in structural and tool materials for operation under conditions of intensive wear.

This work investigates the sensor properties of Cu2O/CuO nanocomposite films, obtained through vacuum deposition followed by atmospheric annealing. It was established that the proposed synthesis method allows for the creation of biphase nanocomposite films with structural element sizes around 20 nm. The obtained film structures are shown to be sensitive to infrared radiation and can even react to human body thermal radiation. Indications of additional sensitivity mechanisms within the sensor layers were observed, and physical considerations regarding these mechanisms are proposed.

The in-plane conductivity of YBa2Cu3–yAlyO7–x single crystals with a defined topology of planar defects has been investigated. It is shown that Al impurities act as effective scattering centers for normal-state carriers. The excess conductivity of the studied samples over a wide temperature range follows an exponential temperature dependence, while near Tc it is satisfactorily described by the Aslamazov–Larkin theoretical model. Partial substitution of Cu with Al leads to a significant expansion of the temperature interval in which the pseudogap anomaly exists in the ab plane. The fractal analysis of the “tweed” structures has been performed.

This paper presents quantitative data on the key parameters of the atomic structure and valence electron energy spectrum of a newly synthesized carbon compound formed by one-electron covalent bonds between atoms. An analysis of these data indicates that this substance can undergo a superconducting transition. Quantitative estimates lead to the conclusion that such a transition can be realized under external pressures of ≈10–2 GPa, with a critical temperature of Tc ≈ 120 K, which is an order of magnitude higher than the average value characteristic of classical metals. It is shown that similar conditions for a superconducting transition can also be realized in a substance with particle sizes R ≤ 10–7 m due to internal (capillary) pressure.

A key physical mechanism that is important when considering energy or semiconductor materials is diffusion. High diffusivity can limit the applicability of semiconductor materials in functional nanoelectronic devices, whereas it is a prerequisite for energy materials used in batteries and solid oxide fuel cells. Typically, diffusion is thermally activated and governed by Arrhenius behavior. In only a few systems, very high diffusivities have been observed with low activation energies of diffusion (less than 0.1 eV). This, in turn, enables atomic diffusion even at low temperatures. We discuss representative examples of such systems and the implications of diffusion at low temperatures. The focus is on low-temperature diffusion mechanisms across multiple material types (oxides and semiconductors), including materials for different applications such as fuel cells, batteries, nanoelectronics, and superconducting devices. This brief review concludes with future perspectives and consideration of recent advances.

The paper presents the results of numerical modeling of the compaction and grain growth processes of powder materials based on zirconium dioxide ZrO2 during electroconsolidation. The theoretical basis is the Skorokhod–Olevskyi–Shtern model, which describes the kinetics of compaction and the evolution of microstructure. The model parameters were identified based on experimental data on the changes in temperature, relative density, and axial stress of samples with a composition ZrO2 + 3 mol % Y2O3. A specialized computer program was developed, which allowed a series of numerical experiments to be performed and the simulation results to be compared with actual sintering data. It was found that the calculated density change curves closely match the experimental results, confirming the adequacy of the model used. It has been shown that grain growth significantly affects the final stages of the process, but the final density is largely determined by the isothermal holding mode and the level of pressure applied. It has been demonstrated that increasing the pressure allows to achieve high density even without significantly prolonging the process time. The developed approach provides the ability to predict the density, grain size, and mechanical properties of sintered samples. The practical significance of the work lies in the fact that the proposed method can be used to optimize electroconsolidation modes, reduce energy consumption, and improve the quality of ceramic zirconium composites.

The heating and evaporation of solid micro- and nanoparticles (MNPs) in a low-temperature plasma, followed by ionization of the resulting vapor, are analyzed theoretically. A scheme is proposed in which MNPs are first charged to a high positive potential outside the plasma and then electrostatically accelerated into it. Within the plasma, collisions with ions and electrons lead to MNPs heating. To compensate for the positive charge introduced by the MNPs, an electron beam is injected into the plasma. This beam simultaneously serves as an additional heating source, promotes evaporation, and enhances vapor ionization. The developed model makes it possible to evaluate how the heating and evaporation rates depend on MNP size and plasma parameters, providing a framework for applications that require controlled injection of solid MNPs into plasma.

The possibilities of producing bilayer substrate tapes based on the paramagnetic Ni–W alloy with the TiN coating, which could be used as a seed layer in the architecture of second-generation high-temperature superconductors (2G HTS), have been explored. The studies were conducted in the following areas: synthesis, mechanical and thermal treatment of Ni–W alloys with an fcc crystal structure, and the investigation of the influence of TiN deposition conditions on the ability to control the properties of the TiN/Ni–9.5 at.% W bilayer system. An effect of mutual interaction between the substrate and the coating was observed, resulting in the correlated enhancement of the degree of cube texture {001}<100 > in both components of the bilayer system.

Based on the wave nature of phonons, we present a study of phonon transmission from superfluid 4He across a bilayered solid, made up of a film layer, of a few tens of nanometers in thickness, deposited on a solid. The main focus is aimed at suppressing the thermal boundary resistance between superfluid helium and the solid by introducing an intermediate film that acts as an antireflection layer. We derive a relationship showing that the maximum transmission is reached for film thicknesses which are comparable to the phonon wavelengths in the film layer. The analysis is carried out within the framework of the acoustic mismatch model and considers phase coherence due to interference effects arising from multiple reflections of phonons inside the intermediate layer.

The paper shows that the use of different energies of plasma irradiation, which is performed simultaneously with copper deposition, leads to changes in the microstructure of condensates. By means of sclerometric studies, the relationship between the microstructure of films and the kinetics of their mechanical fracture was obtained. It is shown that the use of radio-frequency (RF) plasma, which affects the sample during film deposition, leads to the transformation of barrier-free film fracture into a process that requires overcoming some critical force. The influence of RF plasma is in many respects similar to the substrate temperature during condensation-stimulated diffusion. The observed improvement of adhesion properties of films subjected to more energetic RF processing is explained by condensation-stimulated recrystallization of their structure and formation of damping layers due to the embedding of working gas ions. Ion processing of films performed simultaneously with magnetron deposition is used for microstructure modification. The high efficiency of this approach is demonstrated. The effect of the microstructure of the films on their hardness and adhesion to the substrate is shown. An explanation of the change in the strength properties of the films and the mechanism of their fracture is given.

The work is devoted to studying the sensory properties of nanocomposite structures formed by atmospheric annealing of copper films. It is demonstrated that annealing of copper films with a thickness of 50–100 nm at a temperature of 280 °C results in the formation of nanocomposite films consisting of a mixture of two oxides: CuO and Cu2O. The structures obtained in this way retain, or even increase, their dispersibility and exhibit sensitivity to ultraviolet radiation. It has been shown that the studied nanocomposites reversibly reduce their electrical resistance under ultraviolet irradiation; therefore, they can be used as sensor materials. Several mechanisms of photosensitivity have been proposed, among which the most likely involves partial decomposition of the CuO phase under the action of ultraviolet radiation.

The scalability of quantum computing systems based on superconducting qubits is limited by the low integration of quantum modules and the thermal load on cryogenic stages of refrigerators. A key element of scalable architectures is the microwave-magnon-optical (MMO) converter, which coherently connects microwave and optical circuits. We propose an approach to reducing the thermal load on the MMO converter using metasurface-based microwave structures with strong magnetic field localization. This design allows the main microwave circuitry to operate at higher refrigerator stages, leaving only the ferrimagnetic element at ultralow temperatures, thus improving overall scalability and thermal efficiency.

Quantum networks form the most important part of secure post-quantum communication and distributed quantum computing. Realizing long-distance quantum links requires efficient spin-photon interfaces that operate in the telecom range, which can be achieved through optically addressable paramagnetic defects. Hexagonal boron nitride (hBN) represents a very promising platform due to its wide bandgap and the ability to host many single-photon emitters. While intrinsic defects such as the negatively charged boron vacancy have been extensively explored as potential qubit candidates, different dopants are investigated, with carbon attracting considerable attention due to its ability to form complexes and interact with other known spin-defects. In this work, we investigate carbon substitutional occupying boron and nitrogen lattice sites in bulk hBN, using density functional theory with meta-GGA functionals. We compute the formation energies, electronic structure, and density of states across different charge configurations, and identify how substitutional doping influences the defect level in the bandgap. Furthermore, we study the excited state of these defects, which is formed by a defect-bound exciton capable of operating at low temperatures. The present findings highlight the importance of carbon defects and expand the range of candidate systems for quantum communication and sensing.

We have analyzed the doping and temperature dependencies of the physical properties of a square-lattice narrow-band superconductor with a relevant for systems with local Hubbard repulsion nematic pairing due to effective nearest-neighbor attraction. Our focus was a possible crossover of the system from the Bardeen–Cooper–Schrieffer (BCS) superconductivity to the Bose–Einstein condensation (BEC) regimes with increasing temperature. We have found that in the d-channel the crossover does not take place at realistic values of doping and temperature, and even at a large attraction, i.e., the system is always in the BCS regime, unlike the “usual” d-wave superconductor. Contrary to non-nematic d-wave superconductors, the chemical potential μ always grows with temperature increase that might indicate an unexpectedly large size of Cooper pairs. At the same time, in the s-channel, we have found a BCS–BEC crossover with temperature for a wide range of temperatures even at rather high dopings, nf ≈ 0.2. This result is different from the “usual” s-case, where a narrow-band (i.e., at strong coupling) system is always in the BEC state. We have found for a given large value of attraction that there is a value of doping when for a wide range of temperature the system is in a “pre-crossover” regime with μ ≈ 0, i.e., in a regime of coexistence of local and extended pairs. Thus, in narrow-band systems a “usual” s-superconductor is in a BEC state, but in the nematic case one can expect a BCS–to–BEC crossover with temperature growth at realistic system parameters, i.e., the crossover might be measured experimentally.

Disentangling coherent and incoherent effects in the photoemission spectra of strongly correlated materials is generally a challenging problem due to the involvement of numerous parameters. In this study, we employ machine learning techniques, specifically convolutional neural networks (CNNs), to address the long-standing issue of bilayer splitting in superconducting cuprates. We demonstrate the effectiveness of CNN training on modeled spectra and confirm earlier findings that establish the presence of bilayer splitting across the entire doping range. Furthermore, we show that the magnitude of the splitting does not decrease with underdoping, contrary to expectations. This approach not only highlights the potential of machine learning in tackling complex physical problems but also provides a robust framework for advancing the analysis of electronic properties in correlated superconductors.

The electrical resistance and thermal conductivity of the two-phase system TaHx (х ≲ 0.1) were investigated. It was shown that for two-phase metal-hydrogen systems, narrow temperature limits of conductivities can be constructed based on the temperature dependences of the conductivities of individual phases. The corresponding conductivities are within such narrow limits only under conditions of maximum perfection of the hydride phase, i.e., where the precipitations of the hydride phase are well formed, and the bulk particles are stable. Under such conditions, the temperature of the superconducting transition, Tc = 4.4 K, remains unchanged compared to the pure metal.

Pulsed laser deposition (PLD) is a well-established method for synthesizing thin films, enabling precise control over growth parameters while maintaining stoichiometry across the specimen. Integrating PLD with combinatorial studies offers a significant advantage for conducting high-throughput experiments, thereby accelerating discovery. In this work, we introduce a reliable and reproducible approach for combinatorial thin-film synthesis that features spatially controlled thickness gradients on a substrate, achieved using a tilt-enabled stage. This technique exploits the angular distribution of the laser plume on a tilted substrate, which creates varying distances between the plume source and the substrate, resulting in a thickness gradient. While conventional PLD typically produces a plateau-shaped thickness gradient over large wafer-scale areas due to the natural profile of the ablation plume, our study aims to develop a fundamental understanding of how to create controlled thickness gradients. Using boron nitride (BN) as the model system, we investigate the effect of substrate tilt angles on the spatial distribution of thickness. Atomic force microscopy measurements show that a uniform film forms over a 10 mm × 10 mm area of the substrate at 0° tilt. In contrast, substrate tilt angles of ±20° result in a linearly graded film thickness that is asymmetric around the laser plume axis over the same area. We have extended this study to Co and NiCoCr films, which consist of elements with similar atomic numbers to minimize atomic-number-dependent variations during deposition, thereby improving the reproducibility of our approach across different material systems. Using a model having cosine-power dependency that incorporates target-substrate spacing, tilt geometry, and small-axis offsets, we simulate the ablation plasma plume profile to understand film thickness profiles for both 0° and tilted substrate geometries. The results agree with our experimental findings and provide guidance for designing combinatorial experiments that require film thickness gradients.