Phonon Subsystem Research Articles

Impact of Fe-impurity centers on phonon subsystem of wurtzite-type ZnO

The paper addresses the question of the structural and vibrational properties of Fe-doped ZnO in the wurtzite structure with a lattice without an inversion center. The ZnO crystals containing Fe impurities in different charged states are studied by means of density functional theory (DFT) and potential-based methods. A first-principles simulation is performed to determine the local atomic configurations near the considered defects. The DFT results are compared with those obtained using the shell model. Both approaches give a similar pattern of lattice distortion around the Fe-impurity centers occupying cation sites in the isoelectronic charge state Fe2＋ and single-positive-charge state Fe3＋. The phonon local symmetrized densities of states projected onto the displacement of ions surrounding the defects are calculated by a well-established recursion method. The frequencies of defect vibrations of various symmetry types induced by Fe impurities are determined. The calculations made it possible to interpret the structure of the phonon sideband accompanying the zero-phonon line in polarized emission spectra attributed to the Fe3＋ intracenter transitions, as well as to estimate the role of Fe ions in the formation of observed peaks.

In Ukrainian

The influence of the electronic subsystem of nanographene particles modified with titanium dioxide nanoparticles on the thermal destruction of the polymer matrix is analyzed. The variation in the conductivity of the modifiers made it possible to vary the effect of the metallic electronic system of graphene on thermal destruction by attenuating its effect through the transfer of energy to the electronic subsystem throughout the semiconductor film of anatase and excluding its effect on thermal destruction through a wide-band TiO2 dielectric of rutile form, which is opaque for energy transfer to the electronic subsystem. The application of such films does not affect the thermal processes in which the phonon subsystems of polymer, graphene and crystal modifiers are involved. The method of thermally programmable desorption mass spectrometry investigated the thermal destruction of hybrid composites of epoxy resin with graphene, namely the effect of surface modification of multilayer graphene particles by nanoparticles of pyrogenic titanium dioxide (anatase and rutile forms) on the thermal stability of hybrid epoxy composites in the temperature range of 40 − 800 °C. The activation energies Ed of destruction of atomic fragments of the polymer structure with the value m/z in the range of 16 ‒ 140 D were determined. It has been established that the thermal destruction of fragments of the polymer structure of epoxy resin composites with graphene modified with anatase and rutile forms of titanium dioxide is determined by the peculiarities of thermal transport at the polymer-modifier interfaces. Modification of graphene particles by the anatase form of titanium dioxide leads to increased thermal stability of polymer chains and cross-links in epoxycomposites in the entire range of m/z values of desorbed fragments. Filling the resin with graphene particles coated with the rutile form of TiO2 practically does not affect the intensity of thermal destruction of the composite matrix. The Ed values of different destruction fragments in composites with unmodified and modified graphene are in the range of 50 ‒ 130 kJ/mol. The value of Ed increases by 5 ‒ 10 kJ/mol when the resin is filled with graphene nanoparticles, while the coating of particles with titanium dioxide of the anatase and rutile phases practically does not change the value of Ed of the vast majority of fragments.

Effect of thermal motion of transverse domain wall on thermodynamic states of cylindrical iron nanowire

The effect of thermal motion of a transverse domain wall (DW) on the thermodynamic states of a cylindrical iron nanowire (INW) is studied. It is shown that the contribution of the thermal motion of a transverse DW to the entropy and heat capacity of a cylindrical INW can be an actual value, along with the thermodynamic components of its electron, magnon, and phonon (Debye component) subsystems. The critical diameters corresponding to this phenomenon are determined; their magnitudes are lower as compared to the nickel nanowire (Shevchenko et al., 2020). The influence of the thermal motion of a transverse DW on the average magnetization of a cylindrical ferromagnetic nanowire is investigated—it is found out that the maximum magnitude of its relative average magnetization in weak magnetic fields (much lower than 2πM, where M is the magnetization of the nanowire) is ⩽1.2·10-2. It is shown that changes in M affect the temperature of a given thermodynamic state. For example, a reduction of the INW magnetization from M=1.7·103Gs to 102Gs results to a decrease in the temperature (for a given diameter) from 970K to 57K.

Relaxation of photoexcited hot carriers beyond multitemperature models: General theory description verified by experiments on Pb/Si(111)

The equilibration of electronic carriers in metals after excitation by an ultra-short laser pulse provides an important class of non-equilibrium phenomena in metals and allows measuring the effective electron-phonon coupling parameter. Since the observed decay of the electronic distribution is governed by the interplay of both electron-electron and electron-phonon scattering, the interpretation of experimental data must rely on models that ideally should be easy to handle, yet accurate. In this work, an extended rate-equation model is proposed that explicitly includes non-thermal electronic carriers while at the same time incorporating data from first-principles calculations of the electron-phonon coupling via Eliashberg-Migdal theory. The model is verified against experimental data for thin Pb films grown on Si(111). Improved agreement between theory and experiment at short times (<0.3ps) due to non-thermal electron contributions is found. Moreover, the rate equations allow for widely different coupling strength to different phonon subsystems. Consequently, an indirect, electron-mediated energy transfer between strongly and weakly coupled groups of phonons can be observed in the simulations that leads to a retarded equilibration of the subsystems only after several picoseconds.

Electronic, Vibrational and Thermal Properties of α and β Titanium

Titanium and titanium-based alloys are widely used for various technological applications in the aerospace, automotive, and shipbuilding industries because of their good strength-to-weight ratio and good combination of mechanical properties. To understand the fundamental properties of titanium, as well as phase transformations, it is necessary to study the electronic and phonon structure of its main phases. Although such studies have been intensively carried out since the middle of the last century, theoretical studies of the phonon system and properties based on it remain rare. The atomic and electronic structures of α and β titanium are calculated by the projector augmented-wave method within the electron density functional theory. The analysis of the electron energy spectrum and the densities of electronic states is carried out. Using the harmonic approximation and the finite displacement method, the phonon spectrum and a number of related thermodynamic characteristics are calculated. The features of the phonon subsystem of β-Ti, which indicate its dynamic instability, are discussed. The calculation of the vibrational entropy and enthalpy, as well as the lattice heat capacity, showed satisfactory agreement with the experiment. In general, the approach used can be applied to analyze the forming phases of doped titanium.

Charge self-trapping in two strand biomolecules: Adiabatic polaron approach

We investigate the properties of the excess charge (electron, hole) introduced into a two-strand biomolecule. We consider the possibility that the stable soliton excitation can be formed due to interaction of excess charge with the phonon subsystem. The influence of overlap of the molecular orbitals between adjacent structure elements of the macromolecular chain on the soliton properties is discussed. Special attention is paid to the influence of the overlapping of the molecular orbitals between structure elements placed on the different chains. Using the literature values of the basic energy parameters of the two-chain biomolecular structures, possible types of soliton solutions are discussed.

Bilayer Graphene–Stone–Wales Graphene: Structure, Stability, and Interlayer Thermal Conductivity

The interlayer thermal conductivity of two asymmetric bilayer carbon structures has been studied within the nonorthogonal tight binding model. One layer of the first structure proposed in this work for the first time is graphene and the second layer is Stone–Wales graphene, which is recently proposed carbon allotrope. The second asymmetric structure is bilayer graphene, where one layer consists of 12C isotope and the second layer consists of rarer 13C isotope. It has been shown that the interlayer thermal conductivity of asymmetric structures is more than an order of magnitude lower than that for their symmetric analogs, bilayer graphene and Stone–Wales bilayer graphene, with the same isotope composition. A high interlayer thermal conductivity of symmetric structures compared to asymmetric ones is due to the resonant interaction of phonon subsystems of individual layers (phonon spectra of individual layers in symmetric structures coincide, whereas these spectra in asymmetric structures are different). It has been shown that the graphene layer in the unstrained graphene–Stone–Wales graphene structure is flat, whereas the Stone–Wales graphene layer is corrugated. Both layers of this structure biaxially stretched by 5% become flat. The interlayer attraction energy, interlayer distance, activation energy of parallel shear of the layers, and the elastic modulus under vertical compression have been determined for unstrained and deformed structures.

Direct measurement of magnon temperature by magneto-optic Kerr effect in YIG

Magnon–phonon thermal relaxation holds a fundamental role in condensed matter physics, and the difference between local phonon and magnon temperature ΔT mp as an important part of this subfield was theoretically considered responsible for the spin Seebeck effect. Experimental determination of ΔT mp is necessary to give more insight into the magnon–phonon coupling. Here we report spatially resolved measurements of magnon temperature performed by magneto-optic Kerr effect in yttrium iron garnet. Our results indicate a strong interaction between magnon and phonon subsystems in YIG with an upper limit of the phonon-magnon thermal relaxation length of 1.4 mm, and means this method is valid.

Quantum-mechanical model of dielectric losses in nanometer layers of solid dielectrics with hydrogen bonds at ultra-low temperatures

Upon based the finite difference methods construct the solutions for Liouville quantum kinetic equation linearized by the external field, in complex with the stationary Schrodinger equation and the Poisson operator equation, for an ensemble of non-interacting hydrogen ions (protons) migrating in the field of a crystal lattice perturbed by a variable polarizing field. The influence of the phonon subsystem is not taken into account. The equilibrium (non-balanced) proton density matrix is calculated using quantum Boltzmann statistics. The temperature spectra of dielectric losses tangent angle for hydrogen bonded crystals (HBC) in a wide temperature range (50–550 K) are calculated. At the theoretical level detected the effects of nano-crystalline states (1–10 nm) during the polarization of HBC in the region of ultra-low temperatures (4–25 K).

Квантовые свойства диэлектрических потерь в нанометровых слоях твердых диэлектриков при сверхнизких температурах

Using the methods of quasi-classical kinetic theory, continuum electrodynamics, and non-relativistic quantum theory, we construct and study the quantum kinetic equation of proton relaxation, which, together with the Poisson operator equation describes the mechanism of diffusion tunneling transport of hydrogen ions (protons) in the potential field of a crystal lattice perturbed by a polarizing field (quantum diffusion polarization) in crystals with hydrogen bonds. Using the apparatus of the density matrix (statistical matrix), by complete quantum-mechanical averaging of the polarization operator, studies are carried out of the experimental value of the polarization of the dielectric, as a function of the parameters of the external electric field (amplitude, frequency of electromotive force) and temperature. When calculating the equilibrium density matrix for an ensemble of basic relaxers (hydrogen ions), the proton-proton and proton-phonon interactions are not taken into account, and the Hamilton operator for the phonon subsystem is assumed to be a numerical constant for a given crystal under given experimental conditions (calculated by computer method as a parameter for comparing the theory with the experiment). The influence of the phonon subsystem on the kinetics of the relaxation process is reduced to a weak spatially homogeneous force field acting on protons moving in the field of the main forces of hydrogen bonds. The Hamilton of the proton subsystem is constructed for the model of an ideal proton gas in equilibrium with the ionic subsystem of the crystal lattice, and the equilibrium statistical operator of the proton subsystem is written using the Boltzmann quantum statistics. Theoretically, the size effects are found to be manifested in shifts of the low-temperature (50–100 K) maxima of the dielectric loss angle tangent towards ultra-low temperatures (4–25 K) with a decrease in the amplitudes of the maxima by 3-4 orders of magnitude, with a reduction in the thickness of the crystal layer from 1–10 microns to 1–10 nm. The effect of anomalous displacements of low-temperature maxima, which is explained by the abnormally high quantum transparency of the potential barrier for protons (0.8-0.9) in thin films of a crystal with hydrogen bonds (1-10 nm), causes, near the temperatures of the shifted maxima of dielectric losses (4–25 K), a quasi-ferroelectric state, which is also characterized by abnormally high values of the real component of the complete dielectric permittivity (2.5–3.5millions).

Temperature dependent electron–phonon coupling of Au resolved via lattice dynamics measured with sub-picosecond infrared pulses

The detailed understanding of energy transfer between hot electrons and lattice vibrations at non-cryogenic temperatures relies primarily upon the interpretation of ultrafast pump–probe experiments, where thermo-optical models provide insight into the relationship between optical response and temperature of the respective sub-systems; in one of the more studied materials, gold, the Drude model provides this relationship. In this work, we investigate the role of intra- and interband contributions applied to transient optical responses in ultrafast pump–probe experiments using both experiments and first-principle calculations, with probe wavelengths spanning from UV wavelengths into the infrared. We find that during conditions of electron–phonon equilibrium, the Drude model is not applicable to visible wavelengths due to interband transitions. Instead, at probe wavelengths far from these interband transitions (e.g., infrared wavelengths), the optical response is linearly proportional to the temperature of the phonon sub-system and is no longer obfuscated by Fermi-smearing, thus greatly simplifying the extraction of the electron–phonon coupling factor. Our intraband-probe measurements on the electron–phonon coupling factor of Au are in excellent agreement with analytical models and ab initio calculations; we observe a constant electron–phonon coupling factor up to electron temperatures of at least ∼2000 K.

Terahertz Driven Reversible Topological Phase Transition of Monolayer Transition Metal Dichalcogenides.

This paper shows how terahertz light can drive ultrafast topological phase transitions in monolayer transition metal dichalcogenides (TMDs). The phase transition is induced by the light interaction with both electron and phonon subsystems in the material. The mechanism of such a phase transition is formulated by thermodynamics theory: the Gibbs free energy landscape can be effectively modulated under light, and the relative stability between different (meta‐)stable phases can be switched. This mechanism is applied to TMDs and reversible phase transitions between the topologically trivial 2H and nontrivial 1T′ phases are predicted, providing appropriate light frequency, polarization, and intensity are applied. The large energy barrier on the martensitic transformation path can be significantly reduced, yielding a small energy barrier phase transition with fast kinetics. Compared with other phase transition schemes, light illumination has great advantages, such as its non‐contact nature and easy tunability. The reversible topological phase transition can be applicable in high‐resolution fast data storage and in‐memory computing devices.

Effect of electrode temperature on radiofrequency vacuum breakdown characteristics

The results of a numerical simulation of the heating of a microprotrusion on the surface of a copper electrode under the influence of radiofrequency electromagnetic fields, performed for different temperatures of the electrode, are presented. The calculation model describes self-consistently the thermal processes in the electron and phonon subsystems, the electron emission, and the effect of the space charge of emitted electrons. It has been shown that when the initial electrode temperature is increased to 1000 K, the time of heating of the surface microprotrusion to a critical temperature decreases by a factor of 1.5–1.9, namely, to values ranging from a few to tens of nanoseconds. This may significantly increase the probability of radiofrequency vacuum breakdown on the surface of the accelerating structure of an electron-positron collider.

Electronic and thermodynamic properties of lanthanum tetraboride on low-temperature experimental and ab-initio calculation data

The characteristics of the electronic and phonon subsystems of lanthanum tetraboride (LaB4) studied using first-principles calculations. The calculated lattice parameters, as well as the positions of atomic, are satisfactorily consistent with the experimental data. The partial densities of states, band structure, Fermi surface, phonon dispersion curve of LaB4 are calculated and analysed. The reliability of the calculation results is confirmed by a satisfactory agreement between the calculated thermodynamic parameters of LaB4 (temperature changes in heat capacity, entropy, Grüneisen parameter and volume modulus) with experimental data.

Relaxation phenomena in electron plasma of semiconductors

The hydrodynamics of the electron subsystems of semiconductors is studied in the approximations of the ideal and real liquid, taking into account processes of relaxation of temperatures and macroscopic velocities of electrons and phonons without assuming the local equilibrium of the system. A set of integral equations for the electron distribution function of the first order in gradients is obtained, which determines the sources in the hydrodynamic equations of the ideal liquid approximation and the dissipative flows of energy and momentum of electrons. The steady states of the system in the ideal liquid approximation are investigated. The exact formulas for the electron mobility of the semiconductor and its conductivity are derived and kinetic coefficients that determine current in a spatially inhomogeneous state are calculated. In the presence of an electric field, the phenomenon of difference of temperatures of the electron and phonon subsystems is predicted. The obtained expressions are specified for the case of temperatures much higher the Debye temperature.

Hydrodynamic states of electron plasma of semiconductors in the generalized Chapman–Enskog method

The hydrodynamic states of polar semiconductors are investigated. Such states are described by the temperature, mass velocity, and density of the number of particles of the electron subsystem, which is considered as rarefied one. The phonon subsystem of a semiconductor is considered to be equilibrium. The electron-phonon interaction in the system is described by the Fröhlich Hamiltonian. The theory is based on Bogolyubov’s linear kinetic equation for the electron distribution function in the presence of a weak electric field. The reduced description of the system is investigated by the generalized Chapman–Enskog method, which leads to a nonlinear equation for the electron distribution function in the reduced description. The zone structure of the system is neglected, focusing on the study of fundamental issues of relaxation of temperature and velocity. Gradients of hydrodynamic parameters are considered small; the same smallparameters determine the smallness of the electric field. In the basic order of the perturbation theory, the distribution function of the system satisfies a nonlinear equation. The method of spectral theory of the collision integral operator shows that the study of the basic approximation is reduced to an analysis of a first-order quasilinear equation, which, in particular, has a solution that we found earlier.

Electron and phonon thermal conductivity in high entropy carbides with variable carbon content

Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently-bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized, and a combination of changes in microstructure, defect concentration and secondary phase formation, and stiffness influence the phononic contribution to thermal conductivity. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

Magnon-phonon interactions in magnon spintronics (Review article)

Nowadays, the interaction between phonon and magnon subsystems of a magnetic medium is a hot topic of research. The complexity of phonon and magnon spectra, the existence of both bulk and surface modes, the quantization effects, and the dependence of magnon properties on applied magnetic field, make this field very complex and intriguing. Moreover, the recent advances in the fields of spin caloritronics and magnon spintronics as well as the observation of the spin Seebeck effect in magnetic insulators points on the crucial role of magnons in spin-caloric transport processes. In this review, we collect the variety of different studies in which magnon-phonon interaction play important role. The scope of the paper covers the wide range of phenomena starting from the interaction of the coherent magnons with surface acoustic wave and finishing with the formation of magnon supercurrents in the thermal gradients.

Carbon Bonding Controls the Electron and Phonon Thermal Conductivity in High Entropy Carbides

Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus changes as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized and remains at a constant, and a combination of changes in film morphology and phase precipitation systematically lower the phononic contribution to thermal conductivity. This trend in thermal conductivity with carbon content in the HECs is opposite to those observed in binary metal carbide systems. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

The effect of the domain wall on the entropy and the heat capacity of nickel nanowire

The thermodynamic properties of a cylindrical nickel nanowire with a domain wall (DW) were studied. It was found that the entropy and heat capacity of the DW can be higher than or on the order of the corresponding values of the electron, magnon, and phonon (Debye) subsystems. The conditions at which this effect takes place are determined. The methods of its strengthening are proposed. For instance, an increase in the length of nanowire and the number of DWs by a factor of 10 increases its diameter at which the heat capacity of DW has the same order as the phonon subsystem by 33.5%. An increase in this parameter by 38% occurs upon magnetization of the nanowire to M=1.3·102G.