K. V. Kravchenko
National Academy of Sciences of Ukraine
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Featured researches published by K. V. Kravchenko.
Low Temperature Physics | 2009
A. Feher; I. A. Gospodarev; V. I. Grishaev; K. V. Kravchenko; E. V. Manzheliĭ; E. S. Syrkin; S. B. Feodos’ev
It is shown that in graphite the spectral density of phonons polarized along the c axis has a V shaped feature similar to the so-called Dirac singularity characteristic of the electron density of states in graphene. The formation of quasilocal states, which increase the occupation of the quasiparticle levels near this feature, is analyzed from a unified standpoint for the phonon spectrum of metal-intercalated graphite and the electronic spectrum of graphene with vacancies. It is determined that in the electronic spectrum of graphene with an isolated vacancy quasilocal states are characteristic only of atoms belonging to the sublattice that does not contain this vacancy.
Low Temperature Physics | 2009
E. S. Syrkin; S. B. Feodos’ev; K. V. Kravchenko; A. V. Eremenko; B. Ya. Kantor; Yu. A. Kosevich
The reasons for the appearance of flexural rigidity of layers in strongly anisotropic layered crystals are investigated. Structures consisting of loosely bound monatomic layers (specifically, graphite) as well as formed loosely bound structural elements each consisting of several monolayers which are coupled much more strongly with one another (for example, in dichalcogenide transition metals) are examined. The effect of the flexural rigidity on the phonon spectra of these compounds—quasi-flexural bending of the dispersion curves of phonon modes polarized in a direction normal to the layers—and the particularities appearing in the phonon densities of states as a result of quasi-flexural vibrational branches crossing low-frequency optical modes are analyzed.
Low Temperature Physics | 2009
I. A. Gospodarev; K. V. Kravchenko; E. S. Syrkin; S. B. Feodos’ev
The phonon spectrum of graphite is analyzed in detail at the microscopic level and the partial contributions from the displacement of atoms in and perpendicular to the plane of the layers to the phonon density of states are calculated. The main distinctive features of the phonon spectrum of graphite are determined; they are due to the quasi-two-dimensional character of phonon propagation as is characteristic for graphite, specifically, the feature arising in the spectral density as a result of the displacement of atoms along the c axis, analogous to the Dirac singularity in the electron spectrum of graphene. This makes it possible to predict the general changes occurring in the phonon and electron spectra as a result of the intercalation of different metals in graphite as well as to explain the change of the superconducting transition temperature in intercalated graphite.
Low Temperature Physics | 2010
I. A. Gospodarev; V. V. Eremenko; K. V. Kravchenko; V. A. Sirenko; E. S. Syrkin; S. B. Feodos’ev
The phonon spectra of niobium diselenide nanofilms, consisting of several structural elements of this compound, as well as graphite nanofilms starting with bigraphene are analyzed at the microscopic level. The partial contributions of the displacements of atoms along the strong and weak coupling directions (i.e. along and perpendicular to the layers) to the phonon density of states are calculated. The characteristic distinguishing features of the vibrational spectral of these structures are analyzed. The fact that our calculations are practically identical to the data obtained from neutron diffraction, acoustic, and optical experiments confirms that the description of the phonon spectra of the compounds is highly accurate. The temperature dependences of the mean-square shifts for bulk samples and nanofilms along different crystallographic directions, making it possible to evaluate the stability of the graphite and niobium diselenide nanofilms at low temperatures, are calculated for each compound studied.
Low Temperature Physics | 2008
I. A. Gospodarev; V. I. Grishaev; A. V. Kotlyar; K. V. Kravchenko; E. V. Manzheliĭ; E. S. Syrkin; S. B. Feodos’ev
Low-frequency features of the phonon spectra of disordered solid solutions and heterogeneous crystalline structures are analyzed at the microscopic level. It is shown that boson-peak type excitations can arise in disordered solid solutions whose sites have only translational degrees of freedom. Thus it is established that such excitations appear mainly because of the additional positional dispersion of sound waves which is due to the disordering. The influence of boson-peak excitations on the low-temperature specific heat is investigated. It is found that in a number of cases the specific heat is more sensitive to excitations of this kind than the low-frequency density of states is. It is shown that anomalies similar to Ioffe–Regel’ crossover and boson peaks can also arise in disordered heterogeneous crystalline structures with a complicated lattice.
Archive | 2011
A. Feher; E. S. Syrkin; S. B. Feodosyev; I. A. Gospodarev; K. V. Kravchenko
Graphite, graphene, and compounds based on them are of great interest both as objects of fundamental research and as some of the most promising materials for modern technologies. The two-dimensional form of graphite – graphene was prepared only very recently, immediately attracting a great deal of attention. Graphene can be deposited on solid substrates and has been shown to exhibit remarkable properties including large thermal conductivity, mechanical robustness and two-dimensional electronic properties. Note that electrons in graphene obey linear dispersion relation resulting in the observation of a number of very peculiar electronic properties. These properties are essentially changed when different defects are introduced into material. Special interest is devoted to graphite intercalated by metals, since in such graphitic systems the temperature of superconducting transition essentially depends on the type of intercalating metal. Besides, the discovery of superconductors as MgB2 and iron pnictides intensified the search for high-temperature superconductivity in materials other than copper oxides. It is known that in the formation of the superconducting state the electron-phonon interaction plays a crucial role (according to the Bardeen-Cooper-Schrieffer theory). Therefore it is necessary to analyze in detail the phonon spectra of pure graphite and to find out how these spectra are influenced by different defects and by intercalation. This chapter consists of three sections. The first section is devoted to the calculation of the local electronic density of graphene containing a substitutional impurity, vacancy defects due to the substrate surface roughness and adsorbed atoms. The local densities of states for atoms of the sublattice which not contains the vacancy show sharp peaks at energy F e e = ( F e is the energy of the Dirac singularity for ideal graphene). Local spectral densities of atoms of the sublattice which contains the vacancy conserve the same Dirac singularity as is observed in an ideal graphene. The second section will present our model, which allows to quantitatively describe the phonon spectrum of graphite and to determine the relaxation of force constants for the formation of the surface of the sample and the formation of thin films (bigraphene,
Physics of the Solid State | 2013
I. A. Gospodarev; A. V. Eremenko; K. V. Kravchenko; A. F. Sirenko; V. A. Sirenko; E. S. Syrkin; S. B. Feodosyev; Yu. A. Shabakaeva
The characteristic features of thermal expansion of bulk samples of niobium diselenide have been determined experimentally and analyzed theoretically. The manifestation of the so-called membrane effect in this compound due to the significant difference in the temperature dependences of the mean-square amplitudes of atomic displacements in the parallel and perpendicular directions toward the layers has been investigated.
Archive | 2013
A. Feher; E. S. Syrkin; S. B. Feodosyev; I. A. Gospodarev; Elena Manzhelii; Alexander Kotlar; K. V. Kravchenko
It is well known that graphene monolayers cannot exist as planar objects in the free state, because in flat 2D-crystals the mean-square amplitudes of the atoms in the direction normal to the layer plane diverge even at T =0 (see, e.g., [3]). So we can study and practically apply only such graphene, which is deposited on a certain substrate providing the stability of the plane carbon nanofilms (see, e.g., [4-6]). Only small flakes can be detached from the sub‐ strate and these flakes immediately acquire a corrugated shape [7]. When studying the elec‐ tronic properties of graphene a dielectric substrate is often used. The presence of the substrate greatly increases the occurrence of various defects in graphene and carbon nano‐ films. Our investigations make it possible to predict the general properties of phonon and electron spectra for graphene and bigraphene containing different defects.
Defect and Diffusion Forum | 2010
Alexander Feher; S. B. Feodosyev; I. A. Gospodarev; V.I. Grishaev; K. V. Kravchenko; Elena Manzhelii; Eugenyi Syrkin
The calculation of the local density of electronic states of graphene with vacancies, using the method of Jacobi matrix, was performed. It was shown that for atoms in the sublattice with a vacancy the local density of electronic states conserves the Dirac singularity, similarly as in an ideal graphene. A quasi-Dirac singularity was observed also in the phonon spectra of graphite for the atom displacements in the direction perpendicular to layers. Changes of phonon spectra of graphite intercalated with various metals were analyzed. On the basis of our results and using the BCS theory and Eliashberg equation we proposed what dynamic properties an intercalated graphite system should show to obtain an increased Tc.
Archive | 2011
A. Feher; E. S. Syrkin; S. B. Feodosyev; I. A. Gospodarev; Elena Manzhelii; Alexander Kotlar; K. V. Kravchenko
In recent years, the quasi-particle spectra of various condensed systems, crystalline as well as disordered and amorphous, became also the “object” of applications and technical developments and not only of fundamental research. This led to the interest in the theoretical and experimental study of the quasi-particle spectrum of such compounds, which are among the most popular and advanced structural materials. Most of these substances have heterogeneous structure, which is understood as a strong spatial heterogeneity of the location of different atoms and, consequently, the heterogeneity of local physical properties of the system, and not as the coexistence of different phases (i.e. heterophase). To these structures belong disordered solid solutions, crystals with a large number of atoms per unit cell as well as nanoclusters. This chapter is devoted to the study of vibration states in heterogeneous structures. In such systems, the crystalline regularity in the arrangement of atoms is either absent or its effect on the physical properties of the systems is weak, affecting substantially the local spectral functions of different atoms forming this structure. This effect is manifested in the behavior of non-additive thermodynamic properties of different atoms (e.g. mean-square amplitudes of atomic displacements) and in the contribution of individual atoms to the additive thermodynamic and kinetic quantities. The most important elementary excitations appearing in crystalline and disordered systems are acoustic phonons. Moreover, in heterogeneous nanostructures the application of the continuum approximation is significantly restricted; therefore we must take into account the discreteness of the lattice. This chapter contains a theoretical analysis at the microscopic level of the behavior of the spectral characteristics of acoustic phonons as well as their manifestations in the lowtemperature thermodynamic properties. The chapter consists of three sections. The first section contains a detailed analysis at the microscopic level of the propagation of acoustic phonons in crystalline solids and disordered solid solutions. We analyze the changes of phonon spectrum of the broken crystal regularity of the arrangement of atoms in the formation of a disordered solid