A. Feher

Effect of defects on the quasiparticle spectra of graphite and graphene

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.

Resonance absorption, reflection, transmission of phonons and heat transfer through interface between two solids

The different mechanisms of resonant transport of phonons between two media in the presence of an impurity intermediate layer are described. Particular attention is focused on the resonance interaction of elastic waves with a two-dimensional defect on the contact boundary between two solids, on the multichannel interface phonon scattering, and on the experimentally observed nonmonotonic temperature dependence of the reduced heat flux. In the cases when there is a direct interaction between edge atoms of the matrix as non-nearest neighbors or when the impurities do not fill completely the 2D interface layer, an additional channel for the transmission of phonons through the interface opens. This additional transmission channel manifests itself as a transmission (or reflection or absorption) peak with an asymmetric line shape (the so-called Fano-like resonance for phonons due to interference between the two transmission channels). Some applications of the Fano-like interference phenomena in magnon heat condu...

Phonon transport in pressure-made point contacts

A theoretical and experimental analysis of the heat conductivity in metal-dielectric point contacts is given. The contribution of the phonon diffraction effects to the heat conductivity is investigated. The phonon heat transport through the point contacts is measured from 0.1 K to 100 K using the anvil-needle technique. In KBr-KBr and KBr-Cu point contacts, substituting a Cu needle for the dielectric cold edge does not disturb the heat conductivity pattern. Measurements for Si-Cu point contacts reveal well-defined diffraction maxima of reduced heat conductivity at temperatures in the range 0.5 K-1 K.

Quasi-Particle Spectra on Substrate and Embedded Graphene Monolayers

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,

The specific heat of - a quasi-one-dimensional Ising ferromagnet

The specific heat of a layered single crystal, , was measured in the temperature range 0.45 K-6.5 K in zero magnetic field. A -type anomaly found at is associated with a phase transition into the ordered state. The experimental values of the magnetic entropy indicate deviations from two-dimensional behaviour of the magnetic system. Honmuras model of a 2D assembly of coupled S = 1/2 Ising ferromagnetic chains was used for the specific heat data analysis. The estimated value of the intrachain interaction is and that of the interchain interaction is ; these were obtained from the fitting procedure for the ordered phase. The fit for the paramagnetic phase yields and . The comparison of the results of the analysis with theoretical calculations made on the assumption of a pure dipolar character of the magnetic correlations is discussed.

Electronic and Vibrational Properties of Adsorbed and Embedded Graphene and Bigraphene with Defects

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.

The heat capacity of KDy(MoO4)2 near the magnetic phase transition

The results of the heat capacity measurements are compared with theoretical prediction for Ising 2D and 3D models. The behaviour of the heat capacity at temperatures T>Tc is described by the 2D Ising model. At temperatures below Tc the crossover of the magnetic dimensionality was observed from 2D to 3D Ising-like behaviour near Tc.

The Features of Low Frequency Atomic Vibrations and Propagation of Acoustic Waves in Heterogeneous Systems

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