22 research outputs found
Local vibrations in real crystals with combined defects
The conditions of formation and the main characteristics of local vibrations due to the presence of light impurities Al, Mg, and Mn in the bulk and on the close-packed [111] surface of the fcc crystal lattice of silver are investigated. The influence of surface roughness is analyzed. Analytical approximations are obtained which describe the frequency and intensity of the local vibrations at the impurity atom and its nearest neighborhood to high accuracy. The proposed analytical approximations provide a practical way of determining the parameters of the defect structure and the interatomic interactions from the experimentally determined characteristics of the local vibrations
Low-frequency properties of the phonon spectra, and low-temperature thermodynamics of disordered solid solutions
This is an analysis of the properties of quasi-local vibrations, and the conditions of the formation thereof, in a realistic model of the crystal lattice on a microscopic scale. The evolution of quasi-local vibrations with an increase in the concentration of impurity atoms, is examined. It is shown that the formation of boson peaks occurs mainly due to the additional dispersion of high-velocity acoustic phonons (connected to the atomic vibrations of the main lattice), caused by the scattering of these phonons by the quasi-local vibrations localized at the impurities. We demonstrate a connection between the boson peaks in disordered systems, and the first van Hove singularity, in regular crystal structures. We analyze the manifestation of quasi-local vibrations and boson peaks, as it relates to the behavior of low-temperature heat capacity, and how it changes with an increasing impurity concentration
Loffe-Regel' crossover and boson peaks in disordered solid solutions and similar anomalies in heterogeneous crystalline structures
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
Phonon spectrum and vibrational characteristics of linear nanostructures in solid matrices
The atomic dynamics of linear chains embedded in a crystalline matrix or
adsorbed on its surface is studied. A linear chain formed by substitutional
impurities in a surface layer and at the same time offsetting from this layer
was analyzed particularly. This system models the actively studied
experimentally structures in which gas molecules are adsorbed on the walls of
the bundles of carbon nanotubes located in certain medium.
It is shown that the quasi-1D features are typical for the chains in which
the interatomic interaction is higher than the interaction between the atoms of
the chain and the atoms of the crystal matrix. On the local phonon density of
atoms of the chain, the transition to quasi-one-dimensional behavior has the
form of the kink. In other words, it is the first (lowest-frequency) van Hove
singularity, which in 3D structures (the system under consideration is
generally three-dimensional) corresponds to the transition from closed to open
constant-frequency (quasi-plane) surfaces. The local phonon densities of atoms
in the chain have one-dimensional character at frequencies higher than the
frequency of the van Hove singularity.
The rms-amplitude of embedded chains atoms vibrations is calculated and the
behavior of the atomic vibrations contribution in the low-temperature heat
capacity of the system is analyzed.Comment: 14 pages, 7 figure
Oscillatory spectra of surface atoms in strongly anisotropic layered crystal (quasi-1D behaviour)
Vibrations localized near the surface have been analyzed using the Jacobian matrix method taking into account discreteness of the lattice. It has been shown that localized surface vibrations in layered crystals have got quasi-one-dimensional character and their properties are described by exact solutions obtained in the framework of the one-dimensional model
Deuteration effects in the thermal conductivity of molecular glasses
The thermal conductivity κ(T) of pure deuterated ethanol has been measured under its equilibrium vapor pressure in its orientationally-ordered crystal (T = 2 K – Tm), orientational glass and the glass state (T = 2 K – Tg, Tg is the glass transition temperature) solid phases. The temperature dependence of the conductivity is well described by a sum of two contributions: κ(T) = κI(T) + κII(T), where κI(T) account for the heat transport by acoustic phonons and κII(T) for the heat transfer by localized high-frequency excitations respectively. The thermal conductivities of deuterated and hydrogenated ethanols are compared in different phases. The phonon scattering mechanisms in the glasses have been analyzed. In the investigated glasses the effect of complete deuteration shows up as a contribution κII(T)