B.S. Tošić

Thermodynamical properties of ultrathin layered structures

Thermodynamical properties of ultrathin films were analysed using Greens function method. Mean-square of the molecular displacements, mean-square of the velocities and the phonon contribution to the specific heat of the thin films were calculated. A comparison with crystal bulk has shown that thermodynamical properties of thin films are strongly influenced both by the sample dimensions and boundary conditions. The results of the specific heat analyses make the validity of free surface model doubtful, because the specific heat calculated in this model becomes negative at very low temperature.

Kinetics of thin films mechanical oscillations

Thermal conductivity of thin film is analyzed in this paper. It has been shown that at low temperatures, thermal conductivity of film is considerably lower than that of bulk structure. Also, the dependence of activation energy on boundary conditions has been investigated. It has been found that the implantation of light molecules in film boundaries leads to considerably higher activation temperatures. Our theoretical results proved to be in agreement with some of the latest experimental data.

Phonons in ultrathin layered structures

Abstract Phonon subsystem of ultrathin films was analysed using Green’s functions of the displacement–displacement type, including quantum size effect and effect of boundaries onto Hamiltonian parameters. Diagonal components of Green’s functions were evaluated, which enabled determination of the dispersion law, local densities of phonon states and Debye frequencies for thin films. A comparison with crystal bulk has shown that properties of thin films are strongly influenced both by the sample dimensions and boundary conditions. We have stated the conditions under which metal conductors could be made to become better superconductors.

Green's function theory of free electrons in thin films

The electronic subsystem of ultrathin films was analysed using Greens function method including quantum size effect and the effect of boundaries to Hamiltonian parameters. Diagonal components of Greens functions were evaluated which enabled a complete insight into the thermodynamical properties of electrons. A comparison with crystal bulk has shown that electronic properties of the materials are strongly influenced both by the sample dimensions and boundary conditions. The numerical calculations show that the electron distribution may be manipulated by varying the surface parameters which is significant for the operation of devices based on thin films.

Phonon spectra in superconducting ceramics

A model based on the continual deformation of mass distribution by sputtering of thin films is formed. Presented theory shows that due to mass deformation acoustical phonons are eliminated which causes a higher critical superconductive temperatures. The theory and experimental data for ceramic oxides are in good agreement. Presented theory should serve the synthesis of new superconductive materials. Some general considerations concerning specificity of phonon spectra in thin films are also exposed.

Exciton concentrations in thin films

Excitons in thin films are analyzed using Greens function method within the framework of Tyablikovs version of random phase approximation. The spectra of surface and bulk excitons were determined and the distribution of exciton concentrations over film layers was evaluated. It was shown that high surface concentrations could be achieved by decreasing of the dipole-dipole interaction of the molecules in surface layers.

On the problem of exciton-phonon interaction

Abstract The main part of the exciton-phonon interaction is considered to be a consequence of a local deformation of the electromagnetic field due to the molecular vibrations.

Surface deformation in films and exciton concentration

The exciton states in a film which was cut out from an anisotropic cubic structure are analysed whereby the calculations include excitonic concentrations. It is shown that excitonic states in the bulk always exist, without any additional conditions on the type of the deformation or on exciton concentrations. It is found that under certain conditions (the surface concentration and the concentration in the bulk having approximately equal values), the effective mass of the excitons in the bulk is dependent on the azimuth in the k space i.e. the effective mass belonging to bulk excitons can alter its sign when the direction of propagation is altered. The surface excitons can exist also, but only for a specific set of values for the deformation and the concentration given by a formula quoted in this paper. It is shown that by an appropriate deformation of the film one can achieve the surface concentrations of the order of unity (i.e. C = 0.25).

Low-temperature properties of the Ising model in the transverse field and the influence of kinematical effects

We present the low-temperature method for the Ising model in the transverse field based on the approximate boson expression for Pauli-operators. Kinematic effects, which arise through special decoupling of Greens functions, lead to appearance of additional energy levels.

Free electron distribution in thin films

In this paper we analyze the electron states in metallic films. The first part of the paper is devoted to the study of the film where ion-ion interaction changes only in boundary layers. It was concluded that bulk states always appear in such film. Surface states appear only if the variation of the ion-ion interaction in the direction of the symmetry breaking in the boundary layer are neutralized. If the conditions for the creation of surface states are realized, one can obtain dense currents running along the film surface.