Vjekoslav Sajfert

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).

Phonon contribution to heat capacitance of nanolayered crystalline structures

The paper presents the innovated method of two-time dependent Greens functions applied to the bounded and translational perturbed systems. Film-structures and superlattices were analyzed by employing the phonon spectra for the calculation of relevant thermodynamic characteristics. Free energy and the internal energy of the system were found as well as the temperature behavior of layered structures. Heat capacitances of these nanostructures were compared to bulk ones. It was shown that heat capacitances of nanolayered structures in low-temperature region were higher than the same quantities of the corresponding bulk sample. In the middle and the highest temperature region, temperature behavior was inverse: heat capacitance of layered structures was lower than of the corresponding bulk ones. The consequences were discussed with relation to the (super)conductive properties of nanomaterials.

Phonon Thermodynamics in Crystalline Nanofilms

The dispersion law and density of states of phonons in ultrathin films was analyzed in this paper. It turned out that phonons in a thin film require activation energy for exciting. This leads to extremely low specific heat and specific conductivity at low temperatures. Consequences of quoted facts were discussed in detail and their influence on kinetic and thermodynamic properties of thin films is estimated.

THERMODYNAMICS OF MECHANICAL OSCILLATIONS IN CRYSTALLINE SUPERLATTICES

Phonon spectra as well as thermodynamic characteristic of superlattice are analyzed using the method of two-time dependent Greens functions. Free and internal energy of the system as well as specific heat and entropy are found. The temperature behavior of superlattice specific heat is compared with specific heats of bulk structures and thin films. It was shown that at extremely low temperature, superlattice and film specific heats are practically equal and considerably lower than specific heat of bulk sample. The consequences of this fact to the thermal, conducting and superconducting properties of materials was discussed.

Exciton Dispersion Law and States of Bimolecular Thin Films

Dispersion laws and states (i.e. probability of finding) of Frankel excitons in ultra-thin molecular films are found using a well-known method of Green’s functions. Space boundaries and changes of energetic parameters on boundaries are considered as perturbations. The cubic crystalline system with complex cell consisted of two molecules (a and b), i.e. bimolecular film, was analyzed in harmonic approximation, and then compared with the results obtained for simple cubic cell systems (i.e. monomolecular film). In both cases the energy spectra show sharp discrete levels, although the energy spectra of bimolecular films split into two zones with discrete levels. Probability of finding exciton in the mono- or bimolecular ultra-thin films is significantly influenced by the perturbation and the values of on-site energies of molecules a and b. Obtained conditions of the existence of localized exciton states at boundaries are of special interest.

Cylindrical quantum dots with hydrogen-bonded materials

The properties of cylindrical quantum dots are analysed in this paper. Energies of elementary excitations as well as one-particle wavefunctions are found for this structure. For cylindrical quantum wires the temperature of phase transition is found. The behaviour of electric susceptibility in the paraelectric phase is investigated. It is shown that the entropy of cylindrical quantum dots is a quantity depending on two indices. The arithmetical average value of all elements of this matrix is less than the entropy of the corresponding ideal structure.

Perturbation method in the analysis of thin deformed films and a possible application

Abstract The perturbation method for the analysis of thin, manifestly deformed films is given. The application of the method to the excitons in the film has shown that they have an effective mass essentially depending on the propagation direction. The effects of a mechanical deformation of the film were investigated. It was concluded that the film could serve as an emitter of infrared radiation if the mechanical deformation periodically changes in time.

Thermodynamical properties of ferroelectric films

Abstract The elementary excitations in thin ferroelectric films are analysed. It is assumed that the film is deformed only on the boundary surfaces. The dispersion laws of volume as well as the surface excitations are found. The conditions for the surface states existence are discussed. It is shown that two order parameters appear: the volume order parameter and the surface one. It is shown that at T = 0K, when there is full order in the volume, the surface order is destroyed. Both order parameters vanish at the same critical temperature. The temperature behaviours of the order parameters in the film and in the bulk structure of the same material are also compared. It appears that the transition temperature in the film is about 40 per cent higher than in a bulk structure. The more stable ferroelectric order in films makes them more applicable for practical purposes.

CHARGE TUNNELING IN DOPED QUANTUM ROD

This work analyzes the Abrikosovs idea that high temperature conductivity is based on resonant tunneling. A rectangular rod was considered and it was shown that high values of transparency coefficient can be produced in it. It is the consequence of the fact that electrons as well as phonons have energy gap within a rectangular rod.

Excitation on the Coherent States of Pseudoharmonic Oscillator

In the last decades, much attention has been paid to the excitation on coherent states, especially for coherent states of the harmonic oscillator ([1] and references therein). But an interesting anharmonic oscillator with many potential applications is also the pseudoharmonic oscillator (PHO). So, in the present paper we have defined the excitation on the Klauder‐Perelomov coherent states (E‐KP‐CSs) for the PHO. These states are obtained by repeatedly operating the raising operator K+ on a usual Klauder‐Perelomov coherent state (KP‐CS) of the PHO [2]. We have verified that really, the E‐KP‐CSs fulfill all the properties of the coherent states, as stated by Klauder [3]. We have examined the nonclassical properties of the E‐KP‐CSs, by using the density matrix formalism and examining the dependence of the Mandel parameter Qz,k;m(|z|2) on the |z|2 and on the m. It seems that these states can be used in optical communication field and in the physics of quantum information, as signal beams, due to the fact that...