J. Kasperczyk

Low temperature anomalies in polyvinyl alcohol photopolymers

Acoustooptical parameters in a mixture of polyvinyl alcohol (PVA) polymers with modified polyvinyl alcohol (MPA) were investigated as functions of the MPA content, time of exposure and temperature. The maximum value of the acoustooptical quality coefficient was achieved for values of the MPA content above 70%. It was proposed to use the polyvinyl alcohol photopolymers as acoustooptical modulators and deflectors. An essential influence of the polyvinyl alcohol modifications on the acoustooptical parameters was unambiguously shown. Also, a critical dependence of the acoustooptical coefficients on the sample thickness was revealed. The data on the acoustooptical quality coefficient correlate well with the data obtained for the sound losses coefficient.

The structure and electronic properties of silicon oxynitride gate dielectrics

Despite considerable progress achieved over the past few years in understanding ultrathin oxynitrides, several fundamental questions, in particular, the oxynitridation mechanism and the mechanisms behind the beneficial role of nitrogen are still not well understood. To improve understanding of the explanations which have been proposed for the phenomena specific to silicon oxynitride and for the nature of the defects, a study of the electron structure of a MOS system using silicon oxynitrides as the gate oxide and based on a molecular dynamics geometry optimization method, was carried out. Investigations of the band energy parameters versus the SiON film thickness and oxygen to nitrogen ratio were done theoretically as well as experimentally. Theoretical calculations were done by norm-conserving non-local pseudopotentials together with molecular dynamics geometry optimization. Experimental investigations included spectroscopic investigations of the film absorption. Both theoretical and experimental data indicate that the effective band energy gap possess modulated-like dependence versus the film thickness and oxygen/nitrogen ratio. The origin of the observed phenomenon is caused by specific electron-phonon anharmonic interactions between the film and the Si substrate.

Description of spin interactions in model [Fe6S6]4+ supercluster

A theoretical study of Heisenberg exchange and double exchange interactions in iron–sulphur superclusters is presented. The clusters can play an important role in biological systems (proteins and enzymes) acting as so-called active centers. The cluster with valence 4+ can be modeled by four Fe(III) and two Fe(II) ions. An idealized structure of double cubane has been considered instead of a more realistic defected double cubane structure of lower symmetry. Possible spin ground states (S=0,1,2,3,4,5,6) have been predicted. The total spin S=6 is preferred within a partially ferromagnetic region if a Heisenberg exchange parameter [between the nearest Fe(III) ions] J2 0, domination of the spin states with S=0 and S=4 is expected for larger values of the parameter b. The state of S=4 is predicted for J2>0 in agreement with experimental data.

Magnetic field induced ferroelectricity in copper doped CdI2 single crystals

Abstract We have discovered a new type of ferroelectricity in highly anisotropic CdI2 layered crystals doped with copper. A spontaneous electric polarization is created in result of interaction between long-range interlayer polar acoustical vibrations and magnetically oriented copper ions. The phenomenon is strongly enhanced after applying external magnetic field at liquid helium temperatures. The strongest effect has been observed in the case of magnetic field direction perpendicular to the layers. Optical measurements were carried out using a second harmonic generation method.

Electronic and Optical Parameters of the TGM-3 Photopolymer

Molecular properties of the TGM-3 photopolymer have been investigated using a HYPERCHEM 4.0 computer package. The geometry of this photopolymer was successfully optimized and the TGM-3 structure obtained shows a very complicated non-linear shape. The total charge density of TGM-3 is strongly nonuniform. C=O double chemical bonds are roughly perpendicular to a bent central chain. We have calculated a value of the HOMO–LUMO energy gap which is not in agreement with our experimental data for the fundamental absorption edge. A possible origin of this difference is suggested. One can expect that the TGM-3 properties have a strong influence on its solid many-component mixtures with other photopolymers.

Electronic structure and optical spectra of MDF-2 oligoetheracrylate

The molecular spatial structure and electronic energy levels of MDF-2 oligoetheracrylate have been theoretically investigated using a Hyperchem 4.5 computer package. We have optimized the MDF-2 geometry, obtaining a non-linear structure with one benzene ring situated near one of the molecule ends. The electron charge density of the MDF-2 photopolymer is distributed non-uniformly, showing the highest absolute values in the vicinity of the benzene ring. C=O double chemical bonds are perpendicular with regard to the central chain. We have calculated a relatively large value of the HOMO-LUMO energy gap on one hand and, on the other hand, we have estimated a substantially smaller experimental energy gap based on our spectroscopic measurements. An essential influence of the benzene rings on the non-linear optical properties has also been suggested. To check the above semi-empirical results, ab initio calculations have been performed for the molecule fragments as well as for the full MDF-2 structure. An enlarged structure (a tetramer) has also been investigated using the UHF method with a PM3 parametrization.

Quantum chemical modelling of electronic charge density distribution in several tetrathiafulvalene derivatives

We have calculated electronic charge density distributions for typical representatives of tetrathiafulvalene derivatives. We have applied a combined semi-empirical and ab initio method to perform these calculations because of complexity of the mentioned molecules. Our calculations have unambiguously shown an essential influence of backbone aromatic group modifications on distribution of electronic charge density gradients. Moreover, we have observed a substantial change of the charge density gradients under influence of benzene ring modifications that leads to a strong non-uniformity of the electronic charge density distributions. It was shown that the main density change was observed on the opposite (with respect to the structural modification) end of a given molecule.

Influence of domains on nonlinear optical properties of solids

Abstract The investigations of an influence of different macroscopic domain structures on nonlinear optical susceptibilities of several solids were performed. We have chosen a semi-empirical tight-binding calculational method in order to calculate nonlinear optical third rank polar tensor components. The domain walls of different nature (magnetic, ferroelectric, superconducting twins and incommensurate phase) were considered by means of breaking the translational symmetry in the first order perturbation approach. We have predicted an appearance of an oscillatory behaviour of the nonlinear optical susceptibilities as the functions of the domain sizes. The above mentioned theoretical results are in good agreement with our experimental measurements of optical second harmonic generation.

Optimized structure of SiNxCy–Si⟨111⟩ interfaces

SiNxCy films were synthesized using a technique of chemical evaporation at low pressure. We have carried out quantum chemical and molecular dynamics simulations of the film structure versus the nitrogen/carbon ratio. The film thickness varied between 8–24 nm. The stoichiometry of these films was measured after their deposition on Si[111] substrates using an extended x-ray absorption fine structure (EXAFS) method. A comparison was made of the experimentally obtained and molecular dynamics optimized structure. At the same time, pressure derivatives of the total energy within the calculated local density approach agree well with the experimental data. Sufficient agreement between the experimental EXAFS data and the simulated structural data indicates the crucial role of anharmonic electron–phonon interactions.

Electronic charge density distributions in tetrathiafulvalene derivatives

We have calculated electronic charge density distributions for typical representatives of tetrathiafulvalenes. We have applied a combined semi-empirical and ab initio method to perform these calculations because of the complexity of these molecules. Calculations have unambiguously shown an essential influence of modifications of the back-bone aromatic group on the distribution of electronic charge density. Moreover, we have observed a substantial change of the charge density gradients under the influence of aromatic ring modifications. This leads to a strong nonuniformity of the electronic charge density distribution. It is surprising that the main charge density change is observed on the other molecule end than the structural modification (e.g. a substitution) has been made.