Ivan Baník

Barrier–cluster–heating model and photoinduced phenomena in chalcogenide glasses

Barrier–cluster–heating model of disordered semiconductors was originated by synthesis of the barrier–cluster model (which was described in the previous works of authors) and of the local heating model (published short time ago). The synthesis of these two models brought synergic effect enabling deeper understanding of physical processes in amorphous semiconductors. The article gives a new view on the essence of some optical and photoinduced effects in chalcogenide glasses on the base of the barrier–cluster–heating model.

Some open problems in physics of disordered materials (and their possible solutions on the base of the barrier-cluster-heating model)

The paper briefly summarizes the basic problems in physics of disordered materials, namely in physics of non-crystalline semiconductors. Although there was much attention devoted to the research in this area, many questions about the nature of processes taking place in these substances still remain unanswered. This article familiarizes the readers with the barrier-cluster-heating model of disordered semiconductors and discusses the possibility of explanations of the mentioned phenomena on its base. This article offers the explanation of the mid-gap absorption and mid-gap photoluminescence on the base of the barrier-cluster-heating model.

Order and disorder and their influences on optical absorption of glasses in the gap region

The exponential increase of the absorption coefficient near the absorption edge is usually explained by existence of the density-of-states tails. Among the quoted theoretical models which are widely used to explain the manifestations of the Urbach rule in semiconductors, are the Sumi–Toyozava and the Dow–Redfield models and ab initio (from begining) theory. Our barrier-cluster-heating model assumes the different creating mechanism of exponential tails. The energy by optical transition is provided to electrons except from photons also from vibration of microregion. It deals about the replenishment of absented photons energy, which is smaller as gap width. Absented energy needed for the transition by light absorption is acumulated in certain microregions of material in the form of vibrational energy. At absorption sufficiently big package of accumulated energy can be used. Energy of emptied microarea is filled by phonons from surrounding of microarea (as result of temperature status of surrounding), resp. p...

Mid-gap phenomena in chalcogenide glasses and barrier-cluster-heating model

The physical mechanism of photoluminescence spectrum formation of chalcogenide glasses (CHG) belongs to the important unsolved problems in physics of non-crystalline materials. Photoluminescence is an important means of the electron spectrum investigation. PL spectrum in CHG is produced mostly in the middle of the band gap, and its profile is normal - Gaussian. Several features of PL spectra in CHG is still a great mystery. The aim of the paper is to make reader acquainted with the new insight into the problem. In this article we also deal with the issue of clarifying the nature of mid-gap absorption. From the experiments it is known that after excitation of the glass As2S3 (or As2Se3) with primary radiation from Urbach-tail region the glass will be able to absorb the photons of low energy (IR) radiation from mid-gap region of spectra. This low photon absorption without action of the primary excitation radiation of the higher photon energy is impossible. Mid-gap absorption yields boost in the photolumines...

Accelerated ageing in testing bricks used in the conservation of historic buildings

The effect of accelerated climate ageing on historical bricks in the laboratory is investigated in the paper. Differences in thermal properties are experimentally determined and studied before and after bricks exposure to climate ageing, which consists of 60 freeze–thaw cycles. For measuring thermal conductivity, diffusivity and specific heat, pulse method is used.

Optical Properties Of Cd 1-x Zn x Se From Density Functional Theory

Diluted magnetic semiconductor is used in a wide spectrum of optoelectronic devices, photovoltaic solar cells, laser screen materials and various luminescence devices, etc. Cd1-xZnxSe is one of them, especially important due to variation of its electronic and optical properties by changing the Cd:Zn ratio. In this paper we have applied the full-potential linear-augmented plane wave (FP-LAPW) method within the frame work of the density functional theory (DFT) for structural, electronic, and optical properties calculations for Cd1-xZnxSe. For the purpose of exchange-correlation energy (Exc) determination in Kohn-Sham calculation, the standard local density approximation (LDA) formalism has been utilized. Murnaghans equation of state (EOS) has been used for volume optimization by minimizing the total energy with respect to the unit cell volume. With the knowledge of electronic density of states (DOS) optical responses are calculated. The strong interband transitions between the Se s state and Zn 3d states play the main role in the optical response.

Determination of Poroelastic Parameters of Porous Building Materials

Variations of sorption moisture in the capillary porous materials result in strong fluid-skeleton interactions due to molecular and surface forces, which produce moisture-induced deformation. The effect of moisture sorption on the non-linear elastic behavior of hygroscopic porous building materials has been experimentally investigated showing strong influence of moisture especially in the lower moisture content range. In the framework poroelasticity moisture influence on elastic behavior is described by two poroelastic coefficients, which present the fluid-skeleton coupling. This paper presents an application of the procedure for the determination of the coupling coefficients for medieval brick.

Self-Diffusion on Pd(111) from the Point of View of the Band Model of Diffusion

In this article we present a different view on the results of experimental investigation of the self - diffusion on Pd (111) published in „Surface Science“ [1]. Our consideration is based on the band model of diffusion. This model is able to explain the Meyer-Neldel rule (MNR) and to clarify “puzzles” mentioned in [1]. The aim of this article is also to familiarize the readers with this model, to the band model of diffusion.

Optical Transitions in Chalcogenide Glasses From the Point of View of a Barrier-Cluster Model

A barrier-cluster model of chalcogenide glasses is employed to analyze optical transitions near the absorption edge. The influence of temperature on the optical absorption is studied. The model is used to explain the temperature shift of exponential tails of the optical absorption and the temperature dependence of the optical forbidden-band width at low temperatures.