Vladislav A. Bogoyavlenskiy

Diffusion-limited aggregation: A relationship between surface thermodynamics and crystal morphology

We have combined the original diffusion-limited aggregation model introduced by Witten and Sander with the surface thermodynamics of the growing solid aggregate. The theory is based on the consideration of the surface chemical potential as a thermodynamic function of the temperature and nearest-neighbor configuration. The Monte Carlo simulations on a two-dimensional square lattice produce the broad range of shapes such as fractal dendritic structures, densely branching patterns, and compact aggregates. The morphology diagram illustrating the relationship between the model parameters and cluster geometry is presented and discussed.

Experimental study of negative photoconductivity in n-PbTe(Ga) epitaxial films

We report on low-temperature photoconductivity (PC) in n-PbTe(Ga) epitaxial films prepared by the hot-wall technique on -BaF_2 substrates. Variation of the substrate temperature allowed us to change the resistivity of the films from 10^8 down to 10_{-2} Ohm x cm at 4.2 K. The resistivity reduction is associated with a slight excess of Ga concentration, disturbing the Fermi level pinning within the energy gap of n-PbTe(Ga). PC has been measured under continuous and pulse illumination in the temperature range 4.2-300 K. For films of low resistivity, the photoresponse is composed of negative and positive parts. Recombination processes for both effects are characterized by nonexponential kinetics depending on the illumination pulse duration and intensity. Analysis of the PC transient proves that the negative photoconductivity cannot be explained in terms of nonequilibrium charge carriers spatial separation of due to band modulation. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with a negative correlation energy. Specifics of the PC process is determined by the energy levels attributed to donor Ga III, acceptor Ga I, and neutral Ga II states with respect to the crystal surrounding. The energy level corresponding to the metastable state Ga II is supposed to occur above the conduction band bottom, providing fast recombination rates for the negative PC. The superposition of negative and positive PC is considered to be dependent on the ratio of the densities of states corresponding to the donor and acceptor impurity centers.

Single-bubble sonoluminescence: shape stability analysis of collapse dynamics in a semianalytical approach

This paper theoretically analyzes the hydrodynamic shape stability problem for sonoluminescing bubbles. We present a semianalytical approach to describe the evolution of shape perturbations in the strongly nonlinear regime of violent collapse. The proposed approximation estimating the damping rate produced by liquid viscosity is used to elucidate the influence of the collapse phase on subsequent evolution of the Rayleigh-Taylor instability. We demonstrate that time derivatives of shape perturbations grow significantly as the bubble radius vanishes, forming the dominant contribution to destabilization during the ensuing bounce phase. By this effect the Rayleigh-Taylor instability can be enhanced drastically, yielding a viable explanation of the upper threshold of driving pressure experimentally observed by Barber et al. [Phys. Rev. Lett. 72, 1380 (1994)].

Photoconductivity kinetics in high resistivity n-PbTe(Ga) epitaxial films

We present a study of photoelectric and kinetic properties of n-PbTe(Ga) epitaxial films prepared by the hot wall technique on BaF2 and SiO2-Si substrates. The measurements were performed under continuous and pulse illumination with use of a heat source and LED in the temperature range 4.2-300 K. The investigated samples were highly sensitive to infrared illumination at temperatures lower than 100-110 K. Two characteristic regions were clearly resolved in photoconductivity relaxation curves: a fast one at the beginning of relaxation (with lifetimes of about 10-3 s at 4.2 K) and a long duration relaxation tail. The obtained results are discussed in terms of the configuration diagram of a DX-like centre in PbTe(Ga). The experimental data obtained for the films are compared with properties of bulk material.

Differential criterion of a bubble collapse in viscous liquids.

The present work is devoted to a model of bubble collapse in a Newtonian viscous liquid caused by an initial bubble wall motion. The obtained bubble dynamics described by an analytic solution significantly depends on the liquid and bubble parameters. The theory gives two types of bubble behavior: collapse and viscous damping. This results in a general collapse condition proposed as the sufficient differential criterion. The suggested criterion is discussed and successfully applied to the analysis of the void and gas bubble collapse.

Diffusion-limited aggregation: a revised mean-field approach

We propose a revision of the classic mean-field approach of diffusion-limited aggregation (DLA) model originally introduced by Witten and Sander [Phys. Rev. Lett. 47, 1400 (1981)]. The derived nonlinear mean-field equations providing lattice anisotropy are used to model diffusional growth on square lattice in linear and circular source geometries. The overall cluster shapes obtained from the mean-field calculations are found to satisfy the known scaling behavior experimentally observed for DLA simulations.

How to grow isotropic on-lattice diffusion-limited aggregates

We consider the problem of on-lattice cluster anisotropy in the diffusion-limited aggregation (DLA) model. On the basis of a recent paper (Bogoyavlenskiy V A 2001 Phys. Rev. E 64 066303), we derive an isotropic quadratic ratio for a set of aggregation probabilities, in order to grow on-lattice DLA clusters without anisotropy.

Thermally induced currents and instabilities of photoresponse in PbTe(In)-based films

The thermally induced currents (TIC) and instabilities of photoresponse in Pb1-x-ySnxGeyTe(IN) films on BaF2 substrates have been investigated at temperatures 4.2 < T < 30 K. The temperature TM corresponding to maximum values of single TIC peaks appears to be extremely low varying from 6 K up to 14 K. The combined effect of lighting and slight heating of the sample from 4.2 K up to approximately 6 K results in photoresponse instabilities depending on the experimental regime. The results are interpreted in terms of thermal excitation of charge carriers from metastable impurity level.

Photoelectric and kinetic properties of PbTe(Ga) films

The photoelectric and kinetic properties of n-PbTe(Ga) films prepare don the BaF2 and Si substrates by hot wall epitaxy technique have been investigated in the temperature T interval 4.2 divided by 300 K. The photoelectric measurements show that all photosensitive samples may be divided into two groups characterized by positive and negative photosensitivity to IR-illumination at temperatures close to 4.2 K. The kinetics of positively photosensitive films is found to be similar to the kinetics of the bulk n-PbTe(Ga) single crystals of high resistance. The photoresponse of negatively photosensitive films appears to be composed from two parts. They are the dominant negative photosensitivity and a positive photoconductivity signal characterized by significantly faster kinetics. The experimental result are discussed in terms of DX-like behavior of the impurity centers.

Characteristics of diode structures based on the p-Pbte(Ga)–In contact

Abstract A pronounced nonlinearity of the current–voltage characteristics (CVC) accompanied by a high photosensitivity to IR-illumination was observed in In contact–[p-PbTe(Ga)]–Pt contact structures. The experimental data are discussed in terms of energy-band distortion near the Schottky barrier taking into account that at high-ohmic state formation the Fermi level (FL) is pinned by an impurity level within the energy gap. The possible influence of the crystal imperfections is also considered.