Exactly solvable model of a slightly fluctuating ratchet.

We consider the motion of a Brownian particle in a sawtooth potential dichotomously modulated by a spatially harmonic perturbation. An explicit expression for the Laplace transform of the Green function of an extremely asymmetric sawtooth potential is obtained. With this result, within the approximation of small potential-energy fluctuations, the integration of the relations for the average particle velocity is performed in elementary terms. The obtained analytical result, its high-temperature, low-frequency, and high-frequency asymptotics, as well as numerical calculations performed for a sawtooth potential of an arbitrary symmetry, indicate that in such a system, the frequency-temperature controlling the magnitude and direction of the ratchet velocity becomes possible. We clarify the mechanism of the appearance of additional regions of nonmonotonicity in the frequency dependence of the average velocity, which leads to the appearance of additional ratchet stopping points. This mechanism is a consequence of the competition between the sliding time along the steep slope of the highly asymmetric sawtooth potential and the correlation time of the dichotomous noise.

Symmetry of deterministic ratchets.

We consider the overdamped motion of a Brownian particle in an unbiased force field described by a periodic function of coordinate and time. A compact analytical representation has been obtained for the average particle velocity as a series in the inverse friction coefficient, from which follows a simple and clear proof of hidden symmetries of ratchets, reflecting the symmetry of summation indices of the applied force harmonics relative to their numbering from left to right and from right to left. We revealed the conditions under which (i) the ratchet effect is absent; (ii) the ratchet average velocity is an even or odd functional of the applied force, whose dependences on spatial and temporal variables are characterized by periodic functions of the main types of symmetries: shift, symmetric, and antisymmetric, and universal, which combines all three types. These conditions have been specified for forces with those dependences of a multiplicative (or additive-multiplicative) and additive structure describing two main ratchet types, pulsating and forced ratchets. We found the fundamental difference in dependences of the average velocity of pulsating and forced ratchets on parameters of spatial and temporal asymmetry of potential energy of a particle for systems in which the spatial and temporal dependence is described by a sawtooth potential and a deterministic dichotomous process, respectively. In particular, it is shown that a pulsating ratchet with a multiplicative structure of its potential energy cannot move directionally if the energy is of the universal symmetry type in time; this restriction is removed in the inertial regime, but only if the coordinate dependence of the energy does not belong to either symmetric or antisymmetric functions.

High-temperature ratchets driven by deterministic and stochastic fluctuations.

We consider the overdamped dynamics of a Brownian particle in an arbitrary spatial periodic and time-dependent potential on the basis of an exact solution for the probability density in the form of a power series in the inverse friction coefficient. The expression for the average velocity of a Brownian ratchet is simplified in the high-temperature consideration when only the first terms of the series can be used. For the potential of an additive-multiplicative form (a sum of a time-independent contribution and a time-dependent multiplicative perturbation), general explicit expressions are obtained which allow comparative analysis of frequency dependencies of the average velocity, implying deterministic and stochastic potential energy fluctuations. For qualitative and quantitative analysis of these dependences, we choose illustrative examples for spatial harmonic fluctuations: with deterministic time dependences of a relaxation type and stochastic time dependences describing Markovian dichotomous and harmonic noise processes. We explore the influence of fluctuation types on the ratchet effect and demonstrate its enhancement in the case of harmonic noise.

Green light activated hydrogen sensing of nanocrystalline composite ZnO-In2O3 films at room temperature.

The possibility of reducing the operating temperature of H2 gas sensor based on ZnO-In2O3 down to room temperature under green illumination is shown. It is found that sensitivity of ZnO-In2O3 composite to H2 nonmonotonically depends on the oxides' content. The optimal ratio between the components is chosen. The new mechanism of nanocrystalline ZnO-In2O3 sensor sensitivity to H2 under illumination by green light is proposed. The mechanism considers the illumination turns the composite into nonequilibrium state and the photoconductivity change in the H2 atmosphere is linked with alteration of nonequilibrium charge carriers recombination rate.

Pressure and temperature dependence of H-atom tunneling in the Debye approximation. Barrier preparation and media reorganization.

In the frame of the radiationless transitions modified theory, the analytical expression of a rate constant of the chemical reaction with an atom tunneling is found for the case of a continuous spectrum of a phonon subsystem. Two mechanisms of temperature dependence of a rate constant are taken into account, the oscillations of the potential barrier of the reaction at the intermolecular vibrations and media reorganization. The simple expressions for temperature and pressure dependencies of a rate constant are obtained in the special case of lattice motion-the Debye model. The well-known Marcus expression for the rate constant of an electron transfer in the Debye phonon spectrum is deduced first. The pressure dependence of the reorganization energy of the media is derived. Comparison of the theoretical results with the literature experimental data on H-atom tunneling in the fluorene-acridine crystal, taking into account four promotive modes (translational, librational, and two low-frequency intramolecular modes at 95 and 238 cm(-1)) and the frequency dependence of the Grueneisen parameter, is fulfilled. Good agreement of the theory and experiments is observed.

Temperature and pressure dependences of tunneling rate constant: density-functional theory potential-energy surface for H-atom transfer in the fluorene-acridine system.

Temperature and pressure dependences of rate constants for solid phase tunneling reactions are analytically considered within the framework of modified theory of radiationless transitions, taking into account the intermolecular and soft intramolecular promotive vibrations of reagents. This treatment allows us to describe theoretically the process of atomic tunneling and the effect of temperature on the potential barrier and reorganization of the reagents. The influence of external pressure appears in our treatment as a static reduction of widths and heights of the potential barrier with hydrostatic compression of the matrix, and also as an increase of frequencies of promotive vibrational modes owing to anharmonicity. The theoretical results are used to interpret experimental data concerning the effect of temperature and pressure on the hydrogen-atom tunneling in the fluorene-acridine reaction system. It has been shown that by taking into account the contributions from reorganization of the reagents, which statically reduce the tunneling barrier and are related to four types of promotive vibrations (translational, librational, and two low-frequency intramolecular modes at 95 and 238 cm(-1)), one can reproduce the experimental data available in the literature. The parameters of the reaction system required for this analysis are calculated from two-dimensional potential-energy surfaces generated at the DFT-B3LYP/6-31G* level.

[Determination of antioxidative potential of biological fluids by using semiconductor chemical sensors]. / Opredelenie antioksidatnogo potentsiala biologicheskikh zhidkostei s pomoshch'iu poluprovodnikovykh khimicheskikh sensorov.

A procedure has been developed for determining the concentrations of superoxide anion radical (SOAR) in aprotonic fluids by using semiconductor chemical sensors. SOAR life in pure solutions is 6.3 hours. Supplements of biological fluids (blood, urine, plasma) increase the rate of SOAR death. There is a great difference in the reaction rate of SOAR with biological fluids in apparently healthy and ill individuals. It is suggested that the above procedure opens vistas both for studies of vital activity and for diagnosis of abnormalities.