[en] We consider the influence of fluctuations in a screw flow of a conducting liquid on the effect of magnetic field self-excitation; the solution of this problem is important for experimental realization of a turbulent dynamo. We propose a theoretical approach based on the solution of averaged equations obtained in the limit of a short correlation time. The applicability of this approach is confirmed by direct numerical simulation of the initial equations. We demonstrate the influence of the correlation of fluctuations on the dynamo effect threshold. It is shown that the solution of the mean-field equations differs from the solution based on direct numerical simulation for a finite correlation time. The advantages and disadvantages of the two approaches are estimates, as well as the importance of the discovered difference in the context of problems of magnetic field self-excitation. The influence of helicity and intermittency on the type of the solution is considered.

[en] ZnO together with TiO₂ is a main photocatalyst for various redox reactions to convert light energy into a chemical one and to purify the environment. Intrinsic surface defects in ZnO—the vacancies in anionic and cationic sublattices (F-type and V-type centers)—allow creation of long-lived (up to 10³ s) photocatalysis centers and, therefore, tenfold increase in quantum yield of reactions. Slow surface states—the photocatalysis centers—appear via diffusion of electrons and holes generated during the interband transitions in the bulk of a photoactivated sample. The transfer efficiency, however, decreases sharply because of recombination of charge carriers and losses during overcoming the surface Schottky barrier. Neutral energy carriers—excitons—were used in this work to decrease these losses during the energy transfer to a surface. High exciton binding energy in ZnO (60 meV) allows it to move at room temperature without decay. The exciton energy loss for radiation is effectively decreased in our experiments via formation of a 2D surface structure. The results confirm high efficiency of exciton channel to form surface long-lived photocatalysis F-centers and V‑centers during the photoadsorption and photodesorption processes of oxygen, which simulate full cycle of a redox photocatalytic reaction.

[en] The screw dynamo is a critical phenomenon—a self-excitation that occurs only when the magnetic Reynolds number (Rm) reaches some threshold value. Temporal magnetic field growth is shown to occur at Rm slightly smaller than this critical value. The appearance of temporal growth is also limited from below by the second critical value of Rm. If, however, a seed magnetic field is present permanently, and not just at the initial time of evolution, temporal subcritical magnetic field growth gives rise to a stationary magnetic configuration. This stationary magnetic field turns out to be particularly strong if the seed field in the form of a traveling wave coincides in phase velocity with the magnetic field that emerges in the presence of a self-excitation. The possibilities for detecting such a resonance in the subcritical regime of operation of an experimental setup based on the screw dynamo are considered.

[en] 

Abstract—

: The concentration dependences of the magnetic phase transition temperature T_N, determined based on the changes in Mössbauer spectra, were investigated for a BiFeO₃ solid solution with ordered perovskite PbFe_0.5Sb_0.5O₃ and for a disordered BiFe_1 – xCr_xO₃ solid solution. It is established that the magnetic-order type in BiFe_1 – xCr_xO₃ changes from antiferromagnetic to spin-glass at a higher degree of Fe-sublattice dilution in comparison with the PbFe_0.5Nb_0.5O₃-based solid solutions presumably because of the smaller BiFeO₃ lattice parameter. Due to the local ordering of Fe³⁺ and Sb⁵⁺ ions, the T_N value decreases more rapidly (as compared to BiFe_1 – xCr_xO₃) with a decrease in the Fe³⁺ content in the lattice of (1 – x)BiFeO₃– xPbFe_0.5Sb_0.5O₃ solid solutions.