Influence of image forces on charge-dipole interaction in two-layered systems.

Interaction between a fixed point electric charge Q and a freely rotating point electric dipole with the magnitude P pinned near a plane interface between two dispersionless insulators with different dielectric permittivities ɛ1 and ɛ2 has been considered. It was shown that, as a result of this interaction and the interaction of the dipole with the polarization charges induced at the interface by the charge Q and the dipole itself, there arise regions where the dipole can possess either one or two equilibrium orientations. The spatial distributions of the electrostatic dipole energy Wtotal under the combined action of the charge Q and the induced interface polarization charges, as well as the equilibrium dipole orientations (orientation maps), the boundaries between the regions with different numbers of dipole orientations, and their evolution with the variation of problem parameters (the charge and dipole magnitudes, the mismatch between ɛ1 and ɛ2, and the charge-interface distance) were calculated. It was shown that there can emerge local minima of Wtotal, which may play the role of traps for dipoles (in particular, excitons in layered structures), and the corresponding requirements for the problem parameters were found. Most results were obtained in analytical form. The model can be applied to various physical systems, for instance, polar molecules, excitons, and trions in layered structures.

Hierarchical Structuring of Black Silicon Wafers by Ion-Flow-Stimulated Roughening Transition: Fundamentals and Applications for Photovoltaics.

Ion-flow-stimulated roughening transition is a phenomenon that may prove useful in the hierarchical structuring of nanostructures. In this work, we have investigated theoretically and experimentally the surface texturing of single-crystal and multi-crystalline silicon wafers irradiated using ion-beam flows. In contrast to previous studies, ions had relatively low energies, whereas flow densities were high enough to induce a quasi-liquid state in the upper silicon layers. The resulting surface modifications reduced the wafer light reflectance to values characteristic of black silicon, widely used in solar energetics. Features of nanostructures on different faces of silicon single crystals were studied numerically based on the mesoscopic Monte Carlo model. We established that the formation of nano-pyramids, ridges, and twisting dune-like structures is due to the stimulated roughening transition effect. The aforementioned variety of modified surface morphologies arises due to the fact that the effects of stimulated surface diffusion of atoms and re-deposition of free atoms on the wafer surface from the near-surface region are manifested to different degrees on different Si faces. It is these two factors that determine the selection of the allowable "trajectories" (evolution paths) of the thermodynamic system along which its Helmholtz free energy, F, decreases, concomitant with an increase in the surface area of the wafer and the corresponding changes in its internal energy, U (dU>0), and entropy, S (dS>0), so that dF=dU - TdS<0, where T is the absolute temperature. The basic theoretical concepts developed were confirmed in experimental studies, the results of which showed that our method could produce, abundantly, black silicon wafers in an environmentally friendly manner compared to traditional chemical etching.

Electric dipole image forces in three-layer systems: The classical electrostatic model.

General exact analytical expressions have been derived for the image force energy Wi(Z, φ) of a point dipole in a classical three-layer system composed of dispersionless media with arbitrary constant dielectric permittivities εi. Here, i = 1-3 is the layer number, and Z and φ are the dipole coordinate and orientation angle, respectively. It was found that the long-range asymptotics Wi(Z→∞,φ) in both covers (i = 1, 3) are reached unexpectedly far from the interlayer (i = 2). Another specific feature of the solution consists in that the interference of the fields created by polarization charges emerging at both interfaces leads to the appearance of a constant contribution inside the interlayer with a non-standard dependence on the dipole orientation angle φ. It was shown that by changing the dielectric constants of the structure components, one can realize two peculiar regimes of the Wi(Z, φ) behavior in the covers; namely, there arises either a potential barrier preventing adsorption or a well far from the interface, both being of a totally electrostatic origin, i.e., without involving the Pauli exchange repulsion, which is taken into account in the conventional theories of physical adsorption. The results obtained provide a fresh insight into the physics of adsorption in physical electronics, chemical physics, and electrochemistry.

Switch from thermal to force-driven pathways of protein refolding.

The impact of the quenched force on protein folding pathways and free energy landscape was studied in detail. Using the coarse-grain Go model, we have obtained the low, middle, and high force regimes for protein refolding under the quenched force. The folding pathways in the low force regime coincide with the thermal ones. A clear switch from thermal folding pathways to force-driven pathways in the middle force regime was observed. The distance between the denatured state and transition state xf in the temperature-driven regime is smaller than in the force-driven one. The distance xf obtained in the middle force regime is consistent with the available experimental data suggesting that atomic force microscopy experiments deal with the force-regime which is just above the thermal one.

The phase diagram for coexisting d-wave superconductivity and charge-density waves: cuprates and beyond.

Phase diagrams of d-wave superconductivity characterized by an order parameter Δ coexisting with charge-density waves (CDWs) characterized by an order parameter Σ were constructed for the two-dimensional Fermi surface (FS) appropriate to, e.g., cuprates. CDWs were considered as an origin of the pseudogap appearing at antinodal FS sections of the d(x2-y2) superconductor. Two types of the Σ-reentrance were found: with the temperature, T, and with the opening of the CDW sector, 2α. The angular plots in the momentum space for the resulting gap profile over the FS ('gap roses') were obtained. The gap patterns are rather involved, giving insight into the difficulties of the interpretation of photoemission spectra. It was shown that the Σ-Δ coexistence region exists even for the complete dielectric gapping due to the distinction between the superconducting and CDW order parameter symmetries. The checkerboard and unidirectional CDW configurations were examined, and both the phase diagrams and the behavior with T and α of the order parameters were found to differ. A more general case with a non-zero mismatch angle ß between the superconducting lobes and the CDW sectors was analyzed, the case ß = π/4 corresponding to the d(xy) symmetry of the superconducting order parameter. The phase diagrams were found to be sensitive to ß-variations, showing that internal strains and external pressure can drastically affect the behavior of Σ(T) and Δ(T).