AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense (n=4×10¹¹ cm^-2) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ=Γ(ω,H)-Γ(ω,0), and change of velocity of SAW, ΔV/V=(V(H)-V(0))/V(0), were measured in the temperature interval T=1.5–4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f=30–300 MHz. Based on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.     

Low-temperature normal-pressure CVD of carbon nanotubes from CO + H2 gas mixture: Thermodynamic analysis and experimental observation

It has been shown that the deposition of carbon nanotubes from CO could take place at very low temperatures by adding hydrogen to the gas mixture. Adding hydrogen significantly changes the nature of the chemical vapor deposition process. It has been demonstrated that at a certain ratio CO : H₂ this gas mixture on the one hand could be used for the deposition of carbon structures at low temperatures and on the other hand for the etching of carbon structures at high temperatures. Thus, the same gas mixture can be both a source of carbon and its etchant depending on temperature. We have also demonstrated that carbon nanotubes of good quality without impurities of amorphous carbon, soot or graphite can be formed from the gas mixture of CO + H₂ at low temperatures.     

3a,6a-Diaza-1,4-Diphosphapentalene Complexes with Lewis Acids InI3 and SbCl3

Russian Journal of Coordination Chemistry - 3a,6a-Diaza-1,4-diphosphapentalene (DDP) with indium triiodide forms the [In(DDP)I3] complex (I) with the ratio In : DDP = 1 : 1. The DDP ligand reacts...     

Synthetic realization approach to fuzzy global optimization via gamma algorithm

A new approach is proposed for global optimization problems with fuzzy cost functions and fuzzy box and equality constraints. It allows one to avoid complex operations with fuzzy sets and the use of various subjective indices of choice. To resolve the contradiction between economically better solutions with low possibility of realization and a little poorer solution with higher possibility of realization, the synthetic realization is defined as certain fixed -level cut for all membership functions. Consideration of such realizations guarantees a level of credibility not less than given (0, 1] for all globally optimal solutions. Then, so defined -cuts are rectified to cut off realizations with possibility less than and to retain higher possibility realizations which are assigned credibility μ = 1 for the whole interval of possible realizations. This construction results in a set-valued band of credibility not less than for a given fuzzy cost function f̃(x) which band has crisp Lipschitz continuous lower- and upper-value functions f_*(x), f*(x) such that f_*(x) ≤ f̃(x) ≤ f*(x) for all x X̃ Rⁿ. Then, the gamma algorithm is applied to obtain the interval global optimal solution f̄₀(x) = [f⁰_*(x), f*₀(x)]. To further simplify the computations, the fuzziness in the feasible set X̃ is transferred to the function value space transforming X̃ into the crisp unit cube in Rⁿ₊ common for all fuzzy optimization problems in Rⁿ with box and equality constraints.     

Charge transfer between a superconductor and a hopping insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron-Cooper pair conversion process time-reversal reflection, where electrons tunnel into a superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. A specific feature of this problem is the interplay between the time-reversal reflection at the interface and transport through the percolation cluster. To allow for this interplay, we have generalized the connectivity criterion of the percolation theory to include surface effects. We show that the time-reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.     

Mechanical Cooper pair transportation as a source of long-distance superconducting phase coherence

Transportation of Cooper pairs by a movable single Cooper-pair box placed between two remote superconductors is shown to establish coherent coupling between them. This coupling is due to entanglement of the movable box with the leads and is manifested in the suppression of quantum fluctuations of the relative phase of the order parameters of the leads. It can be probed by attaching a high resistance Josephson junction between the leads and measuring the current through this junction. The current is suppressed with increasing temperature.     

Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture

Thin freely suspended films of a mixture of an achiral side-chain liquid crystal polymer and its monomer have been studied with depolarized reflected light microscopy. We observe that regions with an odd number of bilayers exhibit a net spontaneous polarization in the tilt plane of the molecules, while regions with an even number of bilayers have no net polarization. These odd-even effects are direct evidence that the tilted smectic bilayers are anticlinic at the polymer backbone and synclinic at bilayer interface and confirm that the phase is bilayer-by-bilayer antiferroelectric.     

Statistics of deep energy states in Coulomb glasses

We study the statistics of local energy minima in the configuration space and the energy relaxation due to activated hopping in a system of interacting electrons in a random environment. The distribution of the local minima is exponential, which is in agreement with extreme value statistics considerations. The relaxation of the system energy shows logarithmic time dependence reflecting the ultrametric structure of the system.     

Reaction of 3a,6a-Diaza-1,4-diphosphapentalene with Substituted Acetylenes

Russian Journal of General Chemistry - The reaction of 3a,6a-diaza-1,4-diphosphapentalene with 1-bromo-2-phenylacetylene has led to the formation of 1,1-bis(phenylethynyl)- and...