AC Conductance in Dense Array of the Ge0.7_{0.7}Si0.3_{0.3} Quantum Dots in Si

Complex AC-conductance, $\sigma^{AC}$, in the systems with dense Ge$_{0.7}$Si$_{0.3}$ quantum dot (QD) arrays in Si has been determined from simultaneous measurements of attenuation, $\Delta\Gamma=\Gamma(H)-\Gamma(0)$, and velocity, $\Delta V /V=(V(H)-V(0)) / V(0)$, of surface acoustic waves (SAW) with frequencies $f$ = 30-300 MHz as functions of transverse magnetic field $H \leq$ 18 T in the temperature range $T$ = 1-20 K. It has been shown that in the sample with dopant (B) concentration 8.2$\times 10^{11}$ cm$^{-2}$ at temperatures $T \leq$4 K the AC conductivity is dominated by hopping between states localized in different QDs. The observed power-law temperature dependence, $\sigma_1(H=0)\propto T^{2.4}$, and weak frequency dependence, $\sigma_1(H=0)\propto \omega^0$, of the AC conductivity are consistent with predictions of the two-site model for AC hopping conductivity for the case of $\omega \tau_0 \gg$1, where $\omega=2\pi f$ is the SAW angular frequency and $\tau_0$ is the typical population relaxation time. At $T >$ 7 K the AC conductivity is due to thermal activation of the carriers (holes) to the mobility edge. In intermediate temperature region 4$< T<$ 7 K, where AC conductivity is due to a combination of hops between QDs and diffusion on the mobility edge, one succeeded to separate both contributions. Temperature dependence of hopping contribution to the conductivity above $T^*\sim$ 4.5 K saturates, evidencing crossover to the regime where $\omega \tau_0 <$1. From crossover condition, $\omega \tau_0(T^*)$ = 1, the typical value, $\tau_0$, of the relaxation time has been determined.Comment: revtex, 3 pages, 6 figure

Density of States and Conductivity of Granular Metal or Array of Quantum Dots

The conductivity of a granular metal or an array of quantum dots usually has the temperature dependence associated with variable range hopping within the soft Coulomb gap of density of states. This is difficult to explain because neutral dots have a hard charging gap at the Fermi level. We show that uncontrolled or intentional doping of the insulator around dots by donors leads to random charging of dots and finite bare density of states at the Fermi level. Then Coulomb interactions between electrons of distant dots results in the a soft Coulomb gap. We show that in a sparse array of dots the bare density of states oscillates as a function of concentration of donors and causes periodic changes in the temperature dependence of conductivity. In a dense array of dots the bare density of states is totally smeared if there are several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are dropped. Some small changes are given to improve the organizatio

Phonon bottleneck in p-type Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photo- and dark current, photoconductive gain, and hole capture probability in ten-period p-type Ge/Si quantum dot heterostructures. The dot dimensions are varied by changing the Ge coverage and the growth temperature during molecular beam epitaxy of Ge/Si(001) system in the Stranski-Krastanov growth mode. In all samples, we observed the general tendency: with decreasing the size of the dots, the dark current and hole capture probability are reduced, while the photoconductive gain and photoresponse are enhanced. Suppression of the hole capture probability in small-sized quantum dots is attributed to a quenched electron-phonon scattering due to phonon bottleneck

Localization of electrons in dome-shaped GeSi/Si islands

We report on intraband photocurrent spectroscopy of dome-shaped GeSi islands embedded in a Si matrix with n+-type bottom and top Si layers. An in-plane polarized photoresponse in the 85–160 meV energy region has been observed and ascribed to the optical excitation of electrons from states confined in the strained Si near the dome apexes to the continuum states of unstrained Si. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried GeSi quantum dots. Sensitivity of the device to the normal incidence radiation proves a zero-dimensional nature of confined electronic wave functions

Автоматизация проектирования карт раскроя листовых материалов в условиях производства

The article is devoted to the design and use of a software application for the automatic formation of the products layout and the calculation of the criterion "useless material" when cutting sheet materials. A functional model of production for the manufacture of products cut from sheet material is described. This model allows determining the place of the developed application in the production process. The application implements deterministic algorithms and an evolutionary (genetic) cutting algorithm. The results of the study of algorithms for the cutting maps formation according to the time of finding a solution and the criterion of “useless material” are presented. Developed application allows you to find the total area of products, the free area as the difference between the area of sheet material and the total area of products. The software application is written in JavaScript using the React library.Статья посвящена проектированию и применению программного приложения для автоматического формирования схемы расположения изделий и расчета критерия «бесполезный материал» при раскрое листовых материалов. Описана функциональная модель производства по изготовлению продукции, раскраиваемой из листового материала, позволяющая определить место разработанного приложения в производственном процессе. В приложении реализованы детерминированные алгоритмы и эволюционный (генетический) алгоритм раскроя. Представлены результаты исследования алгоритмов формирования карт раскроя по времени нахождения решения и критерию «бесполезный материал». Созданное приложение позволяет находить общую площадь изделий, свободную площадь как разницу между площадью листового материала и общей площадью изделий. Программное приложение написано на языке JavaScript с применением библиотеки React

Ge quantum dot arrays grown by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface: nucleation, morphology and CMOS compatibility

Issues of morphology, nucleation and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (<600 deg C) and high (>600 deg. C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts---pyramids and wedges---are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.Comment: 30 pages, 11 figure

Magnetic Gaps related to Spin Glass Order in Fermionic Systems

We provide evidence for spin glass related magnetic gaps in the fermionic density of states below the freezing temperature. Model calculations are presented and proposed to be relevant for explaining resistivity measurements which observe a crossover from variable-range- to activated behavior. The magnetic field dependence of a hardgap and the low temperature decay of the density of states are given. In models with fermion transport a new metal-insulator transition is predicted to occur due to the spin-glass gap, anteceding the spin glass to quantum paramagnet transition at smaller spin density. Important fluctuation effects due to finite range frustrated interactions are estimated and discussed.Comment: 4 pages, 1 Postscript figure, revised version accepted for publication in Physical Review Letter

Organization of Repair of Locomotives on the Data of Moni-toring Their Technical Condition

The article discusses the structure of the organization of current repairs of modern locomotives equipped with microprocessor control and diagnostic systems. Based on these sys-tems, remote monitoring of the technical condition of locomotive units has been implemented. The block diagram of the recommendatory nature of the organization of current repairs using diagnostic information is presented. The above algorithm allows predicting the residual life by any of the studied parameters, thereby determining the mileage at the moment of which a fail-ure occurs on some parameter. This gives a huge advantage in preventing incidents, as well as in the long-term planning of repair work on wheel turning and rolling, based on the remaining life. Thus, the well-coordinated organization of current repairs using diagnostic information, allows operational methods to eliminate the faults of the nodes. A recommender structure in the form of a single integrated system for organizing repair cycles will allow establishing links between the involved repair personnel in the form of electronic terminals for notification of the actual technical condition of the locomotive. A program has been developed for calculating the residual mileage of wheel sets for the corresponding types of defects, which also allows long-term planning for turning and rolling of wheel-motor blocks wheelsets, as well as to regulate the overhaul runs of a locomotive. © Published under licence by IOP Publishing Ltd