414 research outputs found
AC Conductance in Dense Array of the GeSi Quantum Dots in Si
Complex AC-conductance, , in the systems with dense
GeSi quantum dot (QD) arrays in Si has been determined from
simultaneous measurements of attenuation, ,
and velocity, , of surface acoustic waves (SAW)
with frequencies = 30-300 MHz as functions of transverse magnetic field 18 T in the temperature range = 1-20 K. It has been shown that in the
sample with dopant (B) concentration 8.2 cm at
temperatures 4 K the AC conductivity is dominated by hopping between
states localized in different QDs. The observed power-law temperature
dependence, , and weak frequency dependence,
, of the AC conductivity are consistent with
predictions of the two-site model for AC hopping conductivity for the case of
1, where is the SAW angular frequency and
is the typical population relaxation time. At 7 K the AC
conductivity is due to thermal activation of the carriers (holes) to the
mobility edge. In intermediate temperature region 4 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 4.5 K
saturates, evidencing crossover to the regime where 1. From
crossover condition, = 1, the typical value, , 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
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