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

Correlated electron states and transport in triangular arrays

We study correlated electron states in frustrated geometry of a triangular lattice. The interplay of long range interactions and finite residual entropy of a classical system gives rise to unusual effects in equilibrium ordering as well as in transport. A novel correlated fluid phase is identified in a wide range of densities and temperatures above freezing into commensurate solid phases. The charge dynamics in the correlated phase is described in terms of a height field, its fluctuations, and topological defects. We demonstrate that the height field fluctuations give rise to a ``free'' charge flow and finite dc conductivity. We show that freezing into the solid phase, controlled by the long range interactions, manifests itself in singularities of transport properties.Comment: 19 pages, 10 figure

Absorption of Terahertz Radiation in Ge/Si(001) Heterostructures with Quantum Dots

The terahertz spectra of the dynamic conductivity and radiation absorption coefficient in germanium-silicon heterostructures with arrays of Ge hut clusters (quantum dots) have been measured for the first time in the frequency range of 0.3-1.2 THz at room temperature. It has been found that the effective dynamic conductivity and effective radiation absorption coefficient in the heterostructure due to the presence of germanium quantum dots in it are much larger than the respective quantities of both the bulk Ge single crystal and Ge/Si(001) without arrays of quantum dots. The possible microscopic mechanisms of the detected increase in the absorption in arrays of quantum dots have been discussed.Comment: 9 pages, 4 figures; typos correcte

High representation of archaea across all depths in oxic and low-pH sediment layers underlying an acidic stream

Parys Mountain or Mynydd Parys (Isle of Anglesey, United Kingdom) is a mine-impacted environment, which accommodates a variety of acidophilic organisms. Our previous research of water and sediments from one of the surface acidic streams showed a high proportion of archaea in the total microbial community. To understand the spatial distribution of archaea, we sampled cores (0–20 cm) of sediment and conducted chemical analyses and taxonomic profiling of microbiomes using 16S rRNA gene amplicon sequencing in different core layers. The taxonomic affiliation of sequencing reads indicated that archaea represented between 6.2 and 54% of the microbial community at all sediment depths. Majority of archaea were associated with the order Thermoplasmatales, with the most abundant group of sequences being clustered closely with the phylotype B_DKE, followed by “E-plasma,” “A-plasma,” other yet uncultured Thermoplasmatales with Ferroplasma and Cuniculiplasma spp. represented in minor proportions. Thermoplasmatales were found at all depths and in the whole range of chemical conditions with their abundance correlating with sediment Fe, As, Cr, and Mn contents. The bacterial microbiome component was largely composed in all layers of sediment by members of the phyla Proteobacteria, Actinobacteria, Nitrospirae, Firmicutes, uncultured Chloroflexi (AD3 group), and Acidobacteria. This study has revealed a high abundance of Thermoplasmatales in acid mine drainage-affected sediment layers and pointed at these organisms being the main contributors to carbon, and probably to iron and sulfur cycles in this ecosystem

1/f Noise and Extreme Value Statistics

We study the finite-size scaling of the roughness of signals in systems displaying Gaussian 1/f power spectra. It is found that one of the extreme value distributions (Gumbel distribution) emerges as the scaling function when the boundary conditions are periodic. We provide a realistic example of periodic 1/f noise, and demonstrate by simulations that the Gumbel distribution is a good approximation for the case of nonperiodic boundary conditions as well. Experiments on voltage fluctuations in GaAs films are analyzed and excellent agreement is found with the theory.Comment: 4 pages, 4 postscript figures, RevTe

Granular Electronic Systems

A granular metal is an array of metallic nano-particles imbedded into an insulating matrix. Tuning the intergranular coupling strength a granular system can be transformed into either a good metal or an insulator and, in case of superconducting particles, experience superconductor-insulator transition. The ease of adjusting electronic properties of granular metals makes them most suitable for fundamental studies of disordered solids and assures them a fundamental role for nanotechnological applications. This Review discusses recent important theoretical advances in the study of granular metals, emphasizing on the interplay of disorder, quantum effects, fluctuations and effects of confinement in formation of electronic transport and thermodynamic properties of granular materials.Comment: 51 pages, 23 figures, submitted to Reviews of Modern Physic

Clinical and Epidemiological Features of Hepatitis C in Adults and Children in the City of Tomsk and Tomsk Area for 1994-2009

Epidemiological features of acute HCV infection were studied in adults and children in Tomsk and Tomsk area for the period 1994-2009. The statistics of the Office of Rospotrebnadzor (Federal service on customers' rights protection and human well-being surveillance) for Tomsk area as well as our own research data such as analysis of the epidemiological anamnesis of medical history and epidemiological survey questionnaires, retrospective, dynamic (longitudinal) analysis were used at carrying out of this epidemiological study. The following conclusions are drawn by the team of authors as a result of the study: the severe forms of HCV infection weren’t found in adults and children; hepatomegaly, jaundice, and intoxication were the main clinical symptoms, and splenomagaly was met rarely regardless of age. Intensive morbidity with HCV often exceeds the average for Russia; the incidence in senior age groups with predominance of males among those infected has increased; the proportion of women has increased too; HCV infection in children was more frequently detected in the school-age children of 7-14 years; the predominance of non-working population including socially maladjusted and students at the beginning of the study period; an increase of the proportion of working population, a decrease of infection through intravenous drug use, the increase of nosocomial infection, as well as household and sexual ways of transmission in recent years. HCV infection in children was detected more frequently in school-age children of 7-14 years. The rise of HCV infection both in adults and children was marked at the initial stage of the study.Изучены эпидемиологические особенности острой и хронической HCV инфекции на территории г. Томска и Томской области у взрослого населения и детей за период 1994-2009 гг.. При проведении данного эпидемиологического исследования использовались статистические данные Управления Роспотребнадзора по Томской области, данные собственных исследований: анализ эпидемиологического анамнеза по историям болезней и анкетам эпидемиологического обследования: ретроспективный, динамический (продольный) анализ. В результате проведенного анализа коллективом авторов были сделаны следующие выводы: при 0ВГС у взрослых и детей не встречались тяжелые формы; основными клиническими симптомами были гепатомегалия, желтуха и интоксикация, а спленомегалия встречалась редко независимо от возраста. Интенсивные показатели заболеваемости ОВГС часто превышают среднероссийские; увеличилась заболеваемость в старших возрастных группах, с преобладанием лиц мужского пола среди инфицированных, но возросла и доля женщин; у детей чаще HCV инфекция регистрировалась в школьном возрасте 7-14 лет. преобладание в начале исследуемого периода неработающего населения, в том числе социально не адаптированных и студентов с увеличением доли работающих в последние годы со снижением заражения при внутривенном введении наркотиков и увеличением числа внутрибольничного инфицирования, контактно-бытового и сексуальных путей передачи. У детей чаще HCV инфекция регистрировалась в школьном возрасте 7-14 лет. Как у взрослых, так и у детей подъем заболеваемости ОВГС был отмечен в начальном периоде исследования