Suppressed absolute negative conductance and generation of high-frequency radiation in semiconductor superlattices

We show that space-charge instabilities (electric field domains) in semiconductor superlattices are the attribute of absolute negative conductance induced by small constant and large alternating electric fields. We propose the efficient method for suppression of this destructive phenomenon in order to obtain a generation at microwave and THz frequencies in devices operating at room temperature. We theoretically proved that an unbiased superlattice with a moderate doping subjected to a microwave pump field provides a strong gain at third, fifth, seventh, etc. harmonics of the pump frequency in the conditions of suppressed domains.Comment: 8 pages. Development of cond-mat/0503216 . Version 2: Final version, erratum is include

Intercomparison of five nets used for mesozooplankton sampling

Intercomparison of nets commonly used for mesozooplankton sampling in the Black and Mediterranean seas was attempted within SESAME (Southern European Seas: Assessing and Modelling Ecosystem Changes) project. Five nets were compared: three Juday nets equipped with 150 μm, 180 μm and 200 μm mesh size, Nansen net (100 μm mesh size) and WP2 (200 μm mesh size). Replicated samples were collected at one station in the western Black Sea offshore waters in April 2009. Collected samples were analyzed at species level (except for meroplankton), stages (for copepods) and size length. A decrease of total abundance values was observed with increasing mesh size, due to the significantly higher numbers of animals smaller than 1 mm in the samples obtained by fine mesh size than with coarser nets. Few comparisons were revealed significant for the abundance of animals with 1-2 mm length, while no significance was detected for specimens larger than 2 mm. The above differences resulted in discripancies between nets regarding species and stages composition. Biomass values did not differ significantly between nets, due to the strong contribution to total biomass of the large animals fraction (Calanus euxinus). The smallest and the largest animals revealed high variability between replicates collected by Nansen, Juday- 200 μm and WP2 nets. Correction factors were calculated for the conversion of abundance values between each couple of nets. The detected differences between nets regarding the abundance and biomass, the community taxonomic composition and size structure, as well as the estimated correction factors, provide useful information for the harmonization of data obtained by the above nets in the Black Sea

Subterahertz chaos generation by coupling a superlattice to a linear resonator

We investigate the effects of a linear resonator on the high-frequency dynamics of electrons in devices exhibiting negative differential conductance. We show that the resonator strongly affects both the dc and ac transport characteristics of the device, inducing quasiperiodic and high-frequency chaotic current oscillations. The theoretical findings are confirmed by experimental measurements of a GaAs=AlAs miniband semiconductor superlattice coupled to a linear microstrip resonator. Our results are applicable to other active solid state devices and provide a generic approach for developing modern chaos-based high-frequency technologies including broadband chaotic wireless communication and superfast random-number generation

Transcriptome analysis reveals novel regulatory mechanisms in a genome-reduced bacterium

© 2014 The Author(s). The avian bacterial pathogen Mycoplasma gallisepticum is a good model for systems studies due to small genome and simplicity of regulatory pathways. In this study, we used RNA-Seq and MS-based proteomics to accurately map coding sequences, transcription start sites (TSSs) and transcript 3′-ends (T3Es). We used obtained data to investigate roles of TSSs and T3Es in stress-induced transcriptional responses. We identified 1061 TSSs at a false discovery rate of 10% and showed that almost all transcription in M. gallisepticum is initiated from classic TATAAT promoters surrounded by A/T-rich sequences. Our analysis revealed the pronounced operon structure complexity: on average, each coding operon has one internal TSS and T3Es in addition to the primary ones. Our transcriptomic approach based on the intervals between the two nearest transcript ends allowed us to identify two classes of T3Es: strong, unregulated, hairpin-containing T3Es and weak, heat shock-regulated, hairpinless T3Es. Comparing gene expression levels under different conditions revealed widespread and divergent transcription regulation in M. gallisepticum. Modeling suggested that the core promoter structure plays an important role in gene expression regulation. We have shown that the heat stress activation of cryptic promoters combined with the hairpinless T3Es suppression leads to widespread, seemingly non-functional transcription

Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is non-variational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly non-isotropic due to the selection rules. The results obtained from calculations for Si/Si$_{1-x}$Ge$_x$ quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure

Circularly polarized electroluminescence from silicon nanostructures heavily doped with boron

The circularly polarized electroluminescence (CPEL) from silicon nanostructures which are the p-type ultra-narrow silicon quantum well (Si-QW) confined by {\delta}-barriers heavily doped with boron, 5 10^21 cm^-3, is under study as a function of temperature and excitation levels. The CPEL dependences on the forward current and temperature show the circularly polarized light emission which appears to be caused by the exciton recombination through the negative-U dipole boron centers at the Si-QW {\delta}-barriers interface

A MSFD complementary approach for the assessment of pressures, knowledge and data gaps in Southern European Seas : the PERSEUS experience

PERSEUS project aims to identify the most relevant pressures exerted on the ecosystems of the Southern European Seas (SES), highlighting knowledge and data gaps that endanger the achievement of SES Good Environmental Status (GES) as mandated by the Marine Strategy Framework Directive (MSFD). A complementary approach has been adopted, by a meta-analysis of existing literature on pressure/impact/knowledge gaps summarized in tables related to the MSFD descriptors, discriminating open waters from coastal areas. A comparative assessment of the Initial Assessments (IAs) for five SES countries has been also independently performed. The comparison between meta-analysis results and IAs shows similarities for coastal areas only. Major knowledge gaps have been detected for the biodiversity, marine food web, marine litter and underwater noise descriptors. The meta-analysis also allowed the identification of additional research themes targeting research topics that are requested to the achievement of GES. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license.peer-reviewe

IBD risk loci are enriched in multigenic regulatory modules encompassing putative causative genes.

GWAS have identified >200 risk loci for Inflammatory Bowel Disease (IBD). The majority of disease associations are known to be driven by regulatory variants. To identify the putative causative genes that are perturbed by these variants, we generate a large transcriptome data set (nine disease-relevant cell types) and identify 23,650 cis-eQTL. We show that these are determined by ∼9720 regulatory modules, of which ∼3000 operate in multiple tissues and ∼970 on multiple genes. We identify regulatory modules that drive the disease association for 63 of the 200 risk loci, and show that these are enriched in multigenic modules. Based on these analyses, we resequence 45 of the corresponding 100 candidate genes in 6600 Crohn disease (CD) cases and 5500 controls, and show with burden tests that they include likely causative genes. Our analyses indicate that ≥10-fold larger sample sizes will be required to demonstrate the causality of individual genes using this approach