Self-consistent Coulomb picture of an electron-electron bilayer system

In this work we implement the self-consistent Thomas-Fermi approach and a local conductivity model to an electron-electron bilayer system. The presence of an incompressible strip, originating from screening calculations at the top (or bottom) layer is considered as a source of an external potential fluctuation to the bottom (or top) layer. This essentially yields modifications to both screening properties and the magneto-transport quantities. The effect of the temperature, inter-layer distance and density mismatch on the density and the potential fluctuations are investigated. It is observed that the existence of the incompressible strips plays an important role simply due to their poor screening properties on both screening and the magneto-resistance (MR) properties. Here we also report and interpret the observed MR Hysteresis within our model.Comment: 12 pages, 12 figures, submitted to PR

Realistic modelling of quantum point contacts subject to high magnetic fields and with current bias at out of linear response regime

The electron and current density distributions in the close proximity of quantum point contacts (QPCs) are investigated. A three dimensional Poisson equation is solved self-consistently to obtain the electron density and potential profile in the absence of an external magnetic field for gate and etching defined devices. We observe the surface charges and their apparent effect on the confinement potential, when considering the (deeply) etched QPCs. In the presence of an external magnetic field, we investigate the formation of the incompressible strips and their influence on the current distribution both in the linear response and out of linear response regime. A spatial asymmetry of the current carrying incompressible strips, induced by the large source drain voltages, is reported for such devices in the non-linear regime.Comment: 16 Pages, 9 Figures, submitted to PR

Dynamical mean-field theory of spiking neuron ensembles: response to a single spike with independent noises

Dynamics of an ensemble of $N$-unit FitzHugh-Nagumo (FN) neurons subject to white noises has been studied by using a semi-analytical dynamical mean-field (DMF) theory in which the original $2 N$-dimensional {\it stochastic} differential equations are replaced by 8-dimensional {\it deterministic} differential equations expressed in terms of moments of local and global variables. Our DMF theory, which assumes weak noises and the Gaussian distribution of state variables, goes beyond weak couplings among constituent neurons. By using the expression for the firing probability due to an applied single spike, we have discussed effects of noises, synaptic couplings and the size of the ensemble on the spike timing precision, which is shown to be improved by increasing the size of the neuron ensemble, even when there are no couplings among neurons. When the coupling is introduced, neurons in ensembles respond to an input spike with a partial synchronization. DMF theory is extended to a large cluster which can be divided into multiple sub-clusters according to their functions. A model calculation has shown that when the noise intensity is moderate, the spike propagation with a fairly precise timing is possible among noisy sub-clusters with feed-forward couplings, as in the synfire chain. Results calculated by our DMF theory are nicely compared to those obtained by direct simulations. A comparison of DMF theory with the conventional moment method is also discussed.Comment: 29 pages, 2 figures; augmented the text and added Appendice

Spike-Train Responses of a Pair of Hodgkin-Huxley Neurons with Time-Delayed Couplings

Model calculations have been performed on the spike-train response of a pair of Hodgkin-Huxley (HH) neurons coupled by recurrent excitatory-excitatory couplings with time delay. The coupled, excitable HH neurons are assumed to receive the two kinds of spike-train inputs: the transient input consisting of $M$ impulses for the finite duration ($M$: integer) and the sequential input with the constant interspike interval (ISI). The distribution of the output ISI $T_{\rm o}$ shows a rich of variety depending on the coupling strength and the time delay. The comparison is made between the dependence of the output ISI for the transient inputs and that for the sequential inputs.Comment: 19 pages, 4 figure

Northern Bering Sea tip jets

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L08807, doi:10.1029/2012GL051537.Low-level regions of high wind speed known as tip jets have been identified near Cape Farewell, Greenland's southernmost point. These wind systems contribute to this area being the windiest location on the ocean's surface and play an important role in the regional weather and climate. Here we present the first analysis of the wind systems that make the Siberian coast of the northern Bering Sea the windiest location in the North Pacific Ocean during the boreal winter. In particular we show that tips jets characterized by enhanced northeasterly winds occur in the vicinity of the two prominent headlands along the coast, Cape Navarin and Cape Olyutorsky. The advance of sea ice in the region is shown to impact the frequency and location of the high speed winds in the vicinity of these two capes. Furthermore, we show that these jets are associated with the interaction of extra-tropical cyclones with the high topography of the Koryak Mountain range, situated just inland of the capes. The windstress imparted to the ocean via the tip jets is argued to help drive the formation of dense water in winter in the northern Bering Sea, thus playing an important role in the regional oceanic circulation.GWKM was supported by the Natural Science and Engineering Research Council of Canada. RSP was funded by grant NA08OAR43200895 from the National Oceanic and Atmospheric Administration.2012-10-2

An associative memory of Hodgkin-Huxley neuron networks with Willshaw-type synaptic couplings

An associative memory has been discussed of neural networks consisting of spiking N (=100) Hodgkin-Huxley (HH) neurons with time-delayed couplings, which memorize P patterns in their synaptic weights. In addition to excitatory synapses whose strengths are modified after the Willshaw-type learning rule with the 0/1 code for quiescent/active states, the network includes uniform inhibitory synapses which are introduced to reduce cross-talk noises. Our simulations of the HH neuron network for the noise-free state have shown to yield a fairly good performance with the storage capacity of $\alpha_c = P_{\rm max}/N \sim 0.4 - 2.4$ for the low neuron activity of $f \sim 0.04-0.10$. This storage capacity of our temporal-code network is comparable to that of the rate-code model with the Willshaw-type synapses. Our HH neuron network is realized not to be vulnerable to the distribution of time delays in couplings. The variability of interspace interval (ISI) of output spike trains in the process of retrieving stored patterns is also discussed.Comment: 15 pages, 3 figures, changed Titl

Stochastic Resonance of Ensemble Neurons for Transient Spike Trains: A Wavelet Analysis

By using the wavelet transformation (WT), we have analyzed the response of an ensemble of $N$ (=1, 10, 100 and 500) Hodgkin-Huxley (HH) neurons to {\it transient} $M$-pulse spike trains ($M=1-3$) with independent Gaussian noises. The cross-correlation between the input and output signals is expressed in terms of the WT expansion coefficients. The signal-to-noise ratio (SNR) is evaluated by using the {\it denoising} method within the WT, by which the noise contribution is extracted from output signals. Although the response of a single (N=1) neuron to sub-threshold transient signals with noises is quite unreliable, the transmission fidelity assessed by the cross-correlation and SNR is shown to be much improved by increasing the value of $N$: a population of neurons play an indispensable role in the stochastic resonance (SR) for transient spike inputs. It is also shown that in a large-scale ensemble, the transmission fidelity for supra-threshold transient spikes is not significantly degraded by a weak noise which is responsible to SR for sub-threshold inputs.Comment: 20 pages, 4 figure

Nucleus-encoded plastid sigma factor SIG3 transcribes specifically the psbN gene in plastids

We have investigated the function of one of the six plastid sigma-like transcription factors, sigma 3 (SIG3), by analysing two different Arabidopsis T-DNA insertion lines having disrupted SIG3 genes. Hybridization of wild-type and sig3 plant RNA to a plastid specific microarray revealed a strong reduction of the plastid psbN mRNA. The microarray result has been confirmed by northern blot analysis. The SIG3-specific promoter region has been localized on the DNA by primer extension and mRNA capping experiments. Results suggest tight regulation of psbN gene expression by a SIG3-PEP holoenzyme. The psbN gene is localized on the opposite strand of the psbB operon, between the psbT and psbH genes, and the SIG3-dependent psbN transcription produces antisense RNA to the psbT–psbH intergenic region. We show that this antisense RNA is not limited to the intergenic region, i.e. it does not terminate at the end of the psbN gene but extends as antisense transcript to cover the whole psbT coding region. Thus, by specific transcription initiation at the psbN gene promoter, SIG3-PEP holoenzyme could also influence the expression of the psbB operon by producing psbT antisense RNA

Spectral modeling of scintillator for the NEMO-3 and SuperNEMO detectors

We have constructed a GEANT4-based detailed software model of photon transport in plastic scintillator blocks and have used it to study the NEMO-3 and SuperNEMO calorimeters employed in experiments designed to search for neutrinoless double beta decay. We compare our simulations to measurements using conversion electrons from a calibration source of $\rm ^{207}Bi$ and show that the agreement is improved if wavelength-dependent properties of the calorimeter are taken into account. In this article, we briefly describe our modeling approach and results of our studies.Comment: 16 pages, 10 figure

The Complete Nucleotide Sequences of the 5 Genetically Distinct Plastid Genomes of Oenothera, Subsection Oenothera: II. A Microevolutionary View Using Bioinformatics and Formal Genetic Data

A unique combination of genetic features and a rich stock of information make the flowering plant genus Oenothera an appealing model to explore the molecular basis of speciation processes including nucleus–organelle coevolution. From representative species, we have recently reported complete nucleotide sequences of the 5 basic and genetically distinguishable plastid chromosomes of subsection Oenothera (I–V). In nature, Oenothera plastid genomes are associated with 6 distinct, either homozygous or heterozygous, diploid nuclear genotypes of the 3 basic genomes A, B, or C. Artificially produced plastome–genome combinations that do not occur naturally often display interspecific plastome–genome incompatibility (PGI). In this study, we compare formal genetic data available from all 30 plastome–genome combinations with sequence differences between the plastomes to uncover potential determinants for interspecific PGI. Consistent with an active role in speciation, a remarkable number of genes have high Ka/Ks ratios. Different from the Solanacean cybrid model Atropa/tobacco, RNA editing seems not to be relevant for PGIs in Oenothera. However, predominantly sequence polymorphisms in intergenic segments are proposed as possible sources for PGI. A single locus, the bidirectional promoter region between psbB and clpP, is suggested to contribute to compartmental PGI in the interspecific AB hybrid containing plastome I (AB-I), consistent with its perturbed photosystem II activity