671 research outputs found
Manipulating quantum Hall edge channels in graphene through Scanning Gate Microscopy
We show evidence of the backscattering of quantum Hall edge channels in a
narrow graphene Hall bar, induced by the gating effect of the conducting tip of
a Scanning Gate Microscope, which we can position with nanometer precision. We
show full control over the edge channels and are able, due to the spatial
variation of the tip potential, to separate co-propagating edge channels in the
Hall bar, creating junctions between regions of different charge carrier
density, that have not been observed in devices based on top- or split-gates.
The solution of the corresponding quantum scattering problem is presented to
substantiate these results, and possible follow-up experiments are discussed.Comment: 10 pages, 12 figure
Synchronization of chaotic networks with time-delayed couplings: An analytic study
Networks of nonlinear units with time-delayed couplings can synchronize to a
common chaotic trajectory. Although the delay time may be very large, the units
can synchronize completely without time shift. For networks of coupled
Bernoulli maps, analytic results are derived for the stability of the chaotic
synchronization manifold. For a single delay time, chaos synchronization is
related to the spectral gap of the coupling matrix. For networks with multiple
delay times, analytic results are obtained from the theory of polynomials.
Finally, the analytic results are compared with networks of iterated tent maps
and Lang-Kobayashi equations which imitate the behaviour of networks of
semiconductor lasers
Increasing the active surface of titanium islands on graphene by nitrogen sputtering
Titanium-island formation on graphene as a function of defect density is
investigated. When depositing titanium on pristine graphene, titanium atoms
cluster and form islands with an average diameter of about 10nm and an average
height of a few atomic layers. We show that if defects are introduced in the
graphene by ion bombardment, the mobility of the deposited titanium atoms is
reduced and the average diameter of the islands decreases to 5nm with
monoatomic height. This results in an optimized coverage for hydrogen storage
applications since the actual titanium surface available per unit graphene area
is significantly increased
STM Study of Exfoliated Few Layer Black Phosphorus Annealed in Ultrahigh Vacuum
Black Phosphorus (bP) has emerged as an interesting addition to the category
of two-dimensional materials. Surface-science studies on this material are of
great interest, but they are hampered by bP's high reactivity to oxygen and
water, a major challenge to scanning tunneling microscopy (STM) experiments. As
a consequence, the large majority of these studies were performed by cleaving a
bulk crystal in situ. Here we present a study of surface modifications on
exfoliated bP flakes upon consecutive annealing steps, up to 550 C, well above
the sublimation temperature of bP. In particular, our attention is focused on
the temperature range 375 C - 400 C, when sublimation starts, and a controlled
desorption from the surface occurs alongside with the formation of
characteristic well-aligned craters. There is an open debate in the literature
about the crystallographic orientation of these craters, whether they align
along the zigzag or the armchair direction. Thanks to the atomic resolution
provided by STM, we are able to identify the orientation of the craters with
respect to the bP crystal: the long axis of the craters is aligned along the
zigzag direction of bP. This allows us to solve the controversy, and, moreover,
to provide insight in the underlying desorption mechanism leading to crater
formation
Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene
The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. In particular, we observe anomalous values of the quantized resistance and a peculiar asymmetry with magnetic field which was not observed before for graphene on SiC. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties
Efficacy of repeated intrathecal triamcinolone acetonide application in progressive multiple sclerosis patients with spinal symptoms
BACKGROUND: There are controversial results on the efficacy of the abandoned, intrathecal predominant methylprednisolone application in multiple sclerosis (MS) in contrast to the proven effectiveness in intractable postherpetic neuralgia. METHODS: We performed an analysis of the efficacy of the application of 40 mg of the sustained release steroid triamcinolone acetonide (TCA). We intrathecally injected in sterile saline dissolved TCA six times within three weeks on a regular basis every third day in 161 hospitalized primary and predominant secondary progressive MS patients with spinal symptoms. The MS patients did not experience an acute onset of exacerbation or recent distinct increased progression of symptoms. We simultaneously scored the MS patients with the EDSS and the Barthel index, estimated the walking distance and measured somatosensory evoked potentials. Additionally the MS patients received a standardized rehabilitation treatment. RESULTS: EDSS score and Barthel index improved, walking distance increased, latencies of somatosensory evoked potentials of the median and tibial nerves shortened in all MS patients with serial evaluation (p < 0.0001 for all variables). Side effects were rare, five patients stopped TCA application due to onset of a post lumbar puncture syndrome. CONCLUSIONS: Repeated intrathecal TCA application improves spinal symptoms, walking distance and SSEP latencies in progressive MS patients in this uncontrolled study. Future trials should evaluate the long-term benefit of this invasive treatment
Kalb-Ramond excitations in a thick-brane scenario with dilaton
We compute the full spectrum and eigenstates of the Kalb-Ramond field in a
warped non-compact Randall-Sundrum -type five-dimensional spacetime in which
the ordinary four-dimensional braneworld is represented by a sine-Gordon
soliton. This 3-brane solution is fully consistent with both the warped
gravitational field and bulk dilaton configurations. In such a background we
embed a bulk antisymmetric tensor field and obtain, after reduction, an
infinite tower of normalizable Kaluza-Klein massive components along with a
zero-mode. The low lying mass eigenstates of the Kalb-Ramond field may be
related to the axion pseudoscalar. This yields phenomenological implications on
the space of parameters, particularly on the dilaton coupling constant. Both
analytical and numerical results are given.Comment: 10 pages, 13 figures, and 2 tables. Final version to appear in The
European Physical Journal
A dynamical model reveals gene co-localizations in nucleus
Co-localization of networks of genes in the nucleus is thought to play an important role in determining gene expression patterns. Based upon experimental data, we built a dynamical model to test whether pure diffusion could account for the observed co-localization of genes within a defined subnuclear region. A simple standard Brownian motion model in two and three dimensions shows that preferential co-localization is possible for co-regulated genes without any direct interaction, and suggests the occurrence may be due to a limitation in the number of available transcription factors. Experimental data of chromatin movements demonstrates that fractional rather than standard Brownian motion is more appropriate to model gene mobilizations, and we tested our dynamical model against recent static experimental data, using a sub-diffusion process by which the genes tend to colocalize more easily. Moreover, in order to compare our model with recently obtained experimental data, we studied the association level between genes and factors, and presented data supporting the validation of this dynamic model. As further applications of our model, we applied it to test against more biological observations. We found that increasing transcription factor number, rather than factory number and nucleus size, might be the reason for decreasing gene co-localization. In the scenario of frequency-or amplitude-modulation of transcription factors, our model predicted that frequency-modulation may increase the co-localization between its targeted genes
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