819 research outputs found
Low-frequency Current Fluctuations in Individual Semiconducting Single-Wall Carbon Nanotubes
We present a systematic study on low-frequency current fluctuations of
nano-devices consisting of one single semiconducting nanotube, which exhibit
significant 1/f-type noise. By examining devices with different switching
mechanisms, carrier types (electrons vs. holes), and channel lengths, we show
that the 1/f fluctuation level in semiconducting nanotubes is correlated to the
total number of transport carriers present in the system. However, the 1/f
noise level per carrier is not larger than that of most bulk conventional
semiconductors, e.g. Si. The pronounced noise level observed in nanotube
devices simply reflects on the small number of carriers involved in transport.
These results not only provide the basis to quantify the noise behavior in a
one-dimensional transport system, but also suggest a valuable way to
characterize low-dimensional nanostructures based on the 1/f fluctuation
phenomenon
Graphene Nano-Ribbon Electronics
We have fabricated graphene nano-ribbon field-effect transistor devices and
investigated their electrical properties as a function of ribbon width. Our
experiments show that the resistivity of a ribbon increases as its width
decreases, indicating the impact of edge states. Analysis of temperature
dependent measurements suggests a finite quantum confinement gap opening in
narrow ribbons. The electrical current noise of the graphene ribbon devices at
low frequency is found to be dominated by the 1/f noise.Comment: 6 pages, 7 figure
Saccadic selection and crowding in visual search:stronger lateral masking leads to shorter search times
We investigated the role of crowding in saccadic selection during visual search. To guide eye movements, often information from the visual periphery is used. Crowding is known to deteriorate the quality of peripheral information. In four search experiments, we studied the role of crowding, by accompanying individual search elements by flankers. Varying the difference between target and flankers allowed us to manipulate crowding strength throughout the stimulus. We found that eye movements are biased toward areas with little crowding for conditions where a target could be discriminated peripherally. Interestingly, for conditions in which the target could not be discriminated peripherally, this bias reversed to areas with strong crowding. This led to shorter search times for a target presented in areas with stronger crowding, compared to a target presented in areas with less crowding. These findings suggest a dual role for crowding in visual search. The presence of flankers similar to the target deteriorates the quality of the peripheral target signal but can also attract eye movements, as more potential targets are present over the area
A priori model independent inverse potential mapping: the impact of electrode positioning
__Introduction:__ In inverse potential mapping, local epicardial potentials are computed from recorded body surface potentials (BSP). When BSP are recorded with only a limited number of electrodes, in general biophysical a priori models are applied to facilitate the inverse computation. This study investigated the possibility of deriving epicardial potential information using only 62 torso electrodes in the absence of an a priori model.
__Methods:__ Computer simulations were used to determine the optimal in vivo positioning of 62 torso electrodes. Subsequently, three different electrode configurations, i.e., surrounding the thorax, concentrated precordial (30 mm inter-electrode distance) and super-concentrated precordial (20 mm inter-electrode distance) were used to record BSP from three healthy volunteers. Magnetic resonance imaging (MRI) was performed to register the electrode positions with respect to the anatomy of the patient. Epicardial potentials were inversely computed from the recorded BSP. In order to determine the reconstruction quality, the super-concentrated electrode configuration was applied in four patients with an implanted MRI-conditional pacemaker system. The distance between the position of the ventricular lead tip on MRI and the inversely reconstructed pacing site was determined.
__Results:__ The epicardial potential distribution reconstructed using the super-concentrated electrode configuration demonstrated the highest correlation (R = 0.98; p < 0.01) with the original epicardial source model. A mean localization error of 5.3 mm was found in the pacemaker patients.
__Conclusion:__ This study demonstrated the feasibility of deriving detailed anterior epicardial potential information using only 62 torso electrodes without the use of an a priori model
Low energy excitations in crystalline perovskite oxides: Evidence from noise experiments
In this paper we report measurements of 1/f noise in a crystalline metallic
oxide with perovskite structure down to 4.2K. The results show existence of
localized excitations with average activation energy 70-80 meV which
produce peak in the noise at T 35-40K. In addition, it shows clear
evidence of tunnelling type two-level-systems (as in glasses) which show up in
noise measurements below 30K.Comment: 11 pages, 4 figures, to appear in Phys Rev B, vol 58, 1st Dec issu
Integrated whole-heart computational workflow for inverse potential mapping and personalized simulations
Background: Integration of whole-heart activation simulations and inverse potential mapping (IPM) could benefit the guidance and planning of electrophysiological procedures. Routine clinical application requires a fast and adaptable workflow. These requirements limit clinical translation of existing simulation models. This study proposes a comprehensive finite element model (FEM) based whole-heart computational workflow suitable for IPM and simulations. Methods: Three volunteers and eight patients with premature ventricular contractions underwent body surface potential (BSP) acquisition followed by a cardiac MRI (CMR) scan. The cardiac volumes were segmented from the CMR images using custom written software. The feasibility to integrate tissue-characteristics was assessed by generating meshes with virtual edema and scar. Isochronal activation maps were constructed by identifying the fastest route through the cardiac volume using the Möller-Trumbore and Floyd-Warshall algorithms. IPM's were reconstructed from the BSP's. Results: Whole-heart computational meshes were generated within seconds. The first point of atrial activation on IPM was located near the crista terminalis of the superior vena cave into the right atrium. The IPM demonstrated the ventricular epicardial breakthrough at the attachment of the moderator band with the right ventricular free wall. Simulations of sinus rhythm were successfully performed. The conduction through the virtual edema and scar meshes demonstrated delayed activation or a complete conductional block respectively. Conclusion: The proposed FEM based whole-heart computational workflow offers an integrated platform for cardiac electrical assessment using simulations and IPM. This workflow can incorporate patient-specific electrical parameters, perform whole-heart cardiac activation simulations and accurately reconstruct cardiac activation sequences from BSP's
Long-range potential fluctuations and 1/f noise in hydrogenated amorphous silicon
We present a microscopic theory of the low-frequency voltage noise (known as
"1/f" noise) in micrometer-thick films of hydrogenated amorphous silicon. This
theory traces the noise back to the long-range fluctuations of the Coulomb
potential produced by deep defects, thereby predicting the absolute noise
intensity as a function of the distribution of defect activation energies. The
predictions of this theory are in very good agreement with our own experiments
in terms of both the absolute intensity and the temperature dependence of the
noise spectra.Comment: 8 pages, 3 figures, several new parts and one new figure are added,
but no conceptual revision
Strong Suppression of Electrical Noise in Bilayer Graphene Nano Devices
Low-frequency 1/f noise is ubiquitous, and dominates the signal-to-noise
performance in nanodevices. Here we investigate the noise characteristics of
single-layer and bilayer graphene nano-devices, and uncover an unexpected 1/f
noise behavior for bilayer devices. Graphene is a single layer of graphite,
where carbon atoms form a 2D honeycomb lattice. Despite the similar
composition, bilayer graphene (two graphene monolayers stacked in the natural
graphite order) is a distinct 2D system with a different band structure and
electrical properties. In graphene monolayers, the 1/f noise is found to follow
Hooge's empirical relation with a noise parameter comparable to that of bulk
semiconductors. However, this 1/f noise is strongly suppressed in bilayer
graphene devices, and exhibits an unusual dependence on the carrier density,
different from most other materials. The unexpected noise behavior in graphene
bilayers is associated with its unique band structure that varies with the
charge distribution among the two layers, resulting in an effective screening
of potential fluctuations due to external impurity charges. The findings here
point to exciting opportunities for graphene bilayers in low-noise
applications
Determinants of Dwell Time in Visual Search: Similarity or Perceptual Difficulty?
The present study examined the factors that determine the dwell times in a visual search task, that is, the duration the gaze remains fixated on an object. It has been suggested that an item’s similarity to the search target should be an important determiner of dwell times, because dwell times are taken to reflect the time needed to reject the item as a distractor, and such discriminations are supposed to be harder the more similar an item is to the search target. In line with this similarity view, a previous study shows that, in search for a target ring of thin line-width, dwell times on thin linewidth Landolt C’s distractors were longer than dwell times on Landolt C’s with thick or medium linewidth. However, dwell times may have been longer on thin Landolt C’s because the thin line-width made it harder to detect whether the stimuli had a gap or not. Thus, it is an open question whether dwell times on thin line-width distractors were longer because they were similar to the target or because the perceptual decision was more difficult. The present study de-coupled similarity from perceptual difficulty, by measuring dwell times on thin, medium and thick line-width distractors when the target had thin, medium or thick line-width. The results showed that dwell times were longer on target-similar than target-dissimilar stimuli across all target conditions and regardless of the line-width. It is concluded that prior findings of longer dwell times on thin linewidth-distractors can clearly be attributed to target similarity. As will be discussed towards the end, the finding of similarity effects on dwell times has important implications for current theories of visual search and eye movement control
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