62 research outputs found
Shilnikov Lemma for a nondegenerate critical manifold of a Hamiltonian system
We prove an analog of Shilnikov Lemma for a normally hyperbolic symplectic
critical manifold of a Hamiltonian system. Using this
result, trajectories with small energy shadowing chains of homoclinic
orbits to are represented as extremals of a discrete variational problem,
and their existence is proved. This paper is motivated by applications to the
Poincar\'e second species solutions of the 3 body problem with 2 masses small
of order . As , double collisions of small bodies correspond to
a symplectic critical manifold of the regularized Hamiltonian system
Broken symmetry states and divergent resistance in suspended bilayer graphene
Graphene [1] and its bilayer have generated tremendous excitement in the
physics community due to their unique electronic properties [2]. The intrinsic
physics of these materials, however, is partially masked by disorder, which can
arise from various sources such as ripples [3] or charged impurities [4].
Recent improvements in quality have been achieved by suspending graphene flakes
[5,6], yielding samples with very high mobilities and little charge
inhomogeneity. Here we report the fabrication of suspended bilayer graphene
devices with very little disorder. We observe fully developed quantized Hall
states at magnetic fields of 0.2 T, as well as broken symmetry states at
intermediate filling factors , , and . The
devices exhibit extremely high resistance in the state that grows
with magnetic field and scales as magnetic field divided by temperature. This
resistance is predominantly affected by the perpendicular component of the
applied field, indicating that the broken symmetry states arise from many-body
interactions.Comment: 23 pages, including 4 figures and supplementary information; accepted
to Nature Physic
Ripple Texturing of Suspended Graphene Atomic Membranes
Graphene is the nature's thinnest elastic membrane, with exceptional
mechanical and electrical properties. We report the direct observation and
creation of one-dimensional (1D) and 2D periodic ripples in suspended graphene
sheets, using spontaneously and thermally induced longitudinal strains on
patterned substrates, with control over their orientations and wavelengths. We
also provide the first measurement of graphene's thermal expansion coefficient,
which is anomalously large and negative, ~ -7x10^-6 K^-1 at 300K. Our work
enables novel strain-based engineering of graphene devices.Comment: 15 pages, 4 figure
Phage inducible islands in the gram-positive cocci
The SaPIs are a cohesive subfamily of extremely common phage-inducible chromosomal islands (PICIs) that reside quiescently at specific att sites in the staphylococcal chromosome and are induced by helper phages to excise and replicate. They are usually packaged in small capsids composed of phage virion proteins, giving rise to very high transfer frequencies, which they enhance by interfering with helper phage reproduction. As the SaPIs represent a highly successful biological strategy, with many natural Staphylococcus aureus strains containing two or more, we assumed that similar elements would be widespread in the Gram-positive cocci. On the basis of resemblance to the paradigmatic SaPI genome, we have readily identified large cohesive families of similar elements in the lactococci and pneumococci/streptococci plus a few such elements in Enterococcus faecalis. Based on extensive ortholog analyses, we found that the PICI elements in the four different genera all represent distinct but parallel lineages, suggesting that they represent convergent evolution towards a highly successful lifestyle. We have characterized in depth the enterococcal element, EfCIV583, and have shown that it very closely resembles the SaPIs in functionality as well as in genome organization, setting the stage for expansion of the study of elements of this type. In summary, our findings greatly broaden the PICI family to include elements from at least three genera of cocci
Boron nitride substrates for high-quality graphene electronics
Graphene devices on standard SiO2 substrates are highly disordered,
exhibiting characteristics far inferior to the expected intrinsic properties of
graphene[1-12]. While suspending graphene above the substrate yields
substantial improvement in device quality[13,14], this geometry imposes severe
limitations on device architecture and functionality. Realization of
suspended-like sample quality in a substrate supported geometry is essential to
the future progress of graphene technology. In this Letter, we report the
fabrication and characterization of high quality exfoliated mono- and bilayer
graphene (MLG and BLG) devices on single crystal hexagonal boron nitride (h-BN)
substrates, by a mechanical transfer process. Variable-temperature
magnetotransport measurements demonstrate that graphene devices on h-BN exhibit
enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping
in comparison with SiO2-supported devices. The ability to assemble crystalline
layered materials in a controlled way sets the stage for new advancements in
graphene electronics and enables realization of more complex graphene
heterostructres.Comment: 20 pages (includes supplementary info), 7 figure
Application of Graphene within Optoelectronic Devices and Transistors
Scientists are always yearning for new and exciting ways to unlock graphene's
true potential. However, recent reports suggest this two-dimensional material
may harbor some unique properties, making it a viable candidate for use in
optoelectronic and semiconducting devices. Whereas on one hand, graphene is
highly transparent due to its atomic thickness, the material does exhibit a
strong interaction with photons. This has clear advantages over existing
materials used in photonic devices such as Indium-based compounds. Moreover,
the material can be used to 'trap' light and alter the incident wavelength,
forming the basis of the plasmonic devices. We also highlight upon graphene's
nonlinear optical response to an applied electric field, and the phenomenon of
saturable absorption. Within the context of logical devices, graphene has no
discernible band-gap. Therefore, generating one will be of utmost importance.
Amongst many others, some existing methods to open this band-gap include
chemical doping, deformation of the honeycomb structure, or the use of carbon
nanotubes (CNTs). We shall also discuss various designs of transistors,
including those which incorporate CNTs, and others which exploit the idea of
quantum tunneling. A key advantage of the CNT transistor is that ballistic
transport occurs throughout the CNT channel, with short channel effects being
minimized. We shall also discuss recent developments of the graphene tunneling
transistor, with emphasis being placed upon its operational mechanism. Finally,
we provide perspective for incorporating graphene within high frequency
devices, which do not require a pre-defined band-gap.Comment: Due to be published in "Current Topics in Applied Spectroscopy and
the Science of Nanomaterials" - Springer (Fall 2014). (17 pages, 19 figures
Thermal infrared emission reveals the Dirac point movement in biased graphene
Graphene is a 2-dimensional material with high carrier mobility and thermal
conductivity, suitable for high-speed electronics. Conduction and valence bands
touch at the Dirac point. The absorptivity of single-layer graphene is 2.3%,
nearly independent of wavelength. Here we investigate the thermal radiation
from biased graphene transistors. We find that the emission spectrum of
single-layer graphene follows that of a grey body with constant emissivity (1.6
\pm 0.8)%. Most importantly, we can extract the temperature distribution in the
ambipolar graphene channel, as confirmed by Stokes/anti-Stokes measurements.
The biased graphene exhibits a temperature maximum whose location can be
controlled by the gate voltage. We show that this peak in temperature reveals
the spatial location of the minimum in carrier density, i.e. the Dirac point.Comment: Accepted in principle at Nature Nanotechnolog
Energy gaps, topological insulator state and zero-field quantum Hall effect in graphene by strain engineering
Among many remarkable qualities of graphene, its electronic properties
attract particular interest due to a massless chiral character of charge
carriers, which leads to such unusual phenomena as metallic conductivity in the
limit of no carriers and the half-integer quantum Hall effect (QHE) observable
even at room temperature [1-3]. Because graphene is only one atom thick, it is
also amenable to external influences including mechanical deformation. The
latter offers a tempting prospect of controlling graphene's properties by
strain and, recently, several reports have examined graphene under uniaxial
deformation [4-8]. Although the strain can induce additional Raman features
[7,8], no significant changes in graphene's band structure have been either
observed or expected for realistic strains of approx. 10% [9-11]. Here we show
that a designed strain aligned along three main crystallographic directions
induces strong gauge fields [12-14] that effectively act as a uniform magnetic
field exceeding 10 T. For a finite doping, the quantizing field results in an
insulating bulk and a pair of countercirculating edge states, similar to the
case of a topological insulator [15-20]. We suggest realistic ways of creating
this quantum state and observing the pseudo-magnetic QHE. We also show that
strained superlattices can be used to open significant energy gaps in
graphene's electronic spectrum
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. 1 One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hex a gonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. 2-5 Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system . 6 These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without mag netic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids
Assessment of a continuous blood gas monitoring system in animals during circulatory stress
<p>Abstract</p> <p>Background</p> <p>The study was aimed to determine the measurement accuracy of The CDI™ blood parameter monitoring system 500 (Terumo Cardiovascular Systems Corporation, Ann Arbor MI) in the real-time continuous measurement of arterial blood gases under different cardiocirculatory stress conditions</p> <p>Methods</p> <p>Inotropic stimulation (Dobutamine 2.5 and 5 μg/kg/min), vasoconstriction (Arginine-vasopressin 4, 8 and 16 IU/h), hemorrhage (-10%, -20%, -35%, and -50% of the theoretical volemia), and volume resuscitation were induced in ten swine (57.4 ± 10.7 Kg).Intermittent blood gas assessments were carried out using a routine gas analyzer at any experimental phase and compared with values obtained at the same time settings during continuous monitoring with CDI™ 500 system. The Bland-Altman analysis was employed.</p> <p>Results</p> <p>Bias and precision for pO<sub>2 </sub>were - 0.06 kPa and 0.22 kPa, respectively (r<sup>2 </sup>= 0.96); pCO<sub>2 </sub>- 0.02 kPa and 0.15 kPa, respectively; pH -0.001 and 0.01 units, respectively ( r<sup>2 </sup>= 0.96). The analysis showed very good agreement for SO<sub>2 </sub>(bias 0.04,precision 0.33, r<sup>2 </sup>= 0.95), Base excess (bias 0.04,precision 0.28, r<sup>2 </sup>= 0.98), HCO<sub>3 </sub>(bias 0.05,precision 0.62, r<sup>2 </sup>= 0.92),hemoglobin (bias 0.02,precision 0.23, r<sup>2 </sup>= 0.96) and K<sup>+ </sup>(bias 0.02, precision 0.27, r<sup>2 </sup>= 0.93). The sensor was reliable throughout the experiment during hemodynamic variations.</p> <p>Conclusions</p> <p>Continuous blood gas analysis with the CDI™ 500 system was reliable and it might represent a new useful tool to accurately and timely monitor gas exchange in critically ill patients. Nonetheless, our findings need to be confirmed by larger studies to prove its reliability in the clinical setting.</p
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