243 research outputs found
Testing T Invariance in the Interaction of Slow Neutrons with Aligned Nuclei
The study of five-fold (P even, T odd) correlation in the interaction of slow
polarized neutrons with aligned nuclei is a possible way of testing the time
reversal invariance due to the expected enhancement of T violating effects in
compound resonances. Possible nuclear targets are discussed which can be
aligned both dynamically as well as by the "brute force" method at low
temperature. A statistical estimation is performed of the five-fold correlation
for low lying p wave compound resonances of the Sb, Sb and
I nuclei. It is shown that a significant improvement can be achieved
for the bound on the intensity of the fundamental parity conserving time
violating (PCTV) interaction.Comment: 22 pages, 5 figures, published versio
Dislocations and Grain Boundaries in Two-Dimensional Boron Nitride
A new dislocation structure-square-octagon pair (4|8) is discovered in
two-dimensional boron nitride (h-BN), via first-principles calculations. It has
lower energy than corresponding pentagon-heptagon pairs (5|7), which contain
unfavorable homo-elemental bonds. Based on the structures of dislocations,
grain boundaries (GB) in BN are investigated. Depending on the tilt angle of
grains, GB can be either polar (B-rich or N-rich), constituted by 5|7s, or
un-polar, composed of 4|8s. The polar GBs carry net charges, positive at B-rich
and negative at N-rich ones. In contrast to GBs in graphene which generally
impede the electronic transport, polar GBs have smaller bandgap compared to
perfect BN, which may suggest interesting electronic and optic applications
Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion
To study the shape formation process of carbon nanotubes, a string equation
describing the possible existing shapes of the axis-curve of multishell carbon
tubes (MCTs) is obtained in the continuum limit by minimizing the shape energy,
that is the difference between the MCT energy and the energy of the
carbonaceous mesophase (CM). It is shown that there exists a threshold relation
of the outmost and inmost radii, that gives a parameter regime in which a
straight MCT will be bent or twisted. Among the deformed shapes, the regular
coiled MCTs are shown being one of the solutions of the string equation. In
particular,the optimal ratio of pitch and radius for such a coil is
found to be equal to , which is in good agreement with recent
observation of coil formation in MCTs by Zhang et al.Comment: RevTeX, no figure, 12 pages, to appear in Phys. Rev. Let
Three-component variometer based on a scalar potassium sensor
Abstract A new variometer is developed comprising a fast-response scalar optically pumped potassium magnetometer inside a rotating magnetic field created by a two-dimensional coil system mounted on a quartz frame. The variometer measures three components of the Earth's field: the total field intensity and two transverse components. The theoretically predicted accuracy of the field component measurement is not worse than 0.1 nT. The noise-limited sensitivity measured in a quiet magnetic field has been proved to be not worse than 25 pT rms at 0.2 s and 30 pT rms at 1 min; comparison with a proton vector magnetometer and a fluxgate magnetometer shows 1.5 nT p-t-p daily deviation
Performance of Monolayer Graphene Nanomechanical Resonators with Electrical Readout
The enormous stiffness and low density of graphene make it an ideal material
for nanoelectromechanical (NEMS) applications. We demonstrate fabrication and
electrical readout of monolayer graphene resonators, and test their response to
changes in mass and temperature. The devices show resonances in the MHz range.
The strong dependence of the resonant frequency on applied gate voltage can be
fit to a membrane model, which yields the mass density and built-in strain.
Upon removal and addition of mass, we observe changes in both the density and
the strain, indicating that adsorbates impart tension to the graphene. Upon
cooling, the frequency increases; the shift rate can be used to measure the
unusual negative thermal expansion coefficient of graphene. The quality factor
increases with decreasing temperature, reaching ~10,000 at 5 K. By establishing
many of the basic attributes of monolayer graphene resonators, these studies
lay the groundwork for applications, including high-sensitivity mass detectors
Mechanical and Electronic Properties of MoS Nanoribbons and Their Defects
We present our study on atomic, electronic, magnetic and phonon properties of
one dimensional honeycomb structure of molybdenum disulfide (MoS) using
first-principles plane wave method. Calculated phonon frequencies of bare
armchair nanoribbon reveal the fourth acoustic branch and indicate the
stability. Force constant and in-plane stiffness calculated in the harmonic
elastic deformation range signify that the MoS nanoribbons are stiff quasi
one dimensional structures, but not as strong as graphene and BN nanoribbons.
Bare MoS armchair nanoribbons are nonmagnetic, direct band gap
semiconductors. Bare zigzag MoS nanoribbons become half-metallic as a
result of the (2x1) reconstruction of edge atoms and are semiconductor for
minority spins, but metallic for the majority spins. Their magnetic moments and
spin-polarizations at the Fermi level are reduced as a result of the
passivation of edge atoms by hydrogen. The functionalization of MoS
nanoribbons by adatom adsorption and vacancy defect creation are also studied.
The nonmagnetic armchair nanoribbons attain net magnetic moment depending on
where the foreign atoms are adsorbed and what kind of vacancy defect is
created. The magnetization of zigzag nanoribbons due to the edge states is
suppressed in the presence of vacancy defects.Comment: 11 pages, 5 figures, first submitted at November 23th, 200
Progress in Classical and Quantum Variational Principles
We review the development and practical uses of a generalized Maupertuis
least action principle in classical mechanics, in which the action is varied
under the constraint of fixed mean energy for the trial trajectory. The
original Maupertuis (Euler-Lagrange) principle constrains the energy at every
point along the trajectory. The generalized Maupertuis principle is equivalent
to Hamilton's principle. Reciprocal principles are also derived for both the
generalized Maupertuis and the Hamilton principles. The Reciprocal Maupertuis
Principle is the classical limit of Schr\"{o}dinger's variational principle of
wave mechanics, and is also very useful to solve practical problems in both
classical and semiclassical mechanics, in complete analogy with the quantum
Rayleigh-Ritz method. Classical, semiclassical and quantum variational
calculations are carried out for a number of systems, and the results are
compared. Pedagogical as well as research problems are used as examples, which
include nonconservative as well as relativistic systems
Continuum elastic modeling of graphene resonators
Starting from an atomistic approach we have derived a hierarchy of
successively more simplified continuum elasticity descriptions for modeling the
mechanical properties of suspended graphene sheets. The descriptions are
validated by applying them to square graphene-based resonators with clamped
edges and studying numerically their mechanical responses. Both static and
dynamic responses are treated. We find that already for deflections of the
order of 0.5{\AA} a theory that correctly accounts for nonlinearities is
necessary and that for many purposes a set of coupled Duffing-type equations
may be used to accurately describe the dynamics of graphene membranes.Comment: 7 pages, 5 figure
Electro-Fluidic Shuttle Memory Device: Classical Molecular Dynamics Study
We investigated the internal dynamics of several electro-fluid shuttle memory
elements, consisting of several media encapsulated in C640 nanocapsule. The
systems proposed were (i) bucky shuttle memory devices (C36+ @C420 and C60+
@C420), (ii) encapsulated-ions shuttle memory devices ((13+)@C420, (3+ -C60-2+
)@C640 and (5+ -C60)@C640) and (iii) endo-fullerenes shuttle memory devices
((K+ @C60- F-@C60)@C640). Energetics and operating responses of several
electro-fluidic shuttle memory devices, such as transitions between the two
states of the C640 capsule, were examined by classical molecular dynamics
simulations of the shuttle media in the C640 capsule under the external force
fields. The operating force fields for the stable operations of the shuttle
memory device were investigated
The contribution of large genomic deletions at the CDKN2A locus to the burden of familial melanoma
Mutations in two genes encoding cell cycle regulatory proteins have been shown to cause familial cutaneous malignant melanoma (CMM). About 20% of melanoma-prone families bear a point mutation in the CDKN2A locus at 9p21, which encodes two unrelated proteins, p16INK4a and p14ARF. Rare mutations in CDK4 have also been linked to the disease. Although the CDKN2A gene has been shown to be the major melanoma predisposing gene, there remains a significant proportion of melanoma kindreds linked to 9p21 in which germline mutations of CDKN2A have not been identified through direct exon sequencing. The purpose of this study was to assess the contribution of large rearrangements in CDKN2A to the disease in melanoma-prone families using multiplex ligation-dependent probe amplification. We examined 214 patients from independent pedigrees with at least two CMM cases. All had been tested for CDKN2A and CDK4 point mutation, and 47 were found positive. Among the remaining 167 negative patients, one carried a novel genomic deletion of CDKN2A exon 2. Overall, genomic deletions represented 2.1% of total mutations in this series (1 of 48), confirming that they explain a very small proportion of CMM susceptibility. In addition, we excluded a new gene on 9p21, KLHL9, as being a major CMM gene
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