1,541 research outputs found
Plasmon assisted transport through disordered array of quantum wires
Phononless plasmon assisted thermally activated transport through a long
disordered array of finite length quantum wires is investigated analytically.
Generically strong electron plasmon interaction in quantum wires results in a
qualitative change of the temperature dependence of thermally activated
resistance in comparison to phonon assisted transport. At high temperatures,
the thermally activated resistance is determined by the Luttinger liquid
interaction parameter of the wires.Comment: 7 pages, 1 figure, final version as publishe
Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative
Here we follow, both experimentally and theoretically, the development of
magnetism in Tb clusters from the atomic limit, adding one atom at a time. The
exchange interaction is, surprisingly, observed to drastically increase
compared to that of bulk, and to exhibit irregular oscillations as a function
of the interatomic distance. From electronic structure theory we find that the
theoretical magnetic moments oscillate with cluster size in exact agreement
with experimental data. Unlike the bulk, the oscillation is not caused by the
RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a
competition between wave-function overlap of the 5d shell and the on-site
exchange interaction, which leads to a competition between ferromagnetic
double-exchange and antiferromagnetic super-exchange. This understanding opens
up new ways to tune the magnetic properties of rare-earth based magnets with
nano-sized building blocks
Multi-shell gold nanowires under compression
Deformation properties of multi-wall gold nanowires under compressive loading
are studied. Nanowires are simulated using a realistic many-body potential.
Simulations start from cylindrical fcc(111) structures at T=0 K. After
annealing cycles axial compression is applied on multi-shell nanowires for a
number of radii and lengths at T=300 K. Several types of deformation are found,
such as large buckling distortions and progressive crushing. Compressed
nanowires are found to recover their initial lengths and radii even after
severe structural deformations. However, in contrast to carbon nanotubes
irreversible local atomic rearrangements occur even under small compressions.Comment: 1 gif figure, 5 ps figure
Detection of Coxiella burnetii in complex matrices by using multiplex quantitative PCR during a major Q fever outbreak in the Netherlands
Q fever, caused by Coxiella burnetii, is a zoonosis with a worldwide distribution. A large rural area in the southeast of the Netherlands was heavily affected by Q fever between 2007 and 2009. This initiated the development of a robust and internally controlled multiplex quantitative PCR (qPCR) assay for the detection of C. burnetii DNA in veterinary and environmental matrices on suspected Q fever-affected farms. The qPCR detects three C. burnetii targets (icd, com1, and IS1111) and one Bacillus thuringiensis internal control target (cry1b). Bacillus thuringiensis spores were added to samples to control both DNA extraction and PCR amplification. The performance of the qPCR assay was investigated and showed a high efficiency; a limit of detection of 13.0, 10.6, and 10.4 copies per reaction for the targets icd, com1, and IS1111, respectively; and no crossreactivity with the nontarget organisms tested. Screening for C. burnetii DNA on 29 suspected Q fever-affected farms during the Q fever epidemic in 2008 showed that swabs from dust-accumulating surfaces contained higher levels of C. burnetii DNA than vaginal swabs from goats or sheep. PCR inhibition by coextracted substances was observed in some environmental samples, and 10- or 100-fold dilutions of samples were sufficient to obtain interpretable signals for both the C. burnetii targets and the internal control. The inclusion of an internal control target and three C. burnetii targets in one multiplex qPCR assay showed that complex veterinary and environmental matrices can be screened reliably for the presence of C. burnetii DNA during an outbreak. © 2011, American Society for Microbiology
Theoretical Aspects of the Fractional Quantum Hall Effect in Graphene
We review the theoretical basis and understanding of electronic interactions
in graphene Landau levels, in the limit of strong correlations. This limit
occurs when inter-Landau-level excitations may be omitted because they belong
to a high-energy sector, whereas the low-energy excitations only involve the
same level, such that the kinetic energy (of the Landau level) is an
unimportant constant. Two prominent effects emerge in this limit of strong
electronic correlations: generalised quantum Hall ferromagnetic states that
profit from the approximate four-fold spin-valley degeneracy of graphene's
Landau levels and the fractional quantum Hall effect. Here, we discuss these
effects in the framework of an SU(4)-symmetric theory, in comparison with
available experimental observations.Comment: 12 pages, 3 figures; review for the proceedings of the Nobel
Symposium on Graphene and Quantum Matte
The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction
We present a structural analysis of the multi-layer graphene-4HSiC(000-1})
system using Surface X-Ray Reflectivity. We show for the first time that
graphene films grown on the C-terminated (000-1}) surface have a
graphene-substrate bond length that is very short (0.162nm). The measured
distance rules out a weak Van der Waals interaction to the substrate and
instead indicates a strong bond between the first graphene layer and the bulk
as predicted by ab-initio calculations. The measurements also indicate that
multi-layer graphene grows in a near turbostratic mode on this surface. This
result may explain the lack of a broken graphene symmetry inferred from
conduction measurements on this system [C. Berger et al., Science 312, 1191
(2006)].Comment: 9 pages with 6 figure
Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires
After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.Comment: 4 page
Raman Topography and Strain Uniformity of Large-Area Epitaxial Graphene
We report results from two-dimensional Raman spectroscopy studies of
large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large
variation in Raman peak position across the sample resulting from inhomogeneity
in the strain of the graphene film, which we show to be correlated with
physical topography by coupling Raman spectroscopy with atomic force
microscopy. We report that essentially strain free graphene is possible even
for epitaxial graphene.Comment: 10 pages, 3 figure
Field emission properties of nano-composite carbon nitride films
A modified cathodic arc technique has been used to deposit carbon nitride
thin films directly on n+ Si substrates. Transmission Electron Microscopy
showed that clusters of fullerene-like nanoparticles are embedded in the
deposited material. Field emission in vacuum from as-grown films starts at an
electric field strength of 3.8 V/micron. When the films were etched in an
HF:NH4F solution for ten minutes, the threshold field decreased to 2.6
V/micron. The role of the carbon nanoparticles in the field emission process
and the influence of the chemical etching treatment are discussed.Comment: 22 pages, 8 figures, submitted to J. Vac. Sc. Techn.
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