1,046 research outputs found
Using synchrotron techniques to understand plant diseases stripe rust and wheat
Non-Peer Reviewe
InAs-AlSb quantum wells in tilted magnetic fields
InAs-AlSb quantum wells are investigated by transport experiments in magnetic
fields tilted with respect to the sample normal. Using the coincidence method
we find for magnetic fields up to 28 T that the spin splitting can be as large
as 5 times the Landau splitting. We find a value of the g-factor of about 13.
For small even-integer filling factors the corresponding minima in the
Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt
angles. This indicates the anti-crossing of neighboring Landau and spin levels.
Furthermore we find for particular tilt angles a crossover from even-integer
dominated Shubnikov-de Haas minima to odd-integer minima as a function of
magnetic field
Three-Omega Thermal-Conductivity Measurements with Curved Heater Geometries
The three-omega method, a powerful technique to measure the thermal
conductivity of nanometer-thick films and the interfaces between them, has
historically employed straight conductive wires to act as both heaters and
thermometers. When investigating stochastically prepared samples such as
two-dimensional materials and nanomembranes, residue and excess material can
make it difficult to fit the required millimeter-long straight wire on the
sample surface. There are currently no available criteria for how diverting
three-omega heater wires around obstacles affects the validity of the thermal
measurement. In this Letter, we quantify the effect of wire curvature by
performing three-omega experiments with a wide range of frequencies using both
curved and straight heater geometries on SiO/Si samples. When the heating
wire is curved, we find that the measured Si substrate thermal conductivity
changes by only 0.2%. Similarly, we find that wire curvature has no significant
effect on the determination of the thermal resistance of a 65 nm SiO
layer, even for the sharpest corners considered here, for which the largest
measured ratio of the thermal penetration depth of the applied thermal wave to
radius of curvature of the heating wire is 4.3. This result provides useful
design criteria for three-omega experiments by setting a lower bound for the
maximum ratio of thermal penetration depth to wire radius of curvature.Comment: 4 pages, 3 figure
Density-Functional Theory of Graphene Sheets
We outline a Kohn-Sham-Dirac density-functional-theory (DFT) scheme for
graphene sheets that treats slowly-varying inhomogeneous external potentials
and electron-electron interactions on an equal footing. The theory is able to
account for the the unusual property that the exchange-correlation contribution
to chemical potential increases with carrier density in graphene. Consequences
of this property, and advantages and disadvantages of using the DFT approach to
describe it, are discussed. The approach is illustrated by solving the
Kohn-Sham-Dirac equations self-consistently for a model random potential
describing charged point-like impurities located close to the graphene plane.
The influence of electron-electron interactions on these non-linear screening
calculations is discussed at length, in the light of recent experiments
reporting evidence for the presence of electron-hole puddles in nearly-neutral
graphene sheets.Comment: 11 pages, 9 figures, submitted. High-quality figures can be requested
to the author
Distinct levels in the nanoscale organization of DNA-histone complex revealed by its mechanical unfolding
Mechanical unfolding of nanoscale DNA-histone complex, using an atomic force microscope, shows a stepwise disassembly of histones from the nucleosome. A quantitative analysis of the rupture jump statistics and the length released per jump reveals insights into the possible histone contacts within the octamer complex. The measured ruptures correlate with the breakage of multiple contacts that stabilize the histone octamer. These results provide a mechanistic basis by which stepwise disassembly of histone proteins may result from an external force exerted by the adenosinetriphosphate (ATP) dependent chromatin remodeling machines to access regulatory sites on DNA
Second-order harmonic and combination modes in graphite, single-wall carbon nanotube bundles, and isolated single-wall carbon nanotubes
Long Phonon Mean Free Paths Observed in Cross-plane Thermal-Conductivity Measurements of Exfoliated Hexagonal Boron Nitride
Sub-micron-thick layers of hexagonal boron nitride (hBN) exhibit high
in-plane thermal conductivity, useful optical properties, and serve as
dielectric encapsulation layers with low electrostatic inhomogeneity for
graphene devices. Despite the promising applications of hBN as a heat spreader,
the thickness dependence of the cross-plane thermal conductivity is not known,
and the cross-plane phonon mean free paths in hBN have not been measured. We
measure the cross-plane thermal conductivity of hBN flakes exfoliated from bulk
crystals. We find that the thermal conductivity is extremely sensitive to film
thickness. We measure a forty-fold increase in the cross-plane thermal
conductivity between 7 nm and 585 nm flakes at 295 K. We attribute the large
increase in thermal conductivity with increasing thickness to contributions
from phonons with long mean free paths (MFPs), spanning many hundreds of
nanometers in the thickest flakes. When planar twist interfaces are introduced
into the crystal by mechanically stacking multiple thin flakes, the cross-plane
thermal conductivity of the stack is found to be a factor of seven below that
of individual flakes with similar total thickness, thus providing strong
evidence that phonon scattering at twist boundaries limits the maximum phonon
MFPs. These results have important implications for hBN integration in
nanoelectronics and improve our understanding of thermal transport in
two-dimensional materials.Comment: 4 pages, 3 figure
Polarized resonant Raman study of isolated single-wall carbon nanotubes: Symmetry selection rules, dipolar and multipolar antenna effects
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