862 research outputs found
Hooge's Constant of Carbon Nanotube Field Effect Transistors
The 1/f noise in individual semiconducting carbon nanotubes (s-CNT) in a
field effect transistor configuration has been measured in ultra-high vacuum
and following exposure to air. The amplitude of the normalized current spectral
noise density is independent of source-drain current, indicating the noise is
due to mobility rather than number fluctuations. Hooge's constant for s-CNT is
found to be 9.3 plus minus 0.4x10^-3. The magnitude of the 1/f noise is
substantially degreased by exposing the devices to air
Midgap states and charge inhomogeneities in corrugated graphene
We study the changes induced by the effective gauge field due to ripples on
the low energy electronic structure of graphene. We show that zero energy
Landau levels will form, associated to the smooth deformation of the graphene
layer, when the height corrugation, , and the length of the ripple, , are
such that , where is the lattice constant. The
existence of localized levels gives rise to a large compressibility at zero
energy, and to the enhancement of instabilities arising from electron-electron
interactions including electronic phase separation. The combined effect of the
ripples and an external magnetic field breaks the valley symmetry of graphene
leading to the possibility of valley selection
Conductivity and Fano factor in disordered graphene
Using the recursive Green's function method, we study the problem of electron
transport in a disordered single-layer graphene sheet. The conductivity is of
order and its dependence on the carrier density has a scaling form that
is controlled solely by the disorder strength and the ratio between the sample
size and the correlation length of the disorder potential. The shot noise Fano
factor is shown to have a narrow dip near the neutrality point for weak
disorder and to develop a nearly doping independent behavior at strong
disorder. Our results are in good qualitative and quantitative agreement with
experiments and provide a way for extracting microscopic information about the
magnitude of extrinsic disorder in graphene.Comment: 4 pages, 5 figure
Aligned Molecular Clouds towards SS433 and L=348.5 degrees; Possible Evidence for Galactic "Vapor Trail" Created by Relativistic Jet
We have carried out a detailed analysis of the NANTEN 12CO(J=1-0) dataset in
two large areas of ~25 square degrees towards SS433 (l~40 degree) and of ~18
square degrees towards l~348.5 degree, respectively. We have discovered two
groups of remarkably aligned molecular clouds at |b|~1--5 degree in the two
regions. In SS433, we have detected 10 clouds in total, which are well aligned
nearly along the axis of the X-ray jet emanating from SS433. These clouds have
similar line-of-sight velocities of 42--56 km s^-1 and the total projected
length of the feature is ~300 pc, three times larger than that of the X-ray
jet, at a distance of 3 kpc. Towards l~348.5 degree, we have detected four
clouds named as MJG348.5 at line-of-sight velocities of -80 -- -95 km s^-1 in
V_LSR, which also show alignment nearly perpendicular to the Galactic plane.
The total length of the feature is ~400 pc at a kinematic distance of 6 kpc. In
the both cases, the CO clouds are distributed at high galactic latitudes where
such clouds are very rare. In addition, their alignments and coincidence in
velocity should be even rarer, suggesting that they are physically associated.
We tested a few possibilities to explain these clouds, including protostellar
outflows, supershells, and interactions with energetic jets. Among them, a
favorable scenario is that the interaction between relativistic jet and the
interstellar medium induced the formation of molecular clouds over the last
~10^5-6 yrs. It is suggested that the timescale of the relativistic jet may be
considerably larger, in the order of 10^5-6 yrs, than previously thought in
SS433. The driving engine of the jet is obviously SS433 itself in SS433,
although the engine is not yet identified in MJG348.5 among possible several
candidates detected in the X-rays and TeV gamma rays.Comment: 29 pages, 10 figures, already published in PASJ, 2008,60, 71
Atomic Structure of Graphene on SiO2
We employ scanning probe microscopy to reveal atomic structures and nanoscale
morphology of graphene-based electronic devices (i.e. a graphene sheet
supported by an insulating silicon dioxide substrate) for the first time.
Atomic resolution STM images reveal the presence of a strong spatially
dependent perturbation, which breaks the hexagonal lattice symmetry of the
graphitic lattice. Structural corrugations of the graphene sheet partially
conform to the underlying silicon oxide substrate. These effects are obscured
or modified on graphene devices processed with normal lithographic methods, as
they are covered with a layer of photoresist residue. We enable our experiments
by a novel cleaning process to produce atomically-clean graphene sheets.Comment: 13 pages, 4 figure
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