722 research outputs found
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
Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2
The linear dispersion relation in graphene[1,2] gives rise to a surprising
prediction: the resistivity due to isotropic scatterers (e.g. white-noise
disorder[3] or phonons[4-8]) is independent of carrier density n. Here we show
that acoustic phonon scattering[4-6] is indeed independent of n, and places an
intrinsic limit on the resistivity in graphene of only 30 Ohm at room
temperature (RT). At a technologically-relevant carrier density of 10^12 cm^-2,
the mean free path for electron-acoustic phonon scattering is >2 microns, and
the intrinsic mobility limit is 2x10^5 cm^2/Vs, exceeding the highest known
inorganic semiconductor (InSb, ~7.7x10^4 cm^2/Vs[9]) and semiconducting carbon
nanotubes (~1x10^5 cm^2/Vs[10]). We also show that extrinsic scattering by
surface phonons of the SiO2 substrate[11,12] adds a strong temperature
dependent resistivity above ~200 K[8], limiting the RT mobility to ~4x10^4
cm^2/Vs, pointing out the importance of substrate choice for graphene
devices[13].Comment: 16 pages, 3 figure
Anisotropic spin-density distribution and magnetic anisotropy of strained LaSrMnO thin films: Angle-dependent x-ray magnetic circular dichroism
Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial
strain from the substrate via strain-induced anisotropy in the orbital magnetic
moment and that in the spatial distribution of spin-polarized electrons.
However, the preferential orbital occupation in ferromagnetic metallic
LaSrMnO (LSMO) thin films studied by x-ray linear dichroism
(XLD) has always been found out-of-plane for both tensile and compressive
epitaxial strain and hence irrespective of the magnetic anisotropy. In order to
resolve this mystery, we directly probed the preferential orbital occupation of
spin-polarized electrons in LSMO thin films under strain by angle-dependent
x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density
distribution was found to be in-plane for the tensile strain and out-of-plane
for the compressive strain, consistent with the observed magnetic anisotropy.
The ubiquitous out-of-plane preferential orbital occupation seen by XLD is
attributed to the occupation of both spin-up and spin-down out-of-plane
orbitals in the surface magnetic dead layer.Comment: 20 pages, 4 figure
X-ray absorption spectroscopy and X-ray magnetic circular dichroism studies of transition-metal-co-doped ZnO nano-particles
We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular
dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and
ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive
total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode
have been employed to extract the valence and spin states of the surface and
inner core regions of the nano-particles. XAS spectra reveal that significant
part of the doped Mn and Co atoms are found in the trivalent and tetravalent
state in particular in the surface region while majority of Fe atoms are found
in the trivalent state both in the inner core region and surface region. The
XMCD spectra show that the Fe ions in the surface region give rise to
the ferromagnetism while both the Co and Mn ions in the surface region show
only paramagnetic behaviors. The transition-metal atoms in the inner core
region do not show magnetic signals, meaning that they are
antiferromagnetically coupled. The present result combined with the previous
results on transition-metal-doped ZnO nano-particles and nano-wires suggest
that doped holes, probably due to Zn vacancy formation at the surfaces of the
nano-particles and nano-wires, rather than doped electrons are involved in the
occurrence of ferromagnetism in these systems.Comment: Proceedings of "XAFS theory and nanoparticles
Cr doping-induced ferromagnetism in the spin-glass Cd1-xMnxTe studied by x-ray magnetic circular dichroism
The prototypical diluted magnetic semiconductor Cd1-xMnxTe is a spin glass
(x0.6), but becomes ferromagnetic upon doping
with a small amount of Cr atoms substituting for Mn. In order to investigate
the origin of the ferromagnetism in Cd1-x-yMnxCryTe, we have studied its
element specific magnetic properties by x-ray absorption spectroscopy (XAS) and
x-ray magnetic circular dichroism (XMCD) at the Cr and Mn L2,3 edges. Thin
films were grown by molecular beam epitaxy with a fixed Mn content of x = 0.2
and varying Cr content in the range of y = 0 - 0.04. Measured XAS and XMCD
spectra indicate that both Cr and Mn atoms are divalent and that the
ferromagnetic or superparamagnetic components of Cr and Mn are aligned in the
same directions. The magnetization of Mn increases with increasing Cr content.
These results can be explained if ferromagnetic interaction exists between
neighboring Mn and Cr ions although interaction between Mn atoms is largely
antiferromagnetic. We conclude that each ferromagnetic or superparamagnetic
cluster consists of ferromagnetically coupled several Cr and a much larger
number of Mn ions.Comment: 13 pages, 5 figure
Observation of Wigner cusps in a metallic carbon nanotube
Previous gate-dependent conductance measurements of metallic carbon nanotubes
have revealed unexplainable conductance suppressions, occurring at two
different gate voltages. These were previously attributed to the
gate-dependency of contact resistance. Our gate-dependent conductivity
measurements on a metallic nanotube with known chirality show that these
bimodal conductance suppressions are the manifestations of Wigner cusps, often
seen in atomic and nuclear physics experiments.Comment: 6 pages, 3 figure
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