77,864 research outputs found
Room-temperature ballistic transport in narrow graphene strips
We investigate electron-phonon couplings, scattering rates, and mean free
paths in zigzag-edge graphene strips with widths of the order of 10 nm. Our
calculations for these graphene nanostrips show both the expected similarity
with single-wall carbon nanotubes (SWNTs) and the suppression of the
electron-phonon scattering due to a Dirichlet boundary condition that prohibits
one major backscattering channel present in SWNTs. Low-energy acoustic phonon
scattering is exponentially small at room temperature due to the large phonon
wave vector required for backscattering. We find within our model that the
electron-phonon mean free path is proportional to the width of the nanostrip
and is approximately 70 m for an 11-nm-wide nanostrip.Comment: 5 pages and 5 figure
Ground State Properties of the Doped 3-Leg t-J Ladder
Results for a doped 3-leg t-J ladder obtained using the density matrix
renormalization group are reported. At low hole doping, the holes form a dilute
gas with a uniform density. The momentum occupation of the odd band shows a
sharp decrease at a large value of k_F similar to the behavior of a lightly
doped t-J chain, while the even modes appear gapped. The spin-spin correlations
decay as a power law consistent with the absence of a spin gap, but the pair
field correlations are negligible. At larger doping we find evidence for a spin
gap and as x increases further we find 3-hole diagonal domain walls. In this
regime there are pair field correlations and the internal pair orbital has
d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall
exponentially at large distances.Comment: 14 pages, 11 postscript figure
Effect of nonmagnetic impurities on stripes in high-Tc cuprates
We perform the numerically exact diagonalization study of the t-J model with
nonmagnetic impurities to clarify the relation between Zn impurities and the
stripes. By examining the hole-hole correlation function for a two-hole
\sqrt{18}x\sqrt{18} cluster with a single impurity, we find that the impurity
has a tendency to stabilize vertical charge stripes. This tendency is caused by
the gain of the kinetic energy of holes moving along the stripes that are
formed avoiding the impurity.Comment: 3 pages including 2 figures. Proceedings for ISS2000 (Tokyo, October
2000). To be published in Physica
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A survey of nearby galaxies for CO
We have made a survey of the nuclei of 81 galaxies for the 1-) line of CO. 38 of the galaxies are from a complete sample with recession velocity ≦400 km s-1 and 21-cm line strength ≧10-27 Wm-2, and the remainder represent nearby galaxies with weaker or no HI, early-type galaxies (E/SO/Sa) with detected HI and active/infrared galaxies.
Galaxies with strong CO lines like M82, NGC253 and IC342 are exceedingly rare: all the galaxies we observed are weaker than 0/2K except the irregular galaxy DDO133 with T*A=0.22K. We have new, confirmed detections of two other irregular galaxies, IC10 and Pegasus, at a weaker level, and unconfirmed detections of the irregular NGC3109 and the nearest Type I Seyfert galaxy NGC4051. We have confirmed the existence of CO in the nucleus of NGC6946 and obtained spectra of new positions in M82 and NGC253
Bibliography and summary of methods related to the error analysis of hybrid computers technical note no. 4
Bibliography and summary of methods used in error analysis of hybrid computer
Computation of Casimir Interactions between Arbitrary 3D Objects with Arbitrary Material Properties
We extend a recently introduced method for computing Casimir forces between
arbitrarily--shaped metallic objects [M. T. H. Reid et al., Phys. Rev.
Lett._103_ 040401 (2009)] to allow treatment of objects with arbitrary material
properties, including imperfect conductors, dielectrics, and magnetic
materials. Our original method considered electric currents on the surfaces of
the interacting objects; the extended method considers both electric and
magnetic surface current distributions, and obtains the Casimir energy of a
configuration of objects in terms of the interactions of these effective
surface currents. Using this new technique, we present the first predictions of
Casimir interactions in several experimentally relevant geometries that would
be difficult to treat with any existing method. In particular, we investigate
Casimir interactions between dielectric nanodisks embedded in a dielectric
fluid; we identify the threshold surface--surface separation at which
finite--size effects become relevant, and we map the rotational energy
landscape of bound nanoparticle diclusters
On the Nagaoka polaron in the t-J model
It is widely believed that a single hole in the two (or three) dimensional
t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic
bubble around itself, a finite J remnant of the ferromagnetic groundstate at
J=0 (the infinite U Hubbard model), first established by Nagaoka. We
investigate this phenomenon in two dimensions using the density matrix
renormalization group, for system sizes up to 9x9. We find that the polaron
forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously).
Although finite-size effects appear large, our data seems consistent with the
expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the
Brinkman-Rice model of non-retracing paths in a Neel background, showing that
it is quite accurate, at larger J. Results are also presented in the case where
the
Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a
"dressed polaron" picture in which the hole propagates freely inside a finite
region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure
Ambitious STS-7 mission to feature first landing at Kennedy
The STS-7 press briefing schedule, NASA select television schedule; launch preparations, countdown and liftoff; major countdown milestones; launch window; STS-7 flight sequence of events, landing timeline; STS-7 flight timeline; landing and post landing operations; flight objectives; Telesat's ANIK-C 2; PALAPA-B; STS-7 experiments; and spacecraft tracking and data network are presented
First ALMA Observation of a Solar Plasmoid Ejection from an X-ray Bright Point
Eruptive phenomena such as plasmoid ejections or jets are an important
feature of solar activity with the potential for improving our understanding of
the dynamics of the solar atmosphere. Such ejections are often thought to be
signatures of the outflows expected in regions of fast magnetic reconnection.
The 304 A EUV line of Helium, formed at around 10^5 K, is found to be a
reliable tracer of such phenomena, but the determination of physical parameters
from such observations is not straightforward. We have observed a plasmoid
ejection from an X-ray bright point simultaneously at millimeter wavelengths
with ALMA, at EUV wavelengths with AIA, in soft X-rays with Hinode/XRT. This
paper reports the physical parameters of the plasmoid obtained by combining the
radio, EUV and X-ray data. As a result, we conclude that the plasmoid can
consist either of (approximately) isothermal 10^5 K plasma that is optically
thin at 100 GHz, or else a 10^4 K core with a hot envelope. The analysis
demonstrates the value of the additional temperature and density constraints
that ALMA provides, and future science observations with ALMA will be able to
match the spatial resolution of space-borne and other high-resolution
telescopes.Comment: 10 page, 5 figures, accepted for publication in Astrophysical Journal
Letter. The movie can be seen at the following link:
http://hinode.nao.ac.jp/user/shimojo/data_area/plasmoid/movie5.mp
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