8,761 research outputs found
Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene
Atomic resolution high angle annular dark field imaging of suspended,
single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms
had been deposited was carried out in an aberration corrected scanning
transmission electron microscope. In combination with electron energy loss
spectroscopy, employed to identify individual impurity atoms, it was shown that
nano-scale holes were etched into graphene, initiated at sites where single
atoms of all the metal species except for gold come into close contact with the
graphene. The e-beam scanning process is instrumental in promoting metal atoms
from clusters formed during the original metal deposition process onto the
clean graphene surface, where they initiate the hole-forming process. Our
observations are discussed in the light of calculations in the literature,
predicting a much lowered vacancy formation in graphene when metal ad-atoms are
present. The requirement and importance of oxygen atoms in this process,
although not predicted by such previous calculations, is also discussed,
following our observations of hole formation in pristine graphene in the
presence of Si-impurity atoms, supported by new calculations which predict a
dramatic decrease of the vacancy formation energy, when SiOx molecules are
present.Comment: final version accepted in ACS Nano + supplementary info. 22+6 pages,
4+5 figure
Condensation and Clustering in the Driven Pair Exclusion Process
We investigate particle condensation in a driven pair exclusion process on
one- and two- dimensional lattices under the periodic boundary condition. The
model describes a biased hopping of particles subject to a pair exclusion
constraint that each particle cannot stay at a same site with its pre-assigned
partner. The pair exclusion causes a mesoscopic condensation characterized by
the scaling of the condensate size and the number of
condensates with the total number of sites .
Those condensates are distributed randomly without hopping bias. We find that
the hopping bias generates a spatial correlation among condensates so that a
cluster of condensates appears. Especially, the cluster has an anisotropic
shape in the two-dimensional system. The mesoscopic condensation and the
clustering are studied by means of numerical simulations.Comment: 4 pages, 5 figure
A special irreducible matrix representation of the real Clifford algebra C(3,1)
4x4 Dirac (gamma) matrices (irreducible matrix representations of the
Clifford algebras C(3,1), C(1,3), C(4,0)) are an essential part of many
calculations in quantum physics. Although the final physical results do not
depend on the applied representation of the Dirac matrices (e.g. due to the
invariance of traces of products of Dirac matrices), the appropriate choice of
the representation used may facilitate the analysis. The present paper
introduces a particularly symmetric real representation of 4x4 Dirac matrices
(Majorana representation) which may prove useful in the future. As a byproduct,
a compact formula for (transformed) Pauli matrices is found. The consideration
is based on the role played by isoclinic 2-planes in the geometry of the real
Clifford algebra C(3,0) which provide an invariant geometric frame for it. It
can be generalized to larger Clifford algebras.Comment: 23 pages LaTeX, to appear in the J. Math. Phys. (v2: appendix B on
Pauli matrices and references are added, minor other changes
Infrared Photometry of the Globular Cluster Palomar 6
We present JHK photometry of Palomar 6. Our photometric measurements range
from the RGB-tip to 2 mag below the RHB and our CMDs show that Palomar~6
appears to have a well-defined RHB population. The distance modulus and
interstellar reddening of the cluster are estimated by comparing the magnitude
and color of Palomar 6 RHB stars with respect to those of 47 Tuc. We obtain
(m-M)_0 = 14.28 mag and E(B-V) = 1.30 mag for the cluster and our study
suggests that Palomar~6 is clearly located in the Galaxy's central regions. We
also discuss the metallicity of the cluster using the slope of the RGB. We
obtain [Fe/H] = -1.2 for Palomar~6 and our metallicity estimate is 0.5 - 1.0
dex lower than previous estimates by others.Comment: 14 pages, 4 tables, 12 figures; accepted for publication in AJ (2002
June
Spin relaxation in mesoscopic superconducting Al wires
We studied the diffusion and the relaxation of the polarized quasiparticle
spins in superconductors. To that end, quasiparticles of polarized spins were
injected through an interface of a mesoscopic superconducting Al wire in
proximity contact with an overlaid ferromagnetic Co wire in the single-domain
state. The superconductivity was observed to be suppressed near the
spin-injecting interface, as evidenced by the occurrence of a finite voltage
for a bias current below the onset of the superconducting transition. The spin
diffusion length, estimated from finite voltages over a certain length of Al
wire near the interface, was almost temperature independent in the temperature
range sufficiently below the superconducting transition but grew as the
transition temperature was approached. This temperature dependence suggests
that the relaxation of the spin polarization in the superconducting state is
governed by the condensation of quasiparticles to the paired state. The spin
relaxation in the superconducting state turned out to be more effective than in
the normal state.Comment: 9 pages, 8 figure
Antioxidant responses and lipid peroxidation in gills and erythrocytes of fish (Rhabdosarga sarba) upon exposure to Chattonella marina and hydrogen peroxide : implications on the cause of fish kills
Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Journal of Experimental Marine Biology and Ecology 336 (2006): 230-241, doi:10.1016/j.jembe.2006.05.013.Chattonella marina, a red tide or harmful algal bloom species, has caused mass fish kills and serious economic loss worldwide, and yet its toxic actions remain highly controversial. Previous studies have shown that this species is able to produce reactive oxygen species (ROS), and therefore postulated that ROS are the causative agents of fish kills. The present study investigates antioxidant responses and lipid peroxidation in gills and erythrocytes of fish (Rhabdosarga sarba) upon exposure to C. marina, compared with responses exposed to equivalent and higher levels of ROS exposure. Even though C. marina can produce a high level of ROS, gills and erythrocytes of sea bream exposed to C. marina for 1 to 6 h showed neither significant induction of antioxidant enzymes nor lipid peroxidation. Antioxidant responses and oxidative damage did not occur as fish mortality began to occur, yet could be induced upon exposure to artificially supplied ROS levels an order of magnitude higher. The result of this study implies that ROS produced by C. marina is not the principal cause of fish kills.This study was supported by a CERG grant (CityU 1109/03M / No. 9040864) of the University Grants Committee, Hong Kong SAR government. Support for Don Anderson was also provided by the U.S. National Science Foundation through grant no. OCE-0136861
Interplay between carrier and impurity concentrations in annealed GaMnAs intrinsic anomalous Hall Effect
Investigating the scaling behavior of annealed GaMnAs anomalous
Hall coefficients, we note a universal crossover regime where the scaling
behavior changes from quadratic to linear, attributed to the anomalous Hall
Effect intrinsic and extrinsic origins, respectively. Furthermore, measured
anomalous Hall conductivities when properly scaled by carrier concentration
remain constant, equal to theoretically predicated values, spanning nearly a
decade in conductivity as well as over 100 K in T. Both the qualitative
and quantitative agreement confirms the validity of new equations of motion
including the Berry phase contributions as well as tunablility of the intrinsic
anomalous Hall Effect.Comment: 4 pages, 5 figure
On the distribution of career longevity and the evolution of home run prowess in professional baseball
Statistical analysis is a major aspect of baseball, from player averages to
historical benchmarks and records. Much of baseball fanfare is based around
players exceeding the norm, some in a single game and others over a long
career. Career statistics serve as a metric for classifying players and
establishing their historical legacy. However, the concept of records and
benchmarks assumes that the level of competition in baseball is stationary in
time. Here we show that power-law probability density functions, a hallmark of
many complex systems that are driven by competition, govern career longevity in
baseball. We also find similar power laws in the density functions of all major
performance metrics for pitchers and batters. The use of performance-enhancing
drugs has a dark history, emerging as a problem for both amateur and
professional sports. We find statistical evidence consistent with
performance-enhancing drugs in the analysis of home runs hit by players in the
last 25 years. This is corroborated by the findings of the Mitchell Report [1],
a two-year investigation into the use of illegal steroids in major league
baseball, which recently revealed that over 5 percent of major league baseball
players tested positive for performance-enhancing drugs in an anonymous 2003
survey.Comment: 5 pages, 5 figures, 2-column revtex4 format. Revision has change of
title, a figure added, and minor changes in response to referee comment
Self-Attracting Walk on Lattices
We have studied a model of self-attracting walk proposed by Sapozhnikov using
Monte Carlo method. The mean square displacement
and the mean number of visited sites are calculated for
one-, two- and three-dimensional lattice. In one dimension, the walk shows
diffusive behaviour with . However, in two and three dimension, we
observed a non-universal behaviour, i.e., the exponent varies
continuously with the strength of the attracting interaction.Comment: 6 pages, latex, 6 postscript figures, Submitted J.Phys.
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