4,788 research outputs found
Interpretation of the angular dependence of the de Haas-van Alphen effect in MgB_2
We present detailed results for the amplitude and field dependence of the de
Haas-van Alphen (dHvA) signal arising from the electron-like sheet of
Fermi surface in MgB_2. Our data and analysis show that the dip in dHvA
amplitude when the field is close to the basal plane is caused by a beat
between two very similar dHvA frequencies and not a spin-zero effect as
previously assumed. Our results imply that the Stoner enhancement factors in
MgB_2 are small on both the Sigma and Pi sheets.Comment: 4 pages with figures. Submitted to PR
Reciprocal relationships in collective flights of homing pigeons
Collective motion of bird flocks can be explained via the hypothesis of many
wrongs, and/or, a structured leadership mechanism. In pigeons, previous studies
have shown that there is a well-defined hierarchical structure and certain
specific individuals occupy more dominant positions --- suggesting that
leadership by the few individuals drives the behavior of the collective.
Conversely, by analyzing the same data-sets, we uncover a more egalitarian
mechanism. We show that both reciprocal relationships and a stratified
hierarchical leadership are important and necessary in the collective movements
of pigeon flocks. Rather than birds adopting either exclusive averaging or
leadership strategies, our experimental results show that it is an integrated
combination of both compromise and leadership which drives the group's movement
decisions.Comment: 7 pages, 5 figure
Vertex routing models
A class of models describing the flow of information within networks via
routing processes is proposed and investigated, concentrating on the effects of
memory traces on the global properties. The long-term flow of information is
governed by cyclic attractors, allowing to define a measure for the information
centrality of a vertex given by the number of attractors passing through this
vertex. We find the number of vertices having a non-zero information centrality
to be extensive/sub-extensive for models with/without a memory trace in the
thermodynamic limit. We evaluate the distribution of the number of cycles, of
the cycle length and of the maximal basins of attraction, finding a complete
scaling collapse in the thermodynamic limit for the latter. Possible
implications of our results on the information flow in social networks are
discussed.Comment: 12 pages, 6 figure
De-biased Populations of Kuiper Belt Objects from the Deep Ecliptic Survey
The Deep Ecliptic Survey (DES) discovered hundreds of Kuiper Belt objects
from 1998-2005. Follow-up observations yielded 304 objects with good dynamical
classifications (Classical, Scattered, Centaur, or 16 mean-motion resonances
with Neptune). The DES search fields are well documented, enabling us to
calculate the probability of detecting objects with particular orbital
parameters and absolute magnitudes at a randomized point in each orbit.
Grouping objects together by dynamical class leads, we estimate the orbital
element distributions (a, e, i) for the largest three classes (Classical, 3:2,
and Scattered) using maximum likelihood. Using H-magnitude as a proxy for the
object size, we fit a power law to the number of objects for 8 classes with at
least 5 detected members (246 objects). The best Classical slope is
alpha=1.02+/-0.01 (observed from 5<=H<=7.2). Six dynamical classes (Scattered
plus 5 resonances) are consistent in slope with the Classicals, though the
absolute number of objects is scaled. The exception to the power law relation
are the Centaurs (non-resonant with perihelia closer than Neptune, and thus
detectable at smaller sizes), with alpha=0.42+/-0.02 (7.5<H<11). This is
consistent with a knee in the H-distribution around H=7.2 as reported elsewhere
(Bernstein et al. 2004, Fraser et al. 2014). Based on the Classical-derived
magnitude distribution, the total number of objects (H<=7) in each class are:
Classical (2100+/-300 objects), Scattered (2800+/-400), 3:2 (570+/-80), 2:1
(400+/-50), 5:2 (270+/-40), 7:4 (69+/-9), 5:3 (60+/-8). The independent
estimate for the number of Centaurs in the same H range is 13+/-5. If instead
all objects are divided by inclination into "Hot" and "Cold" populations,
following Fraser et al. (2014), we find that alphaHot=0.90+/-0.02, while
alphaCold=1.32+/-0.02, in good agreement with that work.Comment: 26 pages emulateapj, 6 figures, 5 tables, accepted by A
Damping of the de Haas-van Alphen oscillations in the superconducting state of MgB_2
The de Haas-van Alphen (dHvA) signal arising from orbits on the Fermi
surface sheet of the two-gap superconductor MgB has been observed in the
vortex state below . An extra attenuation of the dHvA signal, beyond
those effects described in the conventional Lifshitz-Kosevich expression, is
seen due to the opening of the superconducting gap. Our data show that the
band gap is still present up to . The data are compared to
current theories of dHvA oscillations in the superconducting state which allow
us to extract estimates for the evolution of the band gap with magnetic
field. Contrary to results for other materials, we find that the most recent
theories dramatically underestimate the damping in MgB.Comment: 10 pages with figures. Submitted to Phys. Rev. B. PDF version with
higher quality figures can be found at
http://www.phy.bris.ac.uk/research/cond_matt/PdfPubs/mgb2RSdhva.pd
Magnetism and local distortions near carbon impurity in -iron
Local perturbations of crystal and magnetic structure of -iron near
carbon interstitial impurity is investigated by {\it ab initio} electronic
structure calculations. It is shown that the carbon impurity creates locally a
region of ferromagnetic ordering with substantial tetragonal distortions.
Exchange integrals and solution enthalpy are calculated, the latter being in a
very good agreement with experimental data. Effect of the local distortions on
the carbon-carbon interactions in -iron is discussed.Comment: 4 pages 3 figures. Final version, accepted to Phys.Rev. Let
How effective are Chinese media in shaping audiences’ attitudes towards China? A survey analysis in Kenya, Nigeria, and South Africa
Purpose
This paper measures how often audiences in Kenya, Nigeria and South Africa consume Chinese media, and investigates whether a positive relationship exists between the consumption of English language Chinese media and attitudes towards China.
Method
We analyze three waves of survey data collected in 2017/2018, 2020 and 2021 (N = 5876) in Kenya, Nigeria, and South Africa.
Findings
We show that, while Chinese State-owned news media have a growing but limited following, the minority of media users who report getting news from Chinese media have better attitudes towards China than those who don’t.
Implications
Our findings suggest that Beijing’s external communication activities might have the potential to fulfil Beijing’s goal of improving the country’s image overseas, provided that media reached a wider audience.
Value
The paper overcomes the limitations of previous studies on the reception of Chinese external communication practices in Africa (i.e., small sample sizes, failure to differentiate between increased presence and increased impact, and use of indirect rather direct measures of media use to infer possible effects on public opinion)
Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions
We report on a sub-stellar companion search utilizing interferometric
fringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the
Hubble Space Telescope. Our targets were Proxima Centauri and Barnard's Star.
We obtain absolute parallax values for Proxima Cen pi_{abs} = 0.7687 arcsecond
and for Barnard's Star pi_{abs} = 0.5454 arcsecond.
Once low-amplitude instrumental systematic errors are identified and removed,
our companion detection sensitivity is less than or equal to one Jupiter mass
for periods longer than 60 days for Proxima Cen. Between the astrometry and the
radial velocity results we exclude all companions with M > 0.8M_{Jup} for the
range of periods 1 < P < 1000 days. For Barnard's Star our companion detection
sensitivity is less than or equal to one Jupiter mass for periods long er than
150 days. Our null results for Barnard's Star are consistent with those of
Gatewood (1995).Comment: 35 pages, 13 figures, to appear in August 1999 A
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