69,531 research outputs found
Electronic structure and magnetic properties of Gd-doped and Eu-rich EuO
The effects of Gd doping and O vacancies on the magnetic interaction and
Curie temperature of EuO are studied using first-principles calculations.
Linear response calculations in the virtual crystal approximation show a broad
maximum in the Curie temperature as a function of doping, which results from
the combination of the saturating contribution from indirect exchange and a
decreasing contribution from the f-d hopping mechanism. Non-Heisenberg
interaction at low doping levels and its effect on the Curie temperature are
examined. The electronic structure of a substitutional Gd and of an O vacancy
in EuO are evaluated. When the 4f spins are disordered, the impurity state goes
from single to double occupation, but correlated bound magnetic polarons are
not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films,
the impurity states broaden into bands and remain partially filled. To go
beyond the homogeneous doping picture, magnetostructural cluster expansions are
constructed, which describe the modified exchange parameters near Gd dopants or
O vacancies. Thermodynamic properties are studied using Monte Carlo
simulations. The Curie temperature for Gd-doped EuO agrees with the results of
the virtual crystal approximation and shows a maximum of about 150 K. At 3.125%
vacancy concentration the Curie temperature increases to 120 K, consistent with
experimental data for Eu-rich film samples.Comment: 15 pages, 13 figures, under review in Physical Review
Why is the nuclear symmetry energy so uncertain at supra-saturation densities?
Within the interacting Fermi gas model for isospin asymmetric nuclear matter,
effects of the in-medium three-body interaction and the two-body short-range
tensor force due to the meson exchange as well as the short-range
nucleon correlation on the high-density behavior of the nuclear symmetry energy
are demonstrated respectively in a transparent way. Possible physics origins of
the extremely uncertain nuclear symmetry energy at supra-saturation densities
are discussed.Comment: Added discussions and revised format. Version to appear in Phys. Rev.
C (2010
Performance Analysis of a Dual-Hop Cooperative Relay Network with Co-Channel Interference
This paper analyzes the performance of a dual-hop amplify-and-forward (AF) cooperative relay network in the presence of direct link between the source and destination and multiple co-channel interferences (CCIs) at the relay. Specifically, we derive the new analytical expressions for the moment generating function (MGF) of the output signal-to-interference-plus-noise ratio (SINR) and the average symbol error rate (ASER) of the relay network. Computer simulations are given to confirm the validity of the analytical results and show the effects of direct link and interference on the considered AF relay network
GRB 060218/SN 2006aj: A Gamma-Ray Burst and Prompt Supernova at z=0.0335
We report the imaging and spectroscopic localization of GRB 060218 to a
low-metallicity dwarf starburst galaxy at z = 0.03345 +/- 0.00006. In addition
to making it the second nearest gamma-ray burst known, optical spectroscopy
reveals the earliest detection of weak, supernova-like Si II near 5720
Angstroms (0.1c), starting 1.95 days after the burst trigger. UBVRI photometry
obtained between 1 and 26 days post-burst confirms the early rise of supernova
light, and suggests a short time delay between the gamma-ray burst and the
onset of SN 2006aj if the early appearance of a soft component in the X-ray
spectrum is understood as a ``shock breakout''. Together, these results verify
the long-hypothesized origin of soft gamma-ray bursts in the deaths of massive
stars.Comment: 5 pages, 2 figure
Vibrational Modes in LiBC: Theory Compared with Experiment
The search for other superconductors in the MgB2 class currently is focussed
on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with
the potential to be a better superconductor than MgB2. Here we present the
calculated phonon spectrum of the parent semiconductor LiBC. The calculated
Raman-active modes are in excellent agreement with a recent observation, and
comparison of calculated IR-active modes with a recent report provides a
prediction of the LO--TO splitting for these four modes, which is small for the
B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes
at 540 cm^{-1} and 620 cm^{-1}.Comment: 4 pages, two embedded figures. Physica B (in press
Phase stability, ordering tendencies, and magnetism in single-phase fcc Au-Fe nanoalloys
Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but
L1 and L1 orderings were reported in single-phase Au-Fe nanoparticles.
Motivated by these observations, we study the structural and ordering
energetics in this alloy by combining density functional theory (DFT)
calculations with effective Hamiltonian techniques: a cluster expansion with
structural filters, and the configuration-dependent lattice deformation model.
The phase separation tendency in Au-Fe persists even if the fcc-bcc
decomposition is suppressed. The relative stability of disordered bcc and fcc
phases observed in nanoparticles is reproduced, but the fully ordered L1
AuFe, L1 AuFe, and L1 AuFe structures are unstable in DFT.
However, a tendency to form concentration waves at the corresponding [001]
ordering vector is revealed in nearly-random alloys in a certain range of
concentrations. This incipient ordering requires enrichment by Fe relative to
the equiatomic composition, which may occur in the core of a nanoparticle due
to the segregation of Au to the surface. Effects of magnetism on the chemical
ordering are also discussed.Comment: 23 pages, 11 figure
Who Contributes to the Knowledge Sharing Economy?
Information sharing dynamics of social networks rely on a small set of
influencers to effectively reach a large audience. Our recent results and
observations demonstrate that the shape and identity of this elite, especially
those contributing \emph{original} content, is difficult to predict.
Information acquisition is often cited as an example of a public good. However,
this emerging and powerful theory has yet to provably offer qualitative
insights on how specialization of users into active and passive participants
occurs.
This paper bridges, for the first time, the theory of public goods and the
analysis of diffusion in social media. We introduce a non-linear model of
\emph{perishable} public goods, leveraging new observations about sharing of
media sources. The primary contribution of this work is to show that
\emph{shelf time}, which characterizes the rate at which content get renewed,
is a critical factor in audience participation. Our model proves a fundamental
\emph{dichotomy} in information diffusion: While short-lived content has simple
and predictable diffusion, long-lived content has complex specialization. This
occurs even when all information seekers are \emph{ex ante} identical and could
be a contributing factor to the difficulty of predicting social network
participation and evolution.Comment: 15 pages in ACM Conference on Online Social Networks 201
Binary pulsars as probes of a Galactic dark matter disk
As a binary pulsar moves through a wind of dark matter particles, the
resulting dynamical friction modifies the binary's orbit. We study this effect
for the double disk dark matter (DDDM) scenario, where a fraction of the dark
matter is dissipative and settles into a thin disk. For binaries within the
dark disk, this effect is enhanced due to the higher dark matter density and
lower velocity dispersion of the dark disk, and due to its co-rotation with the
baryonic disk.We estimate the effect and compare it with observations for two
different limits in the Knudsen number (). First, in the case where DDDM is
effectively collisionless within the characteristic scale of the binary
() and ignoring the possible interaction between the pair of dark
matter wakes. Second, in the fully collisional case (), where a fluid
description can be adopted and the interaction of the pair of wakes is taken
into account. We find that the change in the orbital period is of the same
order of magnitude in both limits. A comparison with observations reveals good
prospects to probe currently allowed DDDM models with timing data from binary
pulsars in the near future. We finally comment on the possibility of extending
the analysis to the intermediate (rarefied gas) case with .Comment: 15 pages, 6 figures. Few comments and references added, version
accepted for publication in Physics of the Dark Universe (PDU
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