611 research outputs found
Compositional optimization of hard-magnetic phases with machine-learning models
Machine Learning (ML) plays an increasingly important role in the discovery
and design of new materials. In this paper, we demonstrate the potential of ML
for materials research using hard-magnetic phases as an illustrative case. We
build kernel-based ML models to predict optimal chemical compositions for new
permanent magnets, which are key components in many green-energy technologies.
The magnetic-property data used for training and testing the ML models are
obtained from a combinatorial high-throughput screening based on
density-functional theory calculations. Our straightforward choice of
describing the different configurations enables the subsequent use of the ML
models for compositional optimization and thereby the prediction of promising
substitutes of state-of-the-art magnetic materials like NdFeB with
similar intrinsic hard-magnetic properties but a lower amount of critical
rare-earth elements.Comment: 12 pages, 6 figure
The prismatic Sigma 3 (10-10) twin bounday in alpha-Al2O3 investigated by density functional theory and transmission electron microscopy
The microscopic structure of a prismatic twin
boundary in \aal2o3 is characterized theoretically by ab-initio
local-density-functional theory, and experimentally by spatial-resolution
electron energy-loss spectroscopy in a scanning transmission electron
microscope (STEM), measuring energy-loss near-edge structures (ELNES) of the
oxygen -ionization edge. Theoretically, two distinct microscopic variants
for this twin interface with low interface energies are derived and analysed.
Experimentally, it is demonstrated that the spatial and energetical resolutions
of present high-performance STEM instruments are insufficient to discriminate
the subtle differences of the two proposed interface variants. It is predicted
that for the currently developed next generation of analytical electron
microscopes the prismatic twin interface will provide a promising benchmark
case to demonstrate the achievement of ELNES with spatial resolution of
individual atom columns
The EVN view of the highly variable TeV active galaxy IC 310
Very-high-energy -ray observations of the active galaxy IC 310 with
the MAGIC telescopes have revealed fast variability with doubling time scales
of less than 4.8min. This implies that the emission region in IC 310 is smaller
than 20% of the gravitational radius of the central supermassive black hole
with a mass of , which poses serious questions on the
emission mechanism and classification of this enigmatic object. We report on
the first quasi-simultaneous multi-frequency VLBI observations of IC 310
conducted with the EVN. We find a blazar-like one-sided core-jet structure on
parsec scales, constraining the inclination angle to be less than but very small angles are excluded to limit the de-projected length
of the large-scale radio jet.Comment: 4 pages, proceedings of the 12th European VLBI Network Symposium and
Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published
online in PoS, ID.10
The Sigma 13 (10-14) twin in alpha-Al2O3: A model for a general grain boundary
The atomistic structure and energetics of the Sigma 13 (10-14)[1-210]
symmetrical tilt grain boundary in alpha-Al2O3 are studied by first-principles
calculations based on the local-density-functional theory with a mixed-basis
pseudopotential method. Three configurations, stable with respect to
intergranular cleavage, are identified: one Al-terminated glide-mirror twin
boundary, and two O-terminated twin boundaries, with glide-mirror and two-fold
screw-rotation symmetries, respectively. Their relative energetics as a
function of axial grain separation are described, and the local electronic
structure and bonding are analysed. The Al-terminated variant is predicted to
be the most stable one, confirming previous empirical calculations, but in
contrast with high-resolution transmission electron microscopy observations on
high-purity diffusion-bonded bicrystals, which resulted in an O-terminated
structure.
An explanation of this discrepancy is proposed, based on the different
relative energetics of the internal interfaces with respect to the free
surfaces
First-principles study of spontaneous polarization in multiferroic BiFeO
The ground-state structural and electronic properties of ferroelectric
BiFeO are calculated using density functional theory within the local
spin-density approximation and the LSDA+U method. The crystal structure is
computed to be rhombohedral with space group , and the electronic
structure is found to be insulating and antiferromagnetic, both in excellent
agreement with available experiments. A large ferroelectric polarization of
90-100 C/cm is predicted, consistent with the large atomic
displacements in the ferroelectric phase and with recent experimental reports,
but differing by an order of magnitude from early experiments. One possible
explanation is that the latter may have suffered from large leakage currents.
However both past and contemporary measurements are shown to be consistent with
the modern theory of polarization, suggesting that the range of reported
polarizations may instead correspond to distinct switching paths in structural
space. Modern measurements on well-characterized bulk samples are required to
confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables
Parametric excitation of plasma waves by gravitational radiation
We consider the parametric excitation of a Langmuir wave and an
electromagnetic wave by gravitational radiation, in a thin plasma on a
Minkowski background. We calculate the coupling coefficients starting from a
kinetic description, and the growth rate of the instability is found. The
Manley-Rowe relations are fulfilled only in the limit of a cold plasma. As a
consequence, it is generally difficult to view the process quantum
mechanically, i.e. as the decay of a graviton into a photon and a plasmon.
Finally we discuss the relevance of our investigation to realistic physical
situations.Comment: 5 pages, REVTe
A general lower bound for collaborative tree exploration
We consider collaborative graph exploration with a set of agents. All
agents start at a common vertex of an initially unknown graph and need to
collectively visit all other vertices. We assume agents are deterministic,
vertices are distinguishable, moves are simultaneous, and we allow agents to
communicate globally. For this setting, we give the first non-trivial lower
bounds that bridge the gap between small () and large () teams of agents. Remarkably, our bounds tightly connect to existing results
in both domains.
First, we significantly extend a lower bound of
by Dynia et al. on the competitive ratio of a collaborative tree exploration
strategy to the range for any . Second,
we provide a tight lower bound on the number of agents needed for any
competitive exploration algorithm. In particular, we show that any
collaborative tree exploration algorithm with agents has a
competitive ratio of , while Dereniowski et al. gave an algorithm
with agents and competitive ratio , for any
and with denoting the diameter of the graph. Lastly, we
show that, for any exploration algorithm using agents, there exist
trees of arbitrarily large height that require rounds, and we
provide a simple algorithm that matches this bound for all trees
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits
We analyze two configurations for laser cooling of neutral atoms whose
internal states store qubits. The atoms are trapped in an optical lattice which
is placed inside a cavity. We show that the coupling of the atoms to the damped
cavity mode can provide a mechanism which leads to cooling of the motion
without destroying the quantum information.Comment: 12 page
- âŠ