896 research outputs found
Exchange Interactions in Paramagnetic Amorphous and Disordered Crystalline CrN-based Systems
We present a first principles supercell methodology for the calculation of
exchange interactions of magnetic materials with arbitrary degrees of
structural and chemical disorder in their high temperature paramagnetic state.
It is based on a projection of the total magnetic energy of the system onto
local pair clusters, allowing the interactions to vary independently as a
response to their local environments. We demonstrate our method by deriving the
distance dependent exchange interactions in vibrating crystalline CrN, a
TiCrN solid solution as well as in amorphous CrN. Our method
reveals strong local environment effects in all three systems. In the amorphous
case we use the full set of exchange interactions in a search for the
non-collinear magnetic ground state.Comment: 5 pages, 3 figure
alphaPDE: A New Multivariate Technique for Parameter Estimation
We present alphaPDE, a new multivariate analysis technique for parameter
estimation. The method is based on a direct construction of joint probability
densities of known variables and the parameters to be estimated. We show how
posterior densities and best-value estimates are then obtained for the
parameters of interest by a straightforward manipulation of these densities.
The method is essentially non-parametric and allows for an intuitive graphical
interpretation. We illustrate the method by outlining how it can be used to
estimate the mass of the top quark, and we explain how the method is applied to
an ensemble of events containing background.Comment: 11 pages, published versio
Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations
Due to the high absorption of solar wind plasma on the lunar dayside, a large
scale wake structure is formed downstream of the Moon. However, recent in-situ
observations have revealed the presence of protons in the near-lunar wake (100
km to 200 km from the surface). The solar wind, either directly or after
interaction with the lunar surface (including magnetic anomalies), is the
source of these protons in the near-wake region. Using the entire data from the
SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed the
velocity distribution of the protons observed in the near-lunar wake. The
average velocity distribution functions, computed in the solar wind rest frame,
were further separated based on the angle between the upstream solar wind
velocity and the IMF. Several proton populations were identified from the
velocity distribution and their possible entry mechanism were inferred based on
the characteristics of the velocity distribution. These entry mechanisms
include (i) diffusion of solar wind protons into the wake along IMF, (ii) the
solar wind protons with finite gyro-radii that are aided by the wake boundary
electric field, (iii) solar wind protons with gyro-radii larger than lunar
radii from the tail of the solar wind velocity distribution, and (iv)
scattering of solar wind protons from the dayside lunar surface or from
magnetic anomalies. In order to gain more insight into the entry mechanisms
associated with different populations, backtracing is carried out for each of
these populations. For most of the populations, the source of the protons
obtained from backtracing is found to be in agreement with that inferred from
the velocity distribution. There are few populations that could not be
explained by the known mechanisms and remain unknown.Comment: 8 figures, paper accepted in Icarus (2016),
http://dx.doi.org/10.1016/j.icarus.2016.01.03
The Fermi Surface Effect on Magnetic Interlayer Coupling
The oscillating magnetic interlayer coupling of Fe over spacer layers
consisting of CuPd alloys is investigated by first principles
density functional theory. The amplitude, period and phase of the coupling, as
well as the disorder-induced decay, are analyzed in detail and the consistency
to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory is discussed. For the first
time an effect of the Fermi surface nesting strength on the amplitude is
established from first principles calculations. An unexpected variation of the
phase and disorder-induced decay is obtained and the results are discussed in
terms of asymptotics
An \emph{ab initio} method for locating characteristic potential energy minima of liquids
It is possible in principle to probe the many--atom potential surface using
density functional theory (DFT). This will allow us to apply DFT to the
Hamiltonian formulation of atomic motion in monatomic liquids [\textit{Phys.
Rev. E} {\bf 56}, 4179 (1997)]. For a monatomic system, analysis of the
potential surface is facilitated by the random and symmetric classification of
potential energy valleys. Since the random valleys are numerically dominant and
uniform in their macroscopic potential properties, only a few quenches are
necessary to establish these properties. Here we describe an efficient
technique for doing this. Quenches are done from easily generated "stochastic"
configurations, in which the nuclei are distributed uniformly within a
constraint limiting the closeness of approach. For metallic Na with atomic pair
potential interactions, it is shown that quenches from stochastic
configurations and quenches from equilibrium liquid Molecular Dynamics (MD)
configurations produce statistically identical distributions of the structural
potential energy. Again for metallic Na, it is shown that DFT quenches from
stochastic configurations provide the parameters which calibrate the
Hamiltonian. A statistical mechanical analysis shows how the underlying
potential properties can be extracted from the distributions found in quenches
from stochastic configurations
A Multi-variate Discrimination Technique Based on Range-Searching
We present a fast and transparent multi-variate event classification
technique, called PDE-RS, which is based on sampling the signal and background
densities in a multi-dimensional phase space using range-searching. The
employed algorithm is presented in detail and its behaviour is studied with
simple toy examples representing basic patterns of problems often encountered
in High Energy Physics data analyses. In addition an example relevant for the
search for instanton-induced processes in deep-inelastic scattering at HERA is
discussed. For all studied examples, the new presented method performs as good
as artificial Neural Networks and has furthermore the advantage to need less
computation time. This allows to carefully select the best combination of
observables which optimally separate the signal and background and for which
the simulations describe the data best. Moreover, the systematic and
statistical uncertainties can be easily evaluated. The method is therefore a
powerful tool to find a small number of signal events in the large data samples
expected at future particle colliders.Comment: Submitted to NIM, 18 pages, 8 figure
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
Intensive care-treated cardiac arrest : a retrospective study on the impact of extended age on mortality, neurological outcome, received treatments and healthcare-associated costs
BackgroundCardiac arrest (CA) is a leading cause of death worldwide. As population ages, the need for research focusing on CA in elderly increases. This study investigated treatment intensity, 12-month neurological outcome, mortality and healthcare-associated costs for patients aged over 75 years treated for CA in an intensive care unit (ICU) of a tertiary hospital.MethodsThis single-centre retrospective study included adult CA patients treated in a Finnish tertiary hospital's ICU between 2005 and 2013. We stratified the study population into two age groups: 75 years. We compared interventions defined by the median daily therapeutic scoring system (TISS-76) between the age groups to find differences in treatment intensity. We calculated cost-effectiveness by dividing the total one-year healthcare-associated costs of all patients by the number of survivors with a favourable neurological outcome. Favourable outcome was defined as a cerebral performance category (CPC) of 1-2 at 12 months after cardiac arrest. Logistic regression analysis was used to identify independent associations between age group, mortality and neurological outcome.ResultsThis study included a total of 1,285 patients, of which 212 (16%) were >= 75 years of age. Treatment intensity was lower for the elderly compared to the younger group, with median TISS scores of 116 and 147, respectively (pPeer reviewe
Liquid state properties from first principles DFT calculations: Static properties
In order to test the Vibration-Transit (V-T) theory of liquid dynamics, ab
initio density functional theory (DFT) calculations of thermodynamic properties
of Na and Cu are performed and compared with experimental data. The
calculations are done for the crystal at T = 0 and T_m, and for the liquid at
T_m. The key theoretical quantities for crystal and liquid are the structural
potential and the dynamical matrix, both as function of volume. The theoretical
equations are presented, as well as details of the DFT computations. The
properties compared with experiment are the equilibrium volume, the isothermal
bulk modulus, the internal energy and the entropy. The agreement of theory with
experiment is uniformly good. Our primary conclusion is that the application of
DFT to V-T theory is feasible, and the resulting liquid calculations achieve
the same level of accuracy as does ab initio lattice dynamics for crystals.
Moreover, given the well established reliability of DFT, the present results
provide a significant confirmation of V-T theory itself.Comment: 9 pages, 3 figures, 5 tables, edited to more closely match published
versio
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