6,533 research outputs found
Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap
The nature of strongly interacting Fermi gases and magnetism is one of the
most important and studied topics in condensed-matter physics. Still, there are
many open questions. A central issue is under what circumstances strong
short-range repulsive interactions are enough to drive magnetic correlations.
Recent progress in the field of cold atomic gases allows to address this
question in very clean systems where both particle numbers, interactions and
dimensionality can be tuned. Here we study fermionic few-body systems in a one
dimensional harmonic trap using a new rapidly converging effective-interaction
technique, plus a novel analytical approach. This allows us to calculate the
properties of a single spin-down atom interacting with a number of spin-up
particles, a case of much recent experimental interest. Our findings indicate
that, in the strongly interacting limit, spin-up and spin-down particles want
to separate in the trap, which we interpret as a microscopic precursor of
one-dimensional ferromagnetism in imbalanced systems. Our predictions are
directly addressable in current experiments on ultracold atomic few-body
systems.Comment: 12 pages, 6 figures, published version including two appendices on
our new numerical and analytical approac
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Exploring thermal signatures in the experimentally heated CM carbonaceous chondrite Allan Hills 83100
Predictable Disruption Tolerant Networks and Delivery Guarantees
This article studies disruption tolerant networks (DTNs) where each node
knows the probabilistic distribution of contacts with other nodes. It proposes
a framework that allows one to formalize the behaviour of such a network. It
generalizes extreme cases that have been studied before where (a) either nodes
only know their contact frequency with each other or (b) they have a perfect
knowledge of who meets who and when. This paper then gives an example of how
this framework can be used; it shows how one can find a packet forwarding
algorithm optimized to meet the 'delay/bandwidth consumption' trade-off:
packets are duplicated so as to (statistically) guarantee a given delay or
delivery probability, but not too much so as to reduce the bandwidth, energy,
and memory consumption.Comment: 9 page
Relativistic many-body calculation of low-energy dielectronic resonances in Be-like carbon
We apply relativistic configuration-interaction method coupled with many-body
perturbation theory (CI+MBPT) to describe low-energy dielectronic
recombination. We combine the CI+MBPT approach with the complex rotation method
(CRM) and compute the dielectronic recombination spectrum for Li-like carbon
recombining into Be-like carbon. We demonstrate the utility and evaluate the
accuracy of this newly-developed CI+MBPT+CRM approach by comparing our results
with the results of the previous high-precision study of the CIII system
[Mannervik et al., Phys. Rev. Lett. 81, 313 (1998)].Comment: 6 pages, 1 figure; v2,v3: fixed reference
Resolving all-order method convergence problems for atomic physics applications
The development of the relativistic all-order method where all single,
double, and partial triple excitations of the Dirac-Hartree-Fock wave function
are included to all orders of perturbation theory led to many important results
for study of fundamental symmetries, development of atomic clocks, ultracold
atom physics, and others, as well as provided recommended values of many atomic
properties critically evaluated for their accuracy for large number of
monovalent systems. This approach requires iterative solutions of the
linearized coupled-cluster equations leading to convergence issues in some
cases where correlation corrections are particularly large or lead to an
oscillating pattern. Moreover, these issues also lead to similar problems in
the CI+all-order method for many-particle systems. In this work, we have
resolved most of the known convergence problems by applying two different
convergence stabilizer methods, reduced linear equation (RLE) and direct
inversion of iterative subspace (DIIS). Examples are presented for B, Al,
Zn, and Yb. Solving these convergence problems greatly expands the
number of atomic species that can be treated with the all-order methods and is
anticipated to facilitate many interesting future applications
A risk assessment scale for the prediction of pressure sore development: reliability and validity
Background. The ability to assess the risk of a patient developing pressure sores is a major issue in pressure sore prevention. Risk assessment scales should be valid, reliable and easy to use in clinical practice.
Aim. To develop further a risk assessment scale, for predicting pressure sore development and, in addition, to present the validity and reliability of this scale.
Methods. The risk assessment pressure sore (RAPS) scale, includes 12 variables, five from the re-modified Norton scale, three from the Braden scale and three from other research results. Five hundred and thirty patients without pressure sores on admission were included in the study and assessed over a maximum period of 12 weeks. Internal consistency was examined by item analysis and equivalence by interrater reliability. To estimate equivalence, 10 pairs of nurses assessed a total of 116 patients. The underlying dimensions of the scale were examined by factor analysis. The predictive validity was examined by determination of sensitivity, specificity and predictive value.
Results. Two variables were excluded as a result of low item–item and item–total correlations. The average percentage of agreement and the intraclass correlation between raters were 70% and 0·83, respectively. The factor analysis gave three factors, with a total variance explained of 65·1%. Sensitivity, specificity and predictive value were high among patients at medical and infection wards.
Conclusions. The RAPS scale is a reliable scale for predicting pressure sore development. The validity is especially good for patients undergoing treatment in medical wards and wards for infectious diseases. This indicates that the RAPS scale may be useful in clinical practice for these groups of patients. For patients undergoing surgical treatment, further analysis will be performed.måsjekke
A scalable parallel finite element framework for growing geometries. Application to metal additive manufacturing
This work introduces an innovative parallel, fully-distributed finite element
framework for growing geometries and its application to metal additive
manufacturing. It is well-known that virtual part design and qualification in
additive manufacturing requires highly-accurate multiscale and multiphysics
analyses. Only high performance computing tools are able to handle such
complexity in time frames compatible with time-to-market. However, efficiency,
without loss of accuracy, has rarely held the centre stage in the numerical
community. Here, in contrast, the framework is designed to adequately exploit
the resources of high-end distributed-memory machines. It is grounded on three
building blocks: (1) Hierarchical adaptive mesh refinement with octree-based
meshes; (2) a parallel strategy to model the growth of the geometry; (3)
state-of-the-art parallel iterative linear solvers. Computational experiments
consider the heat transfer analysis at the part scale of the printing process
by powder-bed technologies. After verification against a 3D benchmark, a
strong-scaling analysis assesses performance and identifies major sources of
parallel overhead. A third numerical example examines the efficiency and
robustness of (2) in a curved 3D shape. Unprecedented parallelism and
scalability were achieved in this work. Hence, this framework contributes to
take on higher complexity and/or accuracy, not only of part-scale simulations
of metal or polymer additive manufacturing, but also in welding, sedimentation,
atherosclerosis, or any other physical problem where the physical domain of
interest grows in time
Third-order many-body perturbation theory calculations for the beryllium and magnesium isoelectronic sequences
Relativistic third-order MBPT is applied to obtain energies of ions with two
valence electrons in the no virtual-pair approximation (NVPA). A total of 302
third-order Goldstone diagrams are organized into 12 one-body and 23 two-body
terms. Only third-order two-body terms and diagrams are presented here, owing
to the fact that the one-body terms are identical to the previously studied
third-order terms in monovalent ions. Dominant classes of diagrams are
identified. The model potential is a Dirac-Hartree-Fock potential,
and B-spline basis functions in a cavity of finite radius are employed in the
numerical calculations. The Breit interaction is taken into account through
second order of perturbation theory and the lowest-order Lamb shift is also
evaluated. Sample calculations are performed for berylliumlike ions with Z =
4--7, and for the magnesiumlike ion P IV. The third-order energies are in
excellent agreement with measurement with an accuracy at 0.2% level for the
cases considered. Comparisons are made with previous second-order MBPT results
and with other calculations. The third-order energy correction is shown to be
significant, improving second-order correlation energies by an order of
magnitude
Relativistic calculations of pionic and kaonic atoms hyperfine structure
We present the relativistic calculation of the hyperfine structure in pionic
and kaonic atoms. A perturbation method has been applied to the Klein-Gordon
equation to take into account the relativistic corrections. The perturbation
operator has been obtained \textit{via} a multipole expansion of the nuclear
electromagnetic potential. The hyperfine structure of pionic and kaonic atoms
provide an additional term in the quantum electrodynamics calculation of the
energy transition of these systems. Such a correction is required for a recent
measurement of the pion mass
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