13,561 research outputs found
Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results
The dispersive interaction between nanotubes is investigated through ab
initio theory calculations and in an analytical approximation. A van der Waals
density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to
determine and compare the binding of a pair of nanotubes as well as in a
nanotube crystal. To analyze the interaction and determine the importance of
morphology, we furthermore compare results of our ab initio calculations with a
simple analytical result that we obtain for a pair of well-separated nanotubes.
In contrast to traditional density functional theory calculations, the vdW-DF
study predicts an intertube vdW bonding with a strength that is consistent with
recent observations for the interlayer binding in graphitics. It also produce a
nanotube wall-to-wall separation which is in very good agreement with
experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal
binding energy can be approximated by a sum of nanotube-pair interactions when
these are calculated in vdW-DF. This observation suggests a framework for an
efficient implementation of quantum-physical modeling of the CNT bundling in
more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure
Numerical thermo-elasto-plastic analysis of residual stresses on different scales during cooling of hot forming parts
In current research, more and more attention is paid to the understanding of residual stress states as well as the application of targeted residual stresses to extend e.g. life time or stiïŹness of a part. In course of that, the numerical simulation and analysis of the forming process of components, which goes along with the evolution of residual stresses, play an important role. In this contribution, we focus on the residual stresses arising from the austenite-to-martensite transformation at microscopic and mesoscopic level of a Cr-alloyed steel. A combination of a Multi-Phase-Field model and a two-scale Finite Element simulation is utilized for numerical analysis. A ïŹrst microscopic simulation considers the lattice change, such that the results can be homogenized and applied on the mesoscale. Based on this result, a polycrystal consisting of a certain number of austenitic grains is built and the phase transformation from austenite to martensite is described with respect to the mesoscale. Afterwards, in a two-scale Finite Element simulation the plastic eïŹects are considered and resulting residual stress states are computed
High-precision epsilon expansions of single-mass-scale four-loop vacuum bubbles
In this article we present a high-precision evaluation of the expansions in
\e=(4-d)/2 of (up to) four-loop scalar vacuum master integrals, using the
method of difference equations developed by S. Laporta. We cover the complete
set of `QED-type' master integrals, i.e. those with a single mass scale only
(i.e. ) and an even number of massive lines at each vertex.
Furthermore, we collect all that is known analytically about four-loop
`QED-type' masters, as well as about {\em all} single-mass-scale vacuum
integrals at one-, two- and three-loop order.Comment: 25 pages, uses axodraw.st
Four-Loop Decoupling Relations for the Strong Coupling
We compute the matching relation for the strong coupling constant within the
framework of QCD up to four-loop order. This allows a consistent five-loop
running (once the function is available to this order) taking into
account threshold effects. As a side product we obtain the effective coupling
of a Higgs boson to gluons with five-loop accuracy.Comment: 11 page
The Automorphism Conjecture for Ordered Sets of Width (Version 2)
We introduce a recursive method to deconstruct the automorphism group of an
ordered set. By connecting this method with deep results for permutation
groups, we prove the Automorphism Conjecture for ordered sets of width less
than or equal to . Subsequent investigations show that the method presented
here could lead to a resolution of the Automorphism Conjecture.Comment: arXiv admin note: substantial text overlap with arXiv:2209.0931
Towards the Automorphism Conjecture I: Combinatorial Control and Compensation for Factorials
This paper exploits adjacencies between the orbits of an ordered set P and a
consequence of the classification of finite simple groups to, in many cases,
exponentially bound the number of automorphisms. Results clearly identify the
structures which currently prevent the proof of such an exponential bound, or
which indeed inflate the number of automorphisms beyond such a bound. This is a
first step towards a possible resolution of the Automorphism Conjecture for
ordered sets
Matching small functions using centroid jitter and two beam position monitors
Matching to small beta functions is required to preserve emittance in plasma
accelerators. The plasma wake provides strong focusing fields, which typically
require beta functions on the mm-scale, comparable to those found in the final
focusing of a linear collider. Such beams can be time consuming to
experimentally produce and diagnose. We present a simple, fast, and noninvasive
method to measure Twiss parameters in a linac using two beam position monitors
only, relying on the similarity of the beam phase space and the jitter phase
space. By benchmarking against conventional quadrupole scans, the viability of
this technique was experimentally demonstrated at the FLASHForward
plasma-accelerator facility.Comment: 8 pages, 7 figure
Multiscale 3D Shape Analysis using Spherical Wavelets
©2005 Springer. The original publication is available at www.springerlink.com:
http://dx.doi.org/10.1007/11566489_57DOI: 10.1007/11566489_57Shape priors attempt to represent biological variations within a population. When variations are global, Principal Component Analysis (PCA) can be used to learn major modes of variation, even from a limited training set. However, when significant local variations exist, PCA typically cannot represent such variations from a small training set. To address this issue, we present a novel algorithm that learns shape variations from data at multiple scales and locations using spherical wavelets and spectral graph partitioning. Our results show that when the training set is small, our algorithm significantly improves the approximation of shapes in a testing set over PCA, which tends to oversmooth data
Habitability of Super-Earth Planets around Main-Sequence Stars including Red Giant Branch Evolution: Models based on the Integrated System Approach
In a previous study published in Astrobiology, we focused on the evolution of
habitability of a 10 M_E super-Earth planet orbiting a star akin to the Sun.
This study was based on a concept of planetary habitability in accordance to
the integrated system approach that describes the photosynthetic biomass
production taking into account a variety of climatological, biogeochemical, and
geodynamical processes. In the present study, we pursue a significant
augmentation of our previous work by considering stars with zero-age main
sequence masses between 0.5 and 2.0 M_sun with special emphasis on models of
0.8, 0.9, 1.2 and 1.5 M_sun. Our models of habitability consider again
geodynamical processes during the main-sequence stage of these stars as well as
during their red giant branch evolution. Pertaining to the different types of
stars, we identify so-called photosynthesis-sustaining habitable zones (pHZ)
determined by the limits of biological productivity on the planetary surface.
We obtain various sets of solutions consistent with the principal possibility
of life. Considering that stars of relatively high masses depart from the
main-sequence much earlier than low-mass stars, it is found that the biospheric
life-span of super-Earth planets of stars with masses above approximately 1.5
M_sun is always limited by the increase in stellar luminosity. However, for
stars with masses below 0.9 M_sun, the life-span of super-Earths is solely
determined by the geodynamic time-scale. For central star masses between 0.9
and 1.5 M_sun, the possibility of life in the framework of our models depends
on the relative continental area of the super-Earth planet.Comment: 25 pages, 6 figures, 2 tables; submitted to: International Journal of
Astrobiolog
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