141,701 research outputs found
Cosmic ray acceleration at supergalactic accretion shocks: a new upper energy limit due to a finite shock extension
Accretion flows onto supergalactic-scale structures are accompanied with
large spatial scale shock waves. These shocks were postulated as possible
sources of ultra-high energy cosmic rays. The highest particle energies were
expected for perpendicular shock configuration in the so-called "Jokipii
diffusion limit", involving weakly turbulent conditions in the large-scale
magnetic field imbedded in the accreting plasma. For such configuration we
discuss the process limiting the highest energy that particles can obtain in
the first-order Fermi acceleration process due to finite shock extensions to
the sides, along and across the mean magnetic field. Cosmic ray outflow along
the shock structure can substantially lower (below ~10^18 eV for protons) the
upper particle energy limit for conditions considered for supergalactic shocks.Comment: A&A, accepte
Dense Regular Packings of Irregular Non-Convex Particles
We present a new numerical scheme to study systems of non-convex, irregular,
and punctured particles in an efficient manner. We employ this method to
analyze regular packings of odd-shaped bodies, not only from a nanoparticle but
also both from a computational geometry perspective. Besides determining
close-packed structures for many shapes, we also discover a new denser
configuration for Truncated Tetrahedra. Moreover, we consider recently
synthesized nanoparticles and colloids, where we focus on the excluded volume
interactions, to show the applicability of our method in the investigation of
their crystal structures and phase behavior. Extensions to the presented scheme
include the incorporation of soft particle-particle interactions, the study of
quasicrystalline systems, and random packings.Comment: 4 pages, 3 figure
Coupled spin states in armchair graphene nanoribbons with asymmetric zigzag edge extensions
Carbon-based magnetic structures promise significantly longer coherence times
than traditional magnetic materials, which is of fundamental importance for
spintronic applications. An elegant way of achieving carbon-based magnetic
moments is the design of graphene nanostructures with an imbalanced occupation
of the two sublattices forming the carbon honeycomb lattice. According to
Lieb's theorem, this induces local magnetic moments that are proportional to
the sublattice imbalance. Exact positioning of sublattice imbalanced
nanostructures in graphene nanomaterials hence offers a route to control
interactions between induced local magnetic moments and to obtain graphene
nanomaterials with magnetically non-trivial ground states. Here, we show that
such sublattice imbalanced nanostructures can be incorporated along a large
band gap armchair graphene nanoribbon on the basis of asymmetric zigzag edge
extensions, which is achieved by incorporating specifically designed precursor
monomers during the bottom-up fabrication of the graphene nanoribbons. Scanning
tunneling spectroscopy of an isolated and electronically decoupled zigzag edge
extension reveals Hubbard-split states in accordance with theoretical
predictions. Investigation of pairs of such zigzag edge extensions reveals
ferromagnetic, antiferromagnetic or quenching of the magnetic interactions
depending on the relative alignment of the asymmetric edge extensions.
Moreover, a ferromagnetic spin chain is demonstrated for a periodic pattern of
zigzag edge extensions along the nanoribbon axis. This work opens a route
towards the design and fabrication of graphene nanoribbon-based spin chains
with complex magnetic ground states
New group structures for Carbon onions and Carbon nanotubes via affine extensions of non-crystallographic Coxeter groups
We present results underlining the conjecture that affine extensions for
non-crystallographic Coxeter groups are suitable mathematical objects for the
description of the symmetries of Carbon onions and Carbon nanotubes. It is the
hope that these considerations will shed new light on open questions concerning
structure, stability and formation of these fullerenes.Comment: 13 pages, submitted to Phys. Lett.
Multi-particle structures in non-sequentially reorganized hard sphere deposits
We have examined extended structures, bridges and arches, in computer
generated, non-sequentially stabilized, hard sphere deposits. The bridges and
arches have well defined distributions of sizes and shapes. The distribution
functions reflect the contraints associated with hard particle packing and the
details of the restructuring process. A subpopulation of string-like bridges
has been identified. Bridges are fundamental microstructural elements in real
granular systems and their sizes and shapes dominate considerations of
structural properties and flow instabilities such as jamming.Comment: 9 pages, 7 figure
Realistic, Extensible DNS and mDNS Models for INET/OMNeT++
The domain name system (DNS) is one of the core services in today's network
structures. In local and ad-hoc networks DNS is often enhanced or replaced by
mDNS. As of yet, no simulation models for DNS and mDNS have been developed for
INET/OMNeT++. We introduce DNS and mDNS simulation models for OMNeT++, which
allow researchers to easily prototype and evaluate extensions for these
protocols. In addition, we present models for our own experimental extensions,
namely Stateless DNS and Privacy-Enhanced mDNS, that are based on the
aforementioned models. Using our models we were able to further improve the
efficiency of our protocol extensions.Comment: Published in: A. F\"orster, C. Minkenberg, G. R. Herrera, M. Kirsche
(Eds.), Proc. of the 2nd OMNeT++ Community Summit, IBM Research - Zurich,
Switzerland, September 3-4, 201
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