1,021 research outputs found
Fracture of complex metallic alloys: An atomistic study of model systems
Molecular dynamics simulations of crack propagation are performed for two
extreme cases of complex metallic alloys (CMAs): In a model quasicrystal the
structure is determined by clusters of atoms, whereas the model C15 Laves phase
is a simple periodic stacking of a unit cell. The simulations reveal that the
basic building units of the structures also govern their fracture behaviour.
Atoms in the Laves phase play a comparable role to the clusters in the
quasicrystal. Although the latter are not rigid units, they have to be regarded
as significant physical entities.Comment: 6 pages, 4 figures, for associated avi file, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/C15.LJ.011.100.av
Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study
Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral
Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage
planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the
STM-images were interpreted as clusters of atoms. These are significant
structural units of quasicrystals. The experiments of Ebert et al. imply that
they are also stable physical entities, a property controversially discussed
currently. For a clarification we performed the first large scale fracture
simulations on three-dimensional complex binary systems. We studied the
propagation of mode I cracks in an icosahedral model quasicrystal by molecular
dynamics techniques at low temperature. In particular we examined how the shape
of the cleavage plane is influenced by the clusters inherent in the model and
how it depends on the plane structure. Brittle fracture with no indication of
dislocation activity is observed. The crack surfaces are rough on the scale of
the clusters, but exhibit constant average heights for orientations
perpendicular to high symmetry axes. From detailed analyses of the fractured
samples we conclude that both, the plane structure and the clusters, strongly
influence dynamic fracture in quasicrystals and that the clusters therefore
have to be regarded as physical entities.Comment: 10 pages, 12 figures, for associated avi files, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/emitted_soundwaves.avi
and
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/dynamic_fracture.av
Superconducting d-wave stripes in cuprates: Valence bond order coexisting with nodal quasiparticles
We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309].Comment: 5 pages, 3 figs, (v2) final version to be published in PR
When resources collide: Towards a theory of coincidence in information spaces
This paper is an attempt to lay out foundations for a general theory of coincidence in information spaces such as the World Wide Web, expanding on existing work on bursty structures in document streams and information cascades. We elaborate on the hypothesis that every resource that is published in an information space, enters a temporary interaction with another resource once a unique explicit or implicit reference between the two is found. This thought is motivated by Erwin Shroedingers notion of entanglement between quantum systems. We present a generic information cascade model that exploits only the temporal order of information sharing activities, combined with inherent properties of the shared information resources. The approach was applied to data from the world's largest online citizen science platform Zooniverse and we report about findings of this case study
Electron-phonon interaction in the three-band model
We study the half-breathing phonon in the three-band model of a high
temperature superconductor, allowing for vibrations of atoms and resulting
changes of hopping parameters. Two different approaches are compared. From the
three-band model a t-J model with phonons can be derived, and phonon properties
can be calculated. To make contact to density functional calculations, we also
study the three-band model in the Hartree-Fock (HF) approximation. The
paramagnetic HF solution, appropriate for the doped cuprates, has similarities
to the local-density approximation (LDA). However, in contrast to the LDA, the
existence of an antiferromagnetic insulating solution for the undoped system
makes it possible to study the softening of the half-breathing phonon under
doping. We find that although the HF approximation and the t-J model give
similar softenings, these softenings happen in quite different ways. We also
find that the HF approximation gives an incorrect doping and q dependence for
the softening and too small a width for the (half-)breathing phonon.Comment: 7 pages, RevTeX, 4 eps figure
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Discrimination between pathogenic and non-pathogenic E. coli strains by means of Raman microspectroscopy
Bacteria can be harmless commensals, beneficial probiotics, or harmful pathogens. Therefore, mankind is challenged to detect and identify bacteria in order to prevent or treat bacterial infections. Examples are identification of species for treatment of infection in clinics and E. coli cell counting for water quality monitoring. Finally, in some instances, the pathogenicity of a species is of interest. The main strategies to investigate pathogenicity are detection of target genes which encode virulence factors. Another strategy could be based on phenotypic identification. Raman spectroscopy is a promising phenotypic method, which offers high sensitivities and specificities for the identification of bacteria species. In this study, we evaluated whether Raman microspectroscopy could be used to determine the pathogenicity of E. coli strains. We used Raman spectra of seven non-pathogenic and seven pathogenic E. coli strains to train a PCA-SVM model. Then, the obtained model was tested by identifying the pathogenicity of three additional E. coli strains. The pathogenicity of these three strains could be correctly identified with a mean sensitivity of 77%, which is suitable for a fast screening of pathogenicity of single bacterial cells. [Figure not available: see fulltext.]. © 2020, The Author(s)
Finite-Temperature Transition in the Spin-Dimer Antiferromagnet BaCuSi2O6
We consider a classical XY-like Hamiltonian on a body-centered tetragonal
lattice, focusing on the role of interlayer frustration. A three-dimensional
(3D) ordered phase is realized via thermal fluctuations, breaking the
mirror-image reflection symmetry in addition to the XY symmetry. A heuristic
field-theoretical model of the transition has a decoupled fixed point in the 3D
XY universality, and our Monte Carlo simulation suggests that there is such a
temperature region where long-wavelength fluctuations can be described by this
fixed point. However, it is shown using scaling arguments that the decoupled
fixed point is unstable against a fluctuation-induced biquadratic interaction,
indicating that a crossover to nontrivial critical phenomena with different
exponents appears as one approaches the critical point beyond the transient
temperature region. This new scenario clearly contradicts the previous notion
of the 3D XY universality.Comment: 16 pages, 7 figure
Photoemission kinks and phonons in cuprates
One of the possible mechanisms of high Tc superconductivity is Cooper pairing
with the help of bosons, which change the slope of the electronic dispersion as
observed by photoemission. Giustino et al. calculated that in the high
temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations
(phonons) should have a negligible effect on photoemission spectra and
concluded that phonons do not play an important role. We show that the
calculations employed by Giustino et al. fail to reproduce huge influence of
electron-phonon coupling on important phonons observed in experiments. Thus one
would expect these calculations to similarly fail in explaining the role of
electron-phonon coupling for the electronic dispersion.Comment: To appear in Nature as a Brief Communiction Arisin
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