847 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
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
Identification of nonlinear heat transfer laws from boundary observations
We consider the problem of identifying a nonlinear heat transfer law at the boundary, or of the temperature-dependent heat transfer coefficient in a parabolic equation from boundary observations. As a practical example, this model applies to the heat transfer coefficient that describes the intensity of heat exchange between a hot wire and the cooling water in which it is placed. We reformulate the inverse problem as a variational one which aims to minimize a misfit functional and prove that it has a solution. We provide a gradient formula for the misfit functional and then use some iterative methods for solving the variational problem. Thorough investigations are made with respect to several initial guesses and amounts of noise in the input data. Numerical results show that the methods are robust, stable and accurate
Kaolinite particles as ice nuclei: learning from the use of different kaolinite samples and different coatings
Kaolinite particles from two different sources (Fluka and Clay
Minerals Society (CMS)) were examined with respect to their ability
to act as ice nuclei (IN). This was done in the water-subsaturated regime
where often deposition ice nucleation is assumed to occur, and for
water-supersaturated conditions, i.e., in the immersion freezing
mode. Measurements were done using a flow tube (the Leipzig Aerosol Cloud Interaction Simulator, LACIS) and
a continuous-flow diffusion chamber (CFDC). Pure and coated
particles were used, with coating thicknesses of a few nanometers or
less, where the coating consisted of levoglucosan, succinic
acid or sulfuric acid. In general, it was found that the coatings
strongly reduced deposition ice nucleation. Remaining ice formation
in the water-subsaturated regime could be attributed to immersion
freezing, with particles immersed in concentrated solutions formed
by the coatings.
In the immersion freezing mode, ice nucleation rate coefficients
jhet from both instruments agreed well with each other, particularly when
the residence times in the instruments were accounted for. Fluka
kaolinite particles coated with either levoglucosan or succinic acid
showed the same IN activity as pure Fluka kaolinite particles;
i.e., it can be assumed that these two types of coating did not alter
the ice-active surface chemically, and that the coatings were
diluted enough in the droplets that were formed prior to the ice
nucleation, so that freezing point depression was
negligible. However, Fluka kaolinite particles, which were either coated
with pure sulfuric acid or were first coated with the
acid and then exposed to additional water vapor, both showed
a reduced ability to nucleate ice compared to the pure
particles. For the CMS kaolinite particles, the ability to nucleate
ice in the immersion freezing mode was similar for all examined
particles, i.e., for the pure ones and the ones with the different
types of coating. Moreover, jhet derived for the CMS
kaolinite particles was comparable to jhet derived for
Fluka kaolinite particles coated with sulfuric acid. This is suggestive
for the Fluka kaolinite possessing a type of ice-nucleating surface
feature which is not present on the CMS kaolinite, and which can be
destroyed by reaction with sulfuric acid. This might be potassium
feldspar
Study of electron-irradiated silicon thin films using transient photocurrent spectroscopy
Electron irradiation of silicon thin films creates localised states, which degrade theiropto-electronic properties. We present a series of transient photocurrent spectroscopy (TPC)measurements on electron-irradiated amorphous and microcrystalline silicon films, annealed atprogressively increasing temperatures. This has enabled localised states associated with bothdangling bonds and conduction band tails to be examined over a wide energy range.Trends inthe evolution of the DOS following electron irradiation followed by isochronal annealing stepsindicate reductions in the deep defect density,which correlate with spin density. We also find asteepening of the conduction band tail slope in amorphous silicon on annealing. Both defectdensity and tail slope may be restored close to as-prepared material values. Earlier CPM dataare re-examined, and a similar trend in the valence band tail slope is indicated. Computersimulations predict that following e-irradiation changes in deep defect density primarily controlsolar cell performance, and will tend to obscure effects related to band tails
Electron-phonon anomaly related to charge stripes: static stripe phase versus optimally-doped superconducting La1.85Sr0.15CuO4
Inelastic neutron scattering was used to study the Cu-O bond-stretching
vibrations in optimally doped La1.85Sr0.15CuO4 (Tc = 35 K) and in two other
cuprates showing static stripe order at low temperatures, i.e.
La1.48Nd0.4Sr0.12CuO4 and La1.875Ba0.125CuO4. All three compounds exhibit a
very similar phonon anomaly, which is not predicted by conventional band
theory. It is argued that the phonon anomaly reflects a coupling to charge
inhomogeneities in the form of stripes, which remain dynamic in superconducting
La1.85Sr0.15CuO4 down to the lowest temperatures. These results show that the
phonon effect indicating stripe formation is not restricted to a narrow region
of the phase diagram around the so-called 1/8 anomaly but occurs in optimally
doped samples as well.Comment: to appear in J. Low Temp. Phy
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