259 research outputs found
On the problem of the relation between phason elasticity and phason dynamics in quasicrystals
It has recently been claimed that the dynamics of long-wavelength phason
fluctuations has been observed in i-AlPdMn quasicrystals. We will show that the
data reported call for a more detailed development of the elasticity theory of
Jaric and Nelsson in order to determine the nature of small phonon-like atomic
displacements with a symmetry that follows the phason elastic constants. We
also show that a simple model with a single diffusing tile is sufficient to
produce a signal that (1) is situated at a "satellite position'' at a distance
q from each Bragg peak, that (2) has an intensity that scales with the
intensity of the corresponding Bragg peak, (3) falls off as 1/q-squared and (4)
has a time decay constant that is proportional to 1/(D q-squared). It is thus
superfluous to call for a picture of "phason waves'' in order to explain such
data, especially as such "waves'' violate many physical principles.Comment: 36 pages, 0 figures, discussion about vacancies, fluctuating Fourier
components, and difference between static and dynamical structure factors
added, other addition
Coherent quasielastic neutron scattering and correlations between rotational jumps of molecules on a periodic lattice
We previously derived a theorem about the {\em coherent} quasielastic
neutron-scattering signal from a -dimensional lattice of molecules that
are undergoing rotational jump diffusion (around an -fold axis), assuming
that there are no correlations between the molecules. In the present paper
molecular correlations are treated, but only in the sense that several
molecules could reorient simultaneously as in a cog-wheel mechanism. Moreover,
we do not examine the possibility that the relaxation times of these combined
reorientations could depend on details of the local environment created by the
neighbouring molecules. Finally also an ergodicity condition has to be
fulfilled. Admitting for all these assumptions we can show that the
correlations do not affect the coherent quasielastic scattering pattern in the
following sense: The functions of that intervene in the description of the
intensities remain unaltered, while the functions of can undergo a
renormalization of the time scales. The latter changes cannot be detected as
the time scales that would occur if the dynamics were independent are not
available for comparison. In other words: Coherent quasielastic neutron
scattering is not able to betray the existence of correlations of the
restricted type that occur in our model.Comment: 15 pages, 0 figure
Comment on "Sound Modes broadening in Quasicrystals"
Recently de Boissieu et al. proposed an explanation for the broadening of
acoustic modes observed in quasicrystals (QC). It is the transcription of a
well-known model used for glasses. We raise two fundamental objections against
applying it to QC. After the text of the Comment we report the methodology that
has been used to thwart the publication of this Comment, which is perfectly
valid.Comment: 7 pages, 0 Figures, literal citations remove
On the problem of the correct interpretation of phason elasticity in quasicrystals
Recently Francoual et al. claimed to have observed the dynamics of
long-wavelength phason fluctuations in i-AlPdMn quasicrystals. We will show
that the data reported call for a more detailed development of the elasticity
theory of Jaric and Nelsson in order to determine the nature of small
phonon-like atomic displacements with a symmetry that follows the phason
elastic constants.Comment: 18 pages, 0 figures, many additions, discussion adde
Porcine intestinal glycosphingolipids recognized by F6-fimbriated enterotoxigenic Escherichia coli
One important virulence factor of enterotoxigenic Escherichia coli is their ability to adhere via fimbrial adhesins to specific receptors located on the intestinal mucosa. Here, the potential glycosphingolipid receptors of enterotoxigenic F6-fimbriated E. coli were examined by binding of purified F6 fimbriae, and F6-expressing bacteria, to glycosphingolipids on thin-layer chromatograms. When intestinal mucosal non-acid glycosphingolipids from single pigs were assayed for F6 binding capacity, a selective interaction with two glycosphingolipids was observed. The binding-active glycosphingolipids were isolated and characterized as lactotriaosylceramide (GlcNAc beta 3Gal beta 4Glc beta 1Cer) and lactotetraosylceramide (Gal beta 3GlcNAc beta 3Gal beta 4Glc beta 1Cer). Further binding assays using a panel of reference glycosphingolipids showed a specific interaction between the F6 fimbriae and a number of neolacto core chain (Gal beta 4GlcNAc) glycosphingolipids. In addition, an occasional binding of the F6 fimbriae to sulfatide, galactosylceramide, lactosylceramide with phytosphingosine and/or hydroxy fatty acids, isoglobotriaosylceramide, gangliotriaosylceramide, and gangliotetraosylceramide was obtained. From the results we conclude that lactotriaosylceramide and lactotetraosylceramide are major porcine intestinal receptors for F6-fimbriated E. coli
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