35,292 research outputs found
A Laplace Transform Method for Molecular Mass Distribution Calculation from Rheometric Data
Polydisperse linear polymer melts can be microscopically described by the
tube model and fractal reptation dynamics, while on the macroscopic side the
generalized Maxwell model is capable of correctly displaying most of the
rheological behavior. In this paper, a Laplace transform method is derived and
different macroscopic starting points for molecular mass distribution
calculation are compared to a classical light scattering evaluation. The
underlying assumptions comprise the modern understanding on polymer dynamics in
entangled systems but can be stated in a mathematically generalized way. The
resulting method is very easy to use due to its mathematical structure and it
is capable of calculating multimodal molecular mass distributions of linear
polymer melts
Structural and magnetic transition in CeFeAsO: separated or connected?
Using an adapted Sn-flux growth technique we obtained comparatively large
CeFeAsO single crystals of better quality than previously reported polycrystals
or single crystals, as evidenced by much sharper anomalies at the structural
and magnetic phase transitions as well as a much higher residual resistivity
ratio of 12. In the magnetically ordered phase we observe a very pronounced
metallic behavior of the in-plane resistivity, which excludes a Mott insulator
regime at low temperature. The separation Delta_T = T_0 - T_N between
structural and magnetic ordering temperatures decreases with increasing sample
quality, from 18 K in the initial reports to 6 K in the present single
crystals, demonstrating that this separation is not an intrinsic property of
the RFeAsO systems. Our results indicate that the coupling between magnetic
ordering and structural distortion is very similar in AFe2As2 and RFeAsO type
of compounds, much more similar than previously thought. The implications of
our experimental results give arguments both in favor and against the nematic
phase model.Comment: published in PRB with the title 'Coupling between the structural and
magnetic transition in CeFeAsO
Effects of the low lying Dirac modes on excited hadrons in lattice QCD
Chiral symmetry breaking in Quantum Chromodynamics is associated with the low
lying spectral modes of the Dirac operator according to the Banks-Casher
relation. Here we study how removal of a variable number of low lying modes
from the valence quark sector affects the masses of the ground states and first
excited states of baryons and mesons in two flavor lattice QCD.Comment: 6 pages, 2 figures. Contribution to proceedings of "Excited QCD
2012", May 6-12, 2012, Peniche, Portuga
Symmetries of hadrons after unbreaking the chiral symmetry
We study hadron correlators upon artificial restoration of the spontaneously
broken chiral symmetry. In a dynamical lattice simulation we remove the lowest
lying eigenmodes of the Dirac operator from the valence quark propagators and
study evolution of the hadron masses obtained. All mesons and baryons in our
study, except for a pion, survive unbreaking the chiral symmetry and their
exponential decay signals become essentially better. From the analysis of the
observed spectroscopic patterns we conclude that confinement still persists
while the chiral symmetry is restored. All hadrons fall into different chiral
multiplets. The broken U(1)_A symmetry does not get restored upon unbreaking
the chiral symmetry. We also observe signals of some higher symmetry that
includes chiral symmetry as a subgroup. Finally, from comparison of the \Delta
- N splitting before and after unbreaking of the chiral symmetry we conclude
that both the color-magnetic and the flavor-spin quark-quark interactions are
of equal importance.Comment: 12 pages, 14 figures; final versio
More effects of Dirac low-mode removal
In previous studies we have shown that hadrons, except for a pion, survive
the removal of the lowest lying Dirac eigenmodes from the valence quark
propagators. The low-modes are tied to the dynamical breaking of chiral
symmetry and we found chiral symmetry to be restored by means of matching
masses of chiral partners, like, e.g., the vector and axial vector currents.
Here we investigate the influence of removing the lowest part of the Dirac
spectrum on the locality of the Dirac operator. Moreover, we analyze the
influence of low-mode truncation on the quark momenta and thereupon on the
hadron spectrum and, finally, introduce a reweighting scheme to extend the
truncation to the sea quark sector.Comment: 7pages, 4 figures. Proceedings of the 31st International Symposium on
Lattice Field Theory, July 29 - August 3, 2013, Mainz, German
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