72,824 research outputs found
Magnetoplasmons excitations in graphene for filling factors
In the frame of the Hartree-Fock approximation, the dispersion of
magnetoplasmons in Graphene is derived for all types of transitions for filling
factors . The optical conductivity components of the magnetoplasmon
curves are calculated. It is shown that the electron-electron interactions lead
to a strong re-normalization of the apparent Fermi velocity of Graphene which
is different for different types of transitions.Comment: 15 pages, 7 figure
Dynamical r-process studies within the neutrino-driven wind scenario and its sensitivity to the nuclear physics input
We use results from long-time core-collapse supernovae simulations to
investigate the impact of the late time evolution of the ejecta and of the
nuclear physics input on the calculated r-process abundances. Based on the
latest hydrodynamical simulations, heavy r-process elements cannot be
synthesized in the neutrino-driven winds that follow the supernova explosion.
However, by artificially increasing the wind entropy, elements up to A=195 can
be made. In this way one can reproduce the typical behavior of high-entropy
ejecta where the r-process is expected to occur. We identify which nuclear
physics input is more important depending on the dynamical evolution of the
ejecta. When the evolution proceeds at high temperatures (hot r-process), an
(n,g)-(g,n) equilibrium is reached. While at low temperature (cold r-process)
there is a competition between neutron captures and beta decays. In the first
phase of the r-process, while enough neutrons are available, the most relevant
nuclear physics input are the nuclear masses for the hot r-process and the
neutron capture and beta-decay rates for the cold r-process. At the end of this
phase, the abundances follow a steady beta flow for the hot r-process and a
steady flow of neutron captures and beta decays for the cold r-process. After
neutrons are almost exhausted, matter decays to stability and our results show
that in both cases neutron captures are key for determining the final
abundances, the position of the r-process peaks, and the formation of the
rare-earth peak. In all the cases studied, we find that the freeze out occurs
in a timescale of several seconds.Comment: 20 pages, 12 figures, submitted to Phys. Rev. C (improved version
Reflectance Transformation Imaging (RTI) System for Ancient Documentary Artefacts
This tutorial summarises our uses of reflectance transformation imaging in archaeological contexts. It introduces the UK AHRC funded project reflectance Transformation Imaging for Anciant Documentary Artefacts and demonstrates imaging methodologies
Theory and simulation of the confined Lebwohl-Lasher model
We discuss the Lebwohl-Lasher model of nematic liquid crystals in a confined
geometry, using Monte Carlo simulation and mean-field theory. A film of
material is sandwiched between two planar, parallel plates that couple to the
adjacent spins via a surface strength . We consider the cases where
the favoured alignments at the two walls are the same (symmetric cell) or
different (asymmetric or hybrid cell). In the latter case, we demonstrate the
existence of a {\it single} phase transition in the slab for all values of the
cell thickness. This transition has been observed before in the regime of
narrow cells, where the two structures involved correspond to different
arrangements of the nematic director. By studying wider cells, we show that the
transition is in fact the usual isotropic-to-nematic (capillary) transition
under confinement in the case of antagonistic surface forces. We show results
for a wide range of values of film thickness, and discuss the phenomenology
using a mean-field model.Comment: 40 pages 19 figures (preprint format). Part of the text and some
figures were modified. New figure was include
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