40,883 research outputs found
Marginal Fermi liquid behavior from 2d Coulomb interaction
A full, nonperturbative renormalization group analysis of interacting
electrons in a graphite layer is performed, in order to investigate the
deviations from Fermi liquid theory that have been observed in the experimental
measures of a linear quasiparticle decay rate in graphite. The electrons are
coupled through Coulomb interactions, which remain unscreened due to the
semimetallic character of the layer. We show that the model flows towards the
noninteracting fixed-point for the whole range of couplings, with logarithmic
corrections which signal the marginal character of the interaction separating
Fermi liquid and non-Fermi liquid regimes.Comment: 7 pages, 2 Postscript figure
High-resolution imaging spectroscopy of two micro-pores and an arch filament system in a small emerging-flux region
Aims. The purpose of this investigation is to characterize the temporal
evolution of an emerging flux region, the associated photospheric and
chromospheric flow fields, and the properties of the accompanying arch filament
system. Methods. This study is based on imaging spectroscopy with the
G\"ottingen Fabry-P\'erot Interferometer at the Vacuum Tower Telescope, on 2008
August 7. Cloud model (CM) inversions of line scans in the strong chromospheric
absorption H line yielded CM parameters, which describe the cool plasma
contained in the arch filament system. Results. The observations cover the
decay and convergence of two micro-pores with diameters of less than one
arcsecond and provide decay rates for intensity and area. The photospheric
horizontal flow speed is suppressed near the two micro-pores indicating that
the magnetic field is sufficiently strong to affect the convective energy
transport. The micro-pores are accompanied by an arch filament system, where
small-scale loops connect two regions with H line-core brightenings
containing an emerging flux region with opposite polarities. The chromospheric
velocity of the cloud material is predominantly directed downwards near the
footpoints of the loops with velocities of up to 12 km/s, whereas loop tops
show upward motions of about 3 km/s. Conclusions. Micro-pores are the smallest
magnetic field concentrations leaving a photometric signature in the
photosphere. In the observed case, they are accompanied by a miniature arch
filament system indicative of newly emerging flux in the form of
-loops. Flux emergence and decay take place on a time-scale of about
two days, whereas the photometric decay of the micro-pores is much more rapid
(a few hours), which is consistent with the incipient submergence of
-loops. The results are representative for the smallest emerging flux
regions still recognizable as such.Comment: 15 pages, 16 figures, 3 tables, published in A&
Biomechanical analysis of a cranial Patient Specific Implant on the interface with the bone using the Finite Element Method
- New advance technologies based on reverse engineering , design and additive
manufacturing, have expanded design capabilities for biomedical applications to
include Patient Specific Implants (PSI). This change in design paradigms needs
advanced tools to assess the mechanical performance of the product, and
simulate the impact on the patient. In this work, we perform a structural
analysis on the interface of a cranial PSI under static loading conditions.
Based on those simulations, we have identified the regions with high stress and
strain and checked the failure criteria both in the implant and the skull. We
evaluate the quality of the design of the implant and determine their response
given different materials, in order to ensure optimality of the final product
to be manufactured
Evidence of several dipolar quasi-invariants in Liquid Crystals
In a closed quantum system of N coupled spins with magnetic quantum number I,
there are about (2I + 1)^N constants of motion. However, the possibility of
observing such quasi-invariant (QI) states in solid-like spin systems in
Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of
this work is to provide experimental evidence of several QI, in the proton NMR
of small spin clusters, besides those already known Zeeman, and dipolar orders
(strong and weak). We explore the spin states prepared with the
Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by
this sequence as a function of the preparation times, in a variety of dipolar
networks. We observe that the signals can be explained with two dipolar QIs
only within a range of short preparation times. At longer times the time-domain
signals have an echo-like behaviour. We study their multiple quantum coherence
content on a basis orthogonal to the z-basis and see that such states involve a
significant number of correlated spins. Then we show that the whole preparation
time-scale can only be reconstructed by assuming the occurrence of multiple QI
which we isolate experimentally
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