6,438 research outputs found
From ergodic to non-ergodic chaos in Rosenzweig-Porter model
The Rosenzweig-Porter model is a one-parameter family of random matrices with
three different phases: ergodic, extended non-ergodic and localized. We
characterize numerically each of these phases and the transitions between them.
We focus on several quantities that exhibit non-analytical behaviour and show
that they obey the scaling hypothesis. Based on this, we argue that non-ergodic
chaotic and ergodic regimes are separated by a continuous phase transition,
similarly to the transition between non-ergodic chaotic and localized phases.Comment: 12 page
Thermodynamics from a scaling Hamiltonian
There are problems with defining the thermodynamic limit of systems with
long-range interactions; as a result, the thermodynamic behavior of these types
of systems is anomalous. In the present work, we review some concepts from both
extensive and nonextensive thermodynamic perspectives. We use a model, whose
Hamiltonian takes into account spins ferromagnetically coupled in a chain via a
power law that decays at large interparticle distance as for
. Here, we review old nonextensive scaling. In addition, we
propose a new Hamiltonian scaled by that
explicitly includes symmetry of the lattice and dependence on the size, , of
the system. The new approach enabled us to improve upon previous results. A
numerical test is conducted through Monte Carlo simulations. In the model,
periodic boundary conditions are adopted to eliminate surface effects.Comment: 12 pages, 2 figures, submitted for publication to Phys. Rev.
Mapping the structural diversity of C60 carbon clusters and their infrared spectra
The current debate about the nature of the carbonaceous material carrying the
infrared (IR) emission spectra of planetary and proto-planetary nebulae,
including the broad plateaus, calls for further studies on the interplay
between structure and spectroscopy of carbon-based compounds of astrophysical
interest. The recent observation of C60 buckminsterfullerene in space suggests
that carbon clusters of similar size may also be relevant. In the present work,
broad statistical samples of C60 isomers were computationally determined
without any bias using a reactive force field, their IR spectra being
subsequently obtained following local optimization with the
density-functional-based tight-binding theory. Structural analysis reveals four
main structural families identified as cages, planar polycyclic aromatics,
pretzels, and branched. Comparison with available astronomical spectra
indicates that only the cage family could contribute to the plateau observed in
the 6-9 micron region. The present framework shows great promise to explore and
relate structural and spectroscopic features in more diverse and possibly
hydrogenated carbonaceous compounds, in relation with astronomical
observations
Size effect in the ionization energy of PAH clusters
We report the first experimental measurement of the near-threshold
photo-ionization spectra of polycyclic aromatic hydrocarbon clusters made of
pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron
photoion coincidence spectrometry with a VUV synchrotron beamline. The
experimental results of the ionization energy are confronted to calculated ones
obtained from simulations using dedicated electronic structure treatment for
large ionized molecular clusters. Experiment and theory consistently find a
decrease of the ionization energy with cluster size. The inclusion of
temperature effects in the simulations leads to a lowering of this energy and
to a quantitative agreement with the experiment. In the case of pyrene, both
theory and experiment show a discontinuity in the IE trend for the hexamer
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