67,017 research outputs found
Ionization of ions
Charged particle binary encounter model modified for evaluating ionization cross section of positive ions by electron impac
Discrepancies between decoherence and the Loschmidt echo
The Loschmidt echo and the purity are two quantities that can provide
invaluable information about the evolution of a quantum system. While the
Loschmidt echo characterizes instability and sensitivity to perturbations,
purity measures the loss of coherence produced by an environment coupled to the
system. For classically chaotic systems both quantities display a number of --
supposedly universal -- regimes that can lead on to think of them as equivalent
quantities. We study the decay of the Loschmidt echo and the purity for systems
with finite dimensional Hilbert space and present numerical evidence of some
fundamental differences between them.Comment: 6 pages, 3 figures. Changed title. Added 1 figure. Published version
Topological phases of fermions in one dimension
In this paper we show how the classification of topological phases in
insulators and superconductors is changed by interactions, in the case of 1D
systems. We focus on the TR-invariant Majorana chain (BDI symmetry class).
While the band classification yields an integer topological index , it is
known that phases characterized by values of in the same equivalence class
modulo 8 can be adiabatically transformed one to another by adding suitable
interaction terms. Here we show that the eight equivalence classes are distinct
and exhaustive, and provide a physical interpretation for the interacting
invariant modulo 8. The different phases realize different Altland-Zirnbauer
classes of the reduced density matrix for an entanglement bipartition into two
half-chains. We generalize these results to the classification of all one
dimensional gapped phases of fermionic systems with possible anti-unitary
symmetries, utilizing the algebraic framework of central extensions. We use
matrix product state methods to prove our results.Comment: 14 pages, 3 figures, v2: references adde
Classification of the phases of 1D spin chains with commuting Hamiltonians
We consider the class of spin Hamiltonians on a 1D chain with periodic
boundary conditions that are (i) translational invariant, (ii) commuting and
(iii) scale invariant, where by the latter we mean that the ground state
degeneracy is independent of the system size. We correspond a directed graph to
a Hamiltonian of this form and show that the structure of its ground space can
be read from the cycles of the graph. We show that the ground state degeneracy
is the only parameter that distinguishes the phases of these Hamiltonians. Our
main tool in this paper is the idea of Bravyi and Vyalyi (2005) in using the
representation theory of finite dimensional C^*-algebras to study commuting
Hamiltonians.Comment: 8 pages, improved readability, added exampl
Circumstellar effects on the Rb abundances in O-rich AGB stars
For the first time we explore the circumstellar effects on the Rb (and Zr)
abundance determination in O-rich asymptotic giant branch (AGB) stars by
considering the presence of a gaseous circumstellar envelope with a radial
wind. A modified version of the spectral synthesis code Turbospectrum was used
to deal with extended atmosphere models and velocity fields. The Rb and Zr
abundances were determined from the resonant 7800A Rb I line and the 6474A ZrO
bandhead, respectively, in five representative O-rich AGB stars with different
expansion velocity and metallicity. By using our new dynamical models, the Rb I
line profile (photospheric and circumstellar components) is very well
reproduced. Interestingly, the derived Rb abundances are much lower (by 1-2
dex) in those O-rich AGB stars showing the higher circumstellar expansion
velocities. The Zr abundances, however, remain close to the solar values. The
Rb abundances and Rb/Zr ratios derived here significantly alleviate the problem
of the present mismatch between the observations of intermediate-mass (4-8
solar masses) Rb-rich AGB stars and the AGB nucleosynthesis theoretical
predictions.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures, and 2 tables); final version (language corrected
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
Non-Markovian Random Walks and Non-Linear Reactions: Subdiffusion and Propagating Fronts
We propose a reaction-transport model for CTRW with non-linear reactions and
non-exponential waiting time distributions. We derive non-linear evolution
equation for mesoscopic density of particles. We apply this equation to the
problem of fronts propagation into unstable state of reaction-transport systems
with anomalous diffusion. We have found an explicit expression for the speed of
propagating front in the case of subdiffusion transport.Comment: 7 page
Synchronization transition of heterogeneously coupled oscillators on scale-free networks
We investigate the synchronization transition of the modified Kuramoto model
where the oscillators form a scale-free network with degree exponent .
An oscillator of degree is coupled to its neighboring oscillators with
asymmetric and degree-dependent coupling in the form of \couplingcoeff
k_i^{\eta-1}. By invoking the mean-field approach, we determine the
synchronization transition point , which is zero (finite) when (). We find eight different synchronization
transition behaviors depending on the values of and , and
derive the critical exponents associated with the order parameter and the
finite-size scaling in each case. The synchronization transition is also
studied from the perspective of cluster formation of synchronized vertices. The
cluster-size distribution and the largest cluster size as a function of the
system size are derived for each case using the generating function technique.
Our analytic results are confirmed by numerical simulations.Comment: 11 pages, 3 figures and two table
- …