2,884 research outputs found
Two-spin entanglement distribution near factorized states
We study the two-spin entanglement distribution along the infinite
chain described by the XY model in a transverse field; closed analytical
expressions are derived for the one-tangle and the concurrences ,
being the distance between the two possibly entangled spins, for values of the
Hamiltonian parameters close to those corresponding to factorized ground
states. The total amount of entanglement, the fraction of such entanglement
which is stored in pairwise entanglement, and the way such fraction distributes
along the chain is discussed, with attention focused on the dependence on the
anisotropy of the exchange interaction. Near factorization a characteristic
length-scale naturally emerges in the system, which is specifically related
with entanglement properties and diverges at the critical point of the fully
isotropic model. In general, we find that anisotropy rule a complex behavior of
the entanglement properties, which results in the fact that more isotropic
models, despite being characterized by a larger amount of total entanglement,
present a smaller fraction of pairwise entanglement: the latter, in turn, is
more evenly distributed along the chain, to the extent that, in the fully
isotropic model at the critical field, the concurrences do not depend on .Comment: 14 pages, 6 figures. Final versio
Tissue location of resistance in apple to the rosy apple aphid established by electrical penetration graphs
A study of the constitutive resistance of the apple cultivar Florina, Malus domestica Borkh. (Rosaceae), to the rosy apple aphid, Dysaphis plantaginea (Passerini) (Homoptera Aphididae), was performed for the first time by the electrical penetration graph (DC-EPG) system, using the susceptible apple cultivar Smoothe as control. All experiments were conducted with apterous adult virginoparae. The results showed a constitutive resistance in Florina due to a much longer period before the first probe reflecting surface factors. Some weak indications were found for pre-phloem resistance and initiating phloem access was not affected as inferred from equal time to show phloem salivation. However, the complete absence of phloem ingestion indicates a major resistance factor in the phloem sieve elements, most likely in the sieve element sap. Surface factors could have affected tissue related variables and this should be studied further. Anyhow, the strong constitutive resistance in Florina, either on the surface alone or in the phloem as well, effectively prevented reliable experiments on induced resistance, previously detected by molecular methods
Tritium -decay in chiral effective field theory
We evaluate the Fermi and Gamow-Teller (GT) matrix elements in tritium
-decay by including in the charge-changing weak current the corrections
up to one loop recently derived in nuclear chiral effective field theory
( EFT). The trinucleon wave functions are obtained from
hyperspherical-harmonics solutions of the Schrodinger equation with two- and
three-nucleon potentials corresponding to either EFT (the N3LO/N2LO
combination) or meson-exchange phenomenology (the AV18/UIX combination). We
find that contributions due to loop corrections in the axial current are, in
relative terms, as large as (and in some cases, dominate) those from one-pion
exchange, which nominally occur at lower order in the power counting. We also
provide values for the low-energy constants multiplying the contact axial
current and three-nucleon potential, required to reproduce the experimental GT
matrix element and trinucleon binding energies in the N3LO/N2LO and AV18/UIX
calculations.Comment: 19 pages,6 figures, corrections to Text as suggested by Referee
added; Erratum: 4 pages, 3 figures, corrections to Eq.(20), Tables I, II,
III, Figures 4, 5, conclusions unchange
Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters
The structural and dynamical properties of OCS molecules solvated in Helium
clusters are studied using reptation quantum Monte Carlo, for cluster sizes
n=3-20 He atoms. Computer simulations allow us to establish a relation between
the rotational spectrum of the solvated molecule and the structure of the He
solvent, and of both with the onset of superfluidity. Our results agree with a
recent spectroscopic study of this system, and provide a more complex and
detailed microscopic picture of this system than inferred from experiments.Comment: 4 pages. TeX (requires revtex4) + 3 ps figures (1 color
A chiral effective field theory study of hadronic parity violation in few-nucleon systems
We reconsider the derivation of the nucleon-nucleon parity-violating (PV)
potential within a chiral effective field theory framework. We construct the
potential up to next-to-next-to-leading order by including one-pion-exchange,
two-pion-exchange, contact, and 1/M (M being the nucleon mass) terms, and use
dimensional regularization to renormalize the pion-loop corrections. A detailed
analysis of the number of independent low-energy constants (LEC's) entering the
potential is carried out. We find that it depends on six LEC's: the
pion-nucleon PV coupling constant and five parameters multiplying
contact interactions. We investigate PV effects induced by this potential on
several few-nucleon observables, including the - longitudinal
asymmetry, the neutron spin rotation in - and -
scattering, and the longitudinal asymmetry in the HeH
charge-exchange reaction. An estimate for the range of values of the various
LEC's is provided by using available experimental data.Comment: 31 pages, 7 figures, submitted to Physical Review
Theoretical determination of the Raman spectra of MgSiO3 perovskite and post-perovskite at high pressure
We use the density functional perturbation theory to determine for the first
time the pressure evolution of the Raman intensities for a mineral, the two
high-pressure structures of MgSiO3 perovskite and post-perovskite. At high
pressures, the Raman powder spectra reveals three main peaks for the perovskite
structure and one main peak for the post-perovskite structure. Due to the large
differences in the spectra of the two phases Raman spectroscopy can be used as
a good experimental indication of the phase transition.Comment: 16 pages, submitted to Geophysical Research Letter
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Achieving descriptive accuracy in explanations via argumentation: the case of probabilistic classifiers
The pursuit of trust in and fairness of AI systems in order to enable human-centric goals has been gathering pace of late, often supported by the use of explanations for the outputs of these systems. Several properties of explanations have been highlighted as critical for achieving trustworthy and fair AI systems, but one that has thus far been overlooked is that of descriptive accuracy (DA), i.e., that the explanation contents are in correspondence with the internal working of the explained system. Indeed, the violation of this core property would lead to the paradoxical situation of systems producing explanations which are not suitably related to how the system actually works: clearly this may hinder user trust. Further, if explanations violate DA then they can be deceitful, resulting in an unfair behavior toward the users. Crucial as the DA property appears to be, it has been somehow overlooked in the XAI literature to date. To address this problem, we consider the questions of formalizing DA and of analyzing its satisfaction by explanation methods. We provide formal definitions of naive, structural and dialectical DA, using the family of probabilistic classifiers as the context for our analysis. We evaluate the satisfaction of our given notions of DA by several explanation methods, amounting to two popular feature-attribution methods from the literature, variants thereof and a novel form of explanation that we propose. We conduct experiments with a varied selection of concrete probabilistic classifiers and highlight the importance, with a user study, of our most demanding notion of dialectical DA, which our novel method satisfies by design and others may violate. We thus demonstrate how DA could be a critical component in achieving trustworthy and fair systems, in line with the principles of human-centric AI
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