4,708 research outputs found

### A simple topological model with continuous phase transition

In the area of topological and geometric treatment of phase transitions and
symmetry breaking in Hamiltonian systems, in a recent paper some general
sufficient conditions for these phenomena in $\mathbb{Z}_2$-symmetric systems
(i.e. invariant under reflection of coordinates) have been found out. In this
paper we present a simple topological model satisfying the above conditions
hoping to enlighten the mechanism which causes this phenomenon in more general
physical models. The symmetry breaking is testified by a continuous
magnetization with a nonanalytic point in correspondence of a critical
temperature which divides the broken symmetry phase from the unbroken one. A
particularity with respect to the common pictures of a phase transition is that
the nonanalyticity of the magnetization is not accompanied by a nonanalytic
behavior of the free energy.Comment: 17 pages, 7 figure

### Adapting robot behavior to user's capabilities: a dance instruction study.

The ALIZ-E1 projects goal is to design a robot companion able to maintain affective interactions with young users over a period of time. One of these interactions consists in teaching a dance to hospitalized children according to their capabilities. We propose a methodology for adapting both, the movements used in the dance based on the users cognitive and physical capabilities through a set of metrics, and the robots interaction based on the users personality traits

### Topological conditions for discrete symmetry breaking and phase transitions

In the framework of a recently proposed topological approach to phase
transitions, some sufficient conditions ensuring the presence of the
spontaneous breaking of a Z_2 symmetry and of a symmetry-breaking phase
transition are introduced and discussed. A very simple model, which we refer to
as the hypercubic model, is introduced and solved. The main purpose of this
model is that of illustrating the content of the sufficient conditions, but it
is interesting also in itself due to its simplicity. Then some mean-field
models already known in the literature are discussed in the light of the
sufficient conditions introduced here

### Accurate quadratic-response approximation for the self-consistent pseudopotential of semiconductor nanostructures

Quadratic-response theory is shown to provide a conceptually simple but
accurate approximation for the self-consistent one-electron potential of
semiconductor nanostructures. Numerical examples are presented for GaAs/AlAs
and InGaAs/InP (001) superlattices using the local-density approximation to
density-functional theory and norm-conserving pseudopotentials without
spin-orbit coupling. When the reference crystal is chosen to be the
virtual-crystal average of the two bulk constituents, the absolute error in the
quadratic-response potential for Gamma(15) valence electrons is about 2 meV for
GaAs/AlAs and 5 meV for InGaAs/InP. Low-order multipole expansions of the
electron density and potential response are shown to be accurate throughout a
small neighborhood of each reciprocal lattice vector, thus providing a further
simplification that is confirmed to be valid for slowly varying envelope
functions. Although the linear response is about an order of magnitude larger
than the quadratic response, the quadratic terms are important both
quantitatively (if an accuracy of better than a few tens of meV is desired) and
qualitatively (due to their different symmetry and long-range dipole effects).Comment: 16 pages, 20 figures; v2: new section on limitations of theor

### 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

### De l'interactivité du récit au récit interactif

À ses origines, la théorie du récit, notamment la narratologie d'inspiration formaliste et structuraliste, s'est développée dans le creuset des études littéraires. De ce fait, le point focal de cette discipline a longtemps été constitué par le récit verbal, généralement sous la forme d'un texte écrit et planifié par un auteur, laissant de côté les formes narratives émergentes, participatives ou interactives. On observe cependant, depuis une vingtaine d'année, une accélération dans le processus d'élargissement des objets d'étude de la narratologie, accompagnée par une mutation épistémologique. Retraçant cette évolution, Jan Christoph Meister observe ainsi un changement d'orientation faisant passer la théorie du récit de l'étude de phénomènes textuels à l'analyse « des fonctions cognitives de récits oraux et non littéraires, ouvrant ainsi un nouveau chapitre dans le projet narratologique » (2009 : 340). Si l'intrigue peut être décrite - en adoptant un point de vue fonctionnaliste et cognitiviste - comme une matrice de possibilités ontologiquement instable, comme un dispositif dont la fonction première est d'ouvrir des virtualités narratives et d'engendrer de la curiosité ou du suspense (Baroni 2007 ; 2009), alors il n'y a pas de raison de la considérer comme un obstacle à l'analyse de récits émergents et participatifs, voire de simulations ou de pratiques ludiques hautement interactives

### Tritium $\beta$-decay in chiral effective field theory

We evaluate the Fermi and Gamow-Teller (GT) matrix elements in tritium
$\beta$-decay by including in the charge-changing weak current the corrections
up to one loop recently derived in nuclear chiral effective field theory
($\chi$ EFT). The trinucleon wave functions are obtained from
hyperspherical-harmonics solutions of the Schrodinger equation with two- and
three-nucleon potentials corresponding to either $\chi$ 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

### 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 $h^1_\pi$ and five parameters multiplying
contact interactions. We investigate PV effects induced by this potential on
several few-nucleon observables, including the $\vec{p}$-$p$ longitudinal
asymmetry, the neutron spin rotation in $\vec{n}$-$p$ and $\vec{n}$-$d$
scattering, and the longitudinal asymmetry in the $^3$He$(\vec{n},p)^3$H
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

### GW quasi-particle spectra from occupied states only

We introduce a method that allows for the calculation of quasi-particle
spectra in the GW approximation, yet avoiding any explicit reference to empty
one-electron states. This is achieved by expressing the irreducible
polarizability operator and the self-energy operator through a set of linear
response equations, which are solved using a Lanczos-chain algorithm. We first
validate our approach by calculating the vertical ionization energies of the
benzene molecule and then show its potential by addressing the spectrum of a
large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure

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