146 research outputs found
Dynamics of a suspended nanowire driven by an ac Josephson current in an inhomogeneous magnetic field
We consider a voltage-biased nanoelectromechanical Josephson junction, where
a suspended nanowire forms a superconducting weak-link, in an inhomogeneous
magnetic field. We show that a nonlinear coupling between the Josephson current
and the magnetic field generates a Laplace force that induces a whirling motion
of the nanowire. By performing an analytical and a numerical analysis, we
demonstrate that at resonance, the amplitude-phase dynamics of the whirling
movement present different regimes depending on the degree of inhomogeneity of
the magnetic field: time independent, periodic and chaotic. Transitions between
these regimes are also discussed.Comment: 7 pages, 5 figure
Fermionic Casimir effect for parallel plates in the presence of compact dimensions with applications to nanotubes
We evaluate the Casimir energy and force for a massive fermionic field in the
geometry of two parallel plates on background of Minkowski spacetime with an
arbitrary number of toroidally compactified spatial dimensions. The bag
boundary conditions are imposed on the plates and periodicity conditions with
arbitrary phases are considered along the compact dimensions. The Casimir
energy is decomposed into purely topological, single plate and interaction
parts. With independence of the lengths of the compact dimensions and the
phases in the periodicity conditions, the interaction part of the Casimir
energy is always negative. In order to obtain the resulting force, the
contributions from both sides of the plates must be taken into account. Then,
the forces coming from the topological parts of the vacuum energy cancel out
and only the interaction term contributes to the Casimir force. Applications of
the general formulae to Kaluza-Klein type models and carbon nanotubes are
given. In particular, we show that for finite length metallic nanotubes the
Casimir forces acting on the tube edges are always attractive, whereas for
semiconducting-type ones they are attractive for small lengths of the nanotube
and repulsive for large lengths.Comment: 20 pages, 3 figure
Induced fermionic current in toroidally compactified spacetimes with applications to cylindrical and toroidal nanotubes
The vacuum expectation value of the fermionic current is evaluated for a
massive spinor field in spacetimes with an arbitrary number of toroidally
compactified spatial dimensions in presence of a constant gauge field. By using
the Abel-Plana type summation formula and the zeta function technique we
present the fermionic current in two different forms. Non-trivial topology of
the background spacetime leads to the Aharonov-Bohm effect on the fermionic
current induced by the gauge field. The current is a periodic function of the
magnetic flux with the period equal to the flux quantum. In the absence of the
gauge field it vanishes for special cases of untwisted and twisted fields.
Applications of the general formulae to Kaluz-Klein type models and to
cylindrical and toroidal carbon nanotubes are given. In the absence of magnetic
flux the total fermionic current in carbon nanotubes vanishes, due to the
cancellation of contributions from two different sublattices of the graphene
hexagonal lattice.Comment: 18 pages, 5 figures, explicit regularization procedure adde
Helium mixtures in nanotube bundles
An analogue to Raoult's law is determined for the case of a 3He-4He mixture
adsorbed in the interstitial channels of a bundle of carbon nanotubes. Unlike
the case of He mixtures in other environments, the ratio of the partial
pressures of the coexisting vapor is found to be a simple function of the ratio
of concentrations within the nanotube bundle.Comment: 3 pages, no figures, submitted to Phys. Rev. Let
Towards the unified description of light and heavy hadrons in the bag model approach
Mass spectra of ground state hadrons containing u-, d-, s-, c-quarks as well
as some lightest hadrons containing b-quarks are calculated on the basis of a
slightly modified bag model. The center-of-mass motion corrections are
incorporated using a wavepacket projection with Gaussian parametrization of the
distribution amplitude. We use running coupling constant and also allow the
effective quark mass to be scale-dependent. The impact of these modifications
on the hadron mass spectrum is investigated. A comparison of the predicted mass
values with the experimental data demonstrates that the modified bag model is
sufficiently flexible to provide a satisfactory description of light and heavy
hadrons (mesons and baryons) in a single consistent framework.Comment: 27 pages, 11 table
ASPECTOS TECNOLÓGICOS NO DESENVOLVIMENTO DE UM OBJETO DE APRENDIZAGEM COM ACESSIBILIDADE: A EXPERIÊNCIA COM O SCRAPBOOK
A utilização de recursos tecnológicos no contexto educacional acessível e inclusivo vem sendo incentivado, e cada vez mais utilizado. Este artigo tem o objetivo de apresentar o Objeto de Aprendizagem (OA) Scrapbook: A arte de decorar álbuns de fotografias. Tal OA foi construído para oferecer a possibilidade de desenvolvimento da arte do scrapbook para aqueles que não possuem materiais ou habilidades artesanais necessárias para a confecção manual do álbum. O OA Scrapbook oferece recursos de acessibilidade para atender necessidades de usuários, como deficiência visual e motora. No decorrer do texto, as etapas de desenvolvimento do OA são descritas, evidenciando a construção e recursos presentes em cada versão do mesmo
Critical Casimir forces and adsorption profiles in the presence of a chemically structured substrate
Motivated by recent experiments with confined binary liquid mixtures near
demixing, we study the universal critical properties of a system, which belongs
to the Ising universality class, in the film geometry. We employ periodic
boundary conditions in the two lateral directions and fixed boundary conditions
on the two confining surfaces, such that one of them has a spatially
homogeneous adsorption preference while the other one exhibits a laterally
alternating adsorption preference, resembling locally a single chemical step.
By means of Monte Carlo simulations of an improved Hamiltonian, so that the
leading scaling corrections are suppressed, numerical integration, and
finite-size scaling analysis we determine the critical Casimir force and its
universal scaling function for various values of the aspect ratio of the film.
In the limit of a vanishing aspect ratio the critical Casimir force of this
system reduces to the mean value of the critical Casimir force for laterally
homogeneous ++ and +- boundary conditions, corresponding to the surface spins
on the two surfaces being fixed to equal and opposite values, respectively. We
show that the universal scaling function of the critical Casimir force for
small but finite aspect ratios displays a linear dependence on the aspect ratio
which is solely due to the presence of the lateral inhomogeneity. We also
analyze the order-parameter profiles at criticality and their universal scaling
function which allows us to probe theoretical predictions and to compare with
experimental data.Comment: revised version, section 5.2 expanded; 53 pages, 12 figures, iopart
clas
Topological confinement in an antisymmetric potential in bilayer graphene in the presence of a magnetic field
We investigate the effect of an external magnetic field on the carrier states that are localized at a potential kink and a kink-antikink in bilayer graphene. These chiral states are localized at the interface between two potential regions with opposite signs
Infrared Properties of QCD from Dyson-Schwinger equations
I review recent results on the infrared properties of QCD from
Dyson-Schwinger equations. The topics include infrared exponents of
one-particle irreducible Green's functions, the fixed point behaviour of the
running coupling at zero momentum, the pattern of dynamical quark mass
generation and properties of light mesons.Comment: 47 pages, 19 figures, Topical Review to be published in J.Phys.G, v2:
typos corrected and some references adde
Exploring the relationship between productive vocabulary knowledge and second language oral ability
The current study investigated the extent to which L2 learners’ productive vocabulary knowledge could predict multiple dimensions of spontaneous speech production. A total of 39 EFL participants with varying L2 proficiency levels first completed a productive vocabulary knowledge task (Lex30). Their spontaneous speech, elicited via a series of picture description task, was then assessed for comprehensibility (i.e., ease of understanding), accentedness (i.e., linguistic nativelikeness), and fluency (i.e., speech rate). The findings showed that the productive vocabulary scores significantly correlated with L2 fluency, but not with comprehensibility or accentedness. Such results might indicate that more proficient L2 learners, as indicated by their productive vocabulary scores, might be able to speak spontaneously without too many pauses and repetitions, and at a faster tempo. Finally, future research directions will be discussed with a focus on the relationships between vocabulary knowledge and speaking
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