12,994 research outputs found
Magnetoasymmetric transport in a mesoscopic interferometer: From the weak to the strong coupling regime
The microreversibility principle implies that the conductance of a
two-terminal Aharonov-Bohm interferometer is an even function of the applied
magnetic flux. Away from linear response, however, this symmetry is not
fulfilled and the conductance phase of the interferometer when a quantum dot is
inserted in one of its arms can be a continuous function of the bias voltage.
Such magnetoasymmetries have been investigated in related mesoscopic systems
and arise as a consequence of the asymetric response of the internal potential
of the conductor out of equilibrium. Here we discuss magnetoasymmetries in
quantum-dot Aharonov-Bohm interferometers when strong electron-electron
interactions are taken into account beyond the mean-field approach. We find
that at very low temperatures the asymmetric element of the differential
conductance shows an abrupt change for voltages around the Fermi level. At
higher temperatures we recover a smooth variation of the magnetoasymmetry as a
function of the bias. We illustrate our results with the aid of the electron
occupation at the dot, demonstrating that its nonequilibrium component is an
asymmetric function of the flux even to lowest order in voltage. We also
calculate the magnetoasymmetry of the current-current correlations (the noise)
and find that it is given, to a good extent, by the magnetoasymmetry of the
weakly nonlinear conductance term. Therefore, both magnetoasymmetries (noise
and conductance) are related to each other via a higher-order
fluctuation-dissipation relation. This result appears to be true even in the
low temperature regime, where Kondo physics and many-body effects dominate the
transport properties.Comment: 17 pages, 9 figure
A Positive Test for Fermi-Dirac Distributions of Quark-Partons
By describing a large class of deep inelastic processes with standard
parameterization for the different parton species, we check the characteristic
relationship dictated by Pauli principle: broader shapes for higher first
moments. Indeed, the ratios between the second and the first moment and the one
between the third and the second moment for the valence partons is an
increasing function of the first moment and agrees quantitatively with the
values found with Fermi-Dirac distributions.Comment: 15 pages LaTeX, 2 eps figures. Final version, to appear in Mod. Phys.
Lett.
The Two-impurity Anderson Model Revisited: Competition between Kondo Effect and Reservoir-mediated Superexchange in Double Quantum Dots
We study a series-coupled double quantum dot in the Kondo regime modeled by
the two-impurity Anderson model and find a new conduction-band mediated
superexchange interaction that competes with Kondo physics in the strong
Coulomb interaction limit. Our numerical renormalization group results,
complemented with the higher-order Rayleigh-Schr\"odinger perturbation theory,
show that the novel exchange mechanism leads to clear experimental consequences
that can be checked in transport measurements through double quantum dots
RKKY and magnetic field interactions in coupled Kondo quantum dots
We investigate theoretically the transport properties of two independent
artificial Kondo impurities. They are coupled together via a tunable
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction. For strong enough
antiferromagnetic RKKY interaction, the impurity density of states increases
with the applied in-plane magnetic field. This effect can be used to
distinguish between antiferromagnetic and ferromagnetic RKKY interactions.
These results may be relevant to explain some features of recent experiments by
Craig et al. (cond-mat/0404213).Comment: 4 pages, 3 figure
Effect of the Gribov horizon on the Polyakov loop and vice versa
We consider finite temperature SU(2) gauge theory in the continuum
formulation, which necessitates the choice of a gauge fixing. Choosing the
Landau gauge, the existing gauge copies are taken into account by means of the
Gribov-Zwanziger (GZ) quantization scheme, which entails the introduction of a
dynamical mass scale (Gribov mass) directly influencing the Green functions of
the theory. Here, we determine simultaneously the Polyakov loop (vacuum
expectation value) and Gribov mass in terms of temperature, by minimizing the
vacuum energy w.r.t. the Polyakov loop parameter and solving the Gribov gap
equation. Inspired by the Casimir energy-style of computation, we illustrate
the usage of Zeta function regularization in finite temperature calculations.
Our main result is that the Gribov mass directly feels the deconfinement
transition, visible from a cusp occurring at the same temperature where the
Polyakov loop becomes nonzero. In this exploratory work we mainly restrict
ourselves to the original Gribov-Zwanziger quantization procedure in order to
illustrate the approach and the potential direct link between the vacuum
structure of the theory (dynamical mass scales) and (de)confinement. We also
present a first look at the critical temperature obtained from the Refined
Gribov-Zwanziger approach. Finally, a particular problem for the pressure at
low temperatures is reported.Comment: 19 pages, 8 .pdf figures. v2: extended section 3 + extra references;
version accepted for publication in EPJ
Transport properties of a molecule embedded in an Aharonov-Bohm interferometer
We theoretically investigate the transport properties of a molecule embedded
in one arm of a mesoscopic Aharonov-Bohm interferometer. Due to the presence of
phonons the molecule level position () and the electron-electron
interaction () undergo a \emph{polaronic shift} which affects dramatically
the electronic transport through the molecular junction. When the
electron-phonon interaction is weak the linear conductance presents Fano-line
shapes as long as the direct channel between the electrodes is opened. The
observed Fano resonances in the linear conductance are originated from the
interference between the spin Kondo state and the direct path. For strong
enough electron-phonon interaction, the electron-electron interaction is
renormalized towards negative values, {\it i.e.} becomes effectively
attractive. This scenario favors fluctuations between the empty and doubly
occupied charge states and therefore promotes a charge Kondo effect. However,
the direct path between the contacts breaks the electron-hole symmetry which
can efficiently suppress this charge Kondo effect. Nevertheless, we show that a
proper tuning of the gate voltage is able to revive the Kondo resonance. Our
results are obtained by using the Numerical Renormalization approximation to
compute the electronic spectral function and the linear conductance.Comment: 17 pages, 12 figure
Brotações epicórmicas no resgate vegetativo de árvores selecionadas de Ilex paraguariensis St. Hil.
Com o objetivo de avaliar a indução de brotações epicórmicas em ramos destacadas de Ilex paraguariensis St. Hil., foram coletados ramos de 60 cm oriundos de um teste combinado de procedências e progênies de 14 anos de idade, em Colombo - PR. Os ramos foram acondicionados em casa de vegetação, nos seguintes tratamentos: T1 = sobre a bancada, T2 = dentro de bandejas com água pura e T3 = dentro de bandeja com areia fina. Foram avaliadas as características sobrevivência e diâmetro do ápice dos ramos, número de tufos e número de brotos produzidos aos 45 e 90 dias. Aos 45 dias, T2 mostrou-se tão eficiente quanto T3 na sobrevivência e número de brotos produzidos, ao passo que aos 90 dias T3 apresentou-se superior em todas as características avaliadas. T1 apresentou baixa produção de tufos e brotos aos 45 dias e mortalidade de 100% dos ramos aos 90 dias. A técnica mostrou-se viável, obtendo-se brotos com tamanho padrão para possível utilização em outras técnicas de propagação vegetativa, sendo a areia superior aos demais tratamentos, ao mesmo tempo em que se mostra menos exigente em manutenção.Secão: Conservação, Melhoramento e Multiplicação. Feira do Agronegócio da Erva-mate, 1., 2003, Chapecó. Integrar para promover o agronegócio da erva-mate
Synthetic spectra of H Balmer and HeI absorption lines. I: Stellar library
We present a grid of synthetic profiles of stellar H Balmer and HeI lines at
optical wavelengths with a sampling of 0.3 A. The grid spans a range of
effective temperature 4000 K < Teff < 50000 K, and gravity 0.0 < log g < 5.0 at
solar metallicity. For Teff > 25000 K, NLTE stellar atmosphere models are
computed using the code TLUSTY (Hubeny 1988). For cooler stars, Kurucz (1993)
LTE models are used to compute thesynthetic spectra. The grid includes the
profiles of the high-order hydrogen Balmer series and HeI lines for effective
temperatures and gravities that have not been previously synthesized. The
behavior of H8 to H13 and HeI 3819 with effective temperature and gravity is
very similar to that of the lower terms of the series (e.g. Hb) and the other
HeI lines at longer wavelengths; therefore, they are suited for the
determination of the atmospheric parameters of stars. These lines are
potentially important to make predictions for these stellar absorption features
in galaxies with active star formation. Evolutionary synthesis models of these
lines for starburst and post-starburst galaxies are presented in a companion
paper. The full set of the synthetic stellar spectra is available for retrieval
at our website http://www.iaa.es/ae/e2.html and
http://www.stsci.edu/science/starburst/ or on request from the authors at
[email protected]: To be published in ApJS. 28 pages and 12 figure
Josephson current in strongly correlated double quantum dots
We study the transport properties of a serial double quantum dot (DQD)
coupled to two superconducting leads, focusing on the Josephson current through
the DQD and the associated 0- transitions which result from the subtle
interplay between the superconductivity, the Kondo physics, and the inter-dot
superexchange interaction. We examine the competition between the
superconductivity and the Kondo physics by tuning the relative strength
of the superconducting gap and the Kondo temperature
, for different strengths of the superexchange coupling determined by the
interdot tunneling relative to the dot level broadening . We find
strong renormalization of , a significant role of the superexchange coupling
, and a rich phase diagram of the 0 and -junction regimes. In
particular, when both the superconductivity and the exchange interaction are in
close competion with the Kondo physics (), there appears
an island of -phase at large values of the superconducting phase
difference.Comment: 4 pages, 4 figure
Fósforo, fosfato de rocha e fluorose em bovinos.
Fosfatos naturais na alimentação animal. Uso e abuso dos fosfatos naturais. Essencialidade do flúor. Metabolismo do flúor. Fluor e produção de leite. Flúor e reprodução. O flúor e a barreira placentária. Recuperação do animal exposto ao flúor. Antagonistas do flúor. Efeitos tóxicos do flúor. Diagnóstico de fluorese. Dentição normal dos bovinos. Fluorose dentária.bitstream/item/104565/1/Fosforo-fosfato-de-rocha.pd
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