552 research outputs found
Resonant tunneling through ultrasmall quantum dots: zero-bias anomalies, magnetic field dependence, and boson-assisted transport
We study resonant tunneling through a single-level quantum dot in the
presence of strong Coulomb repulsion beyond the perturbative regime. The level
is either spin-degenerate or can be split by a magnetic field. We, furthermore,
discuss the influence of a bosonic environment. Using a real-time diagrammatic
formulation we calculate transition rates, the spectral density and the
nonlinear characteristic. The spectral density shows a multiplet of Kondo
peaks split by the transport voltage and the boson frequencies, and shifted by
the magnetic field. This leads to zero-bias anomalies in the differential
conductance, which agree well with recent experimental results for the electron
transport through single-charge traps. Furthermore, we predict that the sign of
the zero-bias anomaly depends on the level position relative to the Fermi level
of the leads.Comment: 27 pages, latex, 21 figures, submitted to Phys. Rev.
Theory of Current and Shot Noise Spectroscopy in Single-Molecular Quantum Dots with Phonon Mode
Using the Keldysh nonequilibrium Green function technique, we study the
current and shot noise spectroscopy of a single molecular quantum dot coupled
to a local phonon mode. It is found that in the presence of electron-phonon
coupling, in addition to the resonant peak associated with the single level of
the dot, satellite peaks with the separation set by the frequency of phonon
mode appear in the differential conductance. In the ``single level'' resonant
tunneling region, the differential shot noise power exhibit two split peaks.
However, only single peaks show up in the ``phonon assisted''
resonant-tunneling region. An experimental setup to test these predictions is
also proposed.Comment: 5 pages, 3 eps figures embedde
Filtering out the cosmological constant in the Palatini formalism of modified gravity
According to theoretical physics the cosmological constant (CC) is expected
to be much larger in magnitude than other energy densities in the universe,
which is in stark contrast to the observed Big Bang evolution. We address this
old CC problem not by introducing an extremely fine-tuned counterterm, but in
the context of modified gravity in the Palatini formalism. In our model the
large CC term is filtered out, and it does not prevent a standard cosmological
evolution. We discuss the filter effect in the epochs of radiation and matter
domination as well as in the asymptotic de Sitter future. The final expansion
rate can be much lower than inferred from the large CC without using a
fine-tuned counterterm. Finally, we show that the CC filter works also in the
Kottler (Schwarzschild-de Sitter) metric describing a black hole environment
with a CC compatible to the future de Sitter cosmos.Comment: 22 pages, 1 figure, discussion extended, references added, accepted
by Gen.Rel.Gra
The powerful in vitro bioactivity of Euterpe oleracea Mart. seeds and related phenolic compounds
The Euterpe oleracea Mart. (açaí) is a plant from the Amazon region, classified as
"super fruit" because of its various functional properties. However, limited investigation
has been performed on açaí by-products, such as seeds. Therefore, the aim of this work
was to characterized the phenolic compounds of the aqueous extract of açaí seeds and
further evaluate its bioactivity (antioxidant and cytotoxic activities. Only
proanthocyanidins were detected, being a B-type (epi)catechin tetramer the most
abundant; however, procyanidin trimmers were the most predominant form. Açaí seeds
extract revealed a high antioxidant (EC50 ranging from 3.6 to 19.4 μg/mL) and cytotoxic
activity, being more effective in the cervical carcinoma cell line (HeLa; GI50 = 18
μg/mL); it did not show toxicity for non-tumor cells. Açaí seeds are considered a waste
and could have an added economic benefit, through the extraction of natural
antioxidants, particularly proanthocyanidins, that could find applications in food and
pharmaceutical industries.The authors are grateful to Foundation for Science and Technology (FCT, Portugal) for
financial support to the research centre CIMO (strategic project PEst
OE/AGR/UI0690/2011) and L. Barros researcher contract under “Programa
Compromisso com Ciência - 2008”. The authors are also grateful to Jamil S. Oliveira by
the initial discussions on the preparation of samples
An Extreme Solar Event of 20 January 2005: Properties of the Flare and the Origin of Energetic Particles
The extreme solar and SEP event of 20 January 2005 is analyzed from two
perspectives. Firstly, we study features of the main phase of the flare, when
the strongest emissions from microwaves up to 200 MeV gamma-rays were observed.
Secondly, we relate our results to a long-standing controversy on the origin of
SEPs arriving at Earth, i.e., acceleration in flares, or shocks ahead of CMEs.
All emissions from microwaves up to 2.22 MeV line gamma-rays during the main
flare phase originated within a compact structure located just above sunspot
umbrae. A huge radio burst with a frequency maximum at 30 GHz was observed,
indicating the presence of a large number of energetic electrons in strong
magnetic fields. Thus, protons and electrons responsible for flare emissions
during its main phase were accelerated within the magnetic field of the active
region. The leading, impulsive parts of the GLE, and highest-energy gamma-rays
identified with pi^0-decay emission, are similar and correspond in time. The
origin of the pi^0-decay gamma-rays is argued to be the same as that of lower
energy emissions. We estimate the sky-plane speed of the CME to be 2000-2600
km/s, i.e., high, but of the same order as preceding non-GLE-related CMEs from
the same active region. Hence, the flare itself rather than the CME appears to
determine the extreme nature of this event. We conclude that the acceleration,
at least, to sub-relativistic energies, of electrons and protons, responsible
for both the flare emissions and the leading spike of SEP/GLE by 07 UT, are
likely to have occurred simultaneously within the flare region. We do not rule
out a probable contribution from particles accelerated in the CME-driven shock
for the leading GLE spike, which seemed to dominate later on.Comment: 34 pages, 14 Postscript figures. Solar Physics, accepted. A typo
corrected. The original publication is available at
http://www.springerlink.co
Nucleonic gamma-ray production in Pulsar Wind Nebulae
Observations of the inner radian of the Galactic disk at very high energy
(VHE) gamma-rays have revealed at least 16 new sources. Besides shell type
super-nova remnants, pulsar wind nebulae (PWN) appear to be a dominant source
population in the catalogue of VHE gamma-ray sources. Except for the Crab
nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially
extended (5-20 pc). Currently, at least 3 middle aged ( kyrs) PWN (Vela
X, G18.0-0.7, and G313.3+0.6 in the ``Kookaburra'' region) and 1 young PWN MSH
15-5{\it2} ( kyrs) have been identified to be VHE emitting PWN
(sometimes called ``TeV Plerions''). Two more candidate ``TeV Plerions'' have
been identifed and have been reported at this conference [1]. In this
contribution, the gamma-ray emission from Vela X is explained by a nucleonic
component in the pulsar wind. The measured broad band spectral energy
distribution is compared with the expected X-ray emission from primary and
secondary electrons. The observed X-ray emission and TeV emission from the
three middle aged PWN are compared with each other.Comment: 6 pages, 3 figures, to appear in proceedings "The Multi-Messenger
Approach to High-Energy Gamma-Ray Sources", Barcelona July 200
A comparative study of chemical composition, antioxidant and antimicrobial properties of Morchella esculenta (L.) Pers. from Portugal and Serbia
A comparative study on chemical composition (nutritional value, primary and secondary metabolites), antioxidant properties (scavenging activity, reducing power and inhibition of lipid peroxidation), and antimicrobial activity (antibacterial and demelanizing properties) of two samples of Morchella esculenta (morel) from different countries (Portugal and Serbia) was performed. This species was chosen for being one of the most highly prized edible mushrooms in the world. Both samples are rich in carbohydrates (including free sugars) and proteins, and contain several bioactive compounds such as organic acids, phenolic compounds and tocopherols. Polyunsaturated fatty acids were the most abundant compounds followed by mono or saturated fatty acids. Sample from Portugal (SP) gave higher radical scavenging activity and reducing power, while sample from Serbia (SS) showed higher lipid peroxidation inhibition. Both samples gave antibacterial activity against five bacteria (in some cases even better than standard antibiotics) and demelanizing activity against four micromycetes, showing SS higher activities. As far as we know, this is the first study reporting chemical compounds and bioactivity of morel samples from Portugal and Serbia. Furthermore, a novel method for evaluation of demelanizing activity was presented.The authors are grateful to Fundacao para a Ciencia e a Tecnologia (FCT, Portugal) and COMPETE/QREN/EU (research project PTDC/AGR-ALI/110062/2009; bilateral cooperation action Portugal/Serbia 2011; strategic projects PEst-OE/AGR/UI0690/2011 and PEst-C/QUI/UI0686/2011), and to Serbian Ministry of Education and Science (grant number 173032) for financial support. S.A. Heleno (BD/70304/2010) and L. Barros (BPD/4609/2008) also thank FCT, POPH-QREN and FSE
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
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