8,399 research outputs found
Indian Bt cotton varieties do not affect the performance of cotton aphids.
Cotton varieties expressing Cry proteins derived from the soil bacterium Bacillus thuringiensis (Bt) are grown worldwide for the management of pest Lepidoptera. To prevent non-target pest outbreaks and to retain the biological control function provided by predators and parasitoids, the potential risk that Bt crops may pose to non-target arthropods is addressed prior to their commercialization. Aphids play an important role in agricultural systems since they serve as prey or host to a number of predators and parasitoids and their honeydew is an important energy source for several arthropods. To explore possible indirect effects of Bt crops we here examined the impact of Bt cotton on aphids and their honeydew. In climate chambers we assessed the performance of cotton aphids, Aphis gossypii Glover (Hemiptera: Aphididae) when grown on three Indian Bt (Cry1Ac) cotton varieties (MECH 12, MECH 162, MECH 184) and their non-transformed near isolines. Furthermore, we examined whether aphids pick up the Bt protein and analyzed the sugar composition of aphid honeydew to evaluate its suitability for honeydew-feeders. Plant transformation did not have any influence on aphid performance. However, some variation was observed among the three cotton varieties which might partly be explained by the variation in trichome density. None of the aphid samples contained Bt protein. As a consequence, natural enemies that feed on aphids are not exposed to the Cry protein. A significant difference in the sugar composition of aphid honeydew was detected among cotton varieties as well as between transformed and non-transformed plants. However, it is questionable if this variation is of ecological relevance, especially as honeydew is not the only sugar source parasitoids feed on in cotton fields. Our study allows the conclusion that Bt cotton poses a negligible risk for aphid antagonists and that aphids should remain under natural control in Bt cotton fields
Low-field microwave absorption and magnetoresistance in iron nanostructures grown by electrodeposition on n-type lightly-doped silicon substrates
In this study we investigate magnetic properties, surface morphology and
crystal structure in iron nanoclusters electrodeposited on lightly-doped (100)
n-type silicon substrates. Our goal is to investigate the spin injection and
detection in the Fe/Si lateral structures. The samples obtained under electric
percolation were characterized by magnetoresistive and magnetic resonance
measurements with cycling the sweeping applied field in order to understand the
spin dynamics in the as-produced samples. The observed hysteresis in the
magnetic resonance spectra, plus the presence of a broad peak in the
non-saturated regime confirming the low field microwave absorption (LFMA), were
correlated to the peaks and slopes found in the magnetoresistance curves. The
results suggest long range spin injection and detection in low resistive
silicon and the magnetic resonance technique is herein introduced as a
promising tool for analysis of electric contactless magnetoresistive samples.Comment: 12 pages, 5 figure
Demonstration of non-Markovian process characterisation and control on a quantum processor
In the scale-up of quantum computers, the framework underpinning
fault-tolerance generally relies on the strong assumption that environmental
noise affecting qubit logic is uncorrelated (Markovian). However, as physical
devices progress well into the complex multi-qubit regime, attention is turning
to understanding the appearance and mitigation of correlated -- or
non-Markovian -- noise, which poses a serious challenge to the progression of
quantum technology. This error type has previously remained elusive to
characterisation techniques. Here, we develop a framework for characterising
non-Markovian dynamics in quantum systems and experimentally test it on
multi-qubit superconducting quantum devices. Where noisy processes cannot be
accounted for using standard Markovian techniques, our reconstruction predicts
the behaviour of the devices with an infidelity of . Our results show
this characterisation technique leads to superior quantum control and extension
of coherence time by effective decoupling from the non-Markovian environment.
This framework, validated by our results, is applicable to any controlled
quantum device and offers a significant step towards optimal device operation
and noise reduction
Self-modulation of nonlinear waves in a weakly magnetized relativistic electron-positron plasma with temperature
We develop a nonlinear theory for self-modulation of a circularly polarized electromagnetic wave in a relativistic hot weakly magnetized electron-positron plasma. The case of parallel propagation along an ambient magnetic field is considered. A nonlinear Schrodinger equation is derived for the complex wave amplitude of a self-modulated wave packet. We show that the maximum growth rate of the modulational instability decreases as the temperature of the pair plasma increases. Depending on the initial conditions, the unstable wave envelope can evolve nonlinearly to either periodic wave trains or solitary waves. This theory has application to high-energy astrophysics and high-power laser physics.CONICyTFONDECyT 1110135 1080658Brazilian agency CNPqBrazilian agency FAPESPMarie Curie International Incoming Fellowshiphospitality of Paris ObservatoryInstitute for Fusion Studie
Vibrational absorption sidebands in the Coulomb blockade regime of single-molecule transistors
Current-driven vibrational non-equilibrium induces vibrational sidebands in
single-molecule transistors which arise from tunneling processes accompanied by
absorption of vibrational quanta. Unlike conventional sidebands, these
absorption sidebands occur in a regime where the current is nominally Coulomb
blockaded. Here, we develop a detailed and analytical theory of absorption
sidebands, including current-voltage characteristics as well as shot noise. We
discuss the relation of our predictions to recent experiments.Comment: 7 pages, 6 figures; revised discussion of relation to experimen
Relaxation processes and entropic traps in the Backgammon model
We examine the density-density correlation function in a model recently
proposed to study the effect of entropy barriers in glassy dynamics. We find
that the relaxation proceeds in two steps with a fast beta process followed by
alpha relaxation. The results are physically interpreted in the context of an
adiabatic approximation which allows to separate the two processes, and to
define an effective temperature in the off-equilibrium dynamics of the model.
We investigate the behavior of the response function associated to the density,
and find violations of the fluctuation dissipation theorem.Comment: 4 Pages including 3 Figures, Revte
Mesoscopic persistent currents in a strong magnetic field
Recent precision measurements of mesoscopic persistent currents in
normal-metal rings rely on the interaction between the magnetic moment
generated by the current and a large applied magnetic field. Motivated by this
technique, we extend the theory of mesoscopic persistent currents to include
the effect of the finite thickness of the ring and the resulting penetration of
the large magnetic field. We discuss both the sample-specific typical current
and the ensemble-averaged current which is dominated by the effects of
electron-electron interactions. We find that the magnetic field strongly
suppresses the interaction-induced persistent current and so provides direct
access to the independent-electron contribution. Moreover, the technique allows
for measurements of the entire distribution function of the persistent current.
We also discuss the consequences of the Zeeman splitting and spin-orbit
scattering, and include a detailed and quantitative comparison of our
theoretical results to experimental data.Comment: 12 pages, 7 figure
Second-order calculation of the local density of states above a nanostructured surface
We have numerically implemented a perturbation series for the scattered
electromagnetic fields above rough surfaces, due to Greffet, allowing us to
evaluate the local density of states to second order in the surface profile
function. We present typical results for thermal near fields of surfaces with
regular nanostructures, investigating the relative magnitude of the
contributions appearing in successive orders. The method is then employed for
estimating the resolution limit of an idealized Near-Field Scanning Thermal
Microscope (NSThM).Comment: 10 pages, 7 figure
Quantum Oscillations Can Prevent the Big Bang Singularity in an Einstein-Dirac Cosmology
We consider a spatially homogeneous and isotropic system of Dirac particles
coupled to classical gravity. The dust and radiation dominated closed
Friedmann-Robertson-Walker space-times are recovered as limiting cases. We find
a mechanism where quantum oscillations of the Dirac wave functions can prevent
the formation of the big bang or big crunch singularity. Thus before the big
crunch, the collapse of the universe is stopped by quantum effects and reversed
to an expansion, so that the universe opens up entering a new era of classical
behavior.
Numerical examples of such space-times are given, and the dependence on
various parameters is discussed. Generically, one has a collapse after a finite
number of cycles. By fine-tuning the parameters we construct an example of a
space-time which is time-periodic, thus running through an infinite number of
contraction and expansion cycles.Comment: 8 pages, LaTeX, 4 figures, statement on energy conditions correcte
- …