1,599 research outputs found
Emission Noise and High Frequency Cut-Off of the Kondo Effect in a Quantum Dot
By coupling on chip a carbon nanotube to a quantum noise detector, a
superconductor-insulator-superconductor junction, via a resonant circuit, we
measure the emission noise of a carbon nanotube quantum dot in the Kondo
regime. The signature of the Kondo effect in the current noise is measured for
different ratios of the Kondo temperature over the measured frequency and for
different asymmetries of the coupling to the contacts, and compared to finite
frequency quantum noise calculations. Our results point towards the existence
of a high frequency cut-off of the electronic emission noise associated with
the Kondo resonance. This cut-off frequency is of the order of a few times the
Kondo temperature when the electronic system is close to equilibrium, which is
the case for a strongly asymmetric coupling. On the other hand, this cut-off is
shifted to lower frequency in a symmetric coupling situation, where the bias
voltage drives the Kondo state out-of-equilibrium. We then attribute the low
frequency cut-off to voltage induced spin relaxation.Comment: 5 pages, 3 figures and appendi
0- quantum transition in a carbon nanotube Josephson junction: universal phase dependence and orbital degeneracy
We investigate experimentally the supercurrent in a clean carbon nanotube
quantum dot, close to orbital degeneracy, connected to superconducting leads in
a regime of strong competition between local electronic correlations and
superconducting proximity effect. For an odd occupancy of the dot and
intermediate coupling to the reservoir, the Kondo effect can develop in the
normal state and screen the local magnetic moment of the dot. This leads to
singlet-doublet transitions that strongly affect the Josephson effect in a
single-level quantum dot: the sign of the supercurrent changes from positive to
negative (0 to -junction). In the regime of strongest competition between
the Kondo effect and proximity effect, meaning that the Kondo temperature
equals the superconducting gap, the magnetic state of the dot undergoes a first
order quantum transition induced by the superconducting phase difference across
the junction. This is revealed experimentally by anharmonic current-phase
relations. In addition, the very specific electronic configuration of clean
carbon nanotubes, with two nearly orbitally degenerated states, leads to
different physics depending whether only one or both quasi-degenerate upper
levels of the dots participate to transport, which is determined by their
occupancy and relative widths. When the transport of Cooper pairs takes place
through only one of these levels, we find that the phase diagram of the
phase-dependent 0- transition is a universal characteristic of a
discontinuous level-crossing quantum transition at zero temperature. In the
case were two levels participate to transport, the nanotube Josephson current
exhibits a continuous 0- transition, independent of the superconducting
phase, revealing a different physical mechanism of the transition.Comment: 14 pages, 12 figure
Manipulating the magnetic state of a carbon nanotube Josephson junction using the superconducting phase
The magnetic state of a quantum dot attached to superconducting leads is
experimentally shown to be controlled by the superconducting phase difference
across the dot. This is done by probing the relation between the Josephson
current and the superconducting phase difference of a carbon nanotube junction
whose Kondo energy and superconducting gap are of comparable size. It exhibits
distinctively anharmonic behavior, revealing a phase mediated singlet to
doublet transition. We obtain an excellent quantitative agreement with
numerically exact quantum Monte Carlo calculations. This provides strong
support that we indeed observed the finite temperature signatures of the phase
controlled zero temperature level-crossing transition originating from strong
local electronic correlations.Comment: 5 pages, 4 figures + supp. material
Physiological, morphological and allocational plasticity in understory deciduous trees: importance of plant size and light availability
In a 4-year study, we investigated changes in leaf physiology, crown morphology and whole-tree biomass allocation in seedlings and saplings of shade-tolerant sugar maple (Acer saccharum Marsh.) and intermediate shade-tolerant yellow birch (Betula alleghaniensis Britt.) growing in natural understory light (0.5 to 35% of full sunlight) or in understory light reduced by 50% with shade nets to simulate the effect of gap closure. Leaf physiological parameters were mainly influenced by the light gradient, whereas crown morphological and whole-tree allocational parameters were mainly influenced by tree size. No single physiological, morphological or allocational trait was identified that could explain the difference in shade tolerance between the species. Yellow birch had higher growth rates, biomass allocation to branches and leaf physiological plasticity and lower crown morphological plasticity in unmodified understory light than sugar maple. Sugar maple did not display significant physiological plasticity, but showed variation with tree size in both crown morphology and whole-tree biomass allocation. When sugar maple was small, a greater proportion of whole-tree biomass was allocated to roots. However, physiological differences between the species decreased with decreasing light and most morphological and allocational differences tended to disappear with increasing tree size, suggesting that many species differences in shade-tolerance are expressed mainly during the seedling stage. Understory trees of both species survived for 4 years under shade nets, possibly because of higher plasticity when small and the use of stored reserves when taller
Do understory saplings respond to both light and below-ground competitio?: a field experiment in a north-eastern American hardwood forest and a literature review
A study was initiated in 1993 to evaluate the potential effects of both above- and below-ground competition exclusion on yellow birch (Betula alleghaniensis Britton), sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.) sapling growth along an understory light gradient ranging from 3% to 50% of full sunlight. We compared four different growth variables between a control and a treatment (trenching and manual removal of nearby vegetation). Height growth, diameter growth, height over stem diameter ratio, and crown area varied with light availability in all three species, whereas trenching treatment had no significant effect. Our results show that light is the main factor affecting understory sapling growth following a selection cut in this northern hardwood forest, at least up to 50% full sunlight. The unresponsiveness of these three species to below-ground competition is discussed in relation to a literature review in which both soil richness and species functional ecology are considered
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