2,725 research outputs found
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
Issues of scale and scope in bio-physical modelling for natural resource management decision making in New South Wales
Natural resource management decision making by Catchment Management Authorities in NSW is being aided by a project involving bio-physical modelling and the development of an alternative decision-making framework. The objective of the bio-physical modelling process is to generate predictions of environmental or natural resource outcomes rather than project outputs. These outcomes can then be used in an investment framework to help priority setting and project decision making. Questions that arise in bio-physical modelling include those relating to scale and scope. Scale issues include how to address the landscape impacts of particular (or a series of local) on-ground works proposals. Scope issues include assessment of multiple-attribute responses to particular changes. In a multi-disciplinary context the challenge is then to translate this information into units that can be adapted to a decision-support framework. Existing Catchment Management Authorities decisions are often based on scoring and weighting of environmental improvements using an environmental benefits index, however other economic frameworks are possible. We discuss the important context for these questions in the decision making framework.environmental benefits, bio-physical models, scale, scope, investment decisions, Resource /Energy Economics and Policy,
Superconductivity in ropes of carbon nanotubes
Recent experimental and theoretical results on intrinsic superconductivity in
ropes of single-wall carbon nanotubes are reviewed and compared. We find strong
experimental evidence for superconductivity when the distance between the
normal electrodes is large enough. This indicates the presence of attractive
phonon-mediated interactions in carbon nanotubes, which can even overcome the
repulsive Coulomb interactions. The effective low-energy theory of rope
superconductivity explains the experimental results on the
temperature-dependent resistance below the transition temperature in terms of
quantum phase slips. Quantitative agreement with only one fit parameter can be
obtained. Nanotube ropes thus represent superconductors in an extreme 1D limit
never explored before.Comment: 19 pages, 9 figures, to appear in special issue of Sol. State Com
Tuning the proximity effect in a superconductor-graphene-superconductor junction
We have tuned in situ the proximity effect in a single graphene layer coupled
to two Pt/Ta superconducting electrodes. An annealing current through the
device changed the transmission coefficient of the electrode/graphene
interface, increasing the probability of multiple Andreev reflections. Repeated
annealing steps improved the contact sufficiently for a Josephson current to be
induced in graphene.Comment: Accepted for publication in Phys. Rev.
Microwave response of an NS ring coupled to a superconducting resonator
A long phase coherent normal (N) wire between superconductors (S) is
characterized by a dense phase dependent Andreev spectrum . We probe this
spectrum in a high frequency phase biased configuration, by coupling an NS ring
to a multimode superconducting resonator. We detect a dc flux and frequency
dependent response whose dissipative and non dissipative components are related
by a simple Debye relaxation law with a characteristic time of the order of the
diffusion time through the N part of the ring. The flux dependence exhibits
periodic oscillations with a large harmonics content at temperatures
where the Josephson current is purely sinusoidal. This is explained considering
that the populations of the Andreev levels are frozen on the time-scale of the
experiments.Comment: 5 pages,4 figure
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