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,

0-π\pi 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 $\pi$-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-$\pi$ 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-$\pi$ transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.Comment: 14 pages, 12 figure

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 $h/2e$ 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