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

Construction of equilibrium networks with an energy function

We construct equilibrium networks by introducing an energy function depending on the degree of each node as well as the product of neighboring degrees. With this topological energy function, networks constitute a canonical ensemble, which follows the Boltzmann distribution for given temperature. It is observed that the system undergoes a topological phase transition from a random network to a star or a fully-connected network as the temperature is lowered. Both mean-field analysis and numerical simulations reveal strong first-order phase transitions at temperatures which decrease logarithmically with the system size. Quantitative discrepancies of the simulation results from the mean-field prediction are discussed in view of the strong first-order nature.Comment: To appear in J. Phys.

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

Modelling nitrogen dynamics at Lochnagar, N.E. Scotland.

International audienceControls on nitrate leaching from upland moorland catchments are not yet fully understood and yet, despite agreements on emission reductions, increased surface water nitrate concentrations may affect significantly the acidity status of these waters in the future. At Lochnagar, an upland moorland catchment in N.E. Scotland, 12 years of surface water chemistry observations have identified a steady increase in nitrate concentration despite no measured change in inorganic nitrogen deposition. The MAGIC model has been applied to simulate a "best case" situation assuming nitrate in surface water represents "hydrological" contributions (direct run-off) and a ?worst case' assuming a nitrogen saturation mechanism in the catchment soil. Only the ?saturation' model is capable of matching the 12 years of observation for nitrate but both model structures match the pH and acid neutralising capacity record. Future predictions to 2040, in response to the agreed emission reductions under the Gothenburg Protocol, are markedly different. The worst case predicts continued surface water acidification whilst the best case predicts a steady recovery. Keywords: nitrogen saturation, modelling, Lochnagar, Gothenburg Protoco

Evaluation Context Impacts Neuropsychological Performance of OEF/OIF Veterans with Reported Combat-Related Concussion

Although soldiers of Operations Iraqi Freedom (OIF) and Enduring Freedom (OEF) encounter combat-related concussion at an unprecedented rate, relatively few studies have examined how evaluation context, insufficient effort, and concussion history impact neuropsychological performances in the years following injury. The current study explores these issues in a sample of 119 U.S. veterans (OEF/OIF forensic concussion, n = 24; non-OEF/OIF forensic concussion, n = 20; OEF/OIF research concussion, n = 38; OEF/OIF research without concussion, n = 37). The OEF/OIF forensic concussion group exhibited significantly higher rates of insufficient effort relative to the OEF/OIF research concussion group, but a comparable rate of insufficient effort relative to the non-OEF/OIF forensic concussion group. After controlling for effort, the research concussion and the research non-concussion groups demonstrated comparable neuropsychological performance. Results highlight the importance of effort assessment among OEF/OIF and other veterans with concussion history, particularly in forensic contexts

Neuropsychological evaluation of blast-related concussion: Illustrating the challenges and complexities through OEF/OIF case studies

Background/objective: Soldiers of Operations Enduring Freedom (OEF) and Iraqi Freedom (OIF) sustain blast-related mild traumatic brain injury (concussion) with alarming regularity. This study discusses factors in addition to concussion, such as co-morbid psychological difficulty (e.g. post-traumatic stress) and symptom validity concerns that may complicate neuropsychological evaluation in the late stage of concussive injury. Case report: The study presents the complexities that accompany neuropsychological evaluation of blast concussion through discussion of three case reports of OEF/OIF personnel. Discussion: The authors emphasize uniform assessment of blast concussion, the importance of determining concussion severity according to acute-injury characteristics and elaborate upon non-concussion-related factors that may impact course of cognitive limitation. The authors conclude with a discussion of the need for future research examining the impact of blast concussion (particularly recurrent concussion) and neuropsychological performance

An Investigation of the Maximum Specimen Thickness for Differential Phase Contrast Lorentz Microscopy

Examination of magnetic domain structure in the transmission electron microscope is generally confined to very thin foils, where the specimen approximates to a pure phase object, and is achieved by the long established methods of Fresnel or Foucault contrast Lorentz microscopy, or by differential phase contrast (DPC) imaging in a scanning transmission electron microscope (STEM). If no quantitative interpretation of the image is required then magnetic contrast can be observed from thicker foils, and in this paper we describe an attempt to determine experimentally the range of foil thickness over which this is possible. To this end we have examined electropolished foils of single crystal Incalloy using an extended VG HB501 STEM to produce both DPC and Fresnel contrast images of the same area. The foil thickness at points along the domain walls was measured from the change in the Lorentz deflection angle as the STEM probe was moved across the domain wall, and this led to an estimate of ~ 700nm for the limiting thickness at which domain contrast was still visible in the DPC images. This value is obviously influenced by a number of factors, including the degree of inelastic scattering and the saturation magnetisation of the material, but it is sufficiently high that there might exist a range of thickness over which both transmission and scanning electron microscopes could be used to study the domain structure in the same areas of specimen

Land use influences on acidification and recovery of freshwaters in Galloway, south-west Scotland

International audienceThe long term response of surface waters to changes in sulphur deposition and afforestation is investigated for three upland river systems in the Galloway region of south-west Scotland. From 1984-1999, these rivers exhibited a statistically significant decline in non-marine sulphate concentrations in response to reduced acid deposition. This reduction in non-marine sulphate was, however, insufficient to induce a pH recovery over the period. A statistically significant increase in river pH was observed between 1956-1970 (0.05 yr-1) when subsidised agricultural lime payments were at a maximum. In 1976, this subsidy ceased and surface waters have progressively acidified. In addition, climatic change is found to influence long-term trends in pH. Mean annual pH was greatest during a dry period between 1969-1973 when total annual discharge was low. Thereafter, pH declined gradually in response to higher rainfall and increased total annual discharge. Overall, surface waters draining the afforested catchments of the Rivers Cree and Bladnoch are more acid than those draining the moorland catchment of the Luce. These results indicate that in afforested catchments, current reductions in sulphur emissions have not led to an observed improvement in the acid status of surface waters. Forestry, therefore, represents a confounding factor with regard to chemical recovery from acidification in this region. Keywords: acidification, afforestation, deposition, rivers, lochs, non-marine sulphate, p