Coherent phenomena in mesoscopic systems

A mesoscopic system of cylindrical geometry made of a metal or a semiconductor is shown to exhibit features of a quantum coherent state. It is shown that magnetostatic interaction can play an important role in mesoscopic systems leading to an ordered ground state. The temperature $T^{*}$ below the system exhibits long-range order is determined. The self-consistent mean field approximation of the magnetostatic interaction is performed giving the effective Hamiltonian from which the self-sustaining currents can be obtained. The relation of quantum coherent state in mesoscopic cylinders to other coherent systems like superconductors is discussed.Comment: REVTeX, 4 figures, in print in Supercond. Sci. Techno

Neighbours matter and the weak succumb: Ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate

The epidemiology and severity of ash dieback (ADB), the disease caused by the ascomycete fungus Hymenoscyphus fraxineus, has been linked to a variety of site conditions; however, there has been a lack of analysis at an individual tree scale.Symptoms of ADB were scored on ca. 400 trees of Fraxinus excelsior (ash) in permanent sample plots during two successive years in a UK natural woodland reserve. Using comprehensive plot records maintained since 1945, and detailed spatial records updated since 1977, we assembled an array of potential explanatory variables, including site environment factors, ash tree density, previous and present tree condition and near neighbourhood summary statistics (NNSS), such as species mingling and size dominance. Their impact on the severity of ADB of focal ash trees was tested with generalised linear mixed effects models (GLMM).The severity of ADB was much greater in the lower slope parts of the site with moister soils and least in a managed area subject to tree thinning in the previous 35 years. Severity of ADB had a negative association with focal ash tree prior relative growth rate over a period of a decade immediately before the disease was detected at the site. Greater ADB severity was also significantly associated with smaller diameter at breast height of ash trees. Additionally, ADB was significantly positively associated with a greater proportion of heterospecific trees amongst the six nearest neighbours of the focal tree.Synthesis. The relationship of the severity of ADB disease with site environment, tree condition and neighbourhood is complex but nevertheless important in the progression of the disease. The findings suggest some silvicultural interventions, such as thinning to increase the vigour of retained ash trees, might reduce the impact of ADB

Possibility of long-range order in clean mesoscopic cylinders

A microscopic Hamiltonian of the magnetostatic interaction is discussed. This long-range interaction can play an important role in mesoscopic systems leading to an ordered ground state. The self-consistent mean field approximation of the magnetostatic interaction is performed to give an effective Hamiltonian from which the spontaneous, self-sustaining currents can be obtained. To go beyond the mean field approximation the mean square fluctuation of the total momentum is calculated and its influence on self-sustaining currents in mesoscopic cylinders with quasi-1D and quasi-2D conduction is considered. Then, by the use of the microscopic Hamiltonian of the magnetostatic interaction for a set of stacked rings, the problem of long-range order is discussed. The temperature $T^{*}$ below which the system is in an ordered state is determined.Comment: 14 pages, REVTeX, 5 figures, in print in Phys. Rev.

Animals can assign novel odours to a known category

The ability to identify a novel stimulus as a member of a known category allows an organism torespond appropriately towards it. Categorisation is thus a fundamental component of cognition andan essential tool for processing and responding to unknown stimuli. Therefore, one might expectto observe it throughout the animal kingdom and across sensory domains. There is much evidenceof visual categorisation in non-human animals, but we currently know little about this process inother modalities. In this experiment, we investigated categorisation in the olfactory domain. Dogswere trained to discriminate between 40 odours; the presence or absence of accelerants formed thecategorical rule. Those in the experimental group were rewarded for responding to substrates withaccelerants (either burnt or un-burnt) and inhibit responses to the same substrates (either burnt or unburnt)without accelerants (S+ counterbalanced). The pseudocategory control group was trained onthe same stimuli without the categorical rule. The experimental group learned the discrimination andanimals were able to generalise to novel stimuli from the same category. None of the control animalswere able to learn the discrimination within the maximum number of trials. This study provides the firstevidence that non-human animals can learn to categorise non-biologically relevant odour information

Fermi Surface of the Electron-doped Cuprate Superconductor Nd_{2-x}Ce_xCuO_{4} Probed by High-Field Magnetotransport

We report on the study of the Fermi surface of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_{4}$ by measuring the interlayer magnetoresistance as a function of the strength and orientation of the applied magnetic field. We performed experiments in both steady and pulsed magnetic fields on high-quality single crystals with Ce concentrations of $x=0.13$ to 0.17. In the overdoped regime of $x > 0.15$ we found both semiclassical angle-dependent magnetoresistance oscillations (AMRO) and Shubnikov-de Haas (SdH) oscillations. The combined AMRO and SdH data clearly show that the appearance of fast SdH oscillations in strongly overdoped samples is caused by magnetic breakdown. This observation provides clear evidence for a reconstructed multiply-connected Fermi surface up to the very end of the overdoped regime at $x\simeq 0.17$. The strength of the superlattice potential responsible for the reconstructed Fermi surface is found to decrease with increasing doping level and likely vanishes at the same carrier concentration as superconductivity, suggesting a close relation between translational symmetry breaking and superconducting pairing. A detailed analysis of the high-resolution SdH data allowed us to determine the effective cyclotron mass and Dingle temperature, as well as to estimate the magnetic breakdown field in the overdoped regime.Comment: 23 pages, 8 figure

Importance of Correlation Effects on Magnetic Anisotropy in Fe and Ni

We calculate magnetic anisotropy energy of Fe and Ni by taking into account the effects of strong electronic correlations, spin-orbit coupling, and non-collinearity of intra-atomic magnetization. The LDA+U method is used and its equivalence to dynamical mean-field theory in the static limit is emphasized. Both experimental magnitude of MAE and direction of magnetization are predicted correctly near U=4 eV for Ni and U=3.5 eV for Fe. Correlations modify one-electron spectra which are now in better agreement with experiments.Comment: 4 pages, 2 figure

Hiding Relations

The present vogue of ‘managing for development results’ is an expression of a historically dominant mode of thought in international aid – ‘substantialism’ – which sees the world primarily in terms of ‘entities’ such as ‘poverty’, ‘basic needs’, ‘rights’, ‘women’, or ‘results’. Another important mode of thought, ‘relationalism’ – in association more generally with ideas of process and complexity – appears to be absent in the thinking of aid institutions. Drawing on my own experiences of working with the UK Department for International Development (DFID), I illustrate how despite formally subscribing to the institution’s substantialist view of the world, some staff are ‘closet relationists’, behaving according to one mode of thought while officially framing their action in terms of the other, more orthodox mode. In so doing, they may be unwittingly keeping international aid sufficiently viable - by the apparent proof of the efficacy of results-based management - to enable the institution as a whole to maintain its substantialist imaginary

Cellobiose Dehydrogenase Aryl Diazonium Modified Single Walled Carbon Nanotubes: Enhanced Direct Electron Transfer through a Positively Charged Surface

One of the challenges in the field of biosensors and biofuel cells is to establish a highly efficient electron transfer rate between the active site of redox enzymes and electrodes to fully access the catalytic potential of the biocatalyst and achieve high current densities. We report on very efficient direct electron transfer (DET) between cellobiose dehydrogenase (CDH) from Phanerochaete sordida (PsCDH) and surface modified single walled carbon nanotubes (SWCNT). Sonicated SWCNTs were adsorbed on the top of glassy carbon electrodes and modified with aryl diazonium salts generated in situ from p-aminobenzoic acid and p-phenylenediamine, thus featuring at acidic pH (3.5 and 4.5) negative or positive surface charges. After adsorption of PsCDH, both electrode types showed excellent long-term stability and very efficient DET. The modified electrode presenting p-aminophenyl groups produced a DET current density of 500,mu A cm(-2) at 200 mV vs normal hydrogen reference electrode (NHE) in a 5 mM lactose solution buffered at pH 3.5. This is the highest reported DET value so far using a CDH modified electrode and comes close to electrodes using mediated electron transfer. Moreover, the onset of the electrocatalytic current for lactose oxidation started at 70 mV vs NHE, a potential which is 50 mV lower compared to when unmodified SWCNTs were used. This effect potentially reduces the interference by oxidizable matrix components in biosensors and increases the open circuit potential in biofuel cells. The stability of the electrode was greatly increased compared with unmodified but cross-linked SWCNTs electrodes and lost only 15% of the initial current after 50 h of constant potential scanning