1,762 research outputs found
The Dimer Model for k-phase Organic Superconductors
We prove that the upper electronic bands of k-phase BEDT-TTF salts are
adequately modeled by an half-filled tight-binding lattice with one site per
cell. The band parameters are derived from recent ab-initio calculations,
getting a very simple but extremely accurate one-electron picture. This picture
allows us to solve the BCS gap equation adopting a real-space pairing
potential. Comparison of the calculated superconducting properties with the
experimental data points to isotropic s_0-pairing. Residual many-body or
phonon-mediated interactions offer a plausible explanation of the large variety
of physical properties observed in k-phase BEDT-TTF salts.Comment: 8 pages, 6 PostScript figures, uses RevTe
Two-color holography concept (T-CHI)
The Material Processing in the Space Program of NASA-MSFC was active in developing numerous optical techniques for the characterization of fluids in the vicinity of various materials during crystallization and/or solidification. Two-color holographic interferometry demonstrates that temperature and concentration separation in transparent (T-CHI) model systems is possible. The experiments were performed for particular (succinonitrile) systems. Several solutions are possible in Microgravity Sciences and Applications (MSA) experiments on future Shuttle missions. The theory of the T-CHI concept is evaluated. Although particular cases are used for explanations, the concepts developed will be universal. A breadboard system design is also presented for ultimate fabrication and testing of theoretical findings. New developments in holography involving optical fibers and diode lasers are also incorporated
Identifying aphid resistance in the ancestral wheat Triticum monococcum under field conditions.
Wheat is an economically, socially, and nutritionally important crop, however, aphid infestation can often reduce wheat yield through feeding and virus transmission. Through field phenotyping, we investigated aphid resistance in ancestral wheat Triticum monococcum (L.). Aphid (Rhopalosiphum padi (L.), Sitobion avenae (F.) and Metopolophium dirhodum (Wlk.)) populations and natural enemy presence (parasitised mummified aphids, ladybird adults and larvae and lacewing eggs and larvae) on two naturally susceptible wheat varieties, Triticum aestivum (L.) var. Solstice and T. monococcum MDR037, and three potentially resistant genotypes T. monococcum MDR657, MDR045 and MDR049 were monitored across three years of field trials. Triticum monococcum MDR045 and MDR049 had smaller aphid populations, whereas MDR657 showed no resistance. Overall, natural enemy presence was positively correlated with aphid populations; however, MDR049 had similar natural enemy presence to MDR037 which is susceptible to aphid infestation. It is hypothesised that alongside reducing aphid population growth, MDR049 also confers indirect resistance by attracting natural enemies. The observed resistance to aphids in MDR045 and MDR049 has strong potential for introgression into commercial wheat varieties, which could have an important role in Integrated Pest Management strategies to reduce aphid populations and virus transmission
A statistical model for the intrinsically broad superconducting to normal transition in quasi-two-dimensional crystalline organic metals
Although quasi-two-dimensional organic superconductors such as
-(BEDT-TTF)Cu(NCS) seem to be very clean systems, with apparent
quasiparticle mean-free paths of several thousand \AA, the superconducting
transition is intrinsically broad (e.g K wide for K).
We propose that this is due to the extreme anisotropy of these materials, which
greatly exacerbates the statistical effects of spatial variations in the
potential experienced by the quasiparticles. Using a statistical model, we are
able to account for the experimental observations. A parameter , which
characterises the spatial potential variations, may be derived from
Shubnikov-de Haas oscillation experiments. Using this value, we are able to
predict a transition width which is in good agreement with that observed in MHz
penetration-depth measurements on the same sample.Comment: 8 pages, 2 figures, submitted to J. Phys. Condens. Matte
Biosynthesis of natural and novel C-glycosylflavones utilising recombinant Oryza sativa C-glycosyltransferase (OsCGT) and Desmodium incanum root proteins
The rice C-glycosyltransferase (OsCGT) is one of only a small number of characterised plant C-glycosyltransferases (CGT) known. The enzyme C-glucosylates a 2-hydroxyflavanone substrate with UDP-glucose as the sugar donor to produce C-glucosyl-2-hydroxyflavanones. We tested substrate specificity of the enzyme, using synthetic 2-hydroxyflavanones, and showed it has the potential to generate known natural CGFs that have been isolated from rice and also other plants. In addition, we synthesised novel, unnatural 2-hydroxyflavanone substrates to test the B-ring chemical space of substrate accepted by the OsCGT and demonstrated the OsCGT capacity as a synthetic reagent to generate significant quantities of known and novel CGFs. Many B-ring analogues are tolerated within a confined steric limit. Finally the OsCGT was used to generate novel mono-C-glucosyl-2-hydroxyflavanones as putative biosynthetic intermediates to examine the potential of Desmodium incanum biosynthetic CGTs to produce novel di-C-glycosylflavones, compounds implicated in the allelopathic biological activity of Desmodium against parasitic weeds from the Striga genus
Popular attitudes to memory, the body, and social identity : the rise of external commemoration in Britain, Ireland, and New England
A comparative analysis of samples of external memorials from burial grounds in Britain, Ireland and New England reveals a widespread pattern of change in monument style and content, and exponential growth in the number of permanent memorials from the 18th century onwards. Although manifested in regionally distinctive styles on which most academic attention has so far been directed, the expansion reflects global changes in social relationships and concepts of memory and the body. An archaeological perspective reveals the importance of external memorials in articulating these changing attitudes in a world of increasing material consumption
Competition between Charge Ordering and Superconductivity in Layered Organic Conductors -(BEDT-TTF)Hg(SCN) (M = K, NH)
While the optical properties of the superconducting salt
-(BEDT-TTF)NHHg(SCN) remain metallic down to 2 K, in the
non-superconducting K-analog a pseudogap develops at frequencies of about 200
cm for temperatures T < 200 K. Based on exact diagonalisation
calculations on an extended Hubbard model at quarter-filling we argue that
fluctuations associated with short range charge ordering are responsible for
the observed low-frequency feature. The different ground states, including
superconductivity, are a consequence of the proximity of these compounds to a
quantum phase charge-ordering transition driven by the intermolecular Coulomb
repulsion.Comment: 4 pages, 3 figure
Heat Conduction in -(BEDT-TTF)Cu(NCS)
The first study of thermal conductivity, , in a quasi-two-dimensional
organic superconductor of the -(BEDT-TTF)X family reveals features
analogous to those already observed in the cuprates. The onset of
superconductivity is associated with a sudden increase in which can be
suppressed by the application of a moderate magnetic field. At low
temperatures, a finite linear term - due to a residual electronic contribution-
was resolved. The magnitude of this term is close to what is predicted by the
theory of transport in unconventional superconductors.Comment: 5 pages, 4 figures include
Pairing due to Spin Fluctuations in Layered Organic Superconductors
I show that for a \kappa-type organic (BEDT-TTF)_2-X molecular crystal, a
superconducting state with T_c ~ 10 K and gap nodes on the Fermi surface can be
caused by short-ranged antiferromagnetic spin fluctuations. Using a two-band
description for the anti-bonding orbitals on a BEDT-TTF dimer of the
\kappa-type salt, and an intermediate local Coulomb repulsion between two holes
on one dimer, the magnetic interaction and the superconducting gap-function are
determined self consistently within the fluctuation exchange approximation. The
pairing interaction is predominantly caused by inter-band coupling and
additionally affected by spin excitations of the quasi one-dimensional band.Comment: 4 pages, 3 figure
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