214 research outputs found
Frustrated multiband superconductivity
We show that a clean multiband superconductor may display one or several
phase transitions with increasing temperature from or to frustrated
configurations of the relative phases of the superconducting order parameters.
These transitions may occur when more than two bands are involved in the
formation of the superconducting phase and when the number of repulsive
interband interactions is odd. These transitions are signalled by slope changes
in the temperature dependence of the superconducting gaps.Comment: 5 pages, 3 figure
Classification of an ocimum germplasm collection (NCRPIS, AMES) and investigation of antifungal activity
The genus Ocimum belongs to the family Labiatae and is distributed worldwide, growing in the warmer parts of both hemispheres. It is an important genus with many medicinal, veterinary, pesticidal and culinary uses. The taxonomic classification of Ocimum is difficult and confusing. Accordingly, a joint effort began in 1999 between the PI Station at Ames, Iowa and SAC (Scottish Agricultural College) Auchincruive, Ayr to classify the Ocimum germplasm collection using classical morphological comparison coupled with chemotaxonomy (GC and GC-MS). Overall, fifty species of O. basilicum, thirteen O. americanum, seven O. tenuiflorum, three O. gratissimum and a single accession of O. selloi were characterised. Novel chemical profiles were discovered in the collection including O. americanum species containing up to 71% fenchone, 47% camphor and 30% limonene and a caryophyllene chemotype of O. tenuiflorum. Considering components ≥5%, limonene was restricted to accessions of O. americanum. Also, on examination of components ≥5%, several appeared to be species specific within this collection. High oil yields (up to 5%) were found in O. americanum species from Zambia. These yields are exceptional for plants of the genus Ocimum. Plants of O. basilicum were found to differ in phenotype while producing a similar chemical profile. Plants grown in both Iowa and the west of Scotland produced oil of almost identical quality and quantity. In contrast, O. americanum plants were phenotypically alike but produced an array of essential oil profiles. In addition, variation between essential oils from leaves and flowers of O. tenuiflorum was also found. Antifungal testing of selected O. basilicum oils and individual components demonstrated activity in vitro with concentrations as low as 2ppm, with methyl chavicol and linalol giving the largest reductions in fungal growth in vitro. In glasshouse experiments, good control of Botrytis fabae and Uromyces viciae-fabae infection of broad bean was also achieved with foliar application of selected Ocimum essential oils and individual chemical components. O. basilicum essential oils and individual components on polyamine biosynthesis, catabolism and excretion were investigated. As an initial approach to determining the mode of action of the essential oil of Ocimum basilicum, it was decided to study effects on fungal polyamine metabolism. Although test compounds did not significantly deplete intracellular levels of the major polyamines spermine, spermidine, putrecine and cadaverine, definite effects were seen on the activity of key biosynthetic and catabolic enzymes. This work provides, for the first time, a detailed chemotype analysis of the genus Ocimum and shows that the whole essential oil and some of its individual components possess fungicidal activity. With increasing interest in the use of plant essential oils as crop protection agents, the essential oil of Ocimum may provide a useful additional means of controlling plant pathogens
Magnetic impurities in the two-band -wave superconductors
We investigate the effects of magnetic impurities in a superconducting state
with pairing symmetry. Within a two-band model, we find that the
intra-band magnetic scattering serves as a pair breaker while the inter-band
magnetic scattering preserves pairing and hardly affects transition temperature
in the Born limit. We also show that the same physics can persist beyond the
weak scattering region. Our results coincide with recent experimental
measurements in iron-based superconductors and thus provides an indirect
evidence of the possible pairing symmetry in these materials.Comment: 5 pages,4 figure
Effect of Impurities with Internal Structure on Multiband Superconductors - Possible Enhancement of Transition Temperature -
We study inelastic (dynamical) impurity scattering effects in two-band
superconductors with the same ( wave) or different ( wave) sign
order parameters. We focus on the enhancement of the superconducting transition
temperature by magnetic interband scattering with the interchange
of crystal-field singlet ground and multiplet excited states. Either the
-wave or -wave state is favored by the impurity-mediated
pairing, which depends on the magnetic and nonmagnetic scattering strengths
derived from the hybridization of the impurity states with the conduction
bands. The details are examined for the singlet-triplet configuration that is
suggestive of Pr impurities in the skutterudite superconductor
LaOsSb.Comment: 14 pages, 5 figures, to appear in J. Phys. Soc. Jpn. Vol. 79, No. 9
(2010
Toward Confined Carbyne with Tailored Properties
Confining carbyne to a space that allows for stability and controlled reactivity is a very appealing approach to have access to materials with tunable optical and electronic properties without rival. Here, we show how controlling the diameter of single-walled carbon nanotubes opens the possibility to grow a confined carbyne with a defined and tunable band gap. The metallicity of the tubes has a minimal influence on the formation of the carbyne, whereas the diameter plays a major role in the growth. It has been found that the properties of confined carbyne can be tailored independently from its length and how these are mostly determined by its interaction with the carbon nanotube. Molecular dynamics simulations have been performed to interpret these findings. Furthermore, the choice of a single-walled carbon nanotube host has been proven crucial even to synthesize an enriched carbyne with the smallest energy gap currently reported and with remarkable homogeneity
Single Impurity Effects in Multiband Superconductors with Different Sign Order Parameters
A single impurity problem is investigated for multiband s-wave
superconductors with different sign order parameters (+-s-wave superconductors)
suggested in Fe-pnictide superconductors. Not only intraband but also interband
scattering is considered at the impurity. The latter gives rise to
impurity-induced local boundstates close to the impurity. We present an exact
form of the energy of the local boundstates as a function of strength of the
two types of impurity scattering. The essential role of the impurity is
unchanged in finite number of impurities. The main conclusions for a single
impurity problem help us understand effects of dense impurities in the +-s-wave
superconductors. Local density of states around the single impurity is also
investigated. We suggest impurity site nuclear magnetic resonance as a suitable
experiment to probe the local boundstates that is peculiar to the +-s-wave
state. We find that the +-s-wave model is mapped to a chiral dx2-y2+-idxy-wave,
reflecting the unconventional nature of the sign reversing order parameter. For
a quantum magnetic impurity, interband scattering destabilizes the Kondo
singlet.Comment: 23 pages, 7 figures, to be published in J. Phys. Soc. Jpn. (2009) No.
Normal State Spin Dynamics of Five-band Model for Iron-pnictides
Normal state spin dynamics of the recently discovered iron-pnictide
superconductors is discussed by calculating spin structure factor S(q, omega)
in an itinerant five-band model within RPA approximation. Due to the
characteristic Fermi surface structure of iron-pnictide, column like response
is found at (pi, 0) in extended Brillouin zone in the undoped case, which is
consistent with the recent neutron scattering experiment. This indicates that
the localized spin model is not necessary to explain the spin dynamics of this
system. Furthermore, we show that the temperature dependence of inelastic
neutron scattering intensity can be well reproduced in the itinerant model. We
also study NMR 1/T_1T in the same footing calculation and show that the
itinerant model can capture the magnetic property of iron-pnictide
superconductors.Comment: 4 page
Distinct transport behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the superconducting and nonsuperconducting metallic y regions divided by y ~ 0.05
Electrical resistivities, Hall coefficients and thermoelectric powers have
been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with
various values of y. The results show that there exists clear distinction of
these transport behaviors between the superconducting and nonsuperconducting
metallic regions of y divided by the boundary value yc~0.05. We have found that
the behaviors in both regions are very similar to those of high-Tc Cu oxides in
the corresponding phases. If they reflect, as in the case of Cu oxides, effects
of strong magnetic fluctuations, the energy scale of the fluctuations is
considered to be smaller than that of the high Cu oxides by a factor of ~1/2.
Arguments on the electronic nature and superconducting symmetry are presented
on the basis of the observed small rate of the Tc suppression rate by the Co
doping.Comment: 8 pages, 4 figures, submitted to J. Phys. Soc. Jp
Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors
The sign-reversing fully gapped superconducting state, which is expected to
be realized in oxypnictide superconductors, can be prominently affected by
nonmagnetic impurities due to the interband scattering of Cooper pairs. We
study this problem based on the isotropic two-band BCS model: In oxypnictide
superconductors, the interband impurity scattering is not equal to the
intraband one . In the Born scattering regime, the reduction in Tc is
sizable and the impurity-induced density of states (DOS) is prominent if , due to the interband scattering. Although impurity-induced DOS can yield a
power-law temperature dependence in , a sizable suppression in Tc is
inevitably accompanied. In the unitary scattering regime, in contrast, impurity
effect is very small for both Tc and DOS except at . By comparing theory
and experiments, we expect that the degree of anisotropy in the -wave
gap function strongly depends on compounds.Comment: 16 pages, 5 figures, to be published in New. J. Phy
How Mistimed and Unwanted Pregnancies Affect Timing of Antenatal Care Initiation in three Districts in Tanzania
Early antenatal care (ANC) initiation is a doorway to early detection and management of potential complications associated with pregnancy. Although the literature reports various factors associated with ANC initiation such as parity and age, pregnancy intentions is yet to be recognized as a possible predictor of timing of ANC initiation. Data originate from a cross-sectional household survey on health behaviour and service utilization patterns. The survey was conducted in 2011 in Rufiji, Kilombero and Ulanga districts in Tanzania on 910 women of reproductive age who had given birth in the past two years. ANC initiation was considered to be early only if it occurred in the first trimester of pregnancy gestation. A recently completed pregnancy was defined as mistimed if a woman wanted it later, and if she did not want it at all the pregnancy was termed as unwanted. Chisquare was used to test for associations and multinomial logistic regression was conducted to examine how mistimed and unwanted pregnancies affect timing of ANC initiation. Although 49.3% of the women intended to become pregnant, 50.7% (34.9% mistimed and 15.8% unwanted) became pregnant unintentionally. While ANC initiation in the 1st trimester was 18.5%, so was 71.7% and 9.9% in the 2nd and 3rd trimesters respectively. Multivariate analysis revealed that ANC initiation in the 2nd trimester was 1.68 (95% CI 1.10‒2.58) and 2.00 (95% CI 1.05‒3.82) times more likely for mistimed and unwanted pregnancies respectively compared to intended pregnancies. These estimates rose to 2.81 (95% CI 1.41‒5.59) and 4.10 (95% CI 1.68‒10.00) respectively in the 3rd trimester. We controlled for gravidity, age, education, household wealth, marital status, religion, district of residence and travel time to a health facility. Late ANC initiation is a significant maternal and child health consequence of mistimed and unwanted pregnancies in Tanzania. Women should be empowered to delay or avoid pregnancies whenever they need to do so. Appropriate counseling to women, especially those who happen to conceive unintentionally is needed to minimize the possibility of delaying ANC initiation.\u
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