2,874 research outputs found
Existence of a Thermodynamic Spin-Glass Phase in the Zero-Concentration Limit of Anisotropic Dipolar Systems
The nature of ordering in dilute dipolar interacting systems dates back to
the work of Debye and is one of the most basic, oldest and as-of-yet unsettled
problems in magnetism. While spin-glass order is readily observed in several
RKKY-interacting systems, dipolar spin-glasses are subject of controversy and
ongoing scrutiny, e.g., in , a rare-earth randomly
diluted uniaxial (Ising) dipolar system. In particular, it is unclear if the
spin-glass phase in these paradigmatic materials persists in the limit of zero
concentration or not. We study an effective model of
using large-scale Monte Carlo simulations that combine parallel tempering with
a special cluster algorithm tailored to overcome the numerical difficulties
that occur at extreme dilutions. We find a paramagnetic to spin-glass phase
transition for all Ho ion concentrations down to the smallest concentration
numerically accessible of 0.1%, and including Ho ion concentrations which
coincide with those studied experimentally up to 16.7%. Our results suggest
that randomly-diluted dipolar Ising systems have a spin-glass phase in the
limit of vanishing dipole concentration, with a critical temperature vanishing
linearly with concentration, in agreement with mean field theory.Comment: 6 pages, 3 figures, 2 table
Xanthomonas campestris pv. campestris race 1 is the main causal agent of black rot of Brassicas in Southern Mozambique
Severe outbreaks of bacterial black rot caused by Xanthomonas campestris pv. campestris (Xcc) were observed in Brassica production fields of Southern Mozambique. The causal agent of the disease in the Mahotas and ChĂČkwĂ© districts was identified and characterised. In total, 83 Xanthomonas-like strains were isolated from seed samples and leaves of cabbage and tronchuda cole with typical symptoms of the disease. Forty-six out of the 83 strains were found to be putative Xcc in at least one of the tests used: Classical biochemical assays, enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies, Biolog identification system, polymerase chain reaction (PCR) with specific primers and pathogenicity tests. The ELISA tests were positive for 43 strains. Biolog identified 43 strains as Xanthomonas, but only 32 as Xcc. PCR tests with primers targeting a fragment of the hrpF gene were positive for all 46 strains tested. Three strains were not pathogenic or weakly pathogenic and all other strains caused typical black rot symptoms in brassicas. Race type differentiation tests revealed the Xcc strains from Mozambique as members of race 1. The prevalence of this pathogenic race of the Xcc pathogen in Mozambique should be considered when black rot resistant cultivars are evaluated or introduced into the production regions of this country
Novel disordering mechanism in ferromagnetic systems with competing interactions
Ferromagnetic Ising systems with competing interactions are considered in the
presence of a random field. We find that in three space dimensions the
ferromagnetic phase is disordered by a random field which is considerably
smaller than the typical interaction strength between the spins. This is the
result of a novel disordering mechanism triggered by an underlying spin-glass
phase. Calculations for the specific case of the long-range dipolar
LiHo_xY_{1-x}F_4 compound suggest that the above mechanism is responsible for
the peculiar dependence of the critical temperature on the strength of the
random field and the broadening of the susceptibility peaks as temperature is
decreased, as found in recent experiments by Silevitch et al. [Nature (London)
448, 567 (2007)]. Our results thus emphasize the need to go beyond the standard
Imry-Ma argument when studying general random-field systems.Comment: 4+2 pages, 3 figure
Antihydrogen studies in ALPHA
he ALPHA experiment studies antihydrogen as a means to investigate the symmetry of matter and antimatter. Spectroscopic studies of the anti-atom hold the promise of the most precise direct comparisons of matter and antimatter possible. ALPHA was the first to trap antihydrogen in a magnetic trap, allowing the first ever detection of atomic transitions in an anti-atom. More recently, through stochastic heating, we have also been able to put a new limit on the charge neutrality of antihydrogen. ALPHA is currently preparing to perform the first laser-spectroscopy of antihydrogen, hoping to excite the 2s state using a two-photon transition from the 1s state. We discuss the recent results as well as the key developments that led to these successes and discuss how we are preparing to perform the first laser-spectroscopy. We will also discuss plans to use our novel technique for gravitational tests on antihydrogen for a direct measurement of the sign of the gravitational force on antihydrogen
Evaluation of the therapeutic potential of ant-TLR4-antibody MTS510 in experimental stroke and significa of different routes of application
Toll-like receptors (TLRs) are central sensors for the inflammatory response in ischemia-reperfusion injury. We therefore investigated whether TLR4 inhibition could be used to treat stroke in a standard model of focal cerebral ischemia. Anti-TLR4/MD2-antibody (mAb clone MTS510) blocked TLR4-induced cell activation in vitro, as reported previously. Here, different routes of MTS510 application in vivo were used to study the effects on stroke outcome up to 2d after occlusion of the middle cerebral artery (MCAO) for 45 min in adult male C57Bl/6 wild-type mice. Improved neurological performance, reduced infarct volumes, and reduced brain swelling showed that intravascular application of MTS510 had a protective effect in the model of 45 min MCAO. Evaluation of potential long-term adverse effects of anti-TLR4-mAb-treament revealed no significant deleterious effect on infarct volumes nor neurological deficit after 14d of reperfusion in a mild model of stroke (15 min MCAO). Interestingly, inhibition of TLR4 resulted in an altered adaptive immune response at 48 hours after reperfusion. We conclude that blocking TLR4 by the use of specific mAb is a promising strategy for stroke therapy. However, long-term studies with increased functional sensitivity, larger sampling sizes and use of other species are required before a clinical use could be envisaged
Comparative genomics and mutagenesis analyses of choline metabolism in the marine Roseobacter clade
Choline is ubiquitous in marine eukaryotes and appears to be widely distributed in surface marine waters; however, its metabolism by marine bacteria is poorly understood. Here, using comparative genomics and molecular genetic approaches, we reveal that the capacity for choline catabolism is widespread in marine heterotrophs of the marine Roseobacter clade (MRC). Using the model bacterium Ruegeria pomeroyi, we confirm that the betA, betB and betC genes, encoding choline dehydrogenase, betaine aldehyde dehydrogenase and choline sulfatase, respectively, are involved in choline metabolism. The betT gene, encoding an organic solute transporter, was essential for the rapid uptake of choline but not glycine betaine (GBT). Growth of choline and GBT as a sole carbon source resulted in the re-mineralization of these nitrogen-rich compounds into ammonium. Oxidation of the methyl groups from choline requires formyltetrahydrofolate synthetase encoded by fhs in R.pomeroyi, deletion of which resulted in incomplete degradation of GBT. We demonstrate that this was due to an imbalance in the supply of reducing equivalents required for choline catabolism, which can be alleviated by the addition of formate. Together, our results demonstrate that choline metabolism is ubiquitous in the MRC and reveal the role of Fhs in methyl group oxidation in R.pomeroyi
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