Efficacy of bifenthrin-impregnated bednets against Anopheles funestus and pyrethroid-resistant Anopheles gambiae in North Cameroon

BACKGROUND: Recent field studies indicated that insecticide-treated bednets (ITNs) maintain their efficacy despite a high frequency of the knock-down resistance (kdr) gene in Anopheles gambiae populations. It was essential to evaluate ITNs efficacy in areas with metabolic-based resistance. METHODS: Bifenthrin was used in this experiment because it is considered a promising candidate for bednets impregnation. Nets were treated at 50 mg/m(2), a dose that has high insecticidal activity on kdr mosquitoes and at 5 mg/m(2), a dose that kills 95% of susceptible mosquitoes under laboratory conditions with 3 minutes exposure. Bednets were holed to mimic physical damage. The trial was conducted in three experimental huts from Pitoa, North-Cameroon where Anopheles gambiae displays metabolic resistance and cohabits with An. funestus. RESULTS: Bifenthrin at 50 mg/m(2 )significantly reduced anophelines' entry rate (>80%). This was not observed at 5 mg/m(2). Both treatments increased exophily in An. gambiae, and to a lesser extent in An. funestus. With bifenthrin at high dosage, over 60% reduction in blood feeding and 75–90% mortality rates were observed in both vectors. Despite presence of holes, only a single An. gambiae and two An. funestus females were collected inside the treated net, and all were found dead. The same trends were observed with low dosage bifenthrin though in most cases, no significant difference was found with the untreated control net. CONCLUSION: Bifenthrin-impregnated bednets at 50 mg/m(2 )were efficient in the reduction of human-vector contact in Pitoa. Considerable personal protection was gained against An. funestus and metabolic pyrethroid resistant An. gambiae populations

Magnetic frustration in a stoichiometric spin-chain compound, Ca3_3CoIrO6_6

The temperature dependent ac and dc magnetization and heat capacity data of Ca$_3$CoIrO$_6$, a spin-chain compound crystallizing in a K$_4$CdCl$_6$-derived rhombohedral structure, show the features due to magnetic ordering of a frustrated-type below about 30 K, however without exhibiting the signatures of the so-called "partially disordered antiferromagnetic structure" encountered in the isostructural compounds, Ca$_3$Co$_2$O$_6$ and Ca$_3$CoRhO$_6$. This class of compounds thus provides a variety for probing the consequences of magnetic frustration due to topological reasons in stoichiometric spin-chain materials, presumably arising from subtle differences in the interchain and intrachain magnetic coupling strengths. This compound presents additional interesting situations in the sense that, ac susceptibility exhibits a large frequency dependence in the vicinity of 30 K uncharacteristic of conventional spin-glasses, with this frustrated magnetic state being robust to the application of external magnetic fields.Comment: Physical Review (Rapid Communications), in pres

Magnetic anomalies in the spin chain system, Sr3_3Cu1−x_{1-x}Znx_xIrO6_6

We report the results of ac and dc magnetization (M) and heat-capacity (C) measurements on the solid solution, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$. While the Zn end member is known to form in a rhombohedral pseudo one-dimensional K$_4$CdCl$_6$ structure with an antiferromagnetic ordering temperature of (T$_N$ =) 19 K, the Cu end member has been reported to form in a monoclinically distorted form with a Curie temperature of (T$_C$ =) 19 K. The magnetism of the Zn compound is found to be robust to synthetic conditions and is broadly consistent with the behavior known in the literature. However, we find a lower magnetic ordering temperature (T$_o$) for our Cu compound (~ 13 K), thereby suggesting that T$_o$ is sensitive to synthetic conditions. The Cu sample appears to be in a spin-glass-like state at low temperatures, judged by a frequency dependence of ac magnetic susceptibility and a broadening of the C anomaly at the onset of magnetic ordering, in sharp contrast to earlier proposals. Small applications of magnetic field, however, drive this system to ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x = 0.75 or 0.25) significantly depress magnetic ordering; in other words, T$_o$ varies non-monotonically with x (T$_o$ ~ 6, 3 and 4 K for x = 0.25, 0.5, and 0.67 respectively). The plot of inverse susceptibility versus temperature is non-linear in the paramagnetic state as if correlations within (or among) the magnetic chains continuously vary with temperature. The results establishComment: 7 pages, 7 figures, Revte

On-site magnetization in open antiferromagnetic chains: a classical analysis versus NMR experiments in a spin-1 compound

The response of an open spin chain with isotropic antiferromagnetic interactions to a uniform magnetic field is studied by classical Monte Carlo simulations. It is observed how the induced on-site magnetization is non uniform, due to the occurrence of edge staggered terms which decay exponentially over a distance equal to the zero field correlation length of the infinite chain. The total magnetic moment associated to each staggered term is found to be about half of the original single-spin magnitude and to decrease as the inverse of temperature (i.e. to behave as a Curie-like moment). The numerical results are compared to recent NMR findings in spinless-doped Y(2)BaNiO(5); the remarkable agreement found shows that, for temperatures above the Haldane gap, the classical approach gives a correct picture of the boundary effects observed in the Heisenberg S=1 chain.Comment: 4 pages, 4 eps figures; minor changes in the text; added reference

Anomalous spectral weight in photoemission spectra of the hole doped Haldane chain Y2-xSrxBaNiO5

In this paper, we present photoemission experiments on the hole doped Haldane chain compound $Y_{2-x}Sr_xBaNiO_5$. By using the photon energy dependence of the photoemission cross section, we identified the symmetry of the first ionisation states (d type). Hole doping in this system leads to a significant increase in the spectral weight at the top of the valence band without any change in the vicinity of the Fermi energy. This behavior, not observed in other charge transfer oxides at low doping level, could result from the inhomogeneous character of the doped system and from a Ni 3d-O 2p hybridization enhancement due to the shortening of the relevant Ni-O distance in the localized hole-doped regions.Comment: 5 pages, 4 figure

Direct Evidence for the Localized Single-Triplet Excitations and the Dispersive Multi-Triplets Excitations in SrCu2(BO3)2

We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5 and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplets excitations at 5 meV exhibit a more dispersive behavior.Comment: 12 pages, with 4 figures, to appear in Phys. Rev. Let

Spin-Wave Description of Haldane-gap antiferromagnets

Modifying the conventional antiferromagnetic spin-wave theory which is plagued by the difficulty of the zero-field sublattice magnetizations diverging in one dimension, we describe magnetic properties of Haldane-gap antiferromagnets. The modified spin waves, constituting a grand canonical bosonic ensemble so as to recover the sublattice symmetry, not only depict well the ground-state correlations but also give useful information on the finite-temperature properties.Comment: to be published in J. Phys. Soc. Jpn. Vol. 72, No. 4 (2003

Comparison of S=0 and S=1/2 Impurities in Haldane Chain Compound, Y2BaNiO5Y_{2}BaNiO_{5}

We present the effect of Zn (S=0) and Cu (S=1/2) substitution at the Ni site of S=1 Haldane chain compound $Y_{2}BaNiO_{5}$. $^{89}$Y NMR allows us to measure the local magnetic susceptibility at different distances from the defects. The $^{89}$Y NMR spectrum consists of one central peak and several less intense satellite peaks. The shift of the central peak measures the uniform susceptibility, which displays a Haldane gap $Delta$$equiv$100 K and it corresponds to an AF coupling J$equiv$260 K between the near-neighbor Ni spins. Zn or Cu substitution does not affect the Haldane gap. The satellites, which are evenly distributed on the two sides of the central peak, probe the antiferromagnetic staggered magnetization near the substituted site, which decays exponentially. Its extension is found identical for both impurities and corresponds accurately to the correlation length $xi$(T) determined by Monte Carlo (QMC) simulations for the pure compound. In the case of non-magnetic Zn, the temperature dependence of the induced magnetization is consistent with a Curie law with an "effective" spin S=0.4 on each side of Zn, which is well accounted by Quantum Monte Carlo computations of the spinless-defect-induced magnetism. In the case of magnetic Cu, the similarity of the induced magnetism to the Zn case implies a weak coupling of the Cu spin to the nearest- neighbor Ni spins. The slight reductionin the induced polarization with respect to Zn is reproduced by QMC computations by considering an antiferromagnetic coupling of strength J'=0.1-0.2 J between the S=1/2 Cu-spin and nearest-neighbor Ni-spin.Comment: 15 pages, 18 figures, submitted to Physical Review

S=1/2S=1/2 Chain-Boundary Excitations in the Haldane Phase of 1D S=1S=1 Systems

The $s=1/2$ chain-boundary excitations occurring in the Haldane phaseof $s=1$ antiferromagnetic spin chains are investigated. The bilinear-biquadratic hamiltonian is used to study these excitations as a function of the strength of the biquadratic term, $\beta$, between $-1\le\beta\le1$. At the AKLT point, $\beta=-1/3$, we show explicitly that these excitations are localized at the boundaries of the chain on a length scale equal to the correlation length $\xi=1/\ln 3$, and that the on-site magnetization for the first site is $=2/3$. Applying the density matrixrenormalization group we show that the chain-boundaryexcitations remain localized at the boundaries for $-1\le\beta\le1$. As the two critical points $\beta=\pm1$ are approached the size of the $s=1/2$ objects diverges and their amplitude vanishes.Comment: 4 Pages, 4 eps figures. Uses RevTeX 3.0. Submitted to PR

Structural properties of Pb3Mn7O15 determined from high-resolution synchrotron powder diffraction

We report on the crystallographic structure of the layered compound Pb3Mn7O15. Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic bulk measurements of this system, we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 K and 295 K. Our results show that the crystal structure of stoichiometric Pb3Mn7O15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.Comment: 6 pages, 4 figures, 2 table