Double exchange magnets: Spin-dynamics in the paramagnetic phase

The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between $t_{2g}$-spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron scattering experiments for the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ . The susceptibility shows Curie-Weiss behaviour and is used to derive a phase diagram. In the paramagnetic phase carriers are characterized as ferromagnetic polarons in an antiferromagnetic spin liquid.Comment: Revtex, 4 pages with 5 postscript figures include

A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La2CuO4 and superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wavevector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale of 22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure

Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined spin waves are observed at all energies and wavevectors, allowing us to determine the parameters of the Hamiltonian of the system. The data are found to be in excellent overall agreement with a minimal model that includes a nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term revealed by our study is broadly consistent with the model developed by Onose et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although our ratio of D=J = 0:18(1) is roughly half of their value and three times larger than calculated by ab initio methods [2].Comment: 5 pages, 4 figure

Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, γ\gamma-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

The frustrated magnet clinoatacamite, $\gamma$-Cu$_2$(OH)$_3$Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagom\'e planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagom\'e planes. Recent work pointed out that this exchange model is inappropriate for $\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a $\mu_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure ($T<6$ K) was canted and involved significant spin ordering on all the Cu$^{2+}$ spins, which is incompatible with the interpretation of simultaneous VBS and N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated Magnetism 2008 (HFM2008

Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP

We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.Comment: 7 pages, 5 figure

Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe$_{3}$O$_{4}$) is a mixed valent system where electronic conductivity occurs on the B-site (octahedral) iron sublattice of the spinel structure. Below $T_{V}=122$ K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B-sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B-site sublattice ($\sim$80 meV) are shifted upwards in energy above $T_{V}$ due to the occurrence of B-B ferromagnetic double exchange in the mixed valent metallic phase. The double exchange interaction affects only spin waves of $\Delta_{5}$ symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by electron correlations above $T_{V}$.Comment: 4 pages, 5 figure

Host-Plant Effects on the Behavior of Eretmocerus sp. nr. Californicus Females Raised from Melon

The behaviors of female Eretmocerus sp. nr. californiens raised from Bemisia argentifolii Bellows & Perring on melon, Cucumis melo L., were analysed on four different host plant species (cotton, melon, sweet potato and Abutilon theophrasti Medic). Comparison with previously published results of similarly treated females reared from sweet potato plants showed performance related differences leading to oviposition. In the present study, the generalized behavioral pathway (walking, host- encounter, antennation, probing and oviposition) did not vary among host plant species for melon- reared parasitoid females. Host assessment by antennation leading to host acceptance for probing varied from 10.5- 12.3 sec among the host plant species with no host stage- related differences in duration. Initial and repeated (multiple probes on the same host nymph) probing events varied from 52.4 to 77.3 sec among host plant species and no stage- related differences were detected. Oviposition occurred under 62 % of the nymphs on cotton, 100 % of the nymphs on melon and 51 % of the nymphs on sweet potato under which the ovipositor was exserted. On cotton, melon and sweet potato proportionally fewer first instar nymphs were encountered than present, a larger proportion of the second instars that were encountered were subsequently antennated, and a larger proportion of second instars on cotton were then subsequently probed. For cotton, melon, and sweet potato, the overall frequency of oviposition was lower than reported in previous studies. Results from these studies showed that the greatest factor in influencing overall parasitism in laboratory experiments was the propensity of females to alight and remain to search for hosts on a particular host plant. The role of “pre- conditioning” parasitoids to be better- suited to a particular host or host plant species is discussed relative to the findings herein

Behavior of Female Eretmocerus sp. nr. Californicus Howard (Aphelinidae: Hymenoptera) Attacking Bemisa Argentifolii Bellows and Perring (Aleyrodidae: Homoptera) on Two Native Californian Weeds

Searching and ovipositional behaviors by female Eretmocems sp. nr. californicus Howard on Bemisia argentifolii Bellows and Perring infesting velvetleaf, Abutilon theophrasti Medic. (Malavaceae), and telegraph weed, Heterotheca grandiflora Nutt. (Asteraceae), were quantified. Adult female behaviors were described and quantified for E. sp. nr. californicus to establish a behavioral time budget analysis. Females departed from leaves of A. theophrasti in 83.3% of the trials, and of those that remained and searched for hosts, walking speeds averaged 0.29 mm/s. Females departed from the leaves of H. grandiflora in 44.4% of the trials, and those remaining readily searched for whitefly hosts with walking speeds averaging 0.26 mm/s. The duration of host assessment by antennation was related to subsequent behaviors; rejecting a host was a shorter process than accepting it for further evaluation irrespective of plant species or nymphal stage. Evidence for a behavioral preference for oviposition under early nymphal instars was documented for female E. sp. nr. californicus on both plant species. Oviposition efficiency in 1-h laboratory trials for nymphs on A. theophrasti was 30% and efficiency on H. grandiflora was 23%. Females spent 61.9 and 53.3% of their total time in searching, host assessment, probing, and oviposition while on A. theophrasti and H. grandiflora, respectively. The remainder of the time was spent grooming, resting, and host feeding, except host feeding on A. theophrasti was not observed. Implications of this study for silverleaf whitefly management through conservation and augmentation of native flora and fauna are discussed

Pseudogap Formation in Models for Manganites

The density-of-states (DOS) and one-particle spectral function $\rm A({\bf k}, \omega)$ of the one- and two-orbital models for manganites, the latter with Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly robust pseudogap (PG) features were found at low- and intermediate-temperatures, particularly at or near regimes where phase-separation occurs as $\rm T$$\to$0. The PG follows the chemical potential and it is caused by the formation of ferromagnetic metallic clusters in an insulating background. It is argued that PG formation should be generic of mixed-phase regimes. The results are in good agreement with recent photoemission experiments for $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4 figures embedde

Spin pseudogap in Ni-doped SrCuO2

The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni-impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap \Delta ~ 8 meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the exprimental data measured in the temperature range 2-300 K, and allows to represent the momentum-integrated dynamic structure factor in a universal scaling form.Comment: 5 pages, 3 figure