72 research outputs found
Quasiparticle-like peaks, kinks, and electron-phonon coupling at the (,0) regions in the CMR oxide LaSrMnO
Using Angle-Resolved Photoemission (ARPES), we present the first observation
of sharp quasiparticle-like peaks in a CMR manganite. We focus on the (,0)
regions of k-space and study their electronic scattering rates and dispersion
kinks, uncovering the critical energy scales, momentum scales, and strengths of
the interactions that renormalize the electrons. To identify these bosons we
measured phonon dispersions in the energy range of the kink by inelastic
neutron scattering (INS), finding a good match in both energy and momentum to
the oxygen bond-stretching phonons
Quasiparticlelike peaks, kinks, and electron phonon coupling at the pi,0 regions in the CMR oxide La2 2xSr1 2xMn2O7
Using Angle Resolved Photoemission ARPES , we present the first observation of sharp quasiparticle like peaks in a CMR manganite. We focus on the pi,0 regions of k space and study their electronic scattering rates and dispersion kinks, uncovering the critical energy scales, momentum scales, and strengths of the interactions that renormalize the electrons. To identify these bosons we measured phonon dispersions in the energy range of the kink by inelastic neutron scattering INS , finding a good match in both energy and momentum to the oxygen bond stretching phonon
New features in the phase diagram of TbMnO
The (H,T)-phase diagram of the multiferroic perovskite TbMnO was studied
by high-resolution thermal expansion and magnetostriction measurements. Below K, TbMnO shows
antiferromagnetic order, which changes at K where
simultaneously a spontaneous polarization develops. Sufficiently high
magnetic fields applied along or induce a polarization flop to .
We find that all of these transitions are strongly coupled to the lattice
parameters. Thus, our data allow for a precise determination of the phase
boundaries and also yield information about their uniaxial pressure
dependencies. The strongly hysteretic phase boundary to the ferroelectric phase
with is derived in detail. Contrary to previous reports, we find that
even in high magnetic fields there are no direct transitions from this phase to
the paraelectric phase. We also determine the various phase boundaries in the
low-temperature region related to complex reordering transitions of the Tb
moments.Comment: 17 pages including 9 figure
Terahertz time-domain spectroscopy of electromagnons in multiferroic perovskite manganites
Recent spectroscopic studies at terahertz frequencies for a variety of
multiferroics endowed with both ferroelectric and magnetic orders have revealed
the possible emergence of a new collective excitation, frequently referred to
as electromagnon. It is magnetic origin, but becomes active in response to the
electric field component of light. Here we give an overview on our recent
advance in the terahertz time-domain spectroscopy of electromagnons or
electric-dipole active magnetic resonances, focused on perovskite
manganites--MnO ( denotes rare-earth ions). The respective electric
and magnetic contributions to the observed magnetic resonance are firmly
identified by the measurements of the light-polarization dependence using a
complete set of the crystal orientations. We extract general optical features
in a variety of the spin ordered phases, including the -type
antiferromagnetic, collinear spin ordered, and ferroelectric and
spiral spin ordered phases, which are realized by tuning the chemical
composition of , temperature, and external magnetic field. In addition to
the antiferromagnetic resonances of Mn ions driven by the magnetic field
component of light, we clarify that the electromagnon appears only for light
polarized along the a-axis even in the collinear spin ordered phase and grows
in intensity with evolution of the spiral spin order, but independent of the
direction of the spiral spin plane ( or ) or equivalently the direction
of the ferroelectric polarization ( or ). A possible origin of the observed magnetic resonances at terahertz
frequencies is discussed by comparing the systematic experimental data
presented here with theoretical considerations based on Heisenberg model.Comment: 19 pages including 15 figures and 2 tables; Invited Paper in Special
Issue on Terahertz Wave Photonics in J. Opt. Soc. Am. B (Accepted for
publication
Field realistic doses of pesticide imidacloprid reduce bumblebee pollen foraging efficiency
Bumblebees and other pollinators provide a vital ecosystem service for the agricultural sector. Recent studies however have suggested that exposure to systemic neonicotinoid insecticides in flowering crops has sub-lethal effects on the bumblebee workforce, and hence in reducing queen production. The mechanism behind reduced nest performance, however, remains unclear. Here we use Radio Frequency Identification (RFID) technology to test whether exposure to a low, field realistic dose (0.7 ppb in sugar water and 6 ppb in pollen) of the neonicotinoid imidacloprid, reduces worker foraging efficiency. Whilst the nectar foraging efficiency of bees treated with imidacloprid was not significantly different than that of control bees, treated bees brought back pollen less often than control bees (40 % of trips vs 63 % trips, respectively) and, where pollen was collected, treated bees brought back 31 % less pollen per hour than controls. This study demonstrates that field-realistic doses of these pesticides substantially impacts on foraging ability of bumblebee workers when collecting pollen, and we suggest that this provides a causal mechanism behind reduced queen production in imidacloprid exposed colonies
Exposure to Sublethal Doses of Fipronil and Thiacloprid Highly Increases Mortality of Honeybees Previously Infected by Nosema ceranae
International audienceBACKGROUND: The honeybee, Apis mellifera, is undergoing a worldwide decline whose origin is still in debate. Studies performed for twenty years suggest that this decline may involve both infectious diseases and exposure to pesticides. Joint action of pathogens and chemicals are known to threaten several organisms but the combined effects of these stressors were poorly investigated in honeybees. Our study was designed to explore the effect of Nosema ceranae infection on honeybee sensitivity to sublethal doses of the insecticides fipronil and thiacloprid. METHODOLOGY/FINDING: Five days after their emergence, honeybees were divided in 6 experimental groups: (i) uninfected controls, (ii) infected with N. ceranae, (iii) uninfected and exposed to fipronil, (iv) uninfected and exposed to thiacloprid, (v) infected with N. ceranae and exposed 10 days post-infection (p.i.) to fipronil, and (vi) infected with N. ceranae and exposed 10 days p.i. to thiacloprid. Honeybee mortality and insecticide consumption were analyzed daily and the intestinal spore content was evaluated 20 days after infection. A significant increase in honeybee mortality was observed when N. ceranae-infected honeybees were exposed to sublethal doses of insecticides. Surprisingly, exposures to fipronil and thiacloprid had opposite effects on microsporidian spore production. Analysis of the honeybee detoxification system 10 days p.i. showed that N. ceranae infection induced an increase in glutathione-S-transferase activity in midgut and fat body but not in 7-ethoxycoumarin-O-deethylase activity. CONCLUSIONS/SIGNIFICANCE: After exposure to sublethal doses of fipronil or thiacloprid a higher mortality was observed in N. ceranae-infected honeybees than in uninfected ones. The synergistic effect of N. ceranae and insecticide on honeybee mortality, however, did not appear strongly linked to a decrease of the insect detoxification system. These data support the hypothesis that the combination of the increasing prevalence of N. ceranae with high pesticide content in beehives may contribute to colony depopulation
Imidacloprid-Induced Impairment of Mushroom Bodies and Behavior of the Native Stingless Bee Melipona quadrifasciata anthidioides
Declines in pollinator colonies represent a worldwide concern. The widespread use of agricultural pesticides is recognized as a potential cause of these declines. Previous studies have examined the effects of neonicotinoid insecticides such as imidacloprid on pollinator colonies, but these investigations have mainly focused on adult honey bees. Native stingless bees (Hymenoptera: Apidae: Meliponinae) are key pollinators in neotropical areas and are threatened with extinction due to deforestation and pesticide use. Few studies have directly investigated the effects of pesticides on these pollinators. Furthermore, the existing impact studies did not address the issue of larval ingestion of contaminated pollen and nectar, which could potentially have dire consequences for the colony. Here, we assessed the effects of imidacloprid ingestion by stingless bee larvae on their survival, development, neuromorphology and adult walking behavior. Increasing doses of imidacloprid were added to the diet provided to individual worker larvae of the stingless bee Melipona quadrifasciata anthidioides throughout their development. Survival rates above 50% were only observed at insecticide doses lower than 0.0056 µg active ingredient (a.i.)/bee. No sublethal effect on body mass or developmental time was observed in the surviving insects, but the pesticide treatment negatively affected the development of mushroom bodies in the brain and impaired the walking behavior of newly emerged adult workers. Therefore, stingless bee larvae are particularly susceptible to imidacloprid, as it caused both high mortality and sublethal effects that impaired brain development and compromised mobility at the young adult stage. These findings demonstrate the lethal effects of imidacloprid on native stingless bees and provide evidence of novel serious sublethal effects that may compromise colony survival. The ecological and economic importance of neotropical stingless bees as pollinators, their susceptibility to insecticides and the vulnerability of their larvae to insecticide exposure emphasize the importance of studying these species
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