Pair Correlations, Short Range Order and Dispersive Excitations in the Quasi-Kagome Quantum Magnet Volborthite

We present spatial and dynamic information on the s=1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD)2.2D2O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest neighbor pair correlations, with short range order at wave vectors Q1=0.65(3) {\AA}^-1 and Q2=1.15(5) {\AA}^-1 emerging below 5 K. The excitation spectrum, S(Q,{\omega}), reveals two steep branches dispersing from Q1 and Q2, and a flat mode at {\omega}=5.0(2) meV. The results allow us to identify the cross-over at T*=1 K in 51V NMR and specific heat measurements as the build-up of correlations at Q_1. We compare our data to theoretical models proposed for volborthite, and demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as also suggested by recent local density calculations.Comment: Rewritten article resubmitted to Phys. Rev. Lett. 021

Structure and magnetic properties of the cubic oxide fluoride BaFeO2F

Fluorination of the parent oxide, BaFeO3- δ, with polyvinylidine fluoride gives rise to a cubic compound with a = 4.0603(4) Å at 298K. 57Fe Mössbauer spectra confirmed that all the iron is present as Fe3+. Neutron diffraction data showed complete occupancy of the anion sites indicating a composition BaFeO2F, with a large displacement of the iron off-site. The magnetic ordering temperature was determined as TN = 645±5K. Neutron diffraction data at 4.2K established G-type antiferromagnetism with a magnetic moment per Fe3+ ion of 3.95μB. However, magnetisation measurements indicated the presence of a weak ferromagnetic moment which is assigned to the canting of the antiferromagnetic structure. 57Fe Mössbauer spectra in the temperature range 10 to 300K were fitted with a model of fluoride ion distribution that retains charge neutrality of the perovskite unit cel

Her9/Hes4 Is Required for Retinal Photoreceptor Development, Maintenance, and Survival

The intrinsic and extrinsic factors that regulate vertebrate photoreceptor specification and differentiation are complex, and our understanding of all the players is far from complete. Her9, the zebrafish ortholog of human HES4, is a basic helix-loop-helix-orange transcriptional repressor that regulates neurogenesis in several developmental contexts. We have previously shown that her9 is upregulated during chronic rod photoreceptor degeneration and regeneration in adult zebrafish, but little is known about the role of her9 during retinal development. To better understand the function of Her9 in the retina, we generated zebrafish her9 CRISPR mutants. Her9 homozygous mutants displayed striking retinal phenotypes, including decreased numbers of rods and red/green cones, whereas blue and UV cones were relatively unaffected. The reduction in rods and red/green cones correlated with defects in photoreceptor subtype lineage specification. The remaining rods and double cones displayed abnormal outer segments, and elevated levels of apoptosis. In addition to the photoreceptor defects, her9 mutants also possessed a reduced proliferative ciliary marginal zone, and decreased and disorganized Müller glia. Mutation of her9 was larval lethal, with no mutants surviving past 13 days post fertilization. Our results reveal a previously undescribed role for Her9/Hes4 in photoreceptor differentiation, maintenance, and survival

Three pillars of sustainability: in search of conceptual origins

The three-pillar conception of (social, economic and environmental) sustainability, commonly represented by three intersecting circles with overall sustainability at the centre, has become ubiquitous. With a view of identifying the genesis and theoretical foundations of this conception, this paper reviews and discusses relevant historical sustainability literature. From this we find that there is no single point of origin of this three-pillar conception, but rather a gradual emergence from various critiques in the early academic literature of the economic status quo from both social and ecological perspectives on the one hand, and the quest to reconcile economic growth as a solution to social and ecological problems on the part of the United Nations on the other. The popular three circles diagram appears to have been first presented by Barbier (Environ Conserv 14:101, doi: 10.1017/s0376892900011449, 1987), albeit purposed towards developing nations with foci which differ from modern interpretations. The conceptualisation of three pillars seems to predate this, however. Nowhere have we found a theoretically rigorous description of the three pillars. This is thought to be in part due to the nature of the sustainability discourse arising from broadly different schools of thought historically. The absence of such a theoretically solid conception frustrates approaches towards a theoretically rigorous operationalisation of ‘sustainability’

Exploring the complex magnetic phase diagram of Ce₂PdGe₃: A neutron powder diffraction and µSR study

The magnetic state of the tetragonal compound Ce2PdGe3, which crystallizes in the space group P4(2)/mmc, a derivative of the alpha-ThSi2 structure, has been investigated by magnetic susceptibility, heat capacity, muon spin relaxation (mu SR), and neutron diffractionmeasurements. Heat capacity data indicate two separate magnetic phase transitions at T-N1 = 10.7 K and T-N2 = 2.3 K. The presence of bulk long-range magnetic order is confirmed by our mu SR study below 11 K, where a drop of nearly 2/3 in the muon initial asymmetry and a sharp increase in the muon depolarization rate were observed. Neutron powder diffraction reveals that only one out of two Ce sites becomes magnetically ordered with magnetic propagation vector kappa = (0) at T-N1, adopting an antiferromagnetic arrangement of magnetic moments mu(Ce3+) = 1.78(1) mu(B) along the c axis. At T-N2 the second Ce site orders similarly, following the same magnetic propagation vector kappa = (0), showing, however, at the same time a significant ferromagnetic component within the tetragonal basal plane. A second propagation vector, kappa = (1/2,0, 1/2), appears concomitantly at T-N2

Spin-liquid behavior in a kagomé antiferromagnet: Deuteronium jarosite

PACS 75.40.Gb -Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) PACS 75.25.+z -Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) PACS 78.70.Nx -Neutron inelastic scattering Abstract -The spin dynamics of the frustrated quasi-two-dimensional spin-5/2 Heisenberg antiferromagnetic kagomé compound deuteronium jarosite, (D3O)Fe3(SO4)2(OD)6, has been studied using inelastic neutron scattering. The dynamic magnetic susceptibility shows a clear spin-liquid behavior with strong dynamic short-range correlations with staggered chirality, as seen from the wave vector dependence at finite-energy transfers. The magnetic scattering at low temperatures is gapless and extends out to at least 20 meV. The dynamics shows a non-exponential relaxation behavior but no dynamical ω/T scaling. The characteristic energy (inverse relaxation rate) increases linearly with temperature

Spin-liquid behavior in a kagome antiferromagnet : Deuteronium jarosite

The spin dynamics of the frustrated quasi-two-dimensional spin-5/2 Heisenberg antiferromagnetic kagome compound deuteronium jarosite, (D3O) Fe-3(SO4)(2)(OD)(6), has been studied using inelastic neutron scattering. The dynamic magnetic susceptibility shows a clear spin-liquid behavior with strong dynamic short-range correlations with staggered chirality, as seen from the wave vector dependence at finite-energy transfers. The magnetic scattering at low temperatures is gapless and extends out to at least 20 meV. The dynamics shows a non-exponential relaxation behavior but no dynamical w/T scaling. The characteristic energy (inverse relaxation rate) increases linearly with temperature. Copyright (C) EPLA, 2008

Inelastic neutron scattering study of magnetic excitations in the kagome antiferromagnet potassium jarosite

We report an inelastic neutron scattering study of coherent magnetic excitations in powder and single-crystal samples of the model kagome antiferromagnet potassium iron jarosite, KFe3(OH)(6)(SO4)(2). Initial measurements on a natural single crystal using a triple-axis spectrometer revealed a mode with a zone-centre gap of 7 meV that showed little dispersion within the kagome layers, as well as some indication of a mode with a zone-boundary energy of approximately 20 meV. However, the high background from hydrogen in the sample made it very difficult to search for other excitations. In the absence of suitable deuterated crystals, measurements were performed on deuterated powders using time-of-flight neutron spectrometers over a range of temperatures that include T-N congruent to 64 K. This confirmed the flat 7 meV mode as well as dispersive modes that reached to higher energies. The origin of these modes is discussed in relation to the most likely Hamiltonian for the magnetic degrees of freedom in this material, and estimates are made of the strength of the nearest-neighbour exchange, J(1), and contributions from a Dzyaloshinsky-Moriya interaction or single-ion anisotropy arising from a crystal field

Neutron diffraction studies of nuclear and magnetic structures in the S=1/2 square Heisenberg antiferromagnets (d6-5CAP)2CuX4 (X=Br and Cl)

We report the neutron scattering studies of the nuclear and magnetic structures of deuterated samples of the model two-dimensional S=1/2 Heisenberg antiferromagnets on a square lattice, (d(6)-5CAP)(2)CuCl4 and (d(6)-5CAP)(2)CuBr4 (where 5CAP is 2-amino-5-chloropyridinium). Interest in these materials stems from the fact that they have relatively weak exchange between the magnetic ions, and it is therefore possible to perturb their magnetic structures and excitations significantly in experimentally accessible magnetic fields, and thereby access new quantum disordered states. We succeeded in growing fully deuterated single crystals and determined the nuclear and magnetic structures of the bromide at 10 and 1.8 K, respectively, confirming the four-sublattice spin structure expected for systems, where both inter- and intraplane exchange interactions are antiferromagnetic. The determination of the full crystal structure of the bromide highlights the possibility that interlayer exchange may also propagate via hydrogen bonds to and through the 5CAP molecule. We also determined the critical exponents for the sublattice magnetization of the bromide and mapped out the H-T phase diagram of the chloride up to 5 T