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

Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet

The search for the resonating valence bond (RVB) state continues to underpin many areas of condensed matter research. The RVB is made from the dimerisation of spins on different sites into fluctuating singlets, and was proposed by Anderson to be the reference state from which the transition to BCS superconductivity occurs. Little is known about the state experimentally, due to the scarcity of model materials. Theoretical work has put forward the S = 1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good approximation to a 2-dimensional kagome antiferromagnet and that susceptibility data indicate a collapse of the magnetic moment below T = 25 K that is compatible with the spins condensing into the non-magnetic RVB state.Comment: Communication, 3 pages, 3 figure

Theory of quasiparticle spectra for Fe, Co, and Ni: bulk and surface

The correlated electronic structure of iron, cobalt and nickel is investigated within the dynamical mean-field theory formalism, using the newly developed full-potential LMTO-based LDA+DMFT code. Detailed analysis of the calculated electron self-energy, density of states and the spectral density are presented for these metals. It has been found that all these elements show strong correlation effects for majority spin electrons, such as strong damping of quasiparticles and formation of a density of states satellite at about -7 eV below the Fermi level. The LDA+DMFT data for fcc nickel and cobalt (111) surfaces and bcc iron (001) surface is also presented. The electron self energy is found to depend strongly on the number of nearest neighbors, and it practically reaches the bulk value already in the second layer from the surface. The dependence of correlation effects on the dimensionality of the problem is also discussed.Comment: 15 pages, 24 figure

Haydeeite: a spin-1/2 kagome ferromagnet

The mineral haydeeite, alpha-MgCu3(OD)6Cl2, is a S=1/2 kagome ferromagnet that displays long-range magnetic order below TC=4.2 K with a strongly reduced moment. Our inelastic neutron scattering data show clear spin-wave excitations that are well described by a Heisenberg Hamiltonian with ferromagnetic nearest-neighbor exchange J1=-38 K and antiferromagnetic exchange Jd=+11 K across the hexagons of the kagome lattice. These values place haydeeite very close to the quantum phase transition between ferromagnetic order and non-coplanar twelve-sublattice cuboc2 antiferromagnetic order. Diffuse dynamic short-range ferromagnetic correlations observed above TC persist well into the ferromagnetically ordered phase with a behavior distinct from critical scattering

What works in advocating for food advertising policy change across an english region - a realist evaluation.

With increasing recognition of the role of commercial determinants of health, local areas in England have sought to restrict the advertising of products high in fat, salt and sugar (HFSS) on council-owned spaces, as part of wider strategies to reduce obesity. While there is some evidence of the impact of such policy change on behaviour, little is known about what works in the process of implementing this policy change. Guided by a realist evaluation framework that explores the interaction between context, mechanism and outcomes, this study aims to investigate the factors that influence the restriction of outdoor advertising of HFSS products in one region in England. It refines a programme theory co-produced with stakeholders from 14 local authorities within a region and uses multiple data sources from each area with an in-depth examination of four case study sites. Data sources include longitudinal realist interviews, focus groups and surveys with policy advocates and policy stakeholders. Data were analysed retroductively to understand the causal link between context, mechanism and outcomes. Outcomes were driven by five dominant mechanisms: a strategic and staggered approach to stakeholder engagement, gathering intelligence, identifying policy champions, building relationships, reframing the issue; and two secondary mechanisms of amplifying the issue and increasing public will. These led to varied outcomes with no changes in formal policy position within the evaluation period but draft policy guidance in place and changes in political will demonstrated. Dominant context factors influencing change included having a named and resourced policy advocate in place supported by an external Community of Improvement and having existing aligned local objectives. Organisational complexity and change, financial concerns, lack of local examples, ideological positions and the pandemic were also influencing contextual factors. Effecting policy change in this area requires the commitment of an extended period and the valuing of short-term policy outcomes, such as increasing political will. The importance of a resourced and well-supported policy advocate to lead this work is fundamental and the commercially sensitive nature of this policy change means that a complex interplay of mechanisms is required which may be dominated by a strategically staggered approach to stakeholder engagement. [Abstract copyright: © 2023. BioMed Central Ltd., part of Springer Nature.

Monitoring of the prompt radio emission from the unusual supernova 2004dj in NGC2403

Supernova 2004dj in the nearby spiral galaxy NGC2403 was detected optically in July 2004. Peaking at a magnitude of 11.2, this is the brightest supernova detected for several years. Here we present Multi-Element Radio Linked Interferometer Network (MERLIN) observations of this source, made over a four month period, which give a position of R.A. = 07h37m17.044s, Dec =+65deg35'57.84" (J2000.0). We also present a well-sampled 5 GHz light curve covering the period from 5 August to 2 December 2004. With the exception of the unusual and very close SN 1987A, these observations represent the first detailed radio light curve for the prompt emission from a Type II-P supernova.Comment: (1) Jodrell Bank Observatory (2) University of Valencia (3) University of Sheffield 6 pages, 1 figure. To appear in ApJ letter