Critical fluctuations in the spin-orbit Mott insulator Sr3_3Ir2_2O7_7

X-ray magnetic critical scattering measurements and specific heat measurements were performed on the perovskite iridate Sr$_3$Ir$_2$O$_7$. We find that the magnetic interactions close to the N\'{e}el temperature $T_N$ = 283.4(2) K are three-dimensional. This contrasts with previous studies which suggest two-dimensional behaviour like Sr$_2$IrO$_4$. Violation of the Harris criterion ($d\nu>2$) means that weak disorder becomes relevant. This leads a rounding of the antiferromagnetic phase transition at $T_N$, and modifies the critical exponents relative to the clean system. Specifically, we determine that the critical behaviour of Sr$_3$Ir$_2$O$_7$ is representative of the diluted 3D Ising universality class.Comment: Accepted in Journal of Physics: Condensed Matte

Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction

This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $\alpha$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H$_{2}$ caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO$_{2}$ overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H$_{2}$-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH$_{3}$. This explains why no diffraction peaks from UH$_{3}$ are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}Comment: Submitted for peer revie

Autosomal recessive primary microcephaly: an analysis of locus heterogeneity and phenotypic variation

BACKGROUND AND OBJECTIVES: Locus heterogeneity is well established in autosomal recessive primary microcephaly (MCPH) and to date five loci have been mapped. However, the relative contributions of these loci have not been assessed and genotype-phenotype correlations have not been investigated. DESIGN: A study population of 56 consanguineous families resident in or originating from northern Pakistan was ascertained and assessed by the authors. A panel of microsatellite markers spanning each of the MCPH loci was designed, against which the families were genotyped. RESULTS: The head circumference of the 131 affected subjects ranged from 4 to 14 SD below the mean, but there was little intrafamilial variation among affecteds (± 1 SD). MCPH5 was the most prevalent, with 24/56 families consistent with linkage; 2/56 families were compatible with linkage to MCPH1, 10/56 to MCPH2, 2/56 to MCPH3, none to MCPH4, and 18/56 did not segregate with any of the loci. CONCLUSIONS: MCPH5 is the most common locus in this population. On clinical grounds alone, the phenotype of families linked to each MCPH locus could not be distinguished. We have also shown that further MCPH loci await discovery with a number of families as yet unlinked

The importance of XY anisotropy in Sr2IrO4 revealed by magnetic critical scattering experiments

The magnetic critical scattering in Sr$_2$IrO$_4$ has been characterized using X-ray resonant magnetic scattering (XRMS) both below and above the 3D antiferromagnetic ordering temperature, T$_{\text{N}}$. The order parameter critical exponent below T$_{\text{N}}$ is found to be \beta=0.195(4), in the range of the 2D XYh$_4$ universality class. Over an extended temperature range above T$_{\text{N}}$, the amplitude and correlation length of the intrinsic critical fluctuations are well described by the 2D Heisenberg model with XY anisotropy. This contrasts with an earlier study of the critical scattering over a more limited range of temperature which found agreement with the theory of the isotropic 2D Heisenberg quantum antiferromagnet, developed to describe the critical fluctuations of the conventional Mott insulator La$_2$CuO$_4$ and related systems. Our study therefore establishes the importance of XY anisotropy in the low-energy effective Hamiltonian of Sr$_2$IrO$_4$, the prototypical spin-orbit Mott insulator.Comment: 6 pages, 4 figure

Spin excitations in a single La2_2CuO4_4 layer

The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart of the mystery of the cuprates \cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}. In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling stabilizes 3D N\'{e}el order up to high temperatures. In a truly 2D system however, thermal spin fluctuations melt long range order at any finite temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer magnetic spectral weight out of a well-defined magnon excitation into a magnetic continuum, the nature of which remains controversial \cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show that coherent magnons persist even in a single layer of \LCO{} despite the loss of magnetic order, with no evidence for resonating valence bond (RVB)-like spin correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these excitations are well described by linear spin wave theory (LSWT). We also observe a high-energy magnetic continuum in the isotropic magnetic response. This high-energy continuum is not well described by 2 magnon LSWT, or indeed any existing theories.Comment: Revised version to appear in Nature Materials; 6 pages,4 figure

Growth and characterisation of uranium multilayers.

This thesis investigates the extents of the 5f-3d and 5f-4f electronic interactions in U/Fe, U/Co and U/Gd thin films, where electronic hybridisation effects are expected to influence the magnetism of the U 5f states. The samples were prepared by DC magnetron sputtering and included niobium buffer and capping layers to instigate crystalline growth and prevent oxidation of the multilayer stack. Layer thicknesses were varied between 5A and 90A with up to 30 bilayer repeats. The majority of the samples were grown at room temperature, but selected compositions were grown at elevated substrate temperatures. Many well-defined Bragg peaks were evident in the X-ray reflectivity spectra of multilayers of all three systems, indicating a low impurity contamination and a well-reproduced bilayer structure. High-angle X-ray diffraction spectra of the transition metal multilayers revealed poorly crystalline, oriented transition metal layers with a non-crystalline component of Fe and Co of 17A. U/Gd X-ray diffraction spectra displayed intensities up to two orders of magnitude greater than those observed for U/Fe or U/Co samples of similar composition and contained satellite peaks, indicating some degree of coherency between the two species. An intense peak was observed, arising from a crystalline uranium component at a position close to that expected for an exotic hep U phase, oriented in the 001 direction. Bulk magnetisation measurements revealed magnetically 'dead' Fe and Co layers of 15A and magnetic moments tending towards the bulk values of 2.2/zb and 1.7/iB for thick Fe and Co layers respectively. A study of the bulk magnetic properties of the U/Gd system did not indicate the presence of any significant 'dead' layer, but reported a saturation magnetic moment for thick Gd layers of 4.5//b> considerably reduced from the bulk value of 7.63//B- Calculations of the magnetic anisotropy for U/Gd samples revealed a possible transition from the gadolinium moments aligned within the plane of the film to samples displaying perpendicular magnetic anisotropy (PMA), at a gadolinium layer thickness of 5A. This transition could be achieved for thicker Gd layers if the thickness of the U layers were increased. A finite-size scaling effect was observed in a gadolinium layer thickness dependent study of the Curie temperature, indicating a transition from three to two dimensional behaviour for very thin Gd layers. Polarised neutron reflectivity spectra were taken in the specular geometry at magnetic saturation, and were simulated with separate, reduced moment and bulk moment ferromagnetic layers for U/Fe and U/Co samples and a simple bilayer structure for U/Gd samples. This simultaneous measurement of both the physical composition and bulk magnetisation measurements supported results obtained by X-ray reflectivity and magnetometry. The hybridisation of the electronic states in U/Fe and U/Gd resulted in an induced polarisation, detected using element selective techniques. X-ray magnetic circular dichroism measurements at the U Mrv and My edges were used to investigate the spin and orbital components of the uranium magnetic moment and a total magnetic moment on the U site of 0.1/b for the case of U/Fe multilayers, 0.01/zb in U/Gd samples and a signal only barely detectable above the statistical noise in the U/Co system were revealed. A uranium layer thickness dependent study of the magnetic moment values was used to indirectly deduce the profile of uranium magnetisation within the layers. X-ray resonant magnetic reflectivity measurements provided a depth dependent measure of the induced U 5f moment for selected U/Fe samples, which indicated a more complicated interfacial structure than that deduced by X- ray and neutron reflectivity techniques and showed that the majority of the U 5f moment was located at the interface region