Infrared evidence of a Slater metal-insulator transition in NaOsO3

The magnetically driven metal-insulator transition (MIT) was predicted by Slater in the fifties. Here a long-range antiferromagnetic (AF) order can open up a gap at the Brillouin electronic band boundary regardless of the Coulomb repulsion magnitude. However, while many low-dimensional organic conductors display evidence for an AF driven MIT, in three-dimensional (3D) systems the Slater MIT still remains elusive. We employ terahertz and infrared spectroscopy to investigate the MIT in the NaOsO3 3D antiferromagnet. From the optical conductivity analysis we find evidence for a continuous opening of the energy gap, whose temperature dependence can be well described in terms of a second order phase transition. The comparison between the experimental Drude spectral weight and the one calculated through Local Density Approximation (LDA) shows that electronic correlations play a limited role in the MIT. All the experimental evidence demonstrates that NaOsO3 is the first known 3D Slater insulator.Comment: 4 figure

Magnetic ordering in the layered oxyhalide Sr2NiO2Cl2

The full characterisation of the high-pressure-synthesised, metastable layered oxyhalide Sr2NiO2Cl2 is reported. It is comprised of infinite NiO2 layers along with double rock salt Sr2Cl2 layers and is closely related to the n = 1 Ruddlesden-Popper phases. At ambient temperature, it crystallises in the tetragonal space group I4/mmm with 2 formula units per unit cell with the basal lattice parameters a = b = 4.03417(2) Å and the stacking direction lattice parameter, c = 15.1058(1) Å. A tiny cusp in the temperature dependence of the magnetic susceptibility in a previous report suggested that this oxyhalide underwent long-range magnetic order and therefore contained high-spin Ni2+ ions. Powder neutron diffraction has confirmed that Sr2NiO2Cl2 is a localised-moment oxyhalide, adopting the high-spin S = 1 configuration with fully occupied dxz, dyz and dxy orbitals and partially occupied dz2 and dx2-y2 orbitals. The Ni2+ magnetic moments order antiferromagnetically below ≈ 180 K in a G-type arrangement on a √2a × √2a × c expansion of the nuclear cell with the propagation vector (½, ½, 0) and with a saturated long-range ordered magnetic moment of 1.57(7) μB per Ni2+ cation which is in line with previously calculated computational results and showing that the ligand field around the Ni2+ cation is not sufficiently anisotropic to drive it into the low-spin, diamagnetic configuration found for this d8 cation in square planar or highly elongated octahedral coordination

Magnetic and structural studies of Sc containing ruthenate double perovskites A2ScRuO6 (A = Ba, Sr).

Ruthenium containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal-insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction, and are described by k = (001) and k = (000) for the Ba and Sr oxides respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner sharing BO6 sites. The magnetic structure is described by k = (½ 0 0) with the basis vector belonging to the irreducible representation Γ3.Australian Synchrotron Australian Research Counci

Evolution of the magnetic excitations in NaOsO3_3 through its metal-insulator transition

The temperature dependence of the excitation spectrum in NaOsO$_{\text{3}}$ through its metal-to-insulator transition (MIT) at 410 K has been investigated using resonant inelastic X-ray scattering (RIXS) at the Os L$_{\text{3}}$ edge. High resolution ($\Delta E \sim$ 56 meV) measurements show that the well-defined, low energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO$_{\text{3}}$.Comment: Accepted in Physical Review Letters, part of a joint submission to Physical Review B. Supersedes arXiv:1707.0555

Unusual effects of magnetic dilution in the ferrimagnetic columnar ordered Sm2MnMnMn4−xTixO12\mathrm{Sm_2MnMnMn_{4-x}Ti_xO_{12}} perovskites

Powder neutron diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the Sm2MnMnMn4-x Tix O12 A-site columnar ordered quadruple perovskite manganites (x = 1, x = 2 and x = 3). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27 K, 62 K and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the x = 1 and x = 2 samples, which indicates a considerable departure from mean field behaviour. This result is corroborated by large reductions in the theoretical ground state magnetic moments observed across the series, which indicate the presence of spin fluctuations and or disorder. We show that long range magnetic order in the x = 3 sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if magnetic order in Sm2MnMnMn4-xTixO12 is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.Comment: 10 pages, 7 figure