Na27Ru14O48: A new mixed-valence sodium ruthenate with magnetic heptameric plaquettes

Na₂₇Ru₁₄O₄₈ has been synthesized in air at 700 °C. The composition and crystal structure of the phase were determined by single crystal X-ray diffraction. The triclinic crystal structure contains isolated planar Ru₇O₂₄ plaquettes made from seven edge-sharing RuO₆ octahedra. The complex Na:Ru ratio is a result of tilting of the plaquettes to disrupt the packing of nominally hexagonal close packed planes made of Na ions and RuO₆ octrahedra. Resistivity measurements show that the material is semiconducting with an activation energy of 0.53 eV. The observed magnetic moment of 3.11 μ_B per Ru is lower than the expected spin only value, but is within the range seen in other compounds and is too large to indicate that the fundamental magnetic entities are the isolated Ru₇O₂₄ plaquettes. Small, reproducible deviations in the Curie-Weiss behavior occur below 200 K and the onset of a broad magnetic transition is seen between 40 and 32 K.     

Quantum spin liquid in frustrated one-dimensional LiCuSbO4

A quantum magnet, LiCuSbO4, with chains of edge-sharing spin-1/2 CuO6 octahedra is reported. While short-range order is observed for T<10 K, no zero-field phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q=(0.47±0.01)π/a and places an upper limit of 70 μeV on any spin gap. Exact diagonalization of 16-spin easy-plane spin-1/2 chains with competing ferro- and antiferromagnetic interactions (J1=-75 K, J2=34 K) accounts for the T>2 K data.     

Low-lying quasiparticle states and hidden collective charge instabilities in parent cobaltate superconductors

We report a state-of-the-art photoemission (angle-resolved photoemission spectroscopy) study of high quality single crystals of NaxCoO2 the series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (<0.5 eV A) and only weakly anisotropic over the Fermi surface at all dopings, setting the size of the pair wave function to be on the order of 10-20 nm. In the low-doping regime, the exchange interlayer splitting vanishes and two-dimensional collective instabilities such as 120 degrees -type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity and the unique symmetry of kinematic instabilities distinguish cobaltates from most other oxide superconductors.     

Successive orbital ordering transitions in NaVO2

Physical property measurements on samples of triangular-lattice NaVO2 reveal two successive orbital ordering transitions. At 300 K, the structure is rhombohedral. At 98 K, the system undergoes a second-order transition to a monoclinic phase in which the in-plane V-V distances separate into four short and two long bonds, corresponding to orbital ordering of one electron per V3+. Below 93 K, there is a first-order transition to a second monoclinic phase with four long and two short V-V bonds, consistent with orbital ordering of two electrons per V3+. Long range magnetic ordering of 0.98(2)mu_(B) per V3+ (3d(2)) sets in at the 93 K structural transition. The orbital ordering relieves the geometric frustration and leads to a magnetically ordered ground state.     

The electronic structure of Fe1+xNb3-xSe10

Fe_1+xNb_3-xSe₁₀ has a crystal structure consisting of prismatic niobium chains and iron/niobium octahedral chains. A Charge Density Wave (CDW) has been observed at low temperatures, as might be expected from the similarity of the prismatic niobium chains to those present in NbSe₃. Here we report tight binding band structure calculations which indicate that the electrons responsible for conduction and the CDW reside primarily on the prismatic chains. The effects of disorder and stoichiometry in the iron/niobium octahedral chains on the electronic structure and CDW wave vector are discussed.     

Palladium mediated dehydrogenation in the photochemical cyclization of heterocyclic analogs of stilbene

A method is described for the photocyclization of heterocyclic analogs of stilbene to the corresponding phenanthrene-like systems under a N₂ atmosphere, using a catalytic amount of Pd on carbon (Pd/C) as the “oxidant”.