Ruben A. Dilanian

Superconductivity in two-dimensional CoO2 layers

Since the discovery of high-transition-temperature (high-Tc) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that NaxCoO2·yH2O (x ≈ 0.35, y ≈ 1.3) is a superconductor with a Tc of about 5u2009K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-Tc copper oxides, suggesting that the two systems have similar underlying physics.

Chemical composition and crystal structure of superconducting sodium cobalt oxide bilayer-hydrate

Sodium cobalt oxide bilayer-hydrate, NaxCoO2·yH2O (x n ≈ 0.35), was characterized by chemical and structure analyses. The formal valence of Co was found to be +3.4 from redox titration, which was much lower than +3.65 that had been estimated from the Na content, x. The deviation from the charge neutralization was considered to be compensated by the presence of oxonium ions in the CoO2 galleries, which was evidenced by Raman spectroscopy. Therefore, the composition of the superconducting phase was determined to be Na0.343(H3O)0.237CoO2·1.19H2O. Structural studies based on Raman spectroscopy and synchrotron X-ray powder diffraction suggested that the oxonium ions occupy the same crystallographic sites as the Na+ ions.

Structural difference between a superconducting sodium cobalt oxide and its related phase

Abstract Monolayer hydrate (MLH) Na x CoO 2 · y ′H 2 O was obtained from superconducting bilayer hydrate (BLH) Na x CoO 2 · y H 2 O by partial extraction of H 2 O molecules between the CoO 2 layers. Magnetization measurements indicated that electron densities in the CoO 2 layer of the MLH phase remained unchanged after the water extraction. Nevertheless, superconductivity was completely suppressed in the MLH phase. This strongly suggests that the highly 2D nature in the BLH phase due to its thick insulating layers consisting of H 2 O molecules and Na + ions plays an important role for inducing superconductivity.

The role of the water molecules in novel superconductor, Na0.35CoO2 · 1.3H2O

Abstract In order to investigate the role of the water molecules in Na 0.35 CoO 2 xa0·xa01.3H 2 O, we synthesized superconducting Na 0.35 CoO 2 xa0·xa01.3H 2 O and nonsuperconducting Na 0.35 CoO 2 xa0·xa00.7H 2 O, and measured their normal-state magnetic susceptibilities. The susceptibility of Na 0.35 CoO 2 xa0·xa01.3H 2 O has an enhancement below ∼150 K probably caused by ferromagnetic fluctuation, whereas no such enhancement was observed in Na 0.35 CoO 2 xa0·xa00.7H 2 O. The water molecules in Na 0.35 CoO 2 xa0·xa01.3H 2 O may work to shield random coulomb potential of the Na ions with smoother potential at the CoO 2 layer. This effect may account for the appearance of superconductivity in Na 0.35 CoO 2 xa0·xa01.3H 2 O.

Correlation between Tc and Lattice Parameters of Novel Superconducting Sodium Co Oxide Hydrate

We synthesized the five batches of the samples of the novel P3 type superconductor, Na

Neutron Powder Diffraction Study of the Order-Disorder Transition in K2Mn2(SO4)3.

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Structures and Superconducting Properties of Sodium Cobalt Oxides

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A New Superconducting Phase of Sodium Cobalt Oxide

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Influences of interlayer distance and cobalt oxidation state on superconductivity of NaxCoO2

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Characterization of Superconducting Sodium Cobalt Oxide Bilayer-Hydrate

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