Eiji Takayama-Muromachi

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 5 K. 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.

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

Abstract This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Rietveld Refinement of the Structure of Ba2YCu3O7-x with Neutron Powder Diffraction Data

The crystal structure of a high Tc superconductor, Ba2YCu3O7-x, has been refined by the Rietveld analysis of TOF neutron powder diffraction data. Two polymorphs of Ba2YCu3O7-x are contained in the sample: orthorhombic (74%) and tetragonal (26%) forms. The two forms have nearly the same structure closely related to that of perovskite-type compounds. In the orthorhombic form, the occupation factor of an oxygen atom on the a axis differed greatly from that on the b axis, which is the main reason why Ba2YCu3O7-x crystallizes in the orthorhombic form.

Assignment of the Powder X-Ray Diffraction Pattern of Superconductor Bi2(Sr, Ca)3-xCu2Oy

The powder X-ray diffraction pattern of superconductor Bi2(Sr, Ca)3-xCu2Oy was assigned taking into account the satellite reflections arising from an incommensurate modulated structure. All reflections are indexable with h a*+k b*+l c*+m k, provided that the wave vector k of the modulated structure is b*/4.8. The result shows that many satellites can be observed and some satellite intensities are strong.

New oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3 (n=3,4) prepared at high pressure

Abstract In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200°C under 5 GPa, we found the new oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3. Two members of the series with n=3 and n=4 were isolated as almost single phases; their structures are closely related to those of Tl (orHg) based 1223 and 1234, respectively. The Tl (Hg) sites are, in the present series, replaced by Cu and C. Disregarding ordering of Cu and C, the present compounds take tetragonal subcells; at=3.859(1), ct=14.766(5) A for n=3 and at=17.930(6) A for n=4. Electron microscopic observation indicated that Cu and C are, in both cases, located alternately along the a-axes of the subcells resulting in superstructures having as=2at, bs=bt and cs=2ct. The n=3 member showed a superconducting transition at 67 K, while the n=4 member at 117 K which is the highest Tc among the oxycarbonate superconductors found thus fur.

Systematic trends in the electronic structure parameters of the4dtransition-metal oxidesSrMO3(M=Zr,Mo, Ru, and Rh)

We investigated the electronic structures of the perovskite-type 4d transition-metal oxides SrMO 3 (M =Zr, Mo, Ru, and Rh) using their optical conductivity spectra σ(ω). The interband transitions in σ(ω) are assigned, andsome important electronic structure parameters, such as charge-transfer energy Δ p d , and crystal-field splitting 10Dq, are estimated. It is observed that Δ p d and 10D q decrease systematically with increasing the atomic number of the 4d transition metal. Compared with the case of 3d oxides, the magnitudes of Δ p d and 10Dq are larger. These behaviors can be explained by the more extended nature of the orbitals in the 4d oxides.

Spin glass behavior in RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10 − δ

The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1 . 5 Ce 0 . 5 Cu 2 O 1 0 - δ (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ∼40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in Χ a c vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization (TRM) measurements at T=60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.

Pressure-Induced Spin-State Transition in BiCoO3

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.

Electronic structure of superconducting Cu oxides

Abstract On the basis of photoemission results on La 2- x Sr x CuO 4- y and YBa 2 Cu 3 O y it is suggested that carriers doped in the otherwise insulating samples are oxygen p holes rather than Cu d holes. The p holes interact with the localized Cu d states through hybridization and may give rise to an anomaly near the Fermi surface as described by a periodic Anderson model with low carrier density.

High Tc Superconductor YBa2Cu3Oy–Oxygen Content vs Tc Relation–

High Tc superconductor YBa2Cu3Oy, having various oxygen contents have been examined by a.c. susceptibility, d.c. conductivity and Raman spectrum measurements. The Tc is greatly affected by the oxygen content, being lowered by decreasing oxygen content. Particularly, the Tc drops markedly when the compound takes a tetragonal form. To confirm the y vs temperature relation, the compound was reexamined by thermogravimetry. The new results cast doubt on our previous y value (6.5>y>5.8).