Kazutaka Kudo

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.

Superconductivity at 38 K in Iron-Based Compound with Platinum--Arsenide Layers Ca10(Pt4As8)(Fe2-xPtxAs2)5

We report superconductivity in novel iron-based compounds Ca 10 (Pt n As 8 )(Fe 2- x Pt x As 2 ) 5 with n = 3 and 4. Both compounds crystallize in triclinic structures (space group P \bar1), in which Fe 2 As 2 layers alternate with Pt n As 8 spacer layers. Superconductivity with a transition temperature of 38 K is observed in the n = 4 compound with a Pt content of x ≃0.36 in the Fe 2 As 2 layers. The compound with n = 3 exhibits superconductivity at 13 K.

Superconductivity in Ca1-xLaxFeAs2: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds

We report superconductivity in the novel 112-type iron-based compound Ca1-xLaxFeAs2. Single-crystal X-ray diffraction analysis revealed that the compound crystallizes in a monoclinic structure (space group \(P2_{1}\)), in which Fe2As2 layers alternate with Ca2As2 spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity with a transition temperature (\(T_{\text{c}}\)) of 34 K was observed for the \(x = 0.1\) sample, while the \(x = 0.21\) sample exhibited trace superconductivity at 45 K. First-principles band calculations demonstrated the presence of almost cylindrical Fermi surfaces, favorable for the high \(T_{\text{c}}\) in La-doped CaFeAs2.

Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2

IrTe 2 , a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. This transition is characterized by the formation of Ir–Ir bonds along the b -axis. We found that the breaking of Ir–Ir bonds that occurs in Ir 1- x Pt x Te 2 results in the appearance of a structural critical point in the T = 0 limit at x c ≃0.035. Although both IrTe 2 and PtTe 2 are paramagnetic metals, superconductivity at T c = 3.1 K is induced by the bond breaking in a narrow range of x ≥ x c in Ir 1- x Pt x Te 2 . This result indicates that structural fluctuations can be involved in the emergence of superconductivity.

Coexistence of Superconductivity and Charge Density Wave in SrPt2As2

SrPt2As2 is a novel arsenide superconductor, which crystallizes in the CaBe2Ge2-type structure as a different polymorphic form of the ThCr2Si2-type structure. SrPt2As2 exhibits a charge-density-wave (CDW) ordering at about 470 K and enters into a superconducting state at Tc = 5.2 K. The coexistence of superconductivity and CDW refers to Peierls instability with a moderately strong electron-phonon interaction. Thus SrPt2As2 can be viewed as a nonmagnetic analog of iron-based superconductors, such as doped BaFe2As2, in which superconductivity emerges in close proximity to spin-density-wave ordering.

Spin gap and hole pairing in the spin-ladder cuprate Sr14-xAxCu24O41 (A=Ca and La) studied by the thermal conductivity

We have systematically measured the thermal conductivity κ of Sr 14- x A x Cu 24 O 41 (A=Ca, La: 0≤ x ( Ca) ≤9: 0 ≤ x ( La) ≤5) single crystals, in order to investigate the spin gap and hole pairin...

Superconductivity in the Honeycomb-Lattice Pnictide SrPtAs

We report superconductivity in the honeycomb-lattice pnictide SrPtAs with Tc = 2.4 K. To our knowledge, SrPtAs is the first superconducting pnictide with a honeycomb lattice structure. Our finding opens up new playground to develop pnictide superconductors with honeycomb lattices, in addition to square lattices known in iron-based superconductors.

Evolution of a pairing-induced pseudogap from the superconducting gap of (Bi,Pb)2Sr2CuO6

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy study of slightly overdoped (Bi,Pb)2Sr2CuO6 to elucidate the origin of the pseudogap. By using a newly developed xenon-plasma light source, we determined the comprehensive momentum and temperature dependencies of the superconducting gap and the pseudogap. We found that the antinodal pseudogap persists far above the superconducting transition temperature and is smoothly connected to the nodal gap. The characteristic temperature of the pseudogap scales well with the superconducting gap size irrespective of the momentum location. The present experimental results point to the pairing origin of the pseudogap.

Orbital degeneracy and Peierls instability in the triangular-lattice superconductor Ir 1-xPt xTe 2

We have studied the electronic structure of the triangular lattice Ir

Enhanced Superconductivity up to 43 K by P/Sb Doping of Ca1−xLaxFeAs2

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