Yoji Koike

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.

Correlation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2O8+δ system

Abstract Correlation between superconducting T c and hole concentration was investigated in the Bi 2 Sr 2 CaCu 2 O 8+δ system. The hole concentration was varied by substituting cations with different valences for Sr or Ca. In order to increase the hole concentration, some samples were annealed at 430°C in 250 bar of oxygen pressure. We find that T c increases, takes a maximum at the hole concentration corresponding to 0.2–0.3 holes per CuO 2 unit, and then decreases, with increasing hole concentration. This correlation is markedly similar to those in the La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− δ systems.

Metal-insulator transition and spin gap in the spin-12 ladder system Sr14−xAxCu24O41 (A Ba and Ca)

Abstract We have investigated the transport and magnetic properties of the spin - 1 2 ladder system with two legs Sr 14− x A x Cu 24 O 41 (A  Ba and Ca). For x = 0, both electrical resistivity and thermoelectric power exhibit semiconductive behavior. It becomes more insulating with the increase in x (Ba), while it becomes more conductive with the increase in x (Ca). A metal-insulator transition is found at x (Ca) = 6.0–8.4 from the thermoelectric power measurements. The magnetic susceptibility exhibits a broad peak around 80 K for x = 0. Subtracting the Curie component at low temperatures, the remainder of the susceptibility χ s ( T ) decreases toward zero with decreasing temperature below ∼80 K, which implies the presence of a spin gap. The temperature at which the broad peak shows the maximum changes little through the partial substitution of Ba and Ca for Sr. However, the value of χ s ( T ) decreases with the increase in x (Ba), while it tends to increase with the increase in x (Ca). These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It appears that the observed spin gap behavior is due to Cu 2+ spins in the CuO 2 chain rather than in the Cu 2 O 3 plane.

Growth, Annealing Effects on Superconducting and Magnetic Properties, and Anisotropy of FeSe1-xTex (0.5≤x≤1) Single Crystals

Single crystals of FeSe 1- x Te x (0.5≤ x ≤1) have been grown by the Bridgman method. After annealing them at 400 °C for 100 h in vacuum, single crystals of x =0.5–0.9 have exhibited bulk superconductivity. Anisotropic properties of the electrical resistivity and upper critical field, H c2 , have been investigated for the single-crystal FeSe 1- x Te x with x =0.6. It has been found that the in-plane resistivity, ρ a b , shows a metallic temperature-dependence, while the out-of-plane resistivity, ρ c , shows a broad maximum around 100 K. The resistivity ratio, ρ c /ρ a b , is 44 and 70 at 290 K and just above the superconducting transition temperature, T c , respectively. The anisotropic parameter, γ≡ H c2 || / H c2 ⊥ (superscripts ∥ and ⊥ indicate field directions parallel and perpendicular to the a b -plane, respectively.), is estimated as 2.7 just below T c .

Recording performances of Ba-ferrite coated perpendicular magnetic tapes

Barium-ferrite fine-particle-coated perpendicular magnetic recording tapes have been experimentally fabricated. Typical tapes have M s = 120 - 130 emu/cc, H c = 600 - 1500 Oe and perpendicular squareness ratio M_{r\perp}/M_{s} larger than 0.9. Tape recording performances were measured with a perpendicular head as well as With a ring-shaped head. A good short wavelength response, D 50 = 76k FRPI, which has never been attained by longitudinally oriented particulate tapes, was obtained; The relationship between perpendicular squareness ratio and output was also investigated to show the importance of perpendicular orientation for attaining a higher recording density.

Growth and very anisotropic upper critical field of single-crystal Bi2Sr2CaCu2Oy

Flaky single crystals of a new high-Tc oxide Bi2Sr2CaCu2Oy with dimensions of the order of 1×1×0.1 mm3 have been prepared using a CuO flux method. The electrical resistivity shows a sharp superconducting transition at 80.6 K. The upper critical field Hc2 was determined to be very anisotropic. The values of -dHc2/dT for H//c-axis and H⊥c-axis are 0.39 T/K and 7.3 T/K, respectively. The ratio of -dHc2⊥/dT to -dHc2///dT is 19. The value of Hc2⊥(0) for H⊥c-axis is estimated to be as large as 400 T. The Ginzburg-Landau coherence length along the c-axis at 0 K ξ//(0) is 2.1 A, suggesting quasi-two-dimensional superconductivity.

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...

Normal and superconducting properties of Pb2Sr2Y1−xCaxCu3O8+δ: II. Dependence on the oxygen concentration

The dependence on the oxygen concentration of normal and superconducting properties of ceramic samples of Pb2Sr2Y1−xCaxCu3O8+δ has been investigated in the composition range of 0⩽x⩽0.5 and 0⩽δ<2. The superconductivity in each sample with different x is suppressed with increasing δ, which is in good correspondence to the increase in resistivity and Hall coefficient with increasing δ, but is contradictory to the speculation by Ohta and Maekawa [Phys. Rev. B41 (1990) 6524]. It is found from chemical analysis that holes introduced by the extra oxygen first enter the CuOδ layer and then the PbO layer rather than the pyramidal CuO2 layer. With increasing δ, a structural phase transition from the orthorhombic phase to the tetragonal phase is observed at δ∼1 in every sample with different x. The dependence on the oxygen concentration of the magnetic susceptibility is also reported.

NMR Study on Spin Gap in Spin Ladder Sr14Cu24O41

We have investigated spin dynamics of both the ladder and the chain sites in Sr 14 Cu 24 O 41 by Cu-NMR and Cu-NQR. From the NMR shift, K spin , and nuclear spin-lattice relaxation time, T 1 , the energy gaps in the spin excitation spectrum are confirmed to be Δ=140 K for the chain site and Δ=470 K for the ladder site for Sr 14 Cu 24 O 41 . Doping effects on the spin gaps and electric field gradient at the Cu sites were also investigated by the substituting of Ca, Y and La for Sr. The spin gap of the ladder site decreases with increasing x for Sr 14- x Ca x Cu 24 O 41 and seems to collapse at around x =13 for Ca substitution.

Superconducting Volume Fraction in Overdoped Regime of La2-xSrxCuO4: Implication for Phase Separation from Magnetic-Susceptibility Measurement

We have grown a single crystal of La 2- x Sr x CuO 4 in which the Sr concentration, x , continuously changes from 0.24 to 0.29 in the overdoped regime and obtained many pieces of single crystals with different x values by slicing the single crystal. From detailed measurements of the magnetic susceptibility, χ, of each piece, it has been found that the absolute value of χ at the measured lowest temperature 2 K, |χ 2 K |, on field cooling rapidly decreases with increasing x as well as the superconducting (SC) transition temperature. As the value of |χ 2 K | is regarded as corresponding to the SC volume fraction in a sample, it has been concluded that a phase separation into SC and normal-state regions occurs in a sample of La 2- x Sr x CuO 4 in the overdoped regime.We have grown a single crystal of La 2- x Sr x CuO 4 in which the Sr concentration, x , continuously changes from 0.24 to 0.29 in the overdoped regime and obtained many pieces of single crystals with different x values by slicing the single crystal. From detailed measurements of the magnetic susceptibility, χ, of each piece, it has been found that the absolute value of χ at the measured lowest temperature 2 K, |χ 2 K |, on field cooling rapidly decreases with increasing x as well as the superconducting (SC) transition temperature. As the value of |χ 2 K | is regarded as corresponding to the SC volume fraction in a sample, it has been concluded that a phase separation into SC and normal-state regions occurs in a sample of La 2- x Sr x CuO 4 in the overdoped regime.