Azusa Matsuda

Large In-Plane Anisotropy on Resistivity and Thermopower in the Misfit Layered Oxide Bi2-xPbxSr2Co2Oy

We investigated the in-plane anisotropy on the resistivity and thermopower of Bi2-xPbxSr2Co2Oy single crystals, which have a misfit structure between the hexagonal CoO2 layer and the rock salt Bi2Sr2O4 layer. The resistivity and thermopower show significant anisotropy, which exceeds two at maximum. This anisotropy is thought to arise from the anisotropic pseudogap formation enhanced by the misfit structure. The thermopower changes with Pb doping to take a maximum at x=0.4. The misfit structure improves the thermoelectric properties through chemical pressure. The power factor is as large as 9 ?W/cm?K2 at 100 K for x=0.6, which is the highest value obtained for thermoelectric oxides at 100 K.

Crystal structure and superconductivity in Ba2Y1-xPrxCu3O7-y

Crystal structures and superconductive properties of the solid solution Ba2Y1-xPrxCu3O7-y (0≤x≤1) are studied. Although Ba2Y1-xPrxCu3O7-y has an orthorhombic structure which is isomorphic with Ba2YCu3O7-y, Tc decreases monotonically with increasing Pr concentration, and superconductivity disappears at around x=0.6. The Rietveld analysis of X-ray powder diffraction data shows that both Cu(1)-O(3) bond length and the structural asymmetry of the Cu(2)O2 planes increase by the substitution of Pr for Y. Such structural changes induce the carrier density reduction and localization effects, which may cause the suppression of superconductivity in this system.

Gap inhomogeneity, phase separation and a pseudogap in Bi2Sr2CaCu2O8+δ

Abstract Gap inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8 has been studied by low-temperature STM. We found a systematic relation between the local gap energy and local background density of states. A model analysis of the dip–hump structure gave evidence of in-plane charge inhomogeneity. The temperature/doping dependences of the pattern shape of inhomogeneity as well as the gap distribution function were measured. We could identify the signature of phase separation into the superconducting and pseudogapped phases as the appearance of a double-peak structure in the distribution function.

Crystal growth and characterization of the 4-leg spin ladder compound La2Cu2O5

Crystallization of La2Cu2O5 in the La2O3-CuO system has been investigated in air, flowing-oxygen, and under high-pressure oxygen [P(O2)=3atm] atmospheres. Large La2Cu2O5 single crystals (0.2x7.0x0.05 mm3) have been grown using modified slow cooling (MSC) method under flowing-oxygen atmosphere. Electrical resistivity parallel to the ladder direction (//b) in as-grown and HIP-treated La2Cu2O5 crystals was measured using the standard four-probe method.

Fermi-Liquid Behavior and BCS s-Wave Pairing of K3C60 Observed by 13C-NMR

This Letter reports on NMR studies of 13 C in K 3 C 60 above and below T c with an applied field of 2.93 T. It is found that non-single-exponential recovery data can be fitted by two components and normalized to an identical shape. With T 1 defined in this way, Fermi-liquid behavior is identified above T c , and a broadened Hebel-Slichter peak observed below T c is well explained by the Eliashberg theory with 2Δ(0)/ k B T c =4.31. The possibility of strong electron correlation and the origin of the multi-exponential recovery are discussed.

Superconductivity in Ba-Ln(lanthanoid)-Cu-O Compounds

Superconductivity observed in the temperature dependence of the resistivity for Ba-Ln(lanthanoid)-Cu-O compounds with a nominal composition of Ba0.6Ln0.4CuO3-x is reported. The compounds are classified into two groups according to the nature of the superconducting transitions and the structures. Those ones with a single perovskite-like phase, where Ln=Nd, Sm, Eu and Gd, showed smeared transitions with relatively low critical temperatures. The other compounds with mixed phases, where Ln=Dy, Ho, Er, Tm, Yb and Lu, have critical temperatures above 90 K with narrow transition widths.

Single crystal growth and superconductivity in La1.87Ca1.13Cu2O6

Abstract Single crystals of La 1.87 Ca 1.13 Cu 2 O 6 have been successfully grown using CuO flux. Although the electrical resistivity of the as-grown crystals is semiconducting, superconductivity appears by high oxygen pressure (∼ 300 atm) annealing. The transition temperature is between 50 K (onset) and 40 K (end point). The full magnetic shielding effect is observed under zero field cooling and the Meissner volume fraction is about 3% of that expected for an ideal diamagnet. Electrical and magnetic measurements confirm that the crystals have bulk nature of superconductivity. The anisotropies of the electrical resistivity and the coherence length are small in comparison with other copper oxide superconductors.

Magnetic phase transition in Gd2CuO4 single crystal

Abstract The magnetic properies of single-crystal Gd2CuO4 are examined in several low magnetic fields between 50 Oe and 2000 Oe. This compound shows a successive magnetic ordering at 7.8 K, ∼15 K and ∼270 K. In addition, magnetic phase transition from antiferromagnetic to weak ferromagnetic is induced by an external magnetic field of 400 Oe in the temperature between 7.8 K and ∼10 K. Gd ions also magnetically order at 6.8 K. The experimental results provide the basis for a discussuion of a possible magnetic structure of Cu spins in Gd2CuO4 in connection with Nd2CuO4.

Preparation and superconducting properties of tetragonal Ba2YCu3O7−y and Ba2EuCu3O7−y with low oxygen-defect concentration (0.05<y<0.20)

Tetragonal phases of Ba2YCu3O7-y and Ba2EuCu3O7-y with low oxygen-defect concentration (0.05<y<0.20) are prepared by synthesis under high oxygen pressure (3–8 atms). As oxygen pressure increases during sintering, the crystal structure of synthesized samples changes from the orthorhombic to tetragonal phase and superconductivity is suppressed. Substitution of Y and Eu for Ba is observed to accompany the change to the tetragonal phase. Disordering of oxygen vacancies and such substitution may be vital for the absence of superconductivity because no strong correlation between Tc and oxygen-defect concentration is observed.

13C spin-lattice relaxation study on the electronic properties of K3C60

Abstract 13 C spin-lattice relaxation was measured for a superconducting fulleride K 3 C 60 . It is found that non-single exponential relaxation (NSER) data at low temperatures ( T T -independent shape above and below T c , are well reproduced by only the anisotropy parameter ( α spin ) for the hyperfine coupling that was derived from a line-shape analysis. From the simulation for various values of α spin , the shape of the NSER, as well as the asymmetric line shapes, is found to be caused by the anisotropy of an electronic single-site rather than electronic multi-sites. By extracting the isotropic part of relaxation time, ( T 1 ) iso , from the NSER, it is found that an extended Korringa relation holds up to 300 K with K ( α ) = 7.4, and that the T dependence of [( T 1 ) iso T ] −1 resultss from a decrease in the density of the states. Below T c , a broadened Hebel-Slichter coherence peak is observed, which means s-wave Cooper pairing. The T dependence of [( T 1 ) iso T ] −1 below T c is well reproduced by a theoretical prediction with the superconducting gap 2 Δ(0)/ k B T c = 4.3. The origin of the isotropic hyperfine coupling is also discussed.