Tsutomu Takata

Preparation and superconducting properties of YBa2Cu4O8

Abstract Samples of YBa2Cu4O8 are prepared by means of a new high oxygen pressure technique employing oxygen-HIP. Both magnetization and resistivity measurements show the superconductivity transition at 82.5 K. The lower and upper critical fields of YBa2Cu4O8 are obtained from the magnetization measurements. The Ginzburg-Landau parameters, ζ(0)=17 A , λ(0)=2400 A , are estimated from these results.

Single crystal growth and characterization of Bi2Sr2Ca1-xYxCu2Oy by TSFZ method

Abstract Single crystals of Bi2Sr2Ca1−xYxCu2Oy were grown by means of the Traveling Solvent Floating Zone (TSFZ) method. Several single crystals with an average size of 0.5mm×2mm× 10μm were obtained by cleaving the boules. Observation by EPMA shows that the ab-plane of the samples are smooth and of single phase. X-ray diffraction data show that the lattice constant along the c-axis decreases with increasing the Y content as reported for sintered materials.

Critical current densities of Y1-xCaxBa2Cu4O8 (x = 0 and 0.1) superconducting ceramics

From magnetization measurements of Y1-xCaxBa2Cu4O8 superconducting ceramics with critical temperatures Tc of 83 K for x=0.0 and 91 K for x=0.1, the critical current densities, Jc(H), at 5 K and 77 K are obtained by employing the Bean critical-state model. The Jc at 77 K is evaluated to be 1.5×103 A/cm3 at 2 kOe for the sample of x=0.1. Compared with YBa2Cu3O7, the Jc(H) values in Y1-xCaxBa2Cu4O8 are lower and more sensitive to the magnetic field. These Jc(H) properties are discussed on the basis of the superconducting properties, i.e., critical fields and Ginzburg-Landau parameters.

High-resolution transmission electron microscopy of defects in YBa2Cu4O8

Microscopic structures of the high-Tc superconductor YBa2Cu4O8 (Tc=80 K) are examined by high-resolution transmission electron microscopy. Planar defects are observed as white spot lines in the micrograph of the single-phase YBa2Cu4O8 and they have a structure with an absence of one Cu-O chain in the double Cu-O chain of YBa2Cu4O8. YBa2Cu3O7 and Y2Ba4Cu7O15-like local structures exist around the planar defects.

Phonon Raman scattering of NbBa2Cu3Oy and Nd1.6Ba1.4Cu3Oy.

Polarized Raman scattering from NdBa{sub 2}Cu{sub 3}O{sub {ital y}} and Nd{sub 1.6}Ba{sub 1.4}Cu{sub 3}O{sub {ital y}} single crystals is reported. Single-phonon peaks have been observed in NdBa{sub 2}Cu{sub 3}O{sub {ital y}} at 136, 168, 316, 448, 508, and 580 cm{sup {minus}1} at room temperature where the 316- and 580-cm{sup {minus}1} peaks are strong in the ({ital x},{ital x}) while the 448- and 508-cm{sup {minus}1} peaks are strong in the ({ital z},{ital z}) configurations. The 316-cm{sup {minus}1} peak is found to separate into two peaks at 290 and 323 cm{sup {minus}1} at 4 K, which are assigned as in-phase and out-of-phase vibration of oxygen atoms in the Cu-O plane, respectively. The polarization dependence of the scattering peaks has been explained on the basis of the characteristics of the electronic transitions. The energy shift of peaks when Ba is substituted by Nd has also been discussed.

Microstructures in (Y0.95Ca0.05)Ba2Cu4O8 prepared by low and high oxygen partial pressure techniques

Abstract Microstructures in Y 0.95 Ca 0.05 Ba 2 Cu 4 O 8 prepared by low and high oxygen partial pressure techniques have been studied by transmission electron microscopy. The crystal lattice of the sample prepared at a high oxygen partial pressure (400 atm) shows a small number of defects compared to the sample prepared at a low oxygen partial pressure of 3 atm. Crystal structure in this latter sample exhibited a large degree of distortion that varied from grain to grain.

HRTEM study of new superconducting lead cuprate (Pb, Cu) (Eu, Ce)2(Eu, Sr)2Cu2Oz

Abstract Crystallographic properties of a new lead cuprate superconducting material (Pb, Cu) (Eu, Ce) 2 (Eu, Sr) 2 Cu 2 O z are examined by electron probe micro-analysis and transmission electron microscopy. It is confirmed that although the sample examined here is not of single-phase, the composition of the superconducting phase is close to the nominal composition. From TEM observations, it follows that the crystal structure of the new phase is similar to that of Tl-1222 which consists of a fluorite type block, a rock salt type layer and a perovskite block.

Observation of the flux line lattice in MPMG-processed YBCO using a decoration technique of ferromagnetic particles

Abstract Observation of the flux line distribution in MPMG-processed YBa2Cu3O7 has been conducted using a decoration technique of ferromagnetic particles. It has been found that the flux lines are trapped mainly at Y2BaCuO5 inclusions or the interface between the inclusion and the superconducting matrix.

Optical Properties of Ln2-xCexCuO4 (Ln=Pr, Nd, and Sm) Single Crystals

Optical properties of Ln2-xCexCuO4 (Ln=Pr, Nd and Sm) single crystals were investigated using both X-ray photoemission and Raman spectroscopies. The Cu 3d core-level shift with the Ce doping in Pr2CuO4 is attributed to the increase in Cu+/Cu2+ ratio. For each parent crystal, two-magnon peak was observed at 2800 cm-1. Two-magnon features persist in the Ce-doped crystals at room temperature, although their peaks are suppressed considerably. The broadening of two-magnon Raman spectra in the Ce-doped samples means that the spin fluctuations remain in the CuO2 layer where spin-half Cu3d9 atoms are diluted with no-spin Cu3d10 atoms.