Hideaki Sawada

The Dependence of the Superconducting Transition upon the Quench Temperature of YBa2Cu3Oy

The superconducting transition properties have been evaluated for YBa2Cu3Oy compound samples prepared after processing from vairous quench temperatures following a sintering stage at 880°C. Two sets of temperatures depicting the onset of the superconducting transition have been observed. One set is found to cover 79-94 K for samples with orthorhombic symmetry, and the onset temperature increases as the quench temperature is lowered. The other set is found to cover the range 52-58 K for samples with tetragonal symmetry.

Study on copper valency of high-tc superconductor YBa2Cu3O7−y by high temperature X-ray absorption spectroscopy

Abstract The X-ray absorption spectra of the Cu K-edge have been obtained for a high-T c superconductor YBa 2 Cu 3 O 7−y by in-situ high temperature measurements between room temperature and 600 °C and for quenched samples prepared by rapid cooling from various temperatures up to 880 °C. The results indicate that the copper valency is predominantly 2+ and the monovalent state appears as the oxygen contents decrease with increasing temperature. The Cu K-edge X-ray absorption spectra of lanthanoide compounds with oxygen-deficient triperovskite structure, LnBa 2 Cu 3 O 7−y (Ln=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu) were also reported.

Magnetic susceptibility of normal state and superconductivity of La2−xSrxCuO4

Abstract The two-dimensional spin-correlation energy, J, was studied in La2−xSrxCuO4 by means of magnetic susceptibility measurements .We found that J, deduced from the susceptibility peak temperature, Tmax, changed drastically with the character of the doped holes, localized or itinerant. In the low carrier density region (0≦x≦0.08), J was higher than 1000 K, representing the nearest neighbor interaction due to the localization of holes. A sudden drop of J and the appearance of bulk superconductivity were observed at x=0.09, which was ascribed to the itinerant holes. The Zn substitution effect for Cu was investigated near the superconductor-to-normal-metal transition. We found that only 1% Zn substitution resulted in 30% decrease of both Tc and Tmax, and Tmax disappeared as Tc became zero. Those results give us direct evidence that the spin correlation relates to the appearance of the superconductivity.

Superconductivity and Crystal Structure of LaBa2Cu3-xOy Compounds

An extensive study of magnetic, electrical transport, and structural properties of the normal and superconducting states of LaBaCuO2BO3 are presented. A ceramic compound of LaBaCuO2BO3 has been synthesized by the solid-state reaction technique under ambient pressure by reacting BaCO3, La2O3, CuO and B2O3. LaBaCuO2BO3 compound was characterized by x-ray diffraction and scanning electron microscope-x-ray energy dispersion spectrum analyses. Rietveld refinement of powder x-ray diffraction data indicated that the compound is P4bm space group symmetry and a=5.57475(8), c=7.5056(l)A. Measured magnetic susceptibilities of powder specimens of LaBaCuO2BO3 compound indicate a transition to a diamagnetic state at 32 K.

Preparation and Property of La1.85Sr0.15CuO4 Single Crystal

A single crystal of the high Tc superconductor, La1.85Sr0.15CuO4, was prepared using the solid phase reaction, and the magnetization was investigated. In the superconducting state, strong anisotropic diamagnetism was observed for external magnetic field parallel and perpendicular to the c-axis. This result supports the two-dimensional character of Sr-La-Cu-O system.

Study of the Infrared Properties of (La1-xSrx)2CuO4

Reflectivity and transmission spectra of (La1-xSrx)2CuO4 were measured in the spectral range of 20-4800 cm-1. The intensity of the absorption peak around 255 cm-1 became twice with lowering temperature from 298 to 19 K accompanying weak softening, while the Cu-O asymmetric stretching mode at 510 cm-1 was almost unchanged both in its energy and intensity. The origin of the soft-phonon mode is discussed.

Infrared properties of the oxygen-deficient triperovskite YBa2Cu3Oy compound

Abstract Infrared absorption spectra have been evaluated for the YBa2Cu3Oy compound samples prepared after rapid cooling from various temperatures in order to maintain the composition existing at the quench temperature. The absorption spectra for the samples with tetragonal symmetry and the reflection spectra for the samples with the orthorhombic symmetry have been determined. The superconducting energy gap was evaluated for a orthorhombic sample.

Identification of a Structure with Two Superconducting Phases in L–Ba–Cu–O System (L=La or Y)

The superconducting transition temperature has been investigated for almost single phase (L1-xBax)aCubOc-y compound samples where L stands for La or Y and (a,b,c) values of (6,6,14) respectively and y the oxygen deficiency has not been evaluated. The structures have tetragonal symmetry and the superconducting transition temperatures of 50 K and 54 K have been determined for the L=La and L=Y samples respectively.

Superconducting Properties of La1.85Sr0.15CuO4 Made by Hot-Press and Sinter Methods

The superconducting transition properties were investigated by resistivity and magnetization in La1.85Sr0.1CuO4 for hot-pressed and sintered samples. The zero resistance at 34.5 K and the midpoint of the transition at 37.7 K were obtained in the sintered one with the onset temperature of 42 K. A single crystal was obtained in the hot-pressed one.

Anomalous enhancement of superconductivity observed in La2CuO4−y

Abstract Superconductivity in undoped La 2 CuO 4−y has been studied by means of magnetic susceptibility measurement. The magnitude of apparent superconductor evaluated at 5 K was found to be enhanced critically depending on cooling process: When a sample was cooled in magnetic field from the temperature higher than 220 K after zero-field cooling to 5 K, the magnitude of 40 K superconductor becomes about 6 times larger than that observed on heating process in the same magnetic field after zero-field cooling. This enhancement of the superconductivity is accompanied with slight increase of the onset temperature of the superconducting transition and these phenomena are observed in wide range of magnetic field of 0.65 mT-3 T.