Yutaka Okabe

Evidence from angle-resolved resonant photoemission for oxygen-2p nature of the Fermi-liquid states in Bi2CaSr2Cu2O8

It has been generally accepted that a strong on-site Coulomb repulsion of the Cu 3d electrons dominates the electronic structure of the high-transition-temperature (high-Tc) superconductors. The on-site Coulomb repulsion has been evaluated as 6–7 eV, comparable with the valence-band width1,2. This strong correlation is thought to cause the Cu 3d electrons to be localized as in a Mott insulator, and doped holes may be transferred to oxygen sites, as the charge transfer energy is small compared with the correlation energy. These doped holes yield a substantial density of states at the Fermi level, characteristic of metals. There has been great effort to find and characterize the electronic states at the Fermi level, because these states relate directly to the mechanism of the high-Tc superconductivity by providing Cooper pairs below Tc. Here we report the first direct evidence for the dominant oxygen-2p nature of the Fermi-liquid state in the high-Tc superconductor, obtained using the technique of angle-resolved resonant photoemission.

Impurity-state-like nature of Fermi-liquid states in Bi2Sr2CaCu2O8 observed by photoemission and x-ray absorption

Abstract Photoemission and X-ray absorption spectroscopy have been performed on Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 in which the hole concentration is controlled by the ratio between Ca and Y atoms. It was found that the density of electronic states at the Fermi level gradually increases as the hole concentration with almost no rigid shift of the electronic structure with respect to the Fermi level. This suggests that the electronic states at the Fermi level (Fermi-liquid states) would not be simple one-electron states but be a kind of impurity states produced through the strong hybridization between doped O2p hole orbitals and empty Cu3d orbitals.

17O NMR study of YBa2Cu3O7−δ (Tc=92 K)

Abstract We have performed 17O NMR in YBa2Cu3O7 oriented powder samples in the temperature range 10–360 K for the orientation H0∥c and H0⊥c. The Electric Field Gradient (EFG) and Magnetic Hyperfine Shift (MHS) tensors have been determined for the four oxygen sites in the structure. The isotropic (axial) part of the MHS tensor were found to be equal to 0.21 (0.08)% for the O(2,3) site in the CuO2 plane and to 0.04(0.05)% for the bridging oxygen O(4). Both were found temperature independent in the normal state. Below Tc, they vanish to zero and the data for the MHS Kcc[O(2,3)] show that this decrease is faster than expected from BCS theory. The coupling between the localized spins at the Cu (2) sites and the mobile oxygen p-holes is discussed in the light of these results.

Oxygen isotope effect in the superconducting BiSrCaCuO system

Abstract An oxygen isotope effect is observed in mixed-phase BiSrCaCuO superconductors when 18 O is substituted for 16 O. The superconducting transition temperature T c , measured by electrical resistivity and magnetic susceptibility, is lowered by about 0.32 K for the higher- T c (110 K) phase and by about 0.34 K for the lower- T c (75 K) phase. These results suggest a measurable contribution to the superconductivity from phonons.

Growth of YBa2Cu3O7-δ Single Crystals

Large single crystals of high-temperature superconductor YBa2Cu3O7-δ (Tc ~ 90 K) have been successfully grown in the cavity of bulk materials with a CuO flux. Crystals with a typical size of 2×2×0.3 mm3 were obtained. As-grown single crystals do not show superconductivity at liquid nitrogen temperature. However, after annealing in O2 flow for three days, they become superconductive at Tc ~ 90 K.

Isotope Effect in Superconducting YBa2Cu3O7-δ System

An oxygen isotope effect is observed by a measurement of the resistivity when 16O is substituted for 18O in the YBa2Cu3O7-δ compound. The superconducting transition temperature Tc is lowered by 0.4 K upon substitution. The results suggest that the phonon related to oxygen in the two-dimensional CuO2 plane contributes to the mechanism of high-Tc superconductivity. When 63Cu is substituted for 65Cu, no appreciable isotope effect is observed.

CORRELATION BETWEEN MAGNETIC PROPERTIES AND COMPOSITION OF ELECTROLESS-PLATED COBALT ALLOY FILMS FOR PERPENDICULAR MAGNETIC RECORDING MEDIA.

The magnetic properties of a perpendicular magnetic anisotropy film prepared by an electroless plating method was investigated, from viewpoints of film composition. The value of saturation magnetization, Ms, of the film decreases (9.4 emu cm-3, 30.8 emu cm-3 and 27.0 emu cm-3) by one atomic percentage codeposition of nickel, rhenium and phosphorus, respectively. The codepositions of nickel and phosphorus decrease the value of the perpendicular anisotropically magnetic field, Hk. Simultaneously, however, for the low-cobalt-content region with high Ni content, the codeposition of rhenium increases Hk while decreasing Ms, making it possible to prepare perpendicular anisotropicallymagnetic films. Therefore, in the electroless plating method, lower Ms conditions with high Ni content is advantageous for making perpendicular anisotropically magnetic films.

A Simple Method of Monte Carlo Renormalization Group

On propose une methode simple du groupe de renormalisation de Monte Carlo basee sur les proprietes de transformation des quantites thermodynamiques sous la transformation de spin bloc

NMR study of 17O in high Tc superconducting oxides

Abstract We present the temperature dependence of the Magnetic Hyperfine Shift tensors of 17 O nuclei for the sites within the CuO 2 plane (site O(I)) and in the BaO, LaO, SrO plane (O(II) site) in the compounds YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.65 , La 1.85 Sr 0.15 CuO 4 and Bi 2 Sr 2 CaCu 2 O 8 . For O(I) sites, the isotropic part of the tensor reflects the polarisation of the Oxygen 2s orbitals by the neigboring Cu d-holes and the temperature dependence of the susceptibility. We show that in YBa 2 Cu 3 O 6.65 , the spins of the oxygen p-holes have their own degree of freedom above 120 K.

High-Temperature Series Analysis for the Classical n-Vector Model with a Free Surface

On presente des developpements en series a haute temperature pour la susceptibilite de couche χ 1 et la susceptibilite locale χ 1,1 du modele n vecteurs classique sur le reseau cubique simple semi-infini