S. Hosoya

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.

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.

Re-investigation of the re-entrant corner effect in twinned crystals

Abstract Morphological characteristics of twinned crystals, such as flattened or elongated morphology have so far been explained in terms of the re-entrant corner effect at twin junction, which was originally proposed as preferential growth site for perfect crystals. The validity of this effect has been critically re-investigated for real crystals. It is shown that this effect in its original sense can operate only under low supersaturation and only for perfect crystals containing no screw dislocations. For real crystals containing screw dislocations, no re-entrant corner effect will be expected whatsoever. Flattened or elongated morphology of twinned crystals is due to either pseudo re-entrant corner effect or one-directional growth along the twin junctions, both resulted from screw dislocation bundles concentrated in the composition plane.

Three-Dimensional Antiferromagnetic Order and AnisotropicMagnetic Properties in Bi2CuO4

The structure and magnetic properties of Bi 2 CuO 4 have been studied by neutron and X-ray diffraction, magnetic susceptibility and magnetization measurements. Three-dimensional (3D) and anisotropic antiferromagnetism is revealed for this compound rather than the previously anticipated one-dimensional one. Long-range antiferromagnetic order associated with a ferromagnetic stacking of Cu 2+ spins along the [001] axis starts to develop below T N =42 K. A magnetic moment of 0.85±0.05 µ B /Cu which is larger than those of Cu 2+ spins in the two-dimensional Heisenberg antiferromagnets such as La 2 CuO 4 and Sr 2 CuCl 2 O 2 reflects the 3D character in Bi 2 CuO 4 . A non-linear magnetization curve under an inplane magnetic field is analyzed quantitatively by a continuous spin rotation in the (001) plane.

Crystal Structures of (La1-xMx)2CuO4-δ(M=Sr and Ba)

Structure determinations of (La1-xSrx)2CuO4-δ and (La1-xBax)2CuO4-δ(x0.075) are carried out by single crystal X-ray diffraction. The former is tetragonal with space group I4/mmm, while the latter is orthorhombic with Pccn. Both compounds have the K2NiF4-type layered perovskite structures formed by the linkage of CuO6 octahedra. The orthorhombic distortion in the compound with Ba is found to be mainly described by the rotation of CuO6 octahedra which corresponds to the R25-mode of cubic perovskite lattice.

Anisotropy of the superconducting critical magnetic field HC2 of LaMCuO system(M = Sr and Ba)

Abstract Preliminary measurement of the superconducting critical magnetic field Hc2 of LaMCuO system (M = Sr and Ba) with layered perovskite structure has been carried out. The results show a quite large anisotropy of Hc2 indicating that the present system has a two dimensional character of the electron transport. The measured conductivities seem to be consistent with the values estimated from the observed Hc2 by use of an ideal two dimensional band with free electron mass.

Incommensurate magnetic correlations in La2NiO4.125: Magnetic phase diagram and two-dimensional hole-order

The magnetic phase diagram of La2NiO4+δ is studied by magnetization and neutron scattering measurements. Three-dimensional (3d) magnetic order with an incommensurate Ni2+ spin modulation in the NiO2 plane appears beyond oxygen doping δ around 0.10. The sharpness of the magnetic phase transition in this phase dramatically depends on δ and the sharpest one is observed at δ≈0.125=18. In contrast to the carrier doping with δ<0.10, which degradates the 3D commensurate magnetic order, the Neel temperature (TN) of this phase is raised by the increase of oxygen doping. Above TN two-dimensional (2D)_incommensurate fluctuations dominate. The magnetic phase diagram and the commensurate-incommensurate phase transition of this system are discussed based on the 2D hole-order in the NiO2 plane.

Electron paramagnetic resonance and polarized optical absorption spectra of Ni2+ in synthetic forsterite

AbstractThe investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn2+ at M2 and Fe3+ at M1, M2, and Si positions. Ni2+ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni2+ spectra are described by the spin Hamiltonian

Spin-Wave Excitations in Two Dimensional Antiferromagnet of Stoichiometric La2NiO4

H = \beta SgB + D\left( {S_{\text{z}}^{\text{2}} - \left( {S + 1} \right)S/3} \right) + E\left( {S_{\text{x}}^{\text{2}} - S_{\text{y}}^{\text{2}} } \right)