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On the idea of material designing, we have succeeded in synthesizing Y 3 Ni 4 B 4 C 3 with a new crystal structure and Y 1 Ni 1 B 1 C 1 phases through phase search experiments. The Y 3 Ni 4 B 4 C 3 phase is a tetragonal layer-like structure that is built up of one half Y 1 Ni 2 B 2 C 1 unit and one full Y 1 Ni 1 B 1 C 1 unit stacked along the c -axis. For the Y 3 Ni 4 B 4 C 3 phase, the possible space group is I4 (No. 79), lattice parameters are a =0.355190(7) nm and c =2.56454(7) nm. For the Y 1 Ni 1 B 1 C 1 phase with a CeFeSi type structure, lattice parameters are a =0.35660(5) nm and c =0.75625(12) nm. The crystal structure of Y 3 Ni 4 B 4 C 3 corresponds to the T * phase of the cuprate high T c superconducting materials and m =3, n =2 in the homologous series (LnC) m (B–Ni 2 –B) n . By magnetic susceptibility measurements, superconducting transitions were observed at around 11.0 and 3.0 K. Although the Y 1 Ni 2 B 2 C 1 phase was included in the sample and the single phase was not obtained yet, it is considered that the Y 3 Ni 4 B 4 C 3 or the Y 1 Ni 1 B 1 C 1 phase show a superconducting transition.

Changes of the dimensionality and Tc through iodine intercalation and oxidation have been studied in Bi2Sr2CaCu2O8+δ single crystals. Both intercalation and oxidation change the dimensionality of the physical properties from pseudo-two-dimensional to three-dimensional and decrease Tc. These effects are explained by the multilayer model, where a high-Tc cuprate is regarded as an alternating stack of CuO2 layers (superconducting layers) and block layers (normal layers), taking into account the proximity effect. The Tc-suppression by Cu-site substitution has also been studied for iodine-intercalated and pristine Bi2Sr2Ca(Cu1−zCoz)2O8+δ polycrystals with different δ-values.

Crystal structure and effects of Fe doping of a non-superconducting phase (B-phase) in the Bi-Sr-Cu-O system are examined by electron-diffraction and high-resolution transmission electron microscope measurements. The B phase has a long-range periodic structure closely related to the modulated structure of the superconducting phase and it crystallizes in a monoclinic superlattice with a s =0.54, b s =2.0, c s =2.5 nm and α =104°. The (BiO) 2 double layer is not a complete continuous sheet but suffers from a periodic displacement along the c -direction to form a terraced (BiO) 2 layer. Each BiO terrace consists of seven Bi atoms, and at every eighth lattice plane a vertical shear plane containing few Bi atoms is inserted. Fe doping has an effect on the periodicity of the terraced structure. Intergrowth of narrow (BiO) 2 terraces is induced in crystals doped with Fe.

Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2 Sr 2 CaCu 2 O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.

The anisotropic resistivity in the a − b plane and Hall coefficient have been measured in a single-crystal Bi 2 Sr 2 CaCU 2 O y (BSCCO ) and their temperature dependences have been analyzed by a two-band model involving holes and electrons. It is confirmed that the calculated values of the resistivities and Hall coefficient reproduce well the data and the carrier concentrations, mobilities and mean-free-paths obtained from the model are reasonable values in physics. It is found that the hole concentration is smaller than the electron one, but the hole mobility is larger than the electron one. The transport properties of BSCCO are discussed in terms of two types of the hole and electron carrier.

The electron tunneling study was done on both Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CaCu 2−x Ni x O 8 with use of STM and temperature dependence of the magnetic field penetration depth was also investigated. The tunneling conductance obtained at the lateral surface of single crystal of Bi 2 Sr 2 CaCu 2 O 8 indicates the finite gap on the whole Fermi surface, while the considerable anisotropy is found. The temperature dependence of the penetration depth is of the thermal activation type. These results indicate symmetry of the pair wave function is the predominant s -wave mixed with the d -wave. For Bi 2 Sr 2 CaCu 2−x Ni x O 8 , any clear gap structure was not found in the tunneling conductance at the cleaved surface. The penetration depth exhibits the T 2 temperature dependence.

High-energy-resolved oxygen K absorption spectra of Bi2Sr2Ca1−xYxCu2O8 (x=0.0 and 0.6) single crystals and YBa2Cu3O7−δ ( δ =0.0−0.7) ceramics were reported. The experimental results show that (1) the electronic states at the Fermi level in superconductor has a dominant O2p x,y symmetry, (2) the lowest unoccupied states in nonsuperconductor is located about 1 eV above the Fermi level and also has a strong in-plane symmetry, and (3) hole-doping causes a transfer of weight of the electronic states from the lowest unoccupied states (upper Hubbard band) to the mid-gap states produced by hole-doping at the Fermi level.

Tunneling measurement was carried out on single crystals of superconducting oxide Bi 2 Sr 2 CaCu 2 O 6−8 with use of the STM. Tunneling conductance varies with the distance between the tip and the sample surface at the outside region of superconducting gap, while the midgap conductance is independent of the tip distance. The superconducting gap parameter was obtained as Δ =26±4 meV, and correspondingly 2 Δ /kT c =7±1. Temperature dependence of gap parameter is consistent with the BCS.

Further investigation on the electronic Raman scattering has been made in Bi 2 Sr 2 CaCu 2 O 8+δ single crystals with special emphasis on two aspects of the temperature variation. The peak energy of the broad spectrum below T c shows a significant variation only in the vicinity of T c , indicating the abrupt opening of the superconducting gap. On the other hand, a redistribution of the whole spectrum occurs in the entire range of the superconducting phase, suggesting a gradual growth of the superconducting condensation with decreasing temperature. Further, we also discuss an anisotropy of the electronic states near the Fermi level from the results of (zz) Raman scattering.