Seiichiro Noguchi

Electrical, Magnetic and Superconducting Properties of the Quenched Bi2Sr2Ca1-xNdxCu2O8+δ System

Structure, resistivity and susceptibility are studied in the quenched Bi2Sr2Ca1-xNdxCu2O8+δ system. The quenched samples with 0x1 have a single phase of the Bi2Sr2CaCu2O8+δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 upon quenching from 800°C to liquid nitrogen temperature. The Tc remains a constant (~90 K) for x0.15, and decreases linearly with x for 0.15x0.45, and superconductivity disappears in the samples with x0.45. Referring to the previous results for the furnace-cooled Bi2Sr2Ca1-xNdxCu2O8+δ system, the relationship between the Tc and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi2Sr2Ca1-xNdxCu2O8+δ systems.

Electrical, Magnetic and Superconducting Properties of the High-Tc Superconductor Ba2Nd1-xPrxO7-δ

Structure, resistivity and susceptibility have been studied in the Ba2Nd1-xPrx Cu3O7-δ system. The samples with 0≤x≤1 have a single phase of the orthorhombic Ba2YCu3O7-δ structure. The Tc decreases linearly with increasing x and superconductivity is not observed for the samples of x≥0.3. The valences of Nd and Pr ions in the samples were determined from the susceptibility measurement as Nd3+ and Pr4+, respectively. It is supposed naturally that the valence of Cu2 ions (Cu ions in the two-dimensional Cu–O plane) is affected strongly by the neighboring rare earth ions. The results show that the valence of Cu2 ions is significant for the superconductivity in the Ba2RECu3O7-δ system.

XPS Study on the Electronic Structure of Bi2Sr2Ca1-xNdxCu2Oy

XPS core level and valence band spectra of sintered Bi2Sr2Ca1-xNdxCu2Oy which manifests a superconducting to semiconducting phase change around x=0.5 have been measured. The fraction of Cu1+ to the total Cu ions evaluated from Cu2p spectra amounts to 7% in Bi2Sr2CaCu2Oy, but decreases to 0% with increasing Nd content. From Ca2p, Sr3d, Nd3d, Bi4f and O1s spectra, it is inferred that Ca2+ and Sr2+ are mixed and distributed over both Ca and SrO layers, whereas Nd3+ and Sr2+ are rather separate. From the valence band spectra, a finite density of states at EF is deduced for Ca-rich oxides, but it diminishes in Bi2Sr2NdCu2Oy. The analysis of ionic configurations based on the mixed valencies of copper and oxygen ions gives a consistent account of the above spectra and the superconductor to semiconductor transition.

Superconducting properties of Y1-xPrxBa2Cu4O8 prepared by the citrate pyrolysis method

Abstract Superconducting properties are examined in Y 1−x Pr x Ba 2 Cu 4 O 8 synthesized by the citrate pyrolysis method followed by sintering in a one atm oxygen flow. Lattice constants increase linearly, while the orthorhombicity decreases linearly with increasing x. Meissner effect measurements indicate that the onset T c decrease monotonically with the Pr substitution, and disappears at x =0.75. This is caused by the change in the hole concentration with the substitution of Pr for Y.

Particle Size and Superconducting Transition Temperature of Aluminum Fine Particles

Aluminum fine particles of free surfaces were made by the evaporation method in helium gas to investigate the relation between the superconducting transition temperature Tc of the fine particle and its size. The fine particles were collected without exposure to the air to obtain clean surfaces, however, this process introduced some undesirable weak contacts among the particles. The size distribution was determined by means of electron micrographs, and the average diameter ranged from 90 to 160 A. The superconducting transition temperature Tc was determined from the change of diamagnetic susceptibility with temperature. The present experiment shows that Tc rises with a decrease in , and becomes about 1.8 K for of 90 A, considerably higher than that of bulk aluminum 1.16 K.

Electrical, Magnetic and Superconducting Properties of the High-Tc Superconductor Bi2Sr2Ca1-xRExCu2O8+δ (RE=Nd and Pr)

Structure, resistivity and susceptibility have been studied in the Bi2Sr2Ca1-xRExCu2O8+δ (RE=Nd and Pr) system. The samples with 0x1 have a single phase of the Bi2Sr2CaCu2O8+δ structure. The oxygen concentration, 8+δ, is estimated as about 8.3 which is independent of the RE composition x. The Tc increases slightly with x in the range of x0.10 and remains constant (~85 K) for 0.10x0.25, then decreases linearly with x for 0.25x0.55. Superconductivity disappears for the samples with x0.55 in both cases. The Nd and Pr ions in the samples were both determined to be trivalent from the susceptibility data. The results show that the hole concentration in [Cu–O] plays a most significant role in the occurrence of superconductivity in the Bi–Sr–Ca–Cu–O system.

Electrical, Magnetic and Superconducting Properties in Bi2-xPbxSr2CaCu2O8+δ

Samples of the Bi2-xPbxSr2CaCu2O8+δ system are prepared to be almost a single phase of the Bi2Sr2CaCu2O8+δ structure for a region of 0x0.15. Substitution of Pb for Bi ions is observed by EPMA at grain level. The Tc decreases linearly with x in this region. Comparison with the results for the Bi2Sr2Ca1-xNdxCu2O8+δ shows that the hole concentration in [Cu-O] plays a most significant role in determining the Tc in the Bi2Sr2CaCu2O8+δ system. The universal relationship between the hole concentration in [Cu-O] and Tc is determined in a wide range of concentration in this system.

Superconductivity and transformation of near-equiatomic M-Ru (M = V, Nb and Ta) alloys

Abstract The superconductivity and structural transformation from cubic to tetragonal phase have been investigated in near-equiatomic M-Ru (M = V, Nb and Ta) alloys through the measurements of X-ray diffraction, electrical resistivity and magnetic susceptibility. The transformation is possibly a second-order phase transition. The electronic specific heat coefficient estimated from the temperature dependence of upper critical field for a transformed sample is smaller than that for a non-transformed sample. The superconducting transition temperature and magnetic susceptibility at 77 K for V-Ru show a peak at the boundary composition between the cubic and tetragonal phases, while the values for Nb-Ru and Ta-Ru vary monotonically without a peak at the boundary composition. Using the McMillans formula, it is found that the electronic density of states for V-Ru is about 2–4 times larger than the values for Nb-Ru and Ta-Ru, while the electron-phonon interaction is smaller.

Carrier Concentration Dependence on Tc in Bi2Sr2-xRExCaCu2O8+δ (RE=Nd and Ce)

Structure, resistivity and susceptibility have been measured in the Bi2Sr2-xRExCaCu2O8+δ (RE=Nd and Ce) system. The samples with 0≤x≤1 (RE=Nd) and 0≤x≤0.25 (RE=Ce) have almost single phases of the Bi2Sr2CaCu2O8+δ structure. The x-dependence on Tc in the Ce-substituted system is almost twice as strong as that in the Nd-substituted one. The valences of Nd and Ce ions in the samples were determined from susceptibility measurement as trivalent and tetravalent, respectively. The hole concentration was determined from the Hall effect measurement. The results confirm that the hole concentration in the Ce-substituted system becomes twice as strong as that in the Nd-substituted one, as expected from the universal relationship between the Tc and the hole concentration in the Bi2Sr2CaCu2O8+δ system.

Superconductivity and structural transformation in V2Hf-based C15 pseudobinary compounds

Superconductivity and structural transformation have been studied through resistivity measurements on V2Hf-based pseudobinary compounds V2(Hf1-xMx) (M=Y, Ti, Nb and Ta). Both the superconducting transition temperature Tc and the structural transformation temperature TL depend strongly on the electron-atom ratio e/a. There is a correlation between TL and the electron-phonon coupling parameter lambda derived from the experimental value of Tc, which suggests that superconductivity is strongly related to lattice instabilities. The value of Tc of 10.7K obtained for V2(Hf0.84Nb0.16) is the highest reported so far for a compound with C15 structure.