Mitsuru Uehara

New high-Tc superconductors without rare earth element

Abstract We have discovered new high-Tc oxide superconductors in the Bi Sr Ca Cu O system with no rare earth elements. The high-Tc phase, with Tc of about 105K, greater than that of YBa 2 Cu 3 O 7 by more than 10K, appears to have the approximate composition of Bi 1 Sr 1 Ca 1 Cu 2 O x . As the composition deviates from this value, a low-Tc phase with Tc of 75–85K tends to appear. In this oxide system, the coexistence of Sr and Ca is essential to realize the high Tc. The chemical compositions of the high and low- Tc phases are estimated to be Bi 4 Sr 3 Ca 3 Cu 6 O x and Bi 4 Sr 3 Ca 2 Cu 4 O y , respectively. The crystal structure of both phases is body-centered orthorhombic with a=0.54,b=5a=2.7and c=3.08nm. This structure is layered structure with such that alternating Bi 2 O 2 and perovskite layers occur along the c-axis. A modulated structure with highly strained a-c lattice planes is observed by high resolution TEM when the electron beam is incident along the a-axis direction.

Magnetization Measurements in Y-Ba-Cu-O Compound System

Magnetization measurements for (Y0.5Ba0.5)CuO3-y compound at 77 K and 4.2 K indicated that the material was type II superconductor. Critical current densities at 1 T derived from magnetization curves were 13 A/cm2 at 77 K and 3×103A/cm2 at 4.2 K. Upper critical field Hc2(mid-point) at 77 K was 15 T. Several parameters such as Hc2 at 0 K, Hc, ζGL, λGL and κ were estimated.

Magnetic Properties of BiSrCaCu2Ox Superconductors

Magnetization measurements have been performed for the newly found superconductor, BiSrCaCu2Ox, which is composed of two phases, i.e., a high-Tc phase (105 K) and a low-Tc phase (75 K). The superconductor is found to be of type II and the lower critical field of the high-Tc phase is as small as 16.2 Oe at 77 K. The volume fraction of the high-Tc phase is as high as 21% in the present sample. The hysteresis observed in the magnetization vs field curve is much smaller in BiSrCaCu2Ox than in YBa2Cu3Oy, implying a small critical current density in the high-Tc phase.

Magnetic properties and upper critical fields of sintered Tl−Ca−Ba−Cu−O superconductors

Magnetic properties and upper critical field Hc2 near Tc (zero field) were measured for sintered Tl-Ca-Ba-Cu-O compound whose zero resistance temperature was 105 K. The magnetic susceptibility curve indicated that the sample contained only one superconducting phase. Hysteresis of magnetization at 77 K was larger than that of Bi-Sr-Ca-Cu-O. Hc2 values at 77 K and 0 K, estimated by extrapolation, were 50 T and 130 T, respectively. These values were slightly higher than those of high-Tc phase in Bi-Sr-Ca-Cu-O compound.

Effect of magnetic field and high pressure on the superconductivity of the new high-Tc oxide Bi-Sr-Ca-Cu-O

The electrical resistances of the multiphase new high-Tc superconducting oxides Bi-Sr-Ca-Cu-O have been measured in the magnetic field and under hydrostatic pressure. The resistances decrease in two steps around 80 K and 110 K, which are believed to correspond to the presence of phases with different Tcs. The effect of the magnetic field on the superconductivity is similar to that in the Y-Ba-Cu-O and La-Ba(Sr)-Cu-O systems. However, the superconducting transition is not affected appreciably by the pressure. The results are deduced to be due to the anisotropic structure of the high-Tc phase.

Magnetic Properties of Pb-Doped Bi-Sr-Ca-Cu-O Superconductors

The D.C. magnetization of Bi-Sr-Ca-Cu-O compound was significantly increased by Pb-doping. However, the hysteresis ΔM in magnetization curves rapidly decreased with increasing magnetic field, and became almost zero above 0.5 T. This small hysteresis may be related to the thin layers of superconducting phases. Low-temperature tails in superconducting resistive transition curves measured in magnetic fields were drastically decreased by the Pb-doping. Hc2 values at 77 K and 0 K obtained by extrapolation using the transition curves were 60 T and 140 T, respectively, when defined at the midpoint of the transition.

Large magnetization in YBa2Cu3O7 prepared by sintering at high-temperature

Magnetization and resistive Jc at 77 K were measured for YBa2Cu3O7 samples prepared by sintering above 950°C. Anomalously large magnetization was observed at a sintering temperature of 1030°C, although resistive Jc remained low. This large magnetization can be explained by both the improvement in pinning Jc of the compound and the coarseness of the YBa2Cu3O7 structure.

Structure and Superconducting Properties of Ba3(1-x)Y3xCu3O9-y Compound System

Structure and superconductivity of the Ba3(1-x)Y3xCu3O9-y compound system prepared by a powder method has been investigated. The zero-resistance temperature of 92.5K has been obtained for x=0.4. Detailed analysis of crystal structure was carried out by X-ray diffraction using Rietveld technique. Critical current densities Jcs were estimated from the magnetizations,which were compared to those obtained by the resistive method.

Anomaly of ac susceptibility in low temperature for over-doped Bi2Sr2CaCu2O8+y

Abstract We studied magnetizations of over-doped Bi 2 Sr 2 CaCu 2 O 8+ y single crystals. The thermoremanent magnetization shows an anomalous random oscillation in low frequency modulation field below 20 K. The result indicates a random motion of the vortices, which suggest there exists a new magnetic phase in the vortex solid phase.

A new phase in the vortex solid region in Bi2Sr2CaCu2O8

Abstract The time decay of the zero field cooled diamagnetic magnetization of Bi 2 Sr 2 CaCu 2 O 8 single crystals can be explained in terms of a characteristic relaxation time τ 0 and a mean activation energy E which is proportional to the reciprocal magnetic field. We find that τ 0 becomes extremely long to be around 10 s at low temperatures below 14 K. This low temperature phase in the vortex solid region is also directly confirmed by the ac susceptibility measurements in the ultra low frequency range.