The initial discovery by Bednorz and Muller1 of 35-K superconductivity in the La-Ba-Cu-O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K superconductivity2–5and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature6. A class of superconducting compounds of the form RBa2Cu307-x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [RI category here) for Y7-13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors14,15. (Zhanget al 14reported 90-K superconductivity in the Th-Ba-Pb(Zr)-Cu-O system. Panetal15reported 50-K superconductivity in the Y-Ba-Ag-O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group 1B as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B—Group 2A-Group 1B—oxygen system. ) Only partial substitutions ha. e led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La2CuO4-x , refs 16, 17). Here we report superconductivity in the rare-earth-free TI-Ba-Cu-O system. We have obsened sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the TI-Ba-Cu-O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.

Superconductivity in the rare-earth-free Tl–Ba–Cu–O system above liquid-nitrogen temperature

Electronic and magnetic properties of rare-earth ions in REBa2Cu3O7−x (RE=Dy, Ho, Er)

A superconducting oxide Y0.4Ba0.6CuO2.22 with Tc higher than 88 K was synthesized. The crystal structure was found to be a derivative of the perovskite structure with orthorhombic unit cell dimensions of a=3.818 (1) A, b=3.888 (1) A, c=11.667 (4) A, and V=173.19 (8) A3. Basic unit cell of the cubic perovskite structure was tripled along the c axis, probably due to the ordering of Y3+ and Ba2+ ions and/or of oxygen vacancy. Apparent short bond length for Cu-O may partly be explained by oxygen vacancy amounting to a quarter, but the observed high Tc suggests actual contraction of Cu-O bonds.

X-Ray and Electron Microscopic Study of a High Temperature Superconductor Y0.4Ba0.6CuO2.22

The crystal structure of a Ba-Y-Cu-O superconductor was refined by the Rietveld analysis of X-ray powder diffraction data. This material is orthorhombic with lattice parameters a = 3.8845 A, b = 3.8293 A, and c = 11.6926 A. Possible space groups are P222, Pmm2, and Pmmm. Metal atoms have essentially the same configuration as that in the perovskite-type structure. A structural model is proposed in which an oxygen atom at (0, 0, 1/2) is deficient.

https://iopscience.iop.org/article/10.1143/JJAP.26.L611/pdf

Crystal Structure of a Ba-Y-Cu-O Superconductor as Revealed by Rietveld Analysis of X-Ray Powder Diffraction Data

The strongly correlated two-dimensional CuO 2 model is studied to identify the pairing mechanism of holes. Detailed studies of the ground state of clusters up to 30 atoms (10 Cu and 20 O atoms) reveal a binding of two holes doped on oxygen sites for particular values of parameters, which appear relevant to high- T c superconductors. The wave function for the ground state shows that a doped hole on oxygen couples strongly with two neighboring Cu spins, which in turn causes a large magnetic distortion in the surrounding Cu spin system. The interaction between two holes becomes attractive through a constructive interference of the magnetic distortions.

Holes in Two-Dimensional CuO2 System –Pairing Mechanism Emerging from Small-Cluster Studies–

New material with high temperature superconductivity is discovered. This material is an oxide of the composition of yttrium, barium and copper. The observation of the superconductivity begins at 85K and completely vanishing resistance occurs at 45 K.

https://iopscience.iop.org/article/10.1143/JJAP.26.L314/pdf

High Transition Temperature Superconductor: Y-Ba-Cu Oxide

A new material, holmium-barium-copper oxide, becomes a superconductor. Its onset temperature of the superconductivity is 93 K and the resistance vanishes below 76 K. By microwave SQUID method, a diamagnetism is found at 4.2 K.

High Tc magnetic superconductor: Ho-Ba-Cu oxide

Critical currents are measured in the title material. At 9.5 K, where an apparent resistance is zero, the critical-current density Jc is found to be about 1.7 A/cm2 in a sintered sample. For currents smaller than Jc the sample resistivity is less than 2.3 µΩ·cm. By X-ray diffraction the crystal structure is found to be essentially of a perovskite type. Its unit cell composed of eight units of the perovskite structure has a face-centered tetragonal symmetry with the lattice parameters a=b=7.83±0.02 A and c=7.72±0.03 A.

Characterization of the High-Tc Superconductor Y-Ba-Cu Oxide –Critical Current and Crystal Structure–

Polycrystalline samples of the high-transition temperature superconductors Y–Ba–Cu, Ho–Ba–Cu and Gd–Ba–Cu oxides are characterized by resistivity, susceptibility, and x-ray measurements. Measured magnetic moments are ascribed mostly to magnetic rare-earth ions, and partly to Cu2+ ions and some other crystal phases. It is found that an oxygen pressure and a sample-preparation temperature dominate the amount of the Cu2+ ions and of other phases, respectively.

Takashi Hirai

Papers

High Transition Temperature Superconductor: Y-Ba-Cu Oxide

High Tc magnetic superconductor: Ho-Ba-Cu oxide

Characterization of the High-Tc Superconductor Y-Ba-Cu Oxide –Critical Current and Crystal Structure–

Characterization of the High-TcSuperconductors Rare-Earth-Ba-Cu Oxides-Resistivity, Susceptibility and Structure-