Superconductivity above 90 K in the square-planar compound systemABa2Cu3

Abstract

We have found superconductivity in the 90-K range in ^Ba2Cu306+x with A = La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The results suggest that the unique square-planar Cu atoms, each surrounded by four or six oxygen atoms, are crucial to the superconductivity of oxides in general. In particular, the high T c of /!Ba2Cu3C>6+x is attributed mainly to the quasi two-dimensional assembly of the Cu02-Ba-CuC>2+x-Ba-Cu02 layers sandwiched between two A layers, with particular emphasis in the Cu02+* layers. Higher-T c oxides are predicted for compounds with bigger assemblies of Cu02 layers coupled by Ba layers.

High Pressure Effects on Superconductivity

“…The origin of this striking difference to the large pressure coefficient of LBCO was attributed to the presence of the large chemical pressure associated with the smaller Y 3+ ions. The superconducting phase in the YBCO system was later identified as the tetragonal YBa 2 Cu 3 O 6+δ and it was shown that a homologous series of isostructural compounds can be derived by replacing Y with different rare earth ions, such as La, Nd, Sm, Eu, Gd, Ho, Er, and Lu, with superconducting T c 's all above 90 K [49]. These results are particularly interesting because they prove that not the size of the A-ion in the structure of A-Ba 2 Cu 3 O 6+δ determines the high T c but it is the layered structure with CuO 2 -Ba-CuO 2+δ -Ba-CuO 2 slabs separated by the A-ions and stacked along the c-axis that gives rise to the large T c values.…”

X-ray absorption spectroscopy of detwinnedPrxY1−

“…͑In the case of TbBa 2 Cu 3 O 7Ϫy and CeBa 2 Cu 3 O 7Ϫy the orthorhombic 1-2-3 structure could not be synthesized.͒ For YBa 2 Cu 3 O 7Ϫy it has been suggested that the block layers act as a charge reservoir and that due to their interaction with adjacent CuO 2 planes they are able to supply holes for the planes when the CuO 3 chains are formed. Since almost all isostructural members of the rare-earth-based 1-2-3 family result in a high-temperature superconductor with transition temperature near 93 K, [3][4][5] it was assumed that the rare-earth atoms serve merely as counterions stabilizing the structure without significantly affecting the superconducting properties. Among those rare-earth-based cuprates that are isostructural to YBa 2 Cu 3 O 7Ϫy , the PrBa 2 Cu 3 O 7Ϫy compound is a puzzling exception since it shows neither metallic nor superconducting behavior.…”

Fluorine Doping Effect in the New Superconducting Y<sub>3</sub>Ba<sub>5</sub>Cu<sub>8</sub>O<sub>y</sub> Compound

“…According to Aliabadi et al [6], to have improvement in T c , one should pump more holes from the chains to the oxygen sites of the planes. Fisk et al [7] and Horet et al [8] have shown that in YBCO system superconductors, substitution at the rare earth site or alkaline-earth site would not change T c considerably. But substitution at the oxygen site is very crucial in the formation of superconductors.…”