Michael I. Larkin

Chemical Signature of the Superconducting Phase in the Nd-Ce-Cu-O System

The electron-doped material Nd2–xCexCuO4 becomes superconducting with a Ce4+ composition around 0.16, but only after removal of a minuscule amount of extraneous oxygen. This enigmatic behavior was addressed here. A small fraction of copper in the CuO2 planes of Nd2–xCexCuO4 was substituted by cobalt-57, which serves as a microprobe of the chemical environment. Deoxygenation brought about little change in the M�ssbauer spectra both above and below the optimal superconducting concentration; however, for x = 0.16 a change was observed. In the latter, a major fraction of the magnetically split, five-coordinate species showed itself as a paramagnetically relaxed doublet upon deoxygenation. The abundance of the paramagnetically relaxed species corresponds closely to the diamagnetic volume fraction and thus provides a microscopic signature of the superconducting phase.

The role of oxygen in the electron-doped superconducting system Nd2−x Ce x CuO4

Mössbauer effect studies suggest the presence of extraneous oxygen in the whole family of title compounds. At certain concentrations of Ce, oxygen is rather tenaciously bound. Such oxygen causes superparamagnetism in the magnetically ordered material, and also brings about the formation of spin clusters in the metallic material.

Anomalous behavior of the Nd1.84Ce0.16CuO4 superconductor

The as-prepared electron-doped system Nd2−xCexCuO4 (where x≈0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mössbauer effect studies were carried out for oxygenated and deoxygenated Nd2−xCexCu(57Co)O4 with x=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe57Co in apical positions, to form 5- and 6-coordinated species. A conventional procedure for deoxygenation brings about little change in the Mössbauer spectra both above and below the optimal superconducting concentration; however, for x=0.16, a dramatic change is observed—a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at x≈0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO2 plane.