Chemical Signature of the Superconducting Phase in the Nd-Ce-Cu-O System
Amar Nath 1,
Nikolai S. Kopelev 1,
Vladimir Chechersky 1,
Jian-Liang Peng 2,
Richard L. Greene 2,
Beom-hoan O 3,
Michael I. Larkin 3, and
John T. Markert 3
1 Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA.
2 Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742, USA.
3 Department of Physics, University of Texas, Austin, TX 78712, USA.
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 Mössbauer 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.
Submitted on January 11, 1994
Accepted on May 12, 1994
