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

The electron-doped material Nd$_{2-x}$Ce$_x$CuO$_4$ becomes superconducting with a Ce$^{4+}$ 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 CuO$_2$ planes of Nd$_{2-x}$Ce$_x$CuO$_4$ was substituted by cobalt-57, which serves as a microprobe of the chemical environment. Deoxygenation brought about little change in the Mossbauer 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.