Comparison of paramagnetic- and nonmagnetic-impurity effects on superconductivity inNd1.85Ce0.15CuO4

The effects of substitution of Co, Ni, or Zn for Cu in the electron-doped superconductors, ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{O}}_4$ (with M=Co, Ni, or Zn, x≤0.05), have been investigated by powder x-ray diffraction analysis and resistivity, magnetic-susceptibility, and Hall-coefficient measurements. It is observed that only 0.7% Co or 0.8% Ni substitution for Cu destroys superconductivity. On the contrary, for the Zn-substituted samples, it is observed that ${\mathit{T}}_{\mathit{c}}$ decreases proportionally to x and the magnitude of ${\mathit{T}}_{\mathit{c}}$ is ∼9 K even for the 2% Zn-substituted sample. This result suggests that the pair-breaking effect plays a dominant role in the suppression of superconductivity in the Co- or Ni-substitute samples. The absolute values of the exchange-interaction energy between an electron and a Co ion and of that between an electron and a Ni ion are estimated at ∼0.09 and ∼0.08 eV, respectively. These values are comparable to those of the conventional BCS-type superconductors containing paramagnetic impurities. Both the absolute value of the Hall coefficient (‖${\mathit{R}}_{\mathit{H}}$‖) and its temperature coefficient (d‖${\mathit{R}}_{\mathit{H}}$‖/dT) are found to decrease as x increases for all the samples at temperatures higher than 120 K. However, for the Co-substituted samples, it is clearly observed that ‖${\mathit{R}}_{\mathit{H}}$‖ increases proportionally to x at 30 K, while ‖${\mathit{R}}_{\mathit{H}}$‖ decreases monotonically with increasing x for the Ni- or Zn-substitute samples. Therefore, it seems that there are no clear relations between the temperature dependence of ${\mathit{R}}_{\mathit{H}}$ and the suppression in the magnitude of ${\mathit{T}}_{\mathit{c}}$ due to the substitutional effects.