Anomalous Low Temperature Behavior of SuperconductingNd1.85Ce0.15CuO4−y

We have measured the temperature dependence of the in-plane London penetration depth ${\lambda }_{\mathrm{ab}}\left(T\right)$ and the maximum Josephson current $I_c\left(T\right)$ using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-y}$. In contrast to the usual monotonous decrease (increase) of ${\lambda }_{\mathrm{ab}}\left(T\right)$ [ $I_c\left(T\right)$] with decreasing temperature, ${\lambda }_{\mathrm{ab}}\left(T\right)$ and $I_c\left(T\right)$ are found to increase and decrease, respectively, with decreasing temperature below 4 K. We attribute this anomalous behavior to the presence of the ${\mathrm{Nd}}^{3+}$ paramagnetic moments. Correcting the measured ${\lambda }_{\mathrm{ab}}\left(T\right)$ dependence for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic $s$-wave pairing.