Effect of magnetic impurities on high-temperature superconductivity in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$

We have investigated the effect of magnetic impurities on the two-dimensional high-$T_c$ superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ by replacing La with Nd. We find a decrease in $T_c$ and ${\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}|}_{T_c}$ and an increase in the normal-state resistivity with increasing Nd content. We use the Werthamer, Helfand, and Hohenberg equations to derive the effect of an exchange field, magnetic polarization, nonmagnetic impurity scattering, and changes in the density of states on $T_c$ and ${\frac{d{H}_{c2\mathrm{}}}{\mathrm{dT}}|}_{T_c}$. We find no evidence of an exchange interaction between the magnetic moments and the conduction electrons. We explain the changes in the superconducting properties in terms of the van Hove singularity in the density of states near the Fermi level.