Electrodynamics of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$: Comparison with Nb and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have measured the surface impedance of ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_4$ (NCCO) thin films and single crystals, ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) single crystals, and Nb single crystals. We find that the electrodynamic properties of Nb are in good agreement with s-wave BCS theory. YBCO single crystals show behavior which has been seen only in very high quality materials and which is clearly not consistent with s-wave BCS theory. Both thin films and single crystals of NCCO have been grown, thoroughly characterized, and measured at microwave frequencies. We find an unusual degree of consistency in the results between thin films and crystals of NCCO. We also find a surprisingly good agreement between the temperature dependence of the surface impedance, and single-gap BCS s-wave calculations over the entire temperature range. These results are puzzling in the light of the fact that NCCO is a cuprate superconductor, and the crystals were grown in a manner very similar to those of YBCO. We discuss these results and consider a variety of ideas currently in the literature which may explain these striking differences.