Commensurate Spin Dynamics in the Superconducting State of an Electron-Doped Cuprate Superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped ($n$-type) superconducting (SC) cuprate ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ ($x=0.15$) with $T_c=18$ and 25 K. Unlike the hole-doped ($p$-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the $n$-type cuprate shows commensurate magnetic fluctuations at the tetragonal ($1/21/20$) reciprocal points both in the SC and in the normal state. A spin gap opens up when the $n$-type cuprate becomes SC, as in the optimally doped $p$-type ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$. The gap energy, however, increases gradually up to about 4 meV as $T$ decreases from $T_c$ to 2 K, which contrasts with the spin pseudogap behavior with a $T$-independent gap energy in the SC state of $p$-type cuprates.