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 ([Formula presented]-type) superconducting (SC) cuprate [Formula presented] ([Formula presented]) with [Formula presented] and 25 K. Unlike the hole-doped ([Formula presented]-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the [Formula presented]-type cuprate shows commensurate magnetic fluctuations at the tetragonal ([Formula presented]) reciprocal points both in the SC and in the normal state. A spin gap opens up when the [Formula presented]-type cuprate becomes SC, as in the optimally doped [Formula presented]-type [Formula presented]. The gap energy, however, increases gradually up to about 4 meV as [Formula presented] decreases from [Formula presented] to 2 K, which contrasts with the spin pseudogap behavior with a [Formula presented]-independent gap energy in the SC state of [Formula presented]-type cuprates.