Novel spin excitations in optimally electron-doped Pr _0.89LaCe_0.11CuO₄

To study the detailed structure of the magnetic excitation spectrum, we performed a neutron scattering experiment on optimally electron-doped Pr _0.89LaCe_0.11CuO4 (T_c = 25:5 K). Well-defined spin excitations existed up to an energy of ω ∼ 180 meV, which is consistent with the recent observation by Wilson et al. [Phys. Rev. Lett. 96 (2006) 157001] for the same cuprate with a different T_c. Better statistics in our experiment revealed a more precise structure of the spectrum. As the energy increased up to ω_c ∼ 60 meV, the peak width (κ) markably increased, but in the high-energy region beyond ω_c the width remained nearly independent of the energy. χ″(ω) showed a sharp peak at ω = 6 meV, which markably diminished as the energy increased until eventually a well-defined magnetic signal was not observed near ω_c. However, a broad magnetic peak at (π, π) reappeared beyond ω ∼ 100 meV. We discuss the nature of the spin excitation spectrum, which is vastly different from that of a conventional spin wave excitation with localized spins.