Novel spin excitations in optimally electron-doped Pr0.89LaCe0.11CuO4

To study the detailed structure of the magnetic excitation spectrum, we performed a neutron scattering experiment on optimally electron-doped Pr0.89LaCe0.11CuO4 (Tc=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 Tc. Better statistics in our experiment revealed a more precise structure of the spectrum. As the energy increased up to ωc - 60 meV, the peak width (κ) markably increased, but in the high-energy region beyond ωc 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 ωc. 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. (author)