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 Pr(0.89)LaCe(0.11)CuO4 (T-c = 25.5 K). Well-defined spin excitations existed up to an energy of omega similar to 180meV, 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 omega(c) similar to 60 meV, the peak width (kappa) markably increased, but in the high-energy region beyond omega(c) the width remained nearly independent of the energy. chi ''(omega) showed a sharp peak at w = 6 meV, which markably diminished as the energy increased until eventually a well-defined magnetic signal was not observed near omega(c). However, a broad magnetic peak at (pi, pi) reappeared beyond omega similar to 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.