Ce-substitution effects on the spin excitation spectra in Pr_1.4-xLa_0.6Ce_xCuO_4+δ

We performed inelastic neutron scattering measurements on as-grown single crystals of electron-doped Pr_1.4-xLa_0.6Ce_xCuO_4+δ with x = 0, 0.08 and 0.18. We succeeded in observing the spin excitation spectra up to 300meV in the all samples. In both parent and electron-doped samples, a magnetic signal was observed at the magnetic zone center (0.5, 0.5) in the low-energy region below 40 meV. In the high-energy region above 100 meV, a double-peak structure reflecting the spin-wave excitation was detected in the constant-energy spectrum for the parent sample, while a single broad peak centered at (0.5, 0.5) was observed for the Ce-doped samples. These results suggest that the dispersion of spin excitation spectra become steeper upon doping. Furthermore, we newly found a drastic reduction of the magnetic intensity with Ce-substitution.