Inelastic neutron scattering study on spin excitations of Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ ($T_c$=27.5K)

We use neutron scattering to study the evolution of spin excitations in electron doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$(PLCCO). For $T_c$ = 24 K PLCCO, Wilson et al. [Nature 442, 59 (2006)] have reported the presence of a resonance mode, a localized magnetic excitations coupled directly to the superconductivity in high-$T_c$ superconductors, similar to hole-doped superconductors such as YBa$_2$Cu$_3$O$_{6+\delta}$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and Tl$_2$Ba$_2$CuO$_ {6+\delta}$. Below the resonance are continuing of magnetic scattering with little temperature dependence. We show here our studies of the magnetic excitations on PLCCO ($T_c$=27.5K) with the highest $T_c$ reported in the literature. We confirm the presence of a resonance mode in this sample, and demonstrate that the low-energy magnetic scattering here is much different from the $T_c$=24 K PLCCO. Our results thus shed new light to the understanding of spin excitations in electron-doped copper oxides.