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We have performed inelastic neutron scattering measurements onoptimally-doped Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ and 10% Cu-dopedFe$_{0.88}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$ to investigate the substitutioneffects on the spin excitations in the whole energy range up to 300 meV. It isfound that substitution of Cu for Fe enhances the low-energy spin excitations($\le$ 100 meV), especially around the (0.5, 0.5) point, and leaves thehigh-energy magnetic excitations intact. In contrast to the expectation that Cuwith spin 1/2 will dilute the magnetic moments contributed by Fe with a largerspin, we find that the 10% Cu doping enlarges the effective fluctuating momentfrom 2.85 to 3.13 $\mu_{\rm B}$/Fe, although there is no long- or short-rangemagnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhancedmagnetic excitations in the 10% Cu doped sample which is in the insulatingstate indicates that the magnetic excitations must have some contributions fromthe local moments, reflecting the dual nature of the magnetism in iron-basedsuperconductors. We attribute the substitution effects to the localization ofthe itinerant electrons induced by Cu dopants. These results also indicate thatthe Cu doping does not act as electron donor as in a rigid-band shift model,but more as scattering centers that localize the system.

Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in an Fe-based superconductor Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$