Localized and delocalized excitons:: Resonant inelastic X-ray scattering in La2-xSrxNiO4 and La2-xSrxCuO4

The dynamics of doped charge in an antiferromagnetic lattice is central to the description of the insulator-metal transition that occurs on doping the parent high T-c compounds. In this work we use high resolution resonant inelastic x-ray scattering to investigate the dynamics of the charge-transfer exciton by measuring its energy dispersion in two prototype compounds, La2CuO4 and La2NiO4. We show that this behavior is radically different in the cuprate with respect to a system known to exhibit strong polaronic behavior, namely, the nickelate: the exciton is mobile in the cuprate while it is well localized in the nickelate. Using a simple Wannier-Mott model we can estimate the total hole plus electron effective mass in the cuprate to be 3.5 +/- 0.3m(e) which would exclude strong localization in the undoped cuprate.