O 1s near-edge x-ray absorption of La2-xSrxNiO4+delta: Holes, polarons, and excitons

Polarization-dependent near-edge x-ray absorption spectroscopy measurements have been performed on the O 1s and Ni 2p edges of La2-xSrxNiO4+delta single crystals for 0 less than or equal to x less than or equal to 0.6. The results are compared with recent data on La2-xSrxCuO4+delta, and NiO and with detailed cluster calculations on a NiO6 cluster. From this, we determine the energetic ordering of the states close to the Fermi level with Ni(Cu) 3d(x2-y2)/O 2p(x,y) and Ni(Cu) 3d(3z2-y2)/apex-O 2p(z) orbital character for the undoped compounds. We find that d-d exciton side bands show up in the final states of La2-xSrxNiO4+delta, giving evidence for the Zhang-Rice character of the doped carriers. The spectral-weight transfer in the O Is spectra shows a significant dynamical component. Thus, we suggest that the polarons in La2-xSrxNiO4+delta can be seen as nonclassical objects on the length scale of a lattice constant and that the formation of polaron domain walls and polaron lattices possibly has to be considered as a result of this large coherence length of the polarons.