Reduction of charge fluctuation energies in ultrathin NiO films on   Ag(001)
  We investigate on-site Coulomb interaction energy between two 3p holes U(Ni 3p) of ultrathin NiO films on Ag(001) by both x-ray photoelectron spectroscopy and Auger electron spectroscopy. As the film becomes thin, U(Ni 3p) monotonically decreases, and the difference of U(Ni 3p) for 1 monolayer (ML) film from that of bulk-like thick film \delta U(Ni 3p) reaches ~ -2.2 eV. The observed \delta U(Ni 3p) for 1 ML film is well reproduced by the differences of both the image potential and polarization energies between 1 ML film and the bulk-like thick film. Hence, the present results provide an evidence for the picture originally proposed by Duffy et al. [J. Phys. C: Solid State Phys., 16, 4087 (1983)] and Altieri et al. [Phys. Rev. B 59, R2517 (1999)] 
Seolun Yang, H. -K. Park, J. -S. Kim, H.-N. Hwang, C. -C. Hwang, S.   -H. Phark, Y. J. Chang, T. W. Noh, and H. -D. Kim
cond-mat.str-el