Reduction of charge fluctuation energies in ultrathin NiO films on Ag(001)

TL;DR

This study measures how the on-site Coulomb interaction energy in ultrathin NiO films on Ag(001) decreases with film thickness, revealing the influence of image potential and polarization effects, and confirming previous theoretical models.

Contribution

It provides experimental evidence for the reduction of charge fluctuation energies in ultrathin NiO films and links this to image potential and polarization energy differences.

Findings

U(Ni 3p) decreases as film becomes thinner

U(Ni 3p) reaches .2 eV for 1 ML film

Results support previous theoretical models of charge fluctuation energy reduction

Abstract

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)]