Surface and thin-film photoemission spectrum of nickel metal: A many-body solution to a two-dimensionally periodic cluster

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Abstract
An exact solution of a four-site crystal model with periodic boundary conditions, a {001} thin film with face-centered-cubic crystal structure, is presented for the nickel density of electron states, as would be measured in surface-sensitive valence-band photoemission. The object is to study modifications of the many-body electronic structure in the presence of surfaces. The model consists of (a) five d orbitals per site per spin, with nearest-neighbor hopping, (b) a one-electron occupation energy for each orbital, and (c) Coulomb repulsions between electrons on the same site. A realistic local-density-approximation one-electron structure and intrasite electron-electron interactions most generally allowed by atomic symmetry are used. Crystal-field effects and single-particle electronic structure in the nickel-film structure are discussed. The photoemission spectrum is calculated and compared with bulk results. Physical conclusions for the true nickel surface many-body states are drawn.