Depolarization and metallization in alkali-metal overlayers

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Abstract
We study the level shift of the alkali-metal valence resonance with increasing adatom coverage (Θ) with a simple formula based on the Gurney model in which alkali-metal adatoms interact only via the long-range dipole interaction. The calculated level shift is in good agreement with the results of first-principles electronic-structure calculations below the onset of the overlayer ‘‘metallization,’’ but breaks down for higher Θ where the orbital overlap among nearby adatoms plays a crucial role in the electronic properties of the overlayer. Our result is also in good accord with the recent experimental data for K/Al(111) at low K coverage.