Sodium overlayers on low-index tungsten surfaces: Field and photofield emission currents and surface electronic structures

摘要

The total-energy distributions (TEDs) of the emission currents in field emission and surface photofield emission and the overlayer-induced modifications in the surface electronic structures from the technologically important W surfaces with the commensurate W(100)/Na c(2 × 2) and W(110)/Na (2 × 2) overlayers are studied from first principle and compared to experiments. The TEDs obtained by our recent numerical method that extends the full-potential linear augmented plane-wave method for the electronic structures to the study of field and photofield emission are used to interpret the shifts of the peaks in the experimental TEDs in field emission and photofield emission from the W(100) and W(l 10) surfaces at submonolayer and monolayer Na coverage. Wave function overlap of the 3_s Na states and the pairs of d_(z~2)-like surface states of the strong Swanson hump in clean W(100) below the Fermi energy shifts these W states by about -1.2 eV, thus stabilizing these states, to yield new.strong peaks in the TEDs in field emission and photofield emission from W(100)/Na c(2 x 2), in agreement with experiments. Na intralayer interactions shift the strong s- and p-like peaks in the surface density of states of W(l 10) below and above the Fermi energy, respectively, to lower energy with increasing coverage.