The electronic structure of surfaces with the matching green function method: II. fcc and bcc transition metal surfaces

Using the matching Green function method, the densities of states at (001) surfaces of Ni (fcc) and Mo (bcc) are calculated. Fixing the wavevector parallel to the surface, the surface density of states at Ni(001) is similar to the bulk, with band edge singularities rounded off and reflected in big surface resonances. Changes are much greater in the case of Mo(001), and the surface density of states has a central peak in the minimum of the bulk density of states. This peak arises from the 4d level in the surface atoms interacting weakly with the substrate and with neighbouring surface atoms. Our results are compared with angular resolved photoemission theory and experiment — in the case of Ni all the experimental peaks correspond to bulk k-conserving transitions, though some surface resonances in the Mo surface density of states show up in photoemission.     

Excitations in charge-transfer atom scattering

The electronic excitations are calculated for a tight-binding model of 25–1000 eV Na atoms scattering off W, assuming a classical trajectory for the Na atoms which interact with a single half-filled band on the substrate. The excitation spectrum consists of substrate electron-hole pairs at low energies, with a jump at the ionization threshold due to electron transfer from the Na to the W. If the Na ionization level crosses the Fermi energy beyond the range of hopping between the Na and the W substrate, the ionization probability is high. As the Na kinetic energy is reduced the ionization probability decreases, but the substrate electron-hole excitations increase in importance, and this is discussed in semi-classical terms.     

Characterization of a surface state on TiN(001)

A surface state seen in normal photoemission from TiN(001), at - 2.9 eV relative to the Fermi energy, is associated with the Δ₅ band in the bulk band structure consisting of N p_x/p_y orbitals. As TiN is somewhat ionic, there is a change in the electrostatic potential at the surface and this is large enough to pull a Tamm surface state off the Δ₅ band.     

Electronic structure and conductance of large molecules and DNA

Using a new technique based on embedding in a local orbital formalism, the electronic structure and electron transmission properties of long biological molecules are calculated, in particular DNA. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present, an extended Hückel scheme is used for the Hamiltonian. The transmission is also calculated within the embedding scheme, taking the molecule–metal contacts into account. The results for transmission depend greatly on the orbitals to which contact is made, and also on energy. The implications of these calculations for conductance are discussed.     

Theory of image states at magnetic surfaces

The interaction of the spin of an electron in an image state with surface magnetism produces a spin-splitting which can be probed experimentally, most directly using spin-polarised inverse photoemission. There has been some debate about whether the spin-splitting is due to the spin-dependence of the surface potential barrier, or to the spin-dependence of the scattering of the surface state by the crystal potential. We have shown that in the case of image states at Fe(110) both effects contribute, but with opposite sign: the major effect is the effect of the crystal, and the potential barrier which has the opposite spin-polarization reduces the spin-splitting. A splitting of 55 meV is found for the n = 1 state, which has been confirmed by experiment. The dispersion of the spin-split states is discussed, particularly their interaction with the spin-split continua which produces different surface resonance behaviour for the two spins.