Final state band gap effects in UV photoemission from Cu(111)

The crystal momentum dependence of the final states involved in bulk photoemission from the sp-bands through a (111) surface of copper was investigated with a photon energy hν = 11.7 eV. Plane wave hybridization was observed and is described in terms of a 2-OPW model Hamiltonian with parameters determined to be V₀=7.1±0.1 eV for the inner potential and E_g = 1.6 ± 0.2 eV for the energy gap. The model is also shown to account for a rapid intensity variation with crystal momentum of the form |a_G(k)|², where a_G(k) is a plane wave amplitude of the final state wave function.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

Direct observation of charge transfer at a MgO(111) surface

Transmission electron diffraction (TED) combined with direct methods have been used to study the sqrt[3]xsqrt[3]R30 degrees reconstruction on the polar (111) surface of MgO and refine the valence charge distribution. The surface is nonstoichiometric and is terminated by a single magnesium atom. A charge-compensating electron hole is localized in the next oxygen layer and there is a nominal charge transfer from the oxygen atoms to the top magnesium atom. The partial charges that we obtain for the surface atoms are in reasonable agreement with empirical bond-valence estimations.     

Lifetimes of electrons in the Shockley surface state band of Ag(111)

We present a theoretical many-body analysis of the electron–electron (e–e) inelastic damping rate Γ of electron-like excitations in the Shockley surface state band of Ag(111). It takes into account ab initio band structures for both bulk and surface states. Γ is found to increase more rapidly as a function of surface state energy E than previously reported, thus leading to an improved agreement with experimental data. PACS 73.20.At; 68.37.Ef; 72.15.Lh     

Photoemission observation of a new surface state band on Cu(110)

From angle-resolved photoelectron energy distribution curves we have identified an occupied surface state band at the symmetry point ? of the two-dimensional Brillouin zone of Cu(110), which has been predicted theoretically by Dempsey and Kleinman. We have also remeasured the dispersion and shape of the surface state band of Cu(111) and compare it with the results from Cu(110).     

Engineering negative differential conductance with the Cu(111) surface state

Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate electron tunneling from a C60-terminated tip into a Cu(111) surface. Tunneling between a C60 orbital and the Shockley surface states of copper is shown to produce negative differential conductance (NDC) contrary to conventional expectations. NDC can be tuned through barrier thickness or C60 orientation up to complete extinction. The orientation dependence of NDC is a result of a symmetry matching between the molecular tip and the surface states.     

Link between adatom resonances and the Cu(111) Shockley surface state

Low-temperature scanning tunneling microscopy and spectroscopy at 7 K was used to assemble and characterize native adatom islands of successive size on the Cu(111) surface. Starting from the single adatom we observe the formation of a series of quantum states which merge into the well known two-dimensional Shockley surface state in the limit of large islands. Our experiments reveal a natural physical link between this fundamental surface property and the sp(z) hybrid resonance associated with the single Cu/Cu(111) adatom.     

Surface state like bulk band emission in the photoelectron energy spectra of Cu(111)

Using a relativistically calculated band structure the presence in Cu(111) UV photoemission spectra of a prominent 3d peak, previously ascribed to a Tamm type surface state, is explained in terms of bulk band transitions.     

Inelastic quasiparticle lifetimes of the Shockley surface state band on Ni(111)

We present a study of the low-energy quasiparticle lifetimes of the Shockley surface state on the Ni(111) surface with scanning tunnelling spectroscopy. By measuring the coherence length of the decaying standing wave pattern at straight step edges electron and hole lifetimes have been determined. The values of the lifetime measured on this ferromagnetic surface show to be considerable smaller than the values obtained from noble metal surfaces. This is explained by differences in the electron density of states at the Fermi energy but has to include substantial spin-flip scattering. Furthermore hole lifetimes appear to be larger than electron lifetimes with the same excitation energy. Although only results for the majority spin component are presented, a spin-dependent selfenergy is expected.     

Enhanced charge transfer in a monolayer of the organic charge transfer complex TTF-TNAP on Au(111)

Electronic doping is a key concept for tuning the properties of organic materials. In bulk structures, the charge transfer between donor and acceptor is mainly given by the respective ionization potential and electron affinity. In contrast, monolayers of charge transfer complexes in contact with a metal are affected by an intriguing interplay of hybridization and screening at the metallic interface, determining the resulting charge state. Using scanning tunneling microscopy and spectroscopy, we characterize the electronic properties of the organic acceptor molecule 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) adsorbed on a Au(111) surface. The ordered islands remain in a weakly physisorbed state with no charge transfer interaction with the substrate. When the electron donor tetrathiafulvalene (TTF) is added, ordered arrays of alternating TNAP and TTF rows are assembled. In these structures, we find the lowest unoccupied molecular orbital (LUMO) of the free TNAP molecule shifted well below the Fermi level of the substrate. The TNAP is thus charged with more than one electron.     

Lifshitz transition across the Ag/Cu(111) superlattice band gap tuned by interface doping

The two-dimensional, free-electron-like band structure of noble metal surfaces can be radically transformed by appropriate nanostructuration. A case example is the triangular dislocation network that characterizes the epitaxial Ag/Cu(111) system, which exhibits a highly featured band topology with a full band gap above E(F) and a hole-pocket-like Fermi surface. Here we show that controlled doping of the Ag/Cu(111) interface with Au allows one to observe a complete Lifshitz transition at 300 K; i.e., the hole pockets fill up, the band gap entirely shifts across E(F), and the Fermi surface becomes electron-pocket-like.     

Angle-resolved photoemission studies of a surface state for Ag overlayers on Cu(111)

We have performed an angle-resolved photoemission study of a surface state on Cu(111) covered by various thicknesses of Ag. The growth mode of Ag on Cu(111) was determined to be layer-by-layer despite the large mismatch between the two lattices. The Cu(111) surface state was observed to evolve monotonically for increasing Ag overlayer thickness to eventually become the Ag(111) surface state. The measured rate of shift of the surface state binding energy can be explained qualitatively in terms of the degree of localization of the surface-state wave functions.     

Localization of the Cu111 surface state by single Cu adatoms

The Cu adatom-induced localization of the two-dimensional Shockley surface state at the Cu(111) surface was identified from experimental and simulated scanning tunneling microscopy spectra. The localization gives rise to a resonance located just below the surface state band edge. The adatom-induced surface state localization is discussed in terms of the existence theorem for bound states in any attractive two-dimensional potential. We also identify adatom-induced resonance states deriving from atomic orbitals in both experimental and simulated spectra.     

Electronic structure of the Cu(111) surface

Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.     

Laser-induced charge transfer and photodesorption of Cs at Cu(111): quantum dynamical model simulations

We present electronic and nuclear quantum model simulations for the direct, laser-induced charge transfer from a Cu(111) surface state to an unoccupied resonance state of an adsorbed Cs atom, and the resulting nuclear dynamics. Based on a modified one-electron model potential adopted from Chulkov et al., we determine energies and lifetimes of electronic states of Cs/Cu(111) at low coverage. In addition, semiempirical nuclear potential energy surfaces of the electronic ground and the antibonding excited states along the Cs-Cu distance are designed and used for nuclear dynamics. For both models, electronic and nuclear, we perform open-system quantum dynamics with the goals (i) to estimate the excitation (charge transfer) and Cs desorption probabilities and (ii) to optimize the charge-transfer process by pulse shaping and the desorption of Cs from the Cu surface by vibrationally exciting the Cs-Cu bond. PACS 79.20.La; 71.10.Li     

On the electronic surface band energy of the Cu(111)⧸Cs adsorption system

As Cs atoms are adsorbed, the electronic surface band of the Cu(111) surface is shifted towards lower energies. Good agreement with experiments is obtained by using a model which takes into account the relation between the surface state energy and the work function. Two other simple models are also discussed.