Unoccupied electronic states in atomic chains on Si(557)-Au : Time-resolved two-photon photoemission investigation

In recent years, it has become possible to create onedimensional 1D structures at vicinal silicon surfaces that consist of atomic chains. Gold atoms produce metallic chains, which may be viewed as the ultimate nanowires.1 The electronic states at the Fermi level lie inside the band gap of silicon and thus cannot hybridize with threedimensional states in the substrate, but the gold atoms are firmly locked to the surface lattice in substitutional sites. These surfaces can be used to systematically explore phases of electrons between one and two dimensions and to search for exotic new phenomena that are caused by the fact that the electron-electron interaction takes a qualitative leap in one dimension because all electrons are moving along the same line.2 As a consequence, the familiar Fermi liquid breaks down in one dimension and is replaced by a TomonagaLuttinger liquid, where single electron excitations are replaced by collective excitations. The Si 557 -Au surface has become a prototype for atomic chain structures. It is one of the simplest chain structures with just a single Au chain per unit cell, and it has been investigated by many techniques, such as electron diffraction,3 x-ray diffraction,4 scanning tunneling microscopy STM and spectroscopy STS ,5–11 angle-resolved photoemission,5,7,12 inverse photoemission,13 electron-energy loss spectroscopy,14 surface conductivity,15 and firstprinciples calculations.16–18 The band structure is characterized by a closely spaced doublet of half-filled bands that is common to all Au-induced chain structures.17 This unusual doublet has been explained by a spin splitting induced by the spin-orbit interaction at a surface.18,19 While the occupied bands have been mapped in detail by photoemission, the unoccupied part of the band structure has remained largely unexplored. An image-potential state has been found by inverse photoemission,13 reflecting the metallic nature of the surface. Theory18 predicts that the bands above EF differ dramatically from the occupied part below EF: The metallic occupied band opens up a gap just above the Fermi level, which is assigned to the splitting between even and odd combinations of two equivalent Si bonds pointing toward a gold atom in the chain. In contrast to the strongly dispersing band below EF the calculation finds a flat, localized band just above EF which is associated with a graphitic Si chain at the step edge, not the Au chains in the middle of the terrace. A wide open territory is the dynamics of carriers in 1D systems. Excited electrons are expected to interact strongly in 1D. Therefore it would be interesting to determine lifetimes and decay mechanisms on a short time scale. This study uses a femtosecond pump-probe technique to access the unoccupied electronic states of Si 557 -Au by bichromatic two-photon photoemission.20 To our knowledge, such studies have not yet been performed on 1D electron systems. Two-photon photoemission 2PPE spectra were obtained using a Ti:sapphire oscillator at a center wavelength IR =800 nm h IR=1.55 eV, pulse duration IR 37 fs . Part of the output intensity was frequency tripled h UV=4.65 eV, UV 55 fs . Both beams were spatially overlapped on a beam splitter after the IR beam passed a delay stage. The polarizations of the IR and UV beams could be rotated to align the electric field vector either perpendicular to the plane of incidence s-pol. or parallel p-pol. . Figure 1 depicts the experimental geometry. In normal emission the beams were incident on the sample at a glancing angle of 10°. The p-type Si 557 -Au samples were prepared in an ultrahigh-vacuum chamber base pressure 5 10−11 mbar following a technique which was described earlier in the literature.17 Photoelectron spectra were recorded using an Omicron EA 125 HR spectrometer with seven channeltron detectors at an angle and energy resolution of 1.6° and 17 meV, respectively. Experiments were carried out at 300 K and 90 K. Apart from a photovoltage arising at low temperatures, no differences could be observed.