Fermi gap stabilization of an incommensurate two-dimensional superstructure

The compressed, incommensurate approximately (9.5 x 9.5) moire superstructure of the Ag monolayer on Cu(111) displays a filled surface state band with a Fermi energy gap at the Brillouin zone boundary. By contrast, the surface band is gapless for the less compressed, commensurate (9 x 9) moire of two Ag layers. A simple estimate of the energy gain rendered by opening this gap gives a value similar to the elastic energy change required to modify the commensurate structure, thereby suggesting that the approximately (9.5 x 9.5) incommensurate phase is stabilized by such a gap opening. The possible presence of a charge density wave state is discussed.     

Continuous transition from two- to one-dimensional states in Si(111)-( 5x2)-Au

A strong, gold-induced surface state is found on single-domain Si(111)-(5x2)-Au at low temperatures. Its band dispersion is one dimensional near the Fermi level E(F) and gradually becomes two dimensional towards the bottom of the band, thus providing a model for a continuous transition in dimensionality. A Peierls-like gap is observed in the one-dimensional portion of the band near E(F).     

Indication of charge-density-wave formation in Bi(111)

Photoemission spectroscopy of Bi(111) reveals a small hexagonal two-dimensional Fermi surface (FS) associated with an electron band centered in the surface Brillouin zone. Along the hexagon the Fermi momentum k(F) ranges from 0.053 to 0.061 A(-1). Temperature dependent valence band spectra show an anisotropic energy gap Delta near the Fermi level. We find a transition temperature of about 75 K. At 11 K, the gap is Delta=4 meV at the corner and Delta=7.5 meV at the side of the hexagon. Arguments based on susceptibility chi(--> q) calculations of a hexagonal FS are used to discuss an incommensurate charge-density-wave (CDW) formation associated with a q(CDW)=0.106 A(-1).     

Role of surface States in scanning tunneling spectroscopy of (111) metal surfaces with Kondo adsorbates

A nearly free-electron model to describe scanning tunneling spectroscopy of (111) metal surfaces with Kondo impurities is presented. Surface states are found to play an important role giving a larger contribution to the conductance of Cu(111) and Au(111) than Ag(111) surfaces. The different line shapes observed when Co is adsorbed on the different substrates are mainly determined by the position of the surface band onset relative to the Fermi energy and the decay length of the surface state into the substrate. The lateral dependence of the line shape amplitude is found to be bulklike for R|| < or approximately 3-5 A and surfacelike at larger distances, in agreement with experimental data.     

Surface-state depopulation on small Ag(111) terraces

The dependence of the local density of states near the Fermi energy E(F) on the width of terraces T is investigated by tunneling scanning spectroscopy on Ag(111) at 7 K. With decreasing T, the electronic density in the occupied surface state shifts monotonically towards E(F), leading to a depopulation at T=3.2 nm in quantitative agreement with a Fabry-Pérot model. Depopulation coincides with a switch from confinement by terrace modulation to step modulation.     

Surface states of cobalt nanoislands on Cu111

The electronic structure of thin Co nanoislands on Cu(111) has been investigated below and above the Fermi level (E(F)) by scanning tunneling spectroscopy at low temperature. Two surface related electronic states are found: a strong localized peak 0.31 eV below E(F) and a mainly unoccupied dispersive state, giving rise to quantum interference patterns of standing electron waves on the Co surface. Ab initio calculations reveal that the electronic states are spin polarized, originating from d3(z(2)-r(2))-minority and sp-majority bands, respectively.     

Surface-state localization at adatoms

Low-temperature scanning tunneling spectroscopy of magnetic and nonmagnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.     

The Cu conduction band gap at the L point determined by photoemission and electron reflectivity measurements

The energies of the Cu conduction band gap edges around the Fermi level at the L point of the Brillouin zone are determined by angle resolved photoemission and electron reflectivity measurements on Na covered Cu(111).     

一种由自由电子能带模型计算费米能级的方法

对面心立方（fcc)、体心立方（bcc)和六角密堆积（hcp)三种不同结构的晶体,在假设它们的原胞中包含8个价电子并将价电子近似为自由电子的情况下,采用“自由电子气理论”和“自由电子能带模型”,研究其根据费米球确定的费米能级EF与根据自由电子能带模型计算的平均键能Em。研究结果表明,由自由电子能带模型计算所得3种不同结构晶体（因而电子密度也不一样）的平均键能Em等于各自自由电子系统的费米能级EF。平均键能Em是我们在异质结带阶理论计算中建议的一种参考能级,研究结果在深化对平均键能Em物理实质认识的同时,提供了一种借助于自由电子能带模型计算自由电子系统费米能级EF的新方法。     

Band gap and electronic structure of an epitaxial, semiconducting Cr0.80Al0.20 thin film

Cr(1-x)Al(x) exhibits semiconducting behavior for x = 0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr(0.80)Al(0.20) thin film show several features in the valence band region, including a gap at the Fermi energy (E(F)) for which the valence band edge is 95 ± 14 meV below E(F). Theory agrees well with the valence band measurements, and shows an incomplete gap at E(F) due to the hole band at M shifting almost below E(F).     

Thin noble metal films on Si (111) investigated by optical second-harmonic generation and photoemission

Thin noble metal films (Ag, Au and Cu) on Si (111) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. The valence band spectra of Ag films show a quantization of the sp-band in the 4-eV energy range from the Fermi level down to the onset of the d-bands. For Cu and Au the corresponding energy range is much narrower and quantization effects are less visible. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness. The oscillations are strongest for Ag and less pronounced for Cu, in agreement with valence band photoemission spectra. In the case of Au, a reacted layer floating on top of the Au film masks the observation of quantum well levels by photoemission. However, SHG shows a well-developed quantization of levels in the Au film below the reacted layer. For Ag films, the relation between film thickness and photon energy of the SHG resonances indicates different types of resonances, some of which involve both quantum well and substrate states. Received: 16 October 2001 / Revised version: 14 March 2002 / Published online: 29 May 2002     

Step-lattice-induced band-gap opening at the fermi level

The interaction of the Shockley surface state with the step lattice of vicinal Cu(111) leads to the formation of an electronic superlattice state. On Cu(443), where the average terrace length forms a "shape resonance" with the Fermi wavelength, we find a step-lattice-induced band-gap opening at the Fermi level. A gap magnitude >200 meV is inferred from high resolution photoemission experiments and line shape analysis. The corresponding energy gain with respect to a gapless case is approximately 11 meV/unit cell, and is a substantial contribution to the stabilization of the step lattice.     

Observing the lateral confinement of surface state electrons in room temperature stable metallic nanostructures

The lateral confinement of the surface state electrons of Cu(111) has been studied by Scanning Tunnelling Microscopy and Spectroscopy at low temperature. The confining nanostructures are Cu(111) islands embedded in a semiconducting Cu₃N(111) film which completely isolate them from each other. The standing wave pattern observed reflect the shape of the edge of the islands, i.e. the positions of the confining potential as long as the islands are larger than twice the Fermi wavelength of the surface electrons. The interference pattern in smaller islands is more complex, reflecting the collective behavior of the electrons. When the width of the islands is, at least in one dimension, smaller than the Fermi wavelength, there is a clear shift in the energy of the bottom of the surface band towards the Fermi level. The depopulation of the surface state produced by lateral confinement might have important consequences with respect to the reactivity of these nanostructures.Received: 15 December 2003, Published online: 10 August 2004PACS: 68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM) - 73.20.At Surface states, band structure, electron density of states - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Single-gap s-Wave superconductivity near the charge-density-wave quantum critical point in CuxTiSe2

The in-plane thermal conductivity kappa of the layered superconductor CuxTiSe2 was measured down to temperatures as low as Tc/40, at x=0.06 near where the charge-density-wave order vanishes. The absence of a residual linear term at T-->0 is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe2, where the superconducting gap is 3 times larger on the Nb 4d band than on the Se 4p band, suggests that in Cu0.06TiSe2 the Se 4p band is below the Fermi level and Cu doping into the Ti 3d band is responsible for the superconductivity.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

Direct observation of the mass renormalization in SrVO3 by angle resolved photoemission spectroscopy

Band dispersions and Fermi surfaces of the three-dimensional Mott-Hubbard system SrVO3 are directly observed by angle-resolved photoemission spectroscopy. An observed spectral weight distribution near the Fermi level (E(F)) shows cylindrical Fermi surfaces as predicted by band-structure calculations. By comparing the experimental results with calculated surface electronic structures, we conclude that the obtained band dispersion reflects the bulk electronic structure. The enhanced effective electron mass obtained from the energy band near E(F) is consistent with the bulk thermodynamic properties and hence with the normal Fermi-liquid behavior of SrVO3.     

Modification of terminating species and band alignment at the interface between alumina films and metal single crystals

Thin epitaxial alumina films were grown on Cu(111), Cu–9 at.%Al(111), Ni(111) and NiAl(110) single crystals. The alumina films grew in such a manner that hexagonal or pseudo-hexagonal oxygen lattices were parallel to the surface of the substrates. Photoelectron spectra were obtained either with synchrotron or Al K-alpha radiation. We measured Al 2p spectra and determined the atomic species that terminated the interface between the alumina films and the substrates. The influence of Al in the substrates on the species that terminated the interface has been discussed based on thermodynamics. From valence band spectra, p-type Schottky barrier height (energy difference between the Fermi level of the metallic substrates and the valence band maximum of the alumina films, band offset) was determined. Differences in interface terminating species resulted in variations in p-type Schottky barrier height, or band alignment.     

The ordinary transport properties of the noble metals

The Fermi Surfaces in Cu, Ag and Au are now known to be greatly distorted, with thick ‘necks’ passing through the zone boundaries. In this paper we enquire whether such an electronic structure is quantitatively consistent with the observed transport coefficients. The mathematical model is quite simple; the shape of the Fermi surface is made to depend on a single parameter which can be interpreted as the pseudo-potential of the {111} atomic planes acting on an orthogonalized plane wave, giving rise to an energy gap of 5–10 ev at the zone boundaries. Various integrals over the Fermi surface can then be evaluated by elementary methods, and compared with the corresponding experimental quantities. The electronic specific heat and optical mass in the pure metals are consistent with the model. The galvanomagnetic effects are shown to depend a great deal on the anisotropy of the electron relaxation time, whose variation with energy is also probably the electron relaxation time, whose variation with energy is also probably the main determinant of the sign of the thermoelectric power. A better theory of electron-phonon interaction is needed before this, and the electrical and thermal conductivities, can be calculated accurately. Generally speaking there is no evidence which directly contradicts the rigid band model, except perhaps the effect of alloying on the optical absorption edges and on the electronic specific heat, but there are still many experimental and theoretical gaps in our knowledge.     

The surface electronic structure of Ag8Au4 superlattice

We present the calculations of electronic structure and photoemission spectra for a lattice-matched Ag-Au(111) superlattice. The selfconsistent band structure exhibits a superlattice gap at about 1 eV below the Fermi level. A surface state is found in this gap and its dispersion properties are investigated. Its energy location is varied with location of surface terminating plane within the superlattice period. The calculated normal photoemission spectra explain well available experimental data.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.