Search

We investigate a number of isostructural, quasi-two dimensional transition metal dichalcogenides with respect to Fermi surface nesting. Using angle-resolved photoemission we find no clear evidence for Fermi surface nesting as a key scenario for charge density wave formation in these materials. However, interesting and unusual behavior has been discovered. For 1T-TaS2 in the charge density wave phase, instead of finding an intact Fermi surface along the rounded parts of its elliptically shaped contours and gaps along the parallel regions, the Fermi surface is essentially completely pseudogapped. For 1T-TiSe2 a model based on an excitonic insulator phase as described by Kohn [Phys. Rev. Lett., 19 (1967) 439] fits well with experimental observations. Bandstructure calculations give interesting insight into the electron configuration indicating the changes in d-band occupation and metal-d chalcogen-p overlap when going from one material to the other.

We demonstrate a very simple and reliable method of manufacturing clean, single-crystalline Y2O3 films on Nb(110) substrates in situ. The method exploits the oxygen bulk contamination of Nb as a source of clean oxygen. For substrate temperatures above 800 K oxygen segregation to the Nb surface is so efficient, that yttrium becomes oxidized during deposition without any background oxygen pressure required in the ultrahigh vacuum system. The crystallinity and stoichiometry of these films can be tuned by the deposition temperature. For Y deposition at 1300 K the formation of well-ordered (111)-oriented Y2O3 films is achieved.

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS2 in the quasicommensurate (metallic) and the commensurate (insulating) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS. Possibly, this Mott localization induced transition represents the underlying instability responsible fur the pseudogapped FS.

Au films, from the submonolayer range up to eight monolayers, have been deposited in situ at 300 K. The electronic structure has been probed by ultra-violet valence-band photoemission. The discussion of our findings is based on a detailed knowledge of the geometrical structure of each film. With increasing Au coverage the valence-band structure changes from Ag- to Au-bulk-like. At intermediate Au coverages an interface state is observed. It appears to be localized between the first and second surface layer of areas where Au and Ag atoms are inverted. Moreover, the sp-type surface state observed for emission along Ȳ in the case of the clean Ag(110) surface is strongly influenced by these inverted Au/Ag areas in that it shifts to higher binding energies. For higher Au coverages this surface state gradually fades out.

Au films, from the submonolayer range up to 11 ML, have been deposited in situ at 300 K. The geometrical structures of these films have been investigated combining full-hemispherical x-ray photoelectron diffraction, low-energy electron diffraction (LEED), low-energy ion-scattering spectroscopy, and scanning tunneling microscopy leading to an intermixed Volmer-Weber growth model. The results demonstrate that below 0.5 ML most Au atoms are buried within the second substrate layer, forming inverted Ag/Au areas on the surface. The ejected Ag atoms and vacancies created during the Au-Ag exchange nucleate into elongated two-dimensional Ag islands and vacancy clusters, respectively, quickly breaking up the surface into smaller terraces. Above about 0.5-ML coverage, the Au-Ag exchange mechanism continues to be active. In addition, due to the reduced mobility of Au atoms deposited on inverted Ag/Au areas, one-dimensional Au stripes as well as elongated three-dimensional (1\ifmmode\times\else\texttimes\fi{}3)-symmetric Au islands are observed already at submonolayer coverages on inverted Ag/Au areas. Only after the deposition of more than 8-ML Au is the Ag substrate completely covered, and missing-row reconstructed terraces extend over regions large enough to yield a well-defined 1\ifmmode\times\else\texttimes\fi{}2 LEED pattern. The growth model is compared to both, published thermodynamic equilibrium predictions and molecular-dynamics simulations, revealing that the Au/Ag(110) growth system is kinetically determined.

A laboratory set-up for ultraviolet (UV) photoelectron spectroscopy combining a high intensity UV source and a toroidal UV monochromator, both being commercially available, is described. The source is modified in order to maximize the solid angle collected by the monochromator. Scanning its exit arm length allows for the optimization of the photon intensity as well as the energy resolution by minimizing the defocus term only. By using several different gases such as He, Ne or H2 one obtains a powerful tool to sample extended regions in k-space. Fermi energy maps are presented for Cu(110) in the photon energy range from 10.2 to 48.3 eV. As a result of the variable photon energy an actual zooming into the well-known Shockley surface state is possible.

The nitrogen-doped (N-doped), type Ib, synthetic diamond (100) surface was investigated by means of X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Photoelectron emission data from the boron-doped (B-doped) and the N-doped diamond (100) surfaces were compared and permitted the energy band diagrams for these differently terminated surfaces to be drawn. We observed emission from energy levels below the conduction band minimum up to the vacuum level and therefore succeed in evaluating the negative electron affinity (NEA) of the hydrogen-terminated diamond surfaces. Both the hydrogen-terminated N- and B-doped diamond (100) surfaces show NEA values of at least −0.2 and −1.0 eV, respectively, while the hydrogen-free surfaces show positive electron affinity. In contrast to the hydrogen-terminated B-doped (100) surface, UPS measurements on the hydrogen-terminated N-doped (100) surface do not reveal a high intensity NEA peak owing to the strong upward band bending. The high intensity NEA peak of B-doped diamond seems to be due to the downward band bending together with the reduced work function because of hydrogen termination. The work function increases for subsequent hydrogen desorption at higher annealing temperatures with associated loss of NEA. For the N-doped diamond (100) surface the work function behaves similarly but the observation of a NEA peak is absent because of the surface barrier formed by the upward band bending.

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the reversibility of hydrogen loading in thin single-crystalline Y films grown by vapor deposition on W(110). Under a ${\mathrm{H}}_{2}$ partial pressure of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ mbar the hexagonal-closed- packed Y films convert to the face-centered-cubic Y dihydride. Unloading is accomplished by annealing the dihydride to 1000 K. No loss of crystallinity is observed during these martensitic transformations of the Y lattice. Moreover, we demonstrate a model to determine the H concentration in Y in situ.

The electronic structure of hydrogen adsorbed on Ni(110) at 150 K has been studied using angle-resolved photoelectron spectroscopy with monochromatized He Iα radiation. At a saturation coverage of 1.5 monolayers, we observe a general shift of the spectral weight away from the Fermi level to higher binding energies indicating the hybridization of hydrogen levels with metal d states. Along selected Ni 3d bands the intensity shift is obvious whereas other bands remain unchanged. In addition, contrary to previous observations, a hydrogen induced state is visible at a binding energy of roughly 1.2 eV.

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the preparation of clean, single-crystalline YHx films (0⩽x⩽2.9). The films have been characterized in situ combining angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction. Direct Y dihydride growth, i.e., Y evaporation under a H2 partial pressures of ≈5×10−6 mbar at 500 K on W(110), is the most convenient starting point for the preparation of clean single-crystalline Y hydride films covering H concentrations from the “clean metal” (x≈0) up to the lower boundary of the pure trihydride phase (x≈2.9). Upon annealing Y dihydride films the desired H concentration can be adjusted within the α-phase or the (α+β) two-phase regime. On the other hand, the extension of our photoelectron spectrometer with an homemade ultrahigh vacuum (UHV) compatible hydrogenation system allows to induce the transition from Y dihydride to Y trihydride within a few minutes. The hydrogenation system combines a high-pressure reaction cell with hydrogen permeation through a Pd–24%Ag tube. The overall design is such that the sample never gets in contact with non-UHV compartments. For direct Y dihydride growth on W(110) two equally populated face-centered- cubic(111) domains rotated by 180° with respect to each other are observed. In the α- and γ-phase the Y atoms form a hexagonal-close-packed(0001) oriented lattice. Furthermore, the previously established model for in situ H concentration estimation in Y [J. Hayoz et al., Phys. Rev. B 58, R4270 (1998)] is extended successfully from the α to β to the β to γ-phase transition. Ultraviolet photoemission spectroscopy data unequivocally reveal the opening of a gap extending as far as 1 eV below EF for normal electron emission upon the phase-transformation from Y dihydride to Y trihydride. It also appears that the H absorption rate strongly depends on the H2 purity. Our experimental results demonstrate the capability of this setup for in situ preparation and investigations on the geometrical and electronic structure of Y hydride films and, more generally, rare-earth hydride films using ARPES.

Pd clusters formed by physical vapor deposition at room temperature (RT) on highly oriented pyrolytic graphite were investigated by a combination of X-ray photoelectron spectroscopy and diffraction, angle-resolved ultraviolet photoelectron spectroscopy, scanning tunneling microscopy and calculations based on the local density functional theory. Different coverages with nominally 3, 5 and 10 A were studied after deposition at RT and after heat treatment at 600°C. Local ordering exhibiting growth with an fcc(1 1 1) orientation is already observed at the lowest coverage, but with no preferred azimuthal orientation in accordance with the substrate itself. However electronic structure features characteristic for a Pd(1 1 1) single crystal appear only after heat treatment.

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we examined the charge density wave (CDW) compound $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ as a function of the photon energy (energy range from 11 up to 33 eV) for both quasicommensurate (QC) and commensurate (C) CDW phases. While the QC-phase normal emission ARPES spectra coincide with previous results, normal emission spectra of the C phase reveal evidence for a fine structure in the CDW-induced features between 1 eV binding energy and the Fermi level. This fine structure may be explained by the subband manifold of the Ta $5d$ band, split off due to the CDW-induced $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure.