J. Hayoz

Remnant Fermi Surface in the Presence of an Underlying Instability in Layered 1T–TaS2

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.

Growth of Ag on Cu(001) studied by full-hemispherical X-ray photoelectron diffraction

Abstract Ag films, from the submonolayer range up to 14 monolayers (ML), have been deposited in situ on two differently cut Cu(001) single crystals. Their geometrical structure has been investigated by full-hemispherical X-ray photoelectron diffraction and by low-energy electron diffraction. Structural models developed on the basis of these data have been compared to single-scattering cluster calculations. Our results indicate the formation of a surface alloy in the initial step of the Ag growth. At coverages below 0.3 ML, we find that the local environment of the Ag atoms remains fourfold. A first indication for small quasi-hexagonal c(10 × 2) clusters, which are known to appear at 1 ML, is already observed at 0.3 ML. Ag films above 1 ML are compact and exhibit a threefold fcc (111) structure. Furthermore, we noticed that on one of the crystals one of the two possible c(10 × 2) orientations is clearly dominant, and mainly one of the four expected fcc (111) domains grows. On the other crystal the two c(10 × 2) orientations are equally present. Moreover, two fcc (111) domains rotated by 90° with respect to each other develop. We have clear evidence that, from the two possibilities of fcc (111) growth on each c(10 × 2) domain, only one occurs. The comparison of the results from the two differently cut crystals indicates that the domain population is related to the absolute miscut of the substrate and hence by the course of the steps.

Preparation and characterization of clean, single-crystalline YHx films (0 <= x <= 2.9) on W(110)

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.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 withi...

Angle-scanned photoemission: Fermi surface mapping and structural determination

Abstract A brief survey of the angle-scanned photoemission technique is given. It incorporates two complementary methods in one: 1. Mapping of X-ray excited photoelectron intensities over virtually the complete hemisphere above the sample surface results in extended data sets where important surface-geometrical structure information is extracted and even “fingerprinting” is possible. This method is known as the very powerful angle-scanned X-ray photoelectron diffraction. 2. Mapping ultraviolet-excited photoelectron intensities as a function of emission angles gives the possibility to do band mapping as well as to study the Fermi surface of single crystals very directly. Therefore, by switching between X-rays and ultraviolet-photons, it is possible to study the geometrical and electronic structure within the same experiment.

Electronic structure of the YH3 phase from angle-resolved photoemission spectroscopy.

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on angle-resolved photoemission experiments of the Y trihydride phase. Most importantly, we find the absence of metal d bands at the Fermi level and a set of flat, H-induced bands located at much higher binding energy than predicted, indicating an increased electron affinity at H sites.

Real space mapping of the surface atomic environment via low energy scattering spectroscopy

Abstract Low-energy ion scattering spectroscopy is used to obtain real space surface imaging of the atomic surroundings of different fcc metal surfaces. Scattered He + ions were mapped over a large solid angle sector. By using an universal shadow cone expression, it is possible to invert the angular maps into two-dimensional real space maps. The inversion procedure is tested on Pt(111), Cu(001) and Al(111) surfaces getting nearest neighbour atoms up to a distance of 4 A. Furthermore, for known surfaces the maps also allow to extract information on the scattering mechanism itself.

Hydrogen-induced changes in the surface states/resonances of Ni(110)

Abstract 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. Thereby, we investigated the surface states/resonances at the S point of the surface Brillouin zone, which had been observed previously by Eberhardt et al. [Phys. Rev. Lett. 45 (1980) 273]. In contrast to their measurements concerning hydrogen adsorption, we detect marked changes in these states, which are either quenched during adsorption or shifted in energy.

Substitution sites of Pb and Y in Bi2Sr2Ca1Cu2O8+δ: X-ray photoelectron diffraction as fingerprinting tool

The substitution site of Y and Pb in the cuprate-type high temperature superconductor Bi2Sr2Ca1Cu2O8+δ is determined in a very direct and unambiguous way by means of angle-scanned x-ray photoelectron diffraction (XPD). Using XPD as a fingerprinting tool, we conclude that Y occupies the Ca sites and Pb the Bi sites, respectively. Furthermore, low-energy electron diffraction data unequivocally show the presence of the incommensurate lattice modulation which is known for pure Bi2212, but not for sufficiently Pb doped Bi2212. We can, therefore, attribute the reappearance of the modulation directly to the Y doping.

NEW ASPECTS OF THE QC-PHASE IN LAYERED 1T-TAS2

Abstract We present Fermi surface (FS) mapping measurements obtained by photoemission for the quasi-commensurate charge density wave (CDW) phase of layered 1 T -TaS 2 . The topology of the FS is rather dominated by the undistorted (1×1) symmetry than by the CDW-induced ( 13 × 13 ) phase. CDW effects can be seen in terms of nesting properties of the FS. Compared to one-electron band structure calculations we observe changed FS contours of Ta 5 d -derived electron pockets around the M , M ′ points of the (1×1) Brillouin zone. Low energy electron diffraction measurements support the view of a quasi-commensurate CDW.

ANGLE-SCANNED PHOTOEMISSION ON YbHx: RELEVANCE FOR SWITCHABLE MIRRORS

Yttrium, lantanum and rare earth elements can be loaded with hydrogen inducing a metal–insulator transition and giving rise to optical switching from reflecting to transparent. We present angle-scanned photoemission experiments characterizing thin YbHx films grown on W(110) at room temperature. Hydrogen loading is performed in an ultrahigh-vacuum-compatible high pressure (1 bar) reaction cell. Via full-hemispherical X-ray photoelectron diffraction and low energy electron diffraction, it is demonstrated that these films grow well-ordered and single-crystalline. Ultraviolet photoemission reveals a gap for the dihydride phase confirming a transition from reflecting to transparent as seen by visual inspection. Ion implantation through additional H+ sputtering allows one to increase the hydrogen content to x ≈ 2.4.