Th. Pillo

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.

On the search for Fermi surface nesting in quasi-2D materials

Abstract 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 -TaS 2 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 -TiSe 2 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.

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.

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.