Philipp Aebi

Evidence for an excitonic insulator phase in 1T-TiSe2.

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2.

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes.

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H(2) activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO{sub 3} films. Our measurements, combined with ab initio calculations, unambiguously demonstrate noncentrosymmetry in films a few unit cells thick, imply that films as thin as three unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface.

Fermi surface mapping with photoelectrons at UV energies

Abstract While photoelectron spectroscopy has been demonstrated to be useful for the determination of Fermi surfaces of two-dimensional systems we discuss the possibility to characterize a fully three-dimensional metal in this manner. Angular distributions of valence photoelectrons at UV energies integrated over a small energy window at the Fermi energy are measured over a large part of the 2π hemisphere for the case of Cu(001), Cu(111) and Cu(110). They are compared to the model of direct transitions from bulk bands crossing the Fermi energy to empty bands. The observed patterns show the proper translational symmetry with respect to the Brillouin zone (BZ). We conclude that the measurements represent a well defined cut through the Fermi surface.

Spontaneous exciton condensation in 1T-TiSe2: BCS-like approach

Recently we found strong evidence in favor of a BCS-like condensation of excitons in 1T-TiSe2 [Cercellier , Phys. Rev. Lett. 99, 146403 (2007)]. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the exciton condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. The scope of this paper is to present the detailed calculations in the framework of this model. They represent an extension of the original excitonic insulator phase model of Jerome [Phys. Rev. 158, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and comparison with experiment is presented. Finally, the disagreement with density-functional theory is discussed.

Angle-scanned photoelectron diffraction

A brief survey is given on the current state-of-the-art of this surface structural technique based on photoelectron spectroscopy, with particular emphasis on the progress that has been made recently by routinely measuring full-hemispherical intensity distributions. We limit the discussion to the photoelectron forward focusing regime, which is attained at electron kinetic energies of a few hundred eV. Surface bond directions are directly revealed as pronounced maxima in the angular distributions from subsurface atoms, while characteristic interference features are observed for surface species. For both cases the dependence on the atomic type is weak enough so that these features provide a fingerprint of the local bonding geometry. For surface and near-surface species, this may then serve as a starting point for a structure refinement using single-scattering cluster calculations. Selected examples are given for illustrating these procedures.

Exciton condensation driving the periodic lattice distortion of 1T-TiSe2.

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase.

Photoelectron emission from the negative electron affinity caesiated natural diamond (100) surface

Abstract The unique property of negative electron affinity (NEA) makes diamond surfaces very interesting for technical applications like cold cathodes and and field emitters. We investigated the hydrogen-terminated and hydrogen-free, caesiated diamond (100) surfaces by means of low-energy electron diffraction, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The photoemission measurements permit us to determine the electron affinity and to draw the band diagrams for these differently terminated surfaces. The NEA peak reappears after caesiation of the hydrogen-free surface, whereas it vanishes after exposure to molecular oxygen. From the shift of the C ls core level, an increasing downward band-bending of 1.2 eV was inferred following caesiation of the hydrogen-free surface and 0.6 eV following caesiation of the hydrogen-terminated surface.

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.

Temperature-induced phase transition observed at a fivefold icosahedral quasicrystalline AlPdMn surface

A switch in symmetry from fivefold to tenfold has been observed at the surface of a monograin icosahedral AlPdMn quasicrystal cut perpendicularly to a fivefold axis as a function of annealing temperature and/or annealing time at a constant temperature. A continuous change in stoichiometry was simultaneously monitored by X-ray photoelectron spectroscopy. The surface structure study was performed using full-hemispherical X-ray photoelectron diffraction and low-energy electron diffraction.