Christian Koitzsch

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-induced lattice distortion in NbTe2

The origin of the monoclinic distortion and domain formation in the quasi-two-dimensional layer compound

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

\mathrm{Nb}{\mathrm{Te}}_{2}

Spin–orbit splitting in the valence bands of 1T-TaS2 and 1T-TaSe2

is investigated. Angle-resolved photoemission shows that the Fermi surface is pseudogapped over large portions of the Brillouin zone. Ab initio calculation of the electron and phonon band structure as well as the static RPA susceptibility lead us to conclude that Fermi surface nesting and electron-phonon coupling play a key role in the lowering of the crystal symmetry and in the formation of the charge density wave phase.

ANGLE-SCANNED PHOTOEMISSION ON YbHx: RELEVANCE FOR SWITCHABLE MIRRORS

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.

Fermi-surface topology of rare-earth dihydrides

We perform angle-resolved photoemission spectroscopy on 1T-TaS2 and 1T-TaSe2 using synchrotron radiation. We observe a characteristic splitting of the chalcogen p-derived valence bands along high symmetry directions. Density functional theory calculation and group theory strongly suggest that this splitting is due to spin–orbit interaction along one direction, and to symmetry along the other direction. We note that, according to the Kramers degeneracy, the spin–orbit interaction leaves every state doubly degenerate. Furthermore, this study allows us to identify a mixing between bands with Ta 5d and Se 4p character, possibly relevant for the different temperature behaviours of the two compounds.

Pseudogapped Fermi surfaces of 1T-TaS2 and 1T-TaSe2: A charge density wave effect

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.