X-ray photoelectron diffraction is used to directly probe the intra-cell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio calculations, unambiguously demonstrate non-centro-symmetry in films a few unit cells thick, imply that films as thin as 3 unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface.

The origin of the monoclinic distortion and domain formation in the quasi two-dimensional layer compound NbTe2 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 bandstructure 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.

We report room temperature angle-resolved photoemission experiments on 1T-TaS 2 and 1T-TaSe 2 complemented by density-functional theory calculations. Fermi-surface mapping experiments in the charge-density wave ͑CDW͒ phase are similar for the two compounds and do not show symmetries due to the CDW-induced new Brillouin zones. However, the band structure a few eV below the Fermi level (E F) displays a clear modulation that we relate, in both cases, to the CDW. At E F , the spectral weight distribution reflects the band structure of the normal state, but no clear quasiparticle crossing is located. Near the zone center ⌫ , CDW-split quasilocalized Ta dz 2 subbands are observed in the vicinity of E F. For 1T-TaS 2 and 1T-TaSe 2 , they are thermally populated slightly above E F and close to E F , respectively. The observed behavior can be understood in terms of the CDW reconstructed, spectral function weighted band structure.

The electronic structure and the Fermi surface of two-dimensional rare-earth silicides epitaxially grown on Si͑111͒, YSi 2 and GdSi 2 , have been studied by a combination of angle-resolved ultraviolet photoemission spectroscopy and density functional theory calculations. Both silicides present a very similar electronic structure, with two characteristic electronic bands below the Fermi energy. One crosses the Fermi energy near the ⌫ point of the surface Brillouin zone ͑hole pocket͒ and the other one close to the M point ͑electron pocket͒. These two bands arise from surface ͑localized͒ states and are responsible for all the Fermi surface features. The theoretical calculations are in good qualitative agreement with the experimental results, and also allow to examine the nature of the bonding between the rare earth and the neighboring silicon atoms. We have found a combination of sp metallic type bond together with covalent bonds involving the rare-earth d states and Si 3p states.

We report high-resolution angle-resolved photoemission experiments on epitaxial thin films of different rare-earth (RE) dihydrides ͑RE= Gd, La͒ and of YH 2 and ScH 2 . It is found through ab initio calculations and confirmed by Fermi surface mapping that the electronic structure becomes very similar upon hydrogenation, rendering the studied dihydrides isoelectronic. We propose that the dihydride phase acts as a common precursor state for the formation of the insulating trihydride phase. For states with higher binding energies (which exhibit considerable H character) the agreement between calculation and measurement is less convincing. Independent of the difficulties to describe these hydrogen related states, we note in the comparison between experiment and calculation a very convincing description of the Fermi surface for the dihydrides. Therefore we trace the apparent inability of density, functional theory to describe the hygrogenation up to the trihydride phase to an insufficient description of hydrogen states in general and, in particular, involving octahedral sites.