Observation and Structure Determination of an Oxide Quasicrystal Approximant

We report on the first observation of an approximant structure to the recently discovered two-dimensional oxide quasicrystal. Using scanning tunneling microscopy, low-energy electron diffraction, and surface x-ray diffraction in combination with ab initio calculations, the atomic structure and the bonding scheme are determined. The oxide approximant follows a 3^{2}.4.3.4 Archimedean tiling. Ti atoms reside at the corners of each tiling element and are threefold coordinated to oxygen atoms. Ba atoms separate the TiO_{3} clusters, leading to a fundamental edge length of the tiling 6.7 Å.

Tuning the Dirac point position in Bi(2)Se(3)(0001) via surface carbon doping

Angular resolved photoemission spectroscopy in combination with ab initio calculations show that trace amounts of carbon doping of the Bi_{2}Se_{3} surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of ≈11% as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.

Tuning the structure of ultrathin BaTiO3 films on Me(001) (Me=Fe, Pd, Pt) surfaces

Using surface x-ray diffraction in combination with ab initio calculations, we demonstrate that the atomic structure of ultrathin BaTiO3 (BTO) films grown on Me(001) surfaces (Me=Fe, Pd, Pt) depends on subtle modifications of the interface chemical composition. A complete reversal of the surface termination from a BaO- [BTO on Fe(001)] to a TiO2-terminated film [BTO on Pt(001)] is observed which goes in parallel with the adsorption of submonolayer amounts of oxygen at metal hollow sites of the interface. Our results may suggest a new route to an overall control of both the surface and the interface geometry in BaTiO3/metal contacts.

Misfit-induced modification of structure and magnetism in O/Fe(001)-p(1×1)

The geometry of oxygen atoms in hollow sites of Fe nanoislands (⊘≈1-2  nm) on Fe(001) is modified by mesoscopic misfit-induced relaxations of the island atoms. Surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations indicate a 0.3 Å increased adsorption height [0.7 Å versus 0.4 Å in O/Fe(001)-p(1×1)] of O atoms going in parallel with a reduced Fe-Fe layer spacing inducing a reduction of the surface magnetic moment (2.85μ(B) versus 3.2μ(B)). Our results demonstrate the importance of the mesoscopic misfit for surface physical properties in general.

BaTiO3(001)-(2×1): surface structure and spin density

Using surface x-ray diffraction and ab initio calculations we present a model of the BaTiO3(001)-(2×1) surface structure, which has not been considered so far. While the crystal is terminated by two TiO2 layers similarly to SrTiO3(001)-(2×1), we find that one out of two surface layer Ti-atoms resides in a tetragonal pyramidal oxygen environment. This peculiar geometry leads to a metallic and magnetic surface involving local magnetic moments up to 2μ(B) in magnitude located at surface Ti and O atoms. Our results are important for the understanding of the intrinsic surface metallicity of insulating oxides in general.

Structural secrets of multiferroic interfaces

We present an experimental and theoretical study of the geometric structure of ultrathin BaTiO(3) films grown on Fe(001). Surface x-ray diffraction reveals that the films are terminated by a BaO layer, while the TiO(2) layer is next to the top Fe layer. Cations in termination layers have incomplete oxygen shells inducing strong vertical relaxations. Onset of polarization is observed at a minimum thickness of two unit cells. Our findings are supported by first-principles calculations providing a quantitative insight into the multiferroic properties on the atomic scale.

Wurtzite-type CoO nanocrystals in ultrathin ZnCoO films

Surface x-ray diffraction experiments reveal that, in cobalt-doped ZnO films two to five monolayers thick, Wurtzite-type CoO nanocrystals are coherently embedded within a hexagonal boron-nitride- (h-BN)-type ZnO matrix, supporting the model of a phase separation. First-principles calculations confirm that, in contrast with ZnO, the formation of h-BN-type CoO is unfavorable in the ultrathin film limit. Our results are important for understanding magnetic properties of transition metal-doped semiconductors in general.