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.

Study of the negative magneto-resistance of single proton-implanted lithium-doped ZnO microwires.

The magneto-transport properties of single proton-implanted ZnO and of Li(7%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7%) doped ZnO microwires showed a non-monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non-magnetic heterostructure in the wire after proton implantation.

Magnetic effects of defect pair formation in ZnO.

In order to gain insight into the so-called d(0)-magnetic properties of defective ZnO we have carried out first principles calculations on various types of defects formed by intrinsic defects and doped atoms as well as pairs of them. The doped atoms include N and H. In agreement with previous works we find several possibilities to create magnetic defects especially by hole formation. Our results also show that two defects which are in the vicinity of each other and that are magnetic when isolated, in general become non-magnetic if one of them is acceptor-like and the other one donor-like. Furthermore, we have investigated the magnetic interaction of different defect pairs via total energy calculations, the results of which show in all cases the stability of ferromagnetic configurations. In order to reproduce the experimentally found localization of the magnetic hole states we have investigated the effect of applying correlation corrections on the p orbitals containing these holes.